View Filing Data

151,907

152,041

(134)

(5,768)

Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax

156

FSIRS amounts reclassified to net income, net of tax

2,075

Dividends declared

(86,231)

Ending balance (in shares) at Dec. 31, 2016

47,482,000

Ending balance at Dec. 31, 2016

1,661,273

1,667,880

$ 49,112

903,123

897,346

(48,008)

(45,639)

759,263

767,061

(2,217)

Beginning balance at Dec. 31, 2015

16,108

Increase (Decrease) in Temporary Equity [Roll Forward]

1,148

5,768

Centuri distribution to redeemable noncontrolling interest

(439)

Ending balance at Dec. 31, 2016

22,590

Increase (Decrease) in Stockholders' Equity [Roll Forward]

Common stock issuances (in shares)

608,000

Common stock issuances

52,817

$ 608

52,209

193,693

156,818

193,841

156,818

(148)

(355)

Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax

1,760

(3,507)

FSIRS amounts reclassified to net income, net of tax

2,073

Stock-based compensation

^[1]

9,278

10,062

(784)

Dividends declared

$ (95,351)

(81,129)

(95,351)

(81,129)

Contributions from Southwest Gas Holdings, Inc.

41,359

Ending balance (in shares) at Dec. 31, 2017

48,090,470

48,090,000

47,482,000

Ending balance at Dec. 31, 2017

$ 1,812,403

1,609,999

$ 49,720

$ 49,112

955,332

948,767

(47,682)

(47,073)

857,398

659,193

(2,365)

Increase (Decrease) in Temporary Equity [Roll Forward]

248

355

Redemption of Centuri shares from noncontrolling parties

(23,000)

Centuri distribution to redeemable noncontrolling interest

(204)

2,073

Ending balance at Dec. 31, 2017

Increase (Decrease) in Stockholders' Equity [Roll Forward]

Common stock issuances (in shares)

4,936,000

Common stock issuances

358,083

$ 4,936

353,147

181,480

138,842

182,277

138,842

(797)

Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax

(3,010)

4,783

FSIRS amounts reclassified to net income, net of tax

2,541

Reclassification of excess deferred taxes

(9,300)

^[2]

(9,300)

^[3]

9,300

^[2]

9,300

^[3]

Change in ownership of noncontrolling interest

^[4]

(2,710)

2,710

Stock-based compensation

^[1]

2,246

2,926

(680)

Dividends declared

$ (104,690)

(89,500)

(104,690)

(89,500)

Contributions from Southwest Gas Holdings, Inc.

113,549

Ending balance (in shares) at Dec. 31, 2018

53,026,848

53,026,000

47,482,000

Ending balance at Dec. 31, 2018

$ 2,251,590

1,782,460

$ 54,656

$ 49,112

$ 1,305,769

$ 1,065,242

$ (52,668)

$ (49,049)

$ 944,285

717,155

$ (452)

Increase (Decrease) in Temporary Equity [Roll Forward]

Redeemable noncontrolling interest acquired

^[5]

81,659

Centuri distribution to redeemable noncontrolling interest

172

$ (182,773)

Ending balance at Dec. 31, 2018

$ 81,831

[1]	Stock-based compensation is based on stock awards of Southwest Gas Corporation to be issued in shares of Southwest Gas Holdings, Inc.
[2]	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, which permitted such release. See Note 1 - Background, Organization, and Summary of Significant Accounting Policies.
[3]	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, which permitted such release.
[4]	Centuri, through its subsidiary, NPL, had historically held a 65% ownership interest in Intellichoice Energy, LLC (“ICE”). A residual interest of 35% has been held by a third party. During the second quarter of 2018, an additional $1 million of capital was contributed by NPL, thereby increasing NPL’s ownership interest to 95%. The carrying amount of the noncontrolling interest has been adjusted with a corresponding charge to Additional paid-in capital on the Company’s Consolidated Balance Sheet.
[5]	The Company, through its subsidiary, Centuri, completed the acquisition of a privately held utility infrastructure services business. Refer to Note 19 - Business Acquisitions.

CONSOLIDATED STATEMENTS OF EQUITY AND REDEEMABLE NONCONTROLLING INTEREST (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Dividends declared (in USD pre share)	$ 2.08	$ 1.98	$ 1.80
Southwest Gas Corporation
Dividends declared (in USD pre share)			$ 1.80
IntelliChoice Energy LLC \| Centuri
Ownership percentage by parent	95.00%

Background, Organization and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Background, Organization and Summary of Significant Accounting Policies

Note 1 - Background, Organization, and Summary of Significant Accounting Policies

Nature of Operations. This is a combined annual report of Southwest Gas Holdings, Inc. and subsidiaries (the “Company”) and Southwest Gas Corporation and its subsidiaries (“Southwest” or the “natural gas operations segment”). The Notes to the Consolidated Financial Statements apply to both entities. Southwest Gas Holdings, Inc. is a holding company, owning all of the shares of common stock of Southwest and all of the shares of common stock of Centuri Construction Group, Inc. (“Centuri” or the “utility infrastructure services” segment). Prior to August 2017, 96.6% of Centuri’s shares were owned by the Company. During August 2017, the Company acquired the remaining 3.4% equity interest in Centuri that was held by the previous owners. Refer to Note 17 - Utility Infrastructure Services Noncontrolling Interests for additional information.

Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. The timing and amount of rate relief can materially impact results of operations. Natural gas purchases and the timing of related recoveries can materially impact liquidity. Results for the natural gas operations segment are higher during winter periods due to the seasonality incorporated in its regulatory rate structures.

Centuri is a comprehensive utility infrastructure services enterprise dedicated to delivering a diverse array of solutions to North America’s gas and electric providers. Centuri derives revenue from installation, replacement, repair, and maintenance of energy distribution systems, and developing industrial construction solutions. Centuri operations are generally conducted under the business names of NPL Construction Co. (“NPL”), Canyon Pipeline Construction, Inc. (“Canyon”), NPL Canada Ltd. (“NPL Canada”), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Canyon Special Projects, Inc. (“Special Projects,” formerly Brigadier Pipelines Inc.). Utility infrastructure services activity is seasonal in most of Centuri’s operating areas. Peak periods are the summer and fall months in colder climate areas, such as the northeastern and midwestern United States (“U.S.”) and in Canada. In warmer climate areas, such as the southwestern and southeastern U.S., utility infrastructure services activity continues year round. Centuri acquired New England Utility Constructors, Inc. (“Neuco”) in November 2017, thereby expanding its core services in the northeast region of the U.S. Additionally, in November 2018, Centuri expanded its operations in the southeast region of the U.S. through the acquisition of an 80% interest in a privately held utility infrastructure services business, Linetec Services, LLC (“Linetec”). See Note 19 - Business Acquisitions for more information.

Basis of Presentation. The Company follows accounting principles generally accepted in the United States (“U.S. GAAP”) in accounting for all of its businesses. Unless specified otherwise, all amounts are in U.S. dollars. Accounting for natural gas utility operations conforms with U.S. GAAP as applied to rate-regulated companies and as prescribed by federal agencies and commissions of the various states in which the utility operates. The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In connection with a holding company reorganization in January 2017, Centuri ceased to be a subsidiary of Southwest and became a subsidiary of the Company. To give effect to this change, the separate consolidated financial statements related to Southwest, which are included in this annual report, depict Centuri-related amounts for periods prior to January 2017 as discontinued operations which are detailed in Note 18 - Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation.

Consolidation. The accompanying financial statements are presented on a consolidated basis for Southwest Gas Holdings, Inc. and all subsidiaries and Southwest Gas Corporation and all subsidiaries as of December 31, 2018 (except those accounted for using the equity method as discussed further below). All significant intercompany balances and transactions have been eliminated with the exception of transactions between Southwest and Centuri in accordance with accounting treatment for rate-regulated entities.

Centuri, through its subsidiaries, historically held a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning technology and products. During the second quarter of 2018, an additional $1 million of capital was contributed, thereby increasing Centuri’s ownership to 95%. The carrying amount of the noncontrolling interest has been adjusted with a corresponding charge to Additional paid-in capital on the Company’s Consolidated Balance Sheet.

Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian infrastructure services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The investment in Western totaled $11.2 million and $12.7 million at December 31, 2018 and 2017, respectively. Both periods include the impacts of foreign currency exchange translation adjustments. Centuri received $1 million in dividends from Western for the year ended December 31, 2018 and no dividends for the year ended December 31, 2017.

The equity method investment in Western is included in Other property and investments in the Consolidated Balance Sheets of the Company. Centuri’s maximum exposure to loss as a result of its involvement with Western was estimated at $44.9 million as of December 31, 2018. The estimated maximum exposure to loss represents the maximum loss that would be absorbed by Centuri in the event that all of the assets of Western were deemed to be worthless. Centuri recorded earnings of approximately $500,000 from this investment in 2018, which is included in Other income (deductions) in the Consolidated Statements of Income.

Fair Value Measurements. Certain assets and liabilities are reported at fair value, which is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.

U.S. GAAP states that a fair value measurement should be based on the assumptions that market participants would use in pricing the asset or liability and establishes a fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to fair values derived from unobservable inputs (Level 3 measurements). Financial assets and liabilities are categorized in their entirety based on the lowest level of input that is significant to the fair value measurement. The three levels of the fair value hierarchy are as follows:

Level 1 – quoted prices (unadjusted) in active markets for identical assets or liabilities that a company has the ability to access at the measurement date.

Level 2 – inputs other than quoted prices included within Level 1 that are observable for similar assets or liabilities, either directly or indirectly.

Level 3 – unobservable inputs for the asset or liability. Unobservable inputs are used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at the measurement date.

The Company primarily used quoted market prices and other observable market pricing information in valuing cash and cash equivalents, derivatives, long-term debt outstanding, and assets of the qualified pension plan and the PBOP required to be disclosed at fair value.

Net Utility Plant. Net utility plant includes gas plant at original cost, less the accumulated provision for depreciation and amortization, plus the unamortized balance of acquisition adjustments. Original cost includes contracted services, material, payroll, and related costs such as taxes and certain benefits, general and administrative expenses, and an allowance for funds used during construction, less contributions in aid of construction.

Other Property and Investments. Other property and investments on Southwest’s and the Company’s Consolidated Balance Sheets includes (thousands of dollars):



	2018			2017
Net cash surrender value of COLI policies	$	114,405		$	117,341
Other property	1,741			1,773
Total Southwest Gas Corporation	116,146			119,114
Centuri property, equipment, and intangibles	792,191			554,730
Centuri accumulated provision for depreciation and amortization	(298,939		)	(258,906		)
Other property	14,153			13,242
Total Southwest Gas Holdings, Inc.	$	623,551		$	428,180

Intangible Assets. Intangible assets (other than goodwill) are amortized using the straight-line method to reflect the pattern of economic benefits consumed over the estimated periods benefited. The recoverability of intangible assets is evaluated when events or circumstances indicate that a revision of estimated useful lives is warranted or that an intangible asset may be impaired. Non-utility intangible assets are associated with utility infrastructure services businesses acquired through 2018, including the Linetec acquisition. All have finite lives. These intangible assets are included in Other property and investments on the Company’s Consolidated Balance Sheets. Centuri has $159.8 million and $80.7 million of intangible assets at December 31, 2018 and 2017, respectively, as detailed in the following table (thousands of dollars):



December 31, 2018	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Customer relationships	$	152,533	$	(11,716	)	$	140,817
Trade names and trademarks	23,013		(5,234		)	17,779
Customer contracts backlog	270		(3		)	267
Noncompete agreements	2,022		(1,064		)	958
Total	$	177,838	$	(18,017	)	$	159,821

December 31, 2017
Customer relationships	$	76,254	$	(6,743	)	$	69,511
Trade names and trademarks	13,754		(4,080		)	9,674
Noncompete agreements	2,060		(543		)	1,517
Total	$	92,068	$	(11,366	)	$	80,702

Amortization expense for the acquired intangible assets listed above for the years ended December 31, 2018, 2017, and 2016 was $7.6 million, $4.1 million, and $3.2 million, respectively.

The estimated future amortization of the intangible assets for the next five years and thereafter is as follows (in thousands):



2019	$	10,622
2020	10,634
2021	10,214
2022	10,127
2023	10,127
Thereafter	108,097
Total	$	159,821

See Note 2 - Utility Plant and Leases for additional information regarding natural gas operations intangible assets. Note 19 - Business Acquisitions includes detailed information about intangible assets purchased in the Linetec acquisition.

Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with maturities of three months or less. Such investments are carried at cost, which approximates market value. Cash and cash equivalents for Southwest and the Company also include money market fund investments totaling approximately $18 million and $59.9 million, respectively at December 31, 2018, and $20.8 million and $22.2 million, respectively, at December 31, 2017, which fall within Level 2 of the fair value hierarchy, due to the asset valuation methods used by money market funds.

Typical non-cash investing activities for Southwest include customer advances applied as contributions toward utility construction activity and capital expenditures that were not paid as of year end that are included in accounts payable. Amounts related to such activities were immaterial for the periods presented herein. Non-cash investing activities for Centuri included $75.6 million of purchase consideration related to the Linetec acquisition in the form of liabilities incurred that remained unpaid as of December 31, 2018; such amounts are included in Other current liabilities on the Consolidated Balance Sheets of the Company. See Note 19 - Business Acquisitions.

Income Taxes. The asset and liability method of accounting is utilized for the recognition of income taxes. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. For regulatory and financial reporting purposes, investment tax credits (“ITC”) related to gas utility operations are deferred and amortized over the life of related fixed assets. As of December 31, 2018, the Company had cumulative book earnings of approximately $25 million in its foreign jurisdiction. Management previously asserted and continues to assert that all the earnings of Centuri’s Canadian subsidiaries will be permanently reinvested in Canada. As a result, no U.S. deferred income taxes have been recorded related to cumulative foreign earnings.

In 2017, the Tax Cuts and Jobs Act (the “TCJA”) was enacted. The TCJA had significant impacts on the taxation of business entities, including specific provisions related to regulated public utilities. The more significant changes that impacted the Company include the reduction in the corporate federal income tax rate from 35% to 21%, and limiting the utilization of net operating losses (“NOLs”) to 80% of taxable income, with the ability to indefinitely carryforward unutilized NOLs to reduce future taxable income.

The Financial Accounting Standards Board (the “FASB”) issued guidance to allow an accounting policy election of either (i) treating taxes attributable to future taxable income related to Global Intangible Low-Taxed Income (“GILTI”) as a current period expense when incurred or (ii) recognizing deferred taxes for temporary differences expected to reverse as GILTI in future years. The Company has elected to treat GILTI as a current period cost when incurred and has considered the estimated 2018 GILTI impact in its 2018 tax expense.

Deferred Purchased Gas Costs. The various regulatory commissions have established procedures to enable Southwest to adjust its billing rates for changes in the cost of natural gas purchased. The difference between the current cost of gas purchased and the cost of gas recovered in billed rates is deferred. Generally, these deferred amounts are recovered or refunded within one year.

Prepaid and other current assets. Prepaid and other current assets for Southwest and the Company include gas pipe materials and operating supplies of $56 million in 2018 and $33 million in 2017 (carried at weighted average cost), and also include $74 million in 2018 and $40 million in 2017 related to a regulatory asset associated with the Arizona decoupling mechanism (an alternative revenue program). In the 2017 Arizona general rate case decision, the decoupled rate design was approved to continue, excluding a winter-period adjustment to rates, making the mechanism fundamentally similar to that which exists in Nevada. This change from a combination of monthly winter-period adjustments to bills (coupled with an annual rate adjustment) to an annual rate adjustment resulted in an increase in the associated regulatory asset noted above.

Goodwill. As required by U.S. GAAP, goodwill is assessed for impairment annually, or more frequently, if circumstances indicate impairment to the carrying value of goodwill may have occurred. The goodwill impairment analysis is conducted as of October 1st each year and may start with an assessment of qualitative factors (commonly referred to as Step 0) to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. If, after assessing the qualitative factors, management determines that it is more likely than not that the fair value of a reporting unit is less than its carrying amount, or if management does not perform a qualitative assessment, a Step 1 impairment test will be performed. Management considered the qualitative factors and the evidence obtained and determined that it is not more likely than not that the fair value of the reporting units are less than their carrying amounts in either 2017 or 2018. Thus, no impairment was recorded in either year. The Linetec acquisition in 2018 (see further discussion in Note 19 - Business Acquisitions) was considered an asset purchase for tax purposes. As a result, goodwill associated with Linetec is expected to be deductible for those same purposes.



	Natural Gas Operations		Utility Infrastructure Services			Total Company
(In thousands of dollars)
Balance, December 31, 2016	$	10,095	$	129,888		$	139,983
Goodwill from Neuco acquisition	—		32,028			32,028
Foreign currency translation adjustment	—		7,303			7,303
Balance, December 31, 2017	10,095		169,219			179,314
Additional goodwill from Neuco acquisition	—		182			182
Goodwill from Linetec acquisition	—		188,494			188,494
Foreign currency translation adjustment	—		(8,945		)	(8,945		)
Balance, December 31, 2018	$	10,095	$	348,950		$	359,045

Other Current Liabilities. Management recognizes in its balance sheets various liabilities that are expected to be settled through future cash payment within the next twelve months, including certain regulatory liabilities (refer to Note 5 - Regulatory Assets and Liabilities), customary accrued expenses for employee compensation and benefits, and declared but unpaid dividends. As of December 31, 2018, this caption on the Company’s Consolidated Balance Sheet also included $75.6 million in unremitted amounts associated with the Linetec acquisition note above.

Accumulated Removal Costs. Approved regulatory practices allow Southwest to include in depreciation expense a component intended to recover removal costs associated with utility plant retirements. In accordance with the Securities and Exchange Commission (“SEC”) position on presentation of these amounts, management reclassifies estimated removal costs from accumulated depreciation to accumulated removal costs within the liabilities section of the Consolidated Balance Sheets. Management regularly updates the estimated accumulated removal costs as amounts fluctuate between periods depending on the level of replacement work performed, the estimated cost of removal in rates, and the actual cost of removal experienced.

Gas Operating Revenues. Southwest recognizes revenue when it satisfies its performance by transferring gas to the customer. Natural gas is delivered and “consumed” by the customer simultaneously. Revenues are recorded when customers are billed. Customer billings are substantially based on monthly meter reads and include certain other charges assessed monthly, and are calculated in accordance with applicable tariffs and state and local laws, regulations, and related agreements. An estimate of the margin associated with natural gas service provided, but not yet billed, to residential and commercial customers from the latest meter read date to the end of the reporting period is also recognized as accrued utility revenue. Revenues also include the net impacts of margin tracker/decoupling accruals based on criteria in U.S. GAAP for rate-regulated entities associated with alternative revenue programs. All of Southwest’s service territories have decoupled rate structures, which are designed to eliminate the direct link between volumetric sales and revenue, thereby mitigating the impacts of unusual weather variability and conservation on margin. See Note 3 - Revenue for additional information regarding Gas operating revenues.

Utility Infrastructure Services Revenues. The majority of Centuri contracts are performed under unit-price contracts. Generally, these contracts state prices per unit of installation. Typical installations are accomplished in a few weeks or less. Revenues are recorded as installations are completed. Revenues are recorded for long-term fixed-price contracts in a pattern that reflects the transfer of control of promised goods and services to the customer over time. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs. Changes in job performance, job conditions, and final contract settlements are factors that influence management’s assessment of total contract value and the total estimated costs to complete those contracts. Revisions in estimates of costs and earnings during the course of work are reflected in the accounting period in which the facts requiring revision become known. If a loss on a contract becomes known or is anticipated, the entire amount of the estimated ultimate loss is recognized at that time in the financial statements. Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model. See Note 3 - Revenue for additional information regarding Utility infrastructure services revenues.

Utility Infrastructure Services Expenses. Utility infrastructure services expenses in the Statements of Income includes payroll expenses, office and equipment rental costs, subcontractor expenses, training, job-related materials, gains and losses on equipment sales, and professional fees of Centuri.

Net Cost of Gas Sold. Components of net cost of gas sold include natural gas commodity costs (fixed-price and variable-rate), pipeline capacity/transportation costs, and actual settled costs of natural gas derivative instruments. Also included are the net impacts of purchased gas adjustment (“PGA”) deferrals and recoveries, which by their inclusion, result in net cost of gas sold overall that is comparable to amounts included in billed gas operating revenues. Differences between amounts incurred with suppliers, transmission pipelines, etc. and those already included in customer rates, are temporarily deferred in PGA accounts pending inclusion in customer rates.

Operations and Maintenance Expense. Operations and maintenance expense includes Southwest’s operating and maintenance costs associated with serving utility customers and maintaining its distribution and transmission systems, uncollectible expense, administrative and general salaries and expense, employee benefits expense excluding relevant non-service cost components (see Note 11 - Pension and Other Postretirement Benefits), and legal expense (including injuries and damages).

Depreciation and Amortization. Utility plant depreciation is computed on the straight-line remaining life method at composite rates considered sufficient to amortize costs over estimated service lives, including components which compensate for removal costs (net of salvage value), and retirements, as approved by the appropriate regulatory agency. When plant is retired from service, the original cost of plant, including cost of removal, less salvage, is charged to the accumulated provision for depreciation. Other regulatory assets, including acquisition adjustments, are amortized when appropriate, over time periods authorized by regulators. See also discussion regarding Accumulated Removal Costs above. Non-utility and utility infrastructure services-related property and equipment are depreciated on a straight-line method based on the estimated useful lives of the related assets. Costs and gains related to refunding utility debt and debt issuance expenses are deferred and amortized over the weighted-average lives of the new issues and become a component of interest expense.

Allowance for Funds Used During Construction (“AFUDC”). AFUDC represents the cost of both debt and equity funds used to finance utility construction. AFUDC is capitalized as part of the cost of utility plant. The debt portion of AFUDC is reported in the Company’s and Southwest’s Consolidated Statements of Income as an offset to Net interest deductions and the equity portion is reported as Other income. Utility plant construction costs, including AFUDC, are recovered in authorized rates through depreciation when completed projects are placed into operation, and general rate relief is requested and granted.



	2018			2017			2016
(In thousands)
AFUDC:
Debt portion	$	3,264		$	1,666		$	1,175
Equity portion	3,627			2,296			2,289
AFUDC capitalized as part of utility plant	$	6,891		$	3,962		$	3,464
AFUDC rate	5.85		%	5.95		%	7.35		%

Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the Consolidated Statements of Income (thousands of dollars):



	2018			2017			2016
Southwest Gas Corporation – natural gas operations segment:
Increase (decrease) in COLI policies	$	(3,200	)	$	10,300		$	7,400
Interest income	6,020			2,784			1,848
Equity AFUDC	3,627			2,296			2,289
Non-service post-retirement benefit cost	(21,059		)	(19,424		)	(19,760		)
Miscellaneous income and (expense)	(2,628		)	(2,344		)	(3,261		)
Southwest Gas Corporation – total other income (deductions)	(17,240		)	(6,388		)	(11,484		)
Utility infrastructure services segment:
Interest income	88			3			1
Foreign transaction gain (loss)	(222		)	(754		)	(22		)
Equity in earnings of unconsolidated investment – Western	531			1,052			69
Miscellaneous income and (expense)	(635		)	44			1,145
Centuri – total other income (deductions)	(238		)	345			1,193
Corporate and administrative	52			13			—
Consolidated Southwest Gas Holdings, Inc. - total other income (deductions)	$	(17,426	)	$	(6,030	)	$	(10,291	)

Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by the Company and Southwest to indemnify against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Current tax regulations provide for tax-free treatment of life insurance (death benefit) proceeds. Therefore, changes in the cash surrender value components of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences.

Foreign Currency Translation. Foreign currency-denominated assets and liabilities of consolidated subsidiaries are translated into U.S. dollars at exchange rates existing at the respective balance sheet dates. Translation adjustments resulting from fluctuations in exchange rates are recorded as a separate component of accumulated other comprehensive income within stockholders’ equity. Results of operations of foreign subsidiaries are translated using the monthly weighted-average exchange rates during the respective periods. Gains and losses resulting from foreign currency transactions are included in Other income (expense) of the Company. Gains and losses resulting from intercompany foreign currency transactions that are of a long-term investment nature are reported in Other comprehensive income, if applicable.

Earnings Per Share. Basic earnings per share (“EPS”) in each period of this report were calculated by dividing net income attributable to Southwest Gas Holdings, Inc. by the weighted-average number of shares during those periods. Diluted EPS includes additional weighted-average common stock equivalents (stock options, performance shares, and restricted stock units). Unless otherwise noted, the term “Earnings Per Share” refers to Basic EPS. A reconciliation of the denominator used in the Basic and Diluted EPS calculations is shown in the following table.



	2018	2017	2016
(In thousands)
Average basic shares	49,419	47,965	47,469
Effect of dilutive securities:
Stock options	—	—	1
Management Incentive Plan shares	25	8	124
Restricted stock units (1)	32	18	220
Average diluted shares	49,476	47,991	47,814


(1)	The number of securities granted for 2018 and 2017 includes 23,000 and 7,000 performance shares, respectively, the total of which was derived by assuming that target performance will be achieved during the relevant performance period.

Recent Accounting Standards Updates.

Accounting pronouncements adopted in 2018:

Effective January 2018, the Company and Southwest adopted the FASB update, Accounting Standards Codification (“ASC”) Topic 606 “Revenue from Contracts with Customers.” The update replaced much of the prior guidance regarding revenue recognition including most industry-specific guidance. See Note 3 - Revenue for more information regarding the adoption of the update.

In January 2018, the FASB issued Accounting Standards Update (“ASU”) No. 2018-02 “Income Statement—Reporting Comprehensive Income (Topic 220)—Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income.” The update addressed issues resulting from the enactment of the TCJA. Stakeholders raised a narrow-scope financial reporting issue that arose as a consequence of the TCJA related to the fact that when deferred tax balances were remeasured in December 2017, those deferred tax balances were to be reduced, but related amounts historically accumulated in Accumulated Other Comprehensive Income (“AOCI”) prior to the enactment of the TCJA, were required to be recognized as income tax expense rather than being relieved from AOCI. The amendments in this update allowed a reclassification from AOCI to retained earnings for those otherwise “stranded” tax effects in AOCI following enactment of the TCJA. The Company and Southwest adopted this update effective January 2018. Accordingly, approximately $9.3 million of previously stranded tax effects resulting from the TCJA were reclassified to retained earnings from AOCI on the Consolidated Balance Sheets of Southwest and the Company during the first quarter of 2018.

In March 2017, the FASB issued ASU 2017-07 “Compensation – Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” The new guidance related to the income statement presentation of net periodic benefit cost for an entity’s sponsored defined benefit pension and other postretirement plans. The Company and Southwest adopted this update effective January 2018. See Note 11 - Pension and Other Postretirement Benefits for additional information.

Accounting pronouncements that will be effective in 2019:

In February 2016, the FASB issued the update “Leases (Topic 842).” Under the update, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date:


•	A lease liability for the obligation to make lease payments, measured on a discounted basis; and


•	A right-of-use asset for the right to use, or control the use of, a specified asset for the lease term.

Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, “Revenue from Contracts with Customers.” Though companies have historically been required to make disclosures regarding leases and associated contractual obligations, leases with terms longer than a year will no longer exist off-balance sheet. In July 2018, the FASB issued narrow-scope improvements to the standard, which include, among other things, guidance on lease classification reassessment and certain circumstances surrounding remeasurement. Also included was clarification that lessor-controlled options to terminate a lease are considered in the lease term.

Management expects to elect various practical expedients and accounting policies regarding the transition method used to implement Topic 842. The Company and Southwest plan to elect the new optional transition method included within the recent FASB update “Leases—Targeted Improvements”, also issued in July 2018, which allows for comparative periods not to be restated. In conjunction with this decision, management currently expects that no retained earnings adjustment will be necessary due to the adoption of Topic 842. At a minimum, management expects the following regarding Topic 842 practical expedients and accounting policy elections:

•

To elect to use the “package”, which is a set of three practical expedients that must be elected as a package and applied consistently to all of the Company’s and Southwest’s leases. These include: not reassessing whether any expired or existing contracts are or contain leases; not reassessing the lease classification for expired or existing leases (that is, existing operating and capital leases in accordance with current lease guidance will in each case be classified as operating and finance leases, respectively, under the updated guidance); and not reassessing initial direct costs for any existing leases.


•	To elect to adopt the practical expedient to exclude all easements in place prior to January 1, 2019 from treatment under Topic 842. However, the Company and Southwest will evaluate any new easements entered into after the effective date of the standard to determine if the arrangements should be accounted for as leases.


•	To make an accounting policy election by asset class to include both the lease and non-lease components (as defined in the guidance) as a single component.


•	To make an accounting policy election to not apply Topic 842 to short-term leases, as permitted.


•	To not elect to use hindsight in determining the lease term and in assessing impairment of right-of-use assets.

Southwest’s leases are comprised primarily of operating leases of buildings, land, and equipment. At this time, Southwest has no finance leases. Centuri leases buildings, land, fleet equipment, and other equipment. The majority of Centuri’s leases are operating leases. The Company and Southwest are currently not the lessor in any significant lease arrangements.

Management has implemented new software systems (one for Southwest and one for Centuri) to facilitate compliance with Topic 842. Management will adopt the update in the first quarter of 2019, as required. Management is finalizing its evaluation of the impact of the adoption on the consolidated financial statements and related disclosures.

Recently issued accounting pronouncements that will be effective after 2019:

In June 2016, the FASB issued ASU 2016-13 update “Financial Instruments — Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The update requires the measurement of all expected credit losses for financial assets held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. The inputs currently used to estimate credit losses will still be used, however they may be adapted to reflect the full amount of expected losses. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. All entities may adopt the amendments in this update earlier as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. Management is evaluating what impact, if any, this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures.

In January 2017, the FASB issued ASU 2017-04 “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” Currently, unless meeting the criteria for qualitative assessment only, an entity is required to perform a two-step test to determine the amount, if any, of goodwill impairment. In Step 1, an entity compares the fair value of a reporting unit with its carrying amount, including goodwill. If the carrying amount of the reporting unit exceeds its fair value, the entity performs Step 2 and compares the implied fair value of goodwill with the carrying amount of that goodwill for that reporting unit. An impairment charge equal to the amount by which the carrying amount of goodwill for the reporting unit exceeds the implied fair value of that goodwill is recorded, limited to the amount of goodwill allocated to that reporting unit. Under the update, an entity will apply a one-step quantitative test and record the amount of goodwill impairment as the excess of a reporting unit's carrying amount over its fair value, not to exceed the total amount of goodwill allocated to the reporting unit. The new guidance does not amend the optional qualitative assessment of goodwill impairment. The amount of any goodwill impairment calculated under the update could vary from the calculation under the existing guidance. The amendments should be applied on a prospective basis. The update is effective for fiscal and interim periods beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures.

In August 2018, the FASB issued ASU 2018-15 “Intangibles—Goodwill and Other—Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” The update generally aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement (that is a service contract) with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. The update also requires the entity to expense the capitalized implementation costs of such hosting arrangements over the term of the hosting arrangement, including reasonably certain renewal periods. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption of the amendments in this update is permitted for interim and related annual fiscal periods after December 15, 2018. Management is evaluating the impacts this update might have on the Company’s and Southwest’s consolidated financial statements and disclosures.

In August 2018, the FASB issued ASU 2018-14 “Compensation—Retirement Benefits—Defined Benefit Plans—General (Subtopic 715-20): Disclosure Framework—Changes to the Disclosure Requirements for Defined Benefit Plans.” This update removes disclosures that are no longer considered cost-beneficial, clarifies the specific requirements of disclosures, and adds disclosure requirements identified as relevant. The update applies to all employers that sponsor defined benefit pension or other post-retirement plans. The update is effective for fiscal years ending after December 15, 2020. Early adoption is permitted. Management is evaluating the impacts this update might have on its disclosures.

In August 2018, the FASB issued ASU 2018-13 “Fair Value Measurement: Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement.” The update is intended to improve the effectiveness of fair value measurement disclosures and removes the following disclosure requirements: the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy; the policy for timing of transfers between levels; and the valuation processes for Level 3 fair value measurements. The update also modifies or clarifies for investments in certain entities that calculate net asset value, a requirement to disclose the timing of liquidation of an investee’s assets and the date when restrictions from redemption might lapse (in cases when the timing has been communicated or announced publicly). It also clarifies communication requirements about measurement uncertainty as of the reporting date. For certain unobservable inputs, an entity may disclose other quantitative information in lieu of the weighted average if it would be a more reasonable and rational method to reflect the distribution of inputs to the measurements. Management is evaluating the impacts this update might have on its disclosures.

Subsequent Events. Management monitors events occurring after the balance sheet date and prior to the issuance of the financial statements to determine the impacts, if any, of events on the financial statements to be issued or disclosures to be made, and has reflected them where appropriate.

Utility Plant and Leases

12 Months Ended

Dec. 31, 2018

Revenue from Contract with Customer [Abstract]

Utility Plant and Leases

Note 2 - Utility Plant and Leases

Net utility plant as of December 31, 2018 and 2017 was as follows (thousands of dollars):



December 31,	2018			2017
Gas plant:
Storage	$	26,825		$	25,019
Transmission	386,159			363,396
Distribution	6,049,380			5,600,769
General	416,643			396,252
Software and software-related intangibles	241,158			230,030
Other	14,074			14,178
	7,134,239			6,629,644
Less: accumulated depreciation and amortization	(2,234,029		)	(2,231,242		)
Construction work in progress	193,028			125,248
Net utility plant	$	5,093,238		$	4,523,650

Utility plant depreciation is computed on the straight-line remaining life method at composite rates considered sufficient to amortize costs over estimated service lives, including components which are intended to compensate for removal costs (net of salvage value), and retirements, based on the processes of regulatory proceedings and related regulatory commission approvals and/or mandates. In 2018, annual utility depreciation and amortization expense averaged 2.7% of the original cost of depreciable and amortizable property. Average rates in 2017 and 2016 approximated 3.0%. Transmission and Distribution plant (combined), associated with core natural gas delivery infrastructure, constitute the majority of gas plant. Annual utility depreciation expense averaged approximately 3.0% of original cost of depreciable transmission and distribution plant during the period 2016 through 2018.

Depreciation and amortization expense on gas plant, including intangibles, was as follows (thousands of dollars):



	2018		2017		2016
Depreciation and amortization expense	$	185,719	$	187,075	$	214,037

Included in the figures above is amortization of utility intangibles of $13.6 million in 2018, $14.3 million in 2017, and $14.8 million in 2016. Additionally, the amounts above exclude regulatory asset and liability amortization.

Operating Leases and Rentals. Certain land, buildings, and construction equipment is leased. The majority of these leases are short-term and accounted for as operating leases. For the natural gas operations segment, these leases are also treated as operating leases for regulatory purposes. Centuri has various short-term operating leases of equipment and temporary office sites. The table below presents Southwest’s and Centuri’s rental and lease payments that are included in operating expenses (in thousands):



	2018		2017		2016
Southwest Gas Corporation	$	4,556	$	4,926	$	4,357
Centuri	59,491		62,310		53,956
Consolidated rental payments/lease expense	$	64,047	$	67,236	$	58,313

The following is a schedule of future minimum lease payments for operating leases (with initial or remaining terms in excess of one year) as of December 31, 2018 (thousands of dollars):



	Southwest		Centuri		Consolidated Total
2019	$	898	$	10,053	$	10,951
2020	363		7,656		8,019
2021	299		5,760		6,059
2022	163		5,163		5,326
2023	79		3,681		3,760
Thereafter	177		10,511		10,688
Total minimum lease payments	$	1,979	$	42,824	$	44,803

Capital Leases. Centuri leases certain construction equipment under capital leases arrangements which are not significant.

Revenue

12 Months Ended

Dec. 31, 2018

Revenue

Note 3 - Revenue

Effective January 2018, the Company and Southwest adopted the FASB’s Topic 606, “Revenue from Contracts with Customers”, using the modified retrospective transition method. Under the modified retrospective approach, the information for periods prior to the adoption date has not been restated and continues to be reported under the accounting standards in effect for those periods (Topic 605, “Revenue Recognition”). As permitted under the standard, the Company and Southwest have elected to apply the guidance retrospectively only to those contracts that were not completed at January 1, 2018. Management assessed the effects the new guidance has on the Company’s (and Southwest’s, in the case of natural gas operations) financial position, results of operations, and cash flows. Based on these assessments, the adoption of Topic 606 had no material impact on any of the financial statements of Southwest or the Company. Certain disclosures included within this note reflect information for 2017 and 2016, during which periods the Company and Southwest followed the guidance in Topic 605. As there were no material changes to the consolidated financial statements as a result of the adoption of Topic 606, as noted above, management elected to disclose information for 2017 and 2016, as the manner in which revenue is recorded has not changed and believes that including such disclosure provides useful information.

The following information about the Company’s revenues is presented by segment. Southwest encompasses one segment – natural gas operations.

Natural Gas Operations Segment:

Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. Southwest recognizes revenue when it satisfies its performance by transferring gas to the customer. Revenues also include the net impacts of margin tracker/decoupling accruals based on criteria in U.S. GAAP for rate-regulated entities associated with alternative revenue programs. Revenues from customer arrangements and from alternative revenue programs are described below.

Southwest acts as an agent for state and local taxing authorities in the collection and remittance of a variety of taxes, including sales and use taxes and surcharges. These taxes are not included in Gas operating revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities.

Southwest generally offers two types of services to its customers: tariff sales and transportation–only service. Tariff sales encompass sales to many types of customers (primarily residential) under various rate schedules, subject to cost-of-service ratemaking, which is based on the rate-regulation of state commissions and the Federal Energy Regulatory Commission (the “FERC”). Southwest provides both the commodity and the related distribution service to nearly all of its approximate 2 million customers, and only several hundred customers (who are eligible to secure their own gas) subscribe to transportation-only service. Also, only a few hundred customers have contracts with stated periods. Southwest recognizes revenue when it satisfies its performance requirement by transferring volumes of gas to the customer. Natural gas is delivered and consumed by the customer simultaneously. The provision of service is represented by the turn of the meter dial and is the primary representation of the satisfaction of performance obligations of Southwest. The amount billable via regulated rates (both volumetric and fixed monthly rates as part of rate design) corresponds to the value to the customer, and management believes that the amount billable under the “invoice practical expedient” (amount Southwest has the right to invoice) is appropriate to utilize for purposes of recognizing revenue. Estimated amounts remaining unbilled since the last meter read date are restricted from being billed due only to the passage of time and therefore are also recognized for service provided through the balance sheet date. While natural gas service is typically recurring, there is generally not a contract term for utility service. Therefore, the contract term is not generally viewed to extend beyond the service provided to date, and customers can generally terminate service at will.

Transportation-only service is also governed by tariff rate provisions. Transportation-only service is generally only available to very large customers under requirements of Southwest’s various tariffs. With this service, customers secure their own gas supply and Southwest provides transportation services to move the customer-supplied gas to the intended location. Southwest concluded that transportation/transmission service is suitable to an “over time” model. Rate structures under Southwest’s regulation for transportation customers include a combination of volumetric charges and monthly “fixed” charges (including charges commonly referred to as capacity charges, demand charges, or reservation charges) as part of the rate design of regulated jurisdictions. These types of fixed charges represent a separate performance obligation associated with standing ready over the period of the month to deliver quantities of gas, regardless of whether the customer takes delivery of any quantity of gas. The performance obligations under these circumstances are satisfied over the course of the month under an output measure of progress based on time, which correlates to the period for which the charges are eligible to be invoiced.

Under its regulation, Southwest enters into negotiated rate contracts for those customers located in proximity to another pipeline, which pose a threat of bypassing its distribution system. Southwest may also enter into similar contracts for customers otherwise able to satisfy their energy needs by means of alternative fuel to natural gas. Less than two dozen customers are party to contracts with rate components subject to negotiation. Many rate provisions and terms of service for these less common types of contracts are also subject to regulatory oversight and tariff provisions. The performance obligations for these customers are satisfied similarly to those for other customers by means of transporting/delivering natural gas to the customer. Many or most of the rate components, and structures, for these types of customers are the same as those for similar customers without negotiated rate components; and the negotiated rates are within the parameters of the tariff guidelines. Management determined that these arrangements qualify for the invoice practical expedient for recognizing revenue. Furthermore, while some of these contracts include contract periods extending over time, including multiple years, as amounts billable under the contract are based on rates in effect for the customer for service provided to date, no significant financing component is deemed to exist.

As indicated above, revenues also include the net impacts of margin tracker/decoupling accruals. All of Southwest’s service territories have decoupled rate structures (also referred to as alternative revenue programs) that are designed to eliminate the direct link between volumetric sales and revenue, thereby mitigating the impacts of unusual weather variability and conservation on margin. The primary alternative revenue programs involve permissible adjustments for differences between stated tariff benchmarks and amounts billable through revenue from contracts with customers via existing rates. Such adjustments are recognized monthly in revenue and in the associated regulatory asset/liability accounts in advance of rate adjustments intended to collect or return amounts recognized. Revenues recognized for the adjustment to the benchmarks noted are required to be presented separately from revenues from contracts with customers, and as such, are provided below and identified as alternative revenue program revenue (which excludes recoveries from customers).

Gas operating revenues on the Consolidated Statements of Income of both the Company and Southwest include revenue from contracts with customers, which is shown below disaggregated by customer type, and various categories of revenue:



	December 31,
(Thousands of dollars)	2018		2017		2016
Residential	$	887,220	$	857,204	$	895,330
Small commercial	255,083		243,513		251,092
Large commercial	53,192		52,379		53,582
Industrial/other	23,489		22,026		19,753
Transportation	86,990		87,759		87,106
Revenue from contracts with customers	1,305,974		1,262,881		1,306,863
Alternative revenue program revenues (deferrals)	45,979		35,347		12,530
Other revenues (a)	5,775		4,080		2,019
Total Gas operating revenues	$	1,357,728	$	1,302,308	$	1,321,412


(a)	Includes various other revenues, and during the first six months of 2018, included $12.5 million as a reserve against revenue associated with a tax reform savings adjustment. During the third quarter of 2018, amounts previously recognized were reclassified to the various categories of revenue from contracts with customers when incorporated in tariff rates.

Utility Infrastructure Services Segment:

The majority of Centuri contracts are performed under unit-price contracts. Generally, these contracts state prices per unit of installation. Typical installations are accomplished in a few weeks or less. Revenues are recorded as installations are completed. Revenues are recorded for long-term fixed-price contracts in a pattern that reflects the transfer of control of promised goods and services to the customer over time. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs (a method of recognition based on inputs). Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model.

Centuri is required to collect taxes imposed by various governmental agencies on the work performed for its customers. These taxes are not included in Utility infrastructure services revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities.

Centuri derives revenue from the installation, replacement, repair, and maintenance of energy distribution systems, and in developing industrial construction solutions. Centuri has operations in the U.S. and Canada. The majority of Centuri’s revenues are related to contracts for natural gas pipeline replacement and installation work for natural gas utilities. In addition, Centuri performs certain industrial construction activities for various customers and industries. Centuri has two types of agreements with its customers: master services agreements (“MSAs”) and bid contracts. Most of Centuri’s customers supply many of their own materials in order for Centuri to complete its work under the contracts.

An MSA identifies most of the terms describing each party’s rights and obligations that will govern future work authorizations. An MSA is often effective for multiple years. A work authorization is issued by the customer to describe the location, timing, and any additional information necessary to complete the work for the customer. The combination of the MSA and the work authorization determines when a contract exists and revenue recognition may begin. Each work authorization is generally a single performance obligation as Centuri is performing a significant integration service. Centuri has elected to use the portfolio method practical expedient at the customer level as the terms and conditions of the work performed under MSAs are similar in nature with each customer but vary significantly between customers.

A bid contract is typically a one-time agreement for a specific project that has all necessary terms defining each party’s rights and obligations. Each bid contract is evaluated for revenue recognition individually. Control of assets created under bid contracts generally passes to the customer over time. Bid contracts often have a single performance obligation as Centuri is providing a significant integration service.

Centuri’s MSA and bid contracts are characterized as either fixed-price contracts or unit-price contracts for revenue recognition purposes. The cost-to-cost input method is used to measure progress towards the satisfaction of a performance obligation for fixed-price contracts. Input methods result in the recognition of revenue based on the entity’s expended effort to satisfy the performance obligation relative to the total expected effort to satisfy the performance obligation. For unit-price contracts, an output method is used to measure progress towards satisfaction of a performance obligation. Also with regard to unit-price contracts, the output measurement will be the completion of each unit that is required under the contract.

Actual revenues and project costs can vary, sometimes substantially, from previous estimates due to changes in a variety of factors, including unforeseen circumstances. These factors, along with other risks inherent in performing fixed-price contracts may cause actual revenues and gross profit for a project to differ from previous estimates and could result in reduced profitability or losses on projects. Changes in these factors may result in revisions to costs and earnings, the impacts for which are recognized in the period in which the changes are identified. Once identified, these types of conditions continue to be evaluated for each project throughout the project term and ongoing revisions in management’s estimates of contract value, contract cost, and contract profit are recognized as necessary in the period determined.

Centuri categorizes work performed under MSAs and bid contracts into three primary service types: gas construction, electrical construction, and other construction. Gas construction includes work involving previously existing gas pipelines and the installation of new pipelines or service lines. Electrical construction includes work involving installation and maintenance of transmission and distribution lines and storm restoration services. Other construction includes all other work and can include industrial and water utility services.

Contracts can have compensation/consideration that is variable. For MSAs, variable consideration is evaluated at the customer level as the terms creating variability in pricing are included within the MSA and are not specific to a work authorization. For multi-year MSAs, variable consideration items are typically determined for each year of the contract and not for the full contract term. For bid contracts, variable consideration is evaluated at the individual contract level. The expected value method or most likely amount method is used based on the nature of the variable consideration. Types of variable consideration include liquidated damages, delay penalties, performance incentives, safety bonuses, payment discounts, and volume rebates. Centuri will typically estimate variable consideration and adjust financial information, as necessary.

Change orders involve the modification in scope, price, or both to the current contract, requiring approval by both parties. The existing terms of the contract continue to be accounted for under the current contract until such time as a change order is approved. Once approved, the change order is either treated as a separate contract or as part of the existing contract, as appropriate, under the circumstances. When the scope is agreed upon in the change order but not the price, Centuri estimates the change to the transaction price.

The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type:



(Thousands of dollars)	December 31,
	2018		2017		2016
Service Types:
Gas infrastructure services	$	1,123,682	$	891,139	$	914,970
Electric power infrastructure services	32,629		18,114		27,915
Other	365,974		337,231		196,193
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078



(Thousands of dollars)	December 31,
	2018		2017		2016
Contract Types:
Master services agreement	$	1,102,412	$	885,513	$	852,472
Bid contract	419,873		360,971		286,606
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078

Unit priced contracts	$	1,258,419	$	968,856	$	886,919
Fixed priced contracts	117,298		127,497		95,494
Time and materials contracts	146,568		150,131		156,665
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078

The following table provides information about contracts receivable and revenue earned on contracts in progress in excess of billings (contract assets), both of which are included within Accounts receivable, net of allowances, and amounts billed in excess of revenue earned on contracts (contract liabilities), which are included in Other current liabilities as of December 31, 2018 and December 31, 2017 on the Company’s Consolidated Balance Sheets:



(Thousands of dollars)	December 31, 2018		December 31, 2017
Contracts receivable, net	$	186,249	$	221,859
Revenue earned on contracts in progress in excess of billings	87,520		5,768
Amounts billed in excess of revenue earned on contracts	4,211		9,602

The revenue earned on contracts in progress in excess of billings (contract asset) primarily relates to Centuri’s rights to consideration for work completed but not billed and/or approved at the reporting date. These contract assets are transferred to contracts receivable when the rights become unconditional. Upon adoption of Topic 606, the Company reclassified $50.6 million to revenue earned on contracts in progress in excess of billings, and during the year ended December 31, 2018, recognized an increase of $30 million, excluding the impact from the adoption, primarily related to normal operating and billing activities. The amounts billed in excess of revenue earned (contract liability) primarily relate to the advance consideration received from customers for which work has not yet been completed. The change in this contract liability balance from January 1, 2018 to December 31, 2018 is due to revenue recognized of $9.6 million that was included in this balance as of January 1, 2018, after which time it became earned and the balance was reduced, and to increases due to cash received, net of revenue recognized during the period related to contracts that commenced during the period.

Prior to the adoption of Topic 606, revenue earned on contracts in progress in excess of billings was only used to recognize contract assets related to fixed-price contracts under previous accounting guidance. This balance now includes any conditional contract assets for both fixed-price contracts and unit-price contracts. Centuri considers retention and unbilled amounts to be conditional contract assets, as payment is contingent on the occurrence of a future event. Contracts receivable, net, includes only amounts that are unconditional in nature, which means only the passage of time remains and Centuri has invoiced the customer. Similarly, amounts billed in excess of revenue earned on contracts was only used to recognize contract liabilities related to fixed-price contracts under previous accounting guidance. This line item now includes contract liabilities related to both fixed-price contracts and unit-price contracts. In the event a contract asset or contract liability is expected to be recognized for greater than one year from the financial statement date, Centuri classifies those amounts as long-term contract assets or contract liabilities, included in Deferred charges and other assets or Other deferred credits and other long-term liabilities on the Company’s Consolidated Balance Sheets.

For contracts that have an original duration of one year or less, Centuri uses the practical expedient applicable to such contracts and does not consider/compute an interest component based on the time value of money. Further, because of the short duration of these contracts, the Company has not disclosed the transaction price for the remaining performance obligations as of the end of each reporting period or when the Company expects to recognize the revenue.

As of December 31, 2018, Centuri has fifteen contracts that had an original duration of more than one year. The aggregate amount of the transaction price allocated to the unsatisfied performance obligations of these contracts as of December 31, 2018 is $38.6 million. Centuri expects to recognize the remaining performance obligations over the next four years; however, the timing of that recognition is largely within the control of the customer, including when the necessary equipment and materials required to complete the work will be provided by the customer.

Utility infrastructure services contracts receivable consists of the following:



(Thousands of dollars)	December 31, 2018			December 31, 2017
Billed on completed contracts and contracts in progress	$	184,100		$	219,298
Other receivables	2,588			2,622
Contracts receivable, gross	186,688			221,920
Allowance for doubtful accounts	(439		)	(61		)
Contracts receivable, net	$	186,249		$	221,859

Receivables and Related Allowances

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Receivables and Related Allowances

Note 4 - Receivables and Related Allowances

Business activity with respect to gas utility operations is conducted with customers located within the three-state region of Arizona, Nevada, and California. The table below contains information about the gas utility customer accounts receivable balance (net of allowance) at December 31, 2018 and 2017, and the percentage of customers in each of the three states, which was consistent with the prior year.



	December 31, 2018		December 31, 2017
Gas utility customer accounts receivable balance (in thousands)	$	138,149	$	119,444



	December 31, 2018
Percent of customers by state
Arizona	53	%
Nevada	37	%
California	10	%

Although Southwest seeks to minimize its credit risk related to utility operations by requiring security deposits from new customers, imposing late fees, and actively pursuing collection on overdue accounts, some accounts are ultimately not collected. Customer accounts are subject to collection procedures that vary by jurisdiction (late fee assessment, noticing requirements for disconnection of service, and procedures for actual disconnection and/or reestablishment of service). After disconnection of service, accounts are generally written off approximately two months after inactivation. Dependent upon the jurisdiction, reestablishment of service requires both payment of previously unpaid balances and additional deposit requirements. Provisions for uncollectible accounts are recorded monthly based on experience, customer and rate composition, and write-off processes. They are included in the ratemaking process as a cost of service. The Nevada jurisdictions have a regulatory mechanism associated with the gas cost-related portion of uncollectible accounts. Such amounts are deferred and collected through a surcharge in the ratemaking process. Activity in the allowance account for uncollectibles is summarized as follows (thousands of dollars):



	Allowance for Uncollectibles
Balance, December 31, 2015	$	2,270
Additions charged to expense	3,264
Accounts written off, less recoveries	(3,010		)
Balance, December 31, 2016	2,524
Additions charged to expense	2,310
Accounts written off, less recoveries	(2,723		)
Balance, December 31, 2017	2,111
Additions charged to expense	2,959
Accounts written off, less recoveries	(2,902		)
Balance, December 31, 2018	$	2,168

At December 31, 2018, the utility infrastructure services segment (Centuri) had $273.8 million in combined customer accounts and contracts receivable. Both the allowance for uncollectibles and write-offs related to Centuri customers have been insignificant and are not reflected in the table above.

The entire disclosure for claims held for amounts due a entity, excluding financing receivables. Examples include, but are not limited to, trade accounts receivables, notes receivables, loans receivables. Includes disclosure for allowance for credit losses.

Regulatory Assets and Liabilities

12 Months Ended

Dec. 31, 2018

Regulatory Assets and Liabilities

Note 5 - Regulatory Assets and Liabilities

Southwest is subject to the regulation of the Arizona Corporation Commission (“ACC”), the Public Utilities Commission of Nevada (“PUCN”), the California Public Utilities Commission (“CPUC”), and the FERC. Accounting policies of Southwest conform to U.S. GAAP applicable to rate-regulated entities and reflect the effects of the ratemaking process. Accounting treatment for rate-regulated entities allows for deferral as regulatory assets, costs that otherwise would be expensed, if it is probable that future recovery from customers will occur. If rate recovery is no longer probable, due to competition or the actions of regulators, Southwest is required to write-off the related regulatory asset. Regulatory liabilities are recorded if it is probable that revenues will be reduced for amounts that will be credited to customers through the ratemaking process.

The following table represents existing regulatory assets and liabilities (thousands of dollars):



December 31,	2018			2017
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	383,170		$	391,403
Unrealized net loss on non-trading derivatives (Swaps) (2)	1,862			5,780
Deferred purchased gas costs (3)	4,928			14,581
Accrued purchased gas costs (4)	29,000			17,000
Unamortized premium on reacquired debt (5)	19,599			20,913
Accrued absence time (8)	14,126			13,870
Margin & interest-tracking (9)	88,290			42,354
Other (10)	32,616			25,997
	573,591			531,898
Regulatory liabilities:
Deferred purchased gas costs (3)	(79,762		)	(6,841		)
Accumulated removal costs	(383,000		)	(315,000		)
Unrealized net gain on non-trading derivatives (Swaps) (2)	(144		)	—
Unamortized gain on reacquired debt (6)	(8,717		)	(9,253		)
Regulatory excess deferred taxes and gross-up (7)	(458,834		)	(433,908		)
Other (10)	(19,911		)	(33,184		)
Net regulatory liabilities	$	(376,777	)	$	(266,288	)


(1)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(2)

Asset balance is included in Deferred charges and other assets and Prepaid and other assets on the Consolidated Balance Sheets. Liability balance is included in Other current liabilities and Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. The actual amounts, when realized at settlement, become a component of purchased gas costs under Southwest’s PGA mechanisms. (For specific details, see Note 14 - Derivatives).


(3)	Balance recovered or refunded on an ongoing basis with interest.


(4)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.


(5)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.


(6)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(7)

The TCJA required a remeasurement and reduction of the net deferred income tax liability. The reduction (excess deferred taxes) became a regulatory liability with appropriate tax gross-up. The excess deferred taxes reduce rate base. The tax benefit will be returned to utility customers in accordance with regulatory requirements. Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. This amount also includes a $2.9 million gross-up related to contributions in aid of construction.


(8)	Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.


(9)	Margin tracking/decoupling mechanisms are alternative revenue programs and revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) are recognized as revenue so long as recovery is expected to take place within 24 months. Included in Prepaid and other current assets on the Consolidated Balance Sheets.

(10)

The following tables detail the components of Other regulatory assets and liabilities. Other regulatory assets are included in either Prepaid and other current assets or Deferred charges and other assets on the Consolidated Balance Sheets (as indicated). Recovery periods vary. Other regulatory liabilities are included in either Other current liabilities or Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets (as indicated).



Other Regulatory Assets	2018		2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	6,253	$	4,832
Infrastructure replacement programs and similar (b) (e)	12,486		9,627
Environmental compliance programs (c) (e)	5,046		9,702
Other (d)	8,831		1,836
	$	32,616	$	25,997


a)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. See Prepaid and other current assets in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.


b)	Included in Deferred charges and other assets on the Consolidated Balance Sheets.


c)	In 2018, approximately $4.5 million included in Prepaid and other current assets and $596,000 included in Deferred charges and other assets on the Consolidated Balance Sheets. In2017, $9.2 million included in Prepaid and other current assets and $527,000 included in Deferred charges and other assets on the Consolidated Balance Sheets.

In 2018, $197,000 included in Prepaid and other current assets and $8.6 million included in Deferred charges and other assets on the Consolidated Balance Sheets. In 2017, $531,000 included in Prepaid and other current assets and $1.3 million included in Deferred charges and other assets on the Consolidated Balance Sheets. The balance in 2018 includes $6 million authorized as part of the recently concluded Nevada general rate case associated with self-insurance cost.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.



Other Regulatory Liabilities	2018			2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	(8,598	)	$	(10,213	)
Margin, interest- and property tax-tracking accounts (b) (e)	(7,273		)	(9,505		)
Environmental compliance programs (e) (f)	—			(8,574		)
Regulatory accounts for differences related to pension funding (c)	(3,221		)	(3,178		)
Other (d) (e)	(819		)	(1,714		)
	$	(19,911	)	$	(33,184	)


a)	Included in Other current liabilities on the Consolidated Balance Sheets.


b)	In 2018, $(539,000) included in Other current liabilities and $(6.7) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, $(6.6) million included in Other current liabilities and $(2.9) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


c)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


d)	In 2018, $(810,000) included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, approximately $(1.7) million included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.


f)	In 2018, included in Prepaid and other current assets on the Consolidated Balance Sheets. In 2017, included in Other current liabilities on the Consolidated Balance Sheets.

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

12 Months Ended

Dec. 31, 2018

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

Note 6 - Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")

The following information provides insight into amounts impacting Other Comprehensive Income (Loss), both before and after-tax, within the Consolidated Statements of Comprehensive Income, which also impact Accumulated Other Comprehensive Income in the Consolidated Balance Sheets and Consolidated Statements of Equity of the Company and Southwest.

Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)



(Thousands of dollars)	2018									2017									2016
	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(20,426	)	$	4,902		$	(15,524	)	$	(43,027	)	$	10,326		$	(32,701	)	$	(22,770	)	$	8,652		$	(14,118	)
Amortization of prior service cost	1,335			(320		)	1,015			1,335			(507		)	828			1,335			(507		)	828
Amortization of net actuarial (gain)/loss	33,617			(8,068		)	25,549			25,445			(9,669		)	15,776			27,066			(10,285		)	16,781
Regulatory adjustment	(8,233		)	1,976			(6,257		)	12,340			250			12,590			(5,584		)	2,122			(3,462		)
Pension plans other comprehensive income (loss)	6,293			(1,510		)	4,783			(3,907		)	400			(3,507		)	47			(18		)	29
FSIRS (designated hedging activities):
Amounts reclassified into net income	3,345			(804		)	2,541			3,344			(1,271		)	2,073			3,345			(1,270		)	2,075
FSIRS other comprehensive income (loss)	3,345			(804		)	2,541			3,344			(1,271		)	2,073			3,345			(1,270		)	2,075
Total other comprehensive income (loss) – Southwest Gas Corporation	9,638			(2,314		)	7,324			(563		)	(871		)	(1,434		)	3,392			(1,288		)	2,104
Foreign currency translation adjustments:
Translation adjustments	(3,010		)	—			(3,010		)	1,771			—			1,771			161			—			161
Foreign currency other comprehensive income (loss)	(3,010		)	—			(3,010		)	1,771			—			1,771			161			—			161
Total other comprehensive income (loss) – Southwest Gas Holdings, Inc.	$	6,628		$	(2,314	)	$	4,314		$	1,208		$	(871	)	$	337		$	3,553		$	(1,288	)	$	2,265

(1) Tax amounts are calculated using a 24% rate following the December 22, 2017 enactment date of the TCJA. For periods prior to the enactment date, tax amounts were calculated using a 38% rate. At December 31, 2017, excess taxes related to pre-tax amounts which accumulated in AOCI prior to tax reform were required to remain in the account until the first quarter of 2018, when ASU 2018-02 was adopted, permitting previously stranded amounts to be released from AOCI and applied to Retained earnings (see table for Accumulated other comprehensive income (loss), including the balance, below). With regard to foreign currency translation adjustments, the Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, the Company is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other comprehensive income (loss), as repatriation of earnings is not anticipated.

With regard to the table above and the roll-forward tables below, management recognizes tax impacts (associated with underlying before-tax amounts in AOCI) in both AOCI and in Deferred income taxes and investment tax credits, net on its balance sheets. U.S. tax reform of the TCJA was enacted on December 22, 2017. U.S. GAAP requires that deferred tax assets and liabilities be adjusted to reflect the effects of a change in tax laws and rates, and also requires that the effect be included in income from continuing operations for the period of enactment. As a result, when deferred tax balances on the balance sheet for the period ending December 31, 2017 were remeasured as a result of the TCJA to reflect the change in enacted rates, those adjustments were also reflected in income tax expense on the Consolidated Statements of Income, as required. However, excess amounts were not able to be released from AOCI until the first quarter of 2018, when upon adoption of ASU 2018-02, previously stranded amounts were released and reflected in Retained earnings on Southwest’s and the Company’s Consolidated Balance Sheets, as required by the ASU.

The estimated amounts that will be amortized from accumulated other comprehensive income or regulatory assets into net periodic benefit cost over the next year are summarized below (in thousands):



Retirement plan net actuarial loss	$	22,000
SERP net actuarial loss	1,000
PBOP prior service cost	1,300

Approximately $2.5 million of realized losses (net of tax) related to the FSIRS, included in AOCI at December 31, 2018, will be reclassified into interest expense within the next twelve months as the related interest payments on long-term debt occur. The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity:

AOCI—Rollforward

(Thousands of dollars)



	Defined Benefit Plans									FSIRS									Foreign Currency Items
	Before-Tax			Tax (Expense) Benefit (5)			After-Tax			Before- Tax			Tax (Expense) Benefit (5)			After-Tax			Before- Tax			Tax (Expense) Benefit		After-Tax			Other			AOCI
Beginning Balance AOCI December 31, 2017	$	(61,520	)	$	22,293		$	(39,227	)	$	(12,655	)	$	4,809		$	(7,846	)	$	(609	)	$	—	$	(609	)	$	—		$	(47,682	)
Net actuarial gain/(loss)	(20,426		)	4,902			(15,524		)	—			—			—			—			—		—			—			(15,524		)
Translation adjustments	—			—			—			—			—			—			(3,010		)	—		(3,010		)	—			(3,010		)
Other comprehensive income before reclassifications	(20,426		)	4,902			(15,524		)	—			—			—			(3,010		)	—		(3,010		)	—			(18,534		)
FSIRS amounts reclassified from AOCI (1)	—			—			—			3,345			(804		)	2,541			—			—		—			—			2,541
Amortization of prior service cost (2)	1,335			(320		)	1,015			—			—			—			—			—		—			—			1,015
Amortization of net actuarial loss (2)	33,617			(8,068		)	25,549			—			—			—			—			—		—			—			25,549
Regulatory adjustment (3)	(8,233		)	1,976			(6,257		)	—			—			—			—			—		—			—			(6,257		)
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	6,293			(1,510		)	4,783			3,345			(804		)	2,541			(3,010		)	—		(3,010		)	—			4,314
Reclassification of excess deferred taxes (4)	—			—			—			—			—			—			—			—		—			(9,300		)	(9,300		)
Ending Balance AOCI December 31, 2018	$	(55,227	)	$	20,783		$	(34,444	)	$	(9,310	)	$	4,005		$	(5,305	)	$	(3,619	)	$	—	$	(3,619	)	$	(9,300	)	$	(52,668	)


(1)	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income.


(2)	These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).


(3)	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets).


(4)	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02 during the first quarter of 2018, which permitted such release.


(5)	Tax amounts related to the before-tax balance are calculated using a 24% effective rate after the release of previously stranded excess deferred taxes existing as a result of the TCJA; amounts prior to the December 22, 2017 enactment of the TCJA were calculated using a 38% effective rate.

The following table represents a rollforward of AOCI, presented on Southwest’s Consolidated Balance Sheets:

AOCI—Rollforward

(Thousands of dollars)



	Defined Benefit Plans									FSIRS
	Before-Tax			Tax (Expense) Benefit (10)			After- Tax			Before- Tax			Tax (Expense) Benefit (10)			After- Tax			Other			AOCI
Beginning Balance AOCI December 31, 2017	$	(61,520	)	$	22,293		$	(39,227	)	$	(12,655	)	$	4,809		$	(7,846	)	$	—		$	(47,073	)
Net actuarial gain/(loss)	(20,426		)	4,902			(15,524		)	—			—			—			—			(15,524		)
Other comprehensive income before reclassifications	(20,426		)	4,902			(15,524		)	—			—			—			—			(15,524		)
FSIRS amounts reclassified from AOCI (6)	—			—			—			3,345			(804		)	2,541			—			2,541
Amortization of prior service cost (7)	1,335			(320		)	1,015			—			—			—			—			1,015
Amortization of net actuarial loss (7)	33,617			(8,068		)	25,549			—			—			—			—			25,549
Regulatory adjustment (8)	(8,233		)	1,976			(6,257		)	—			—			—			—			(6,257		)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation	6,293			(1,510		)	4,783			3,345			(804		)	2,541			—			7,324
Reclassification of excess deferred taxes (9)	—			—			—			—			—			—			(9,300		)	(9,300		)
Ending Balance AOCI December 31, 2018	$	(55,227	)	$	20,783		$	(34,444	)	$	(9,310	)	$	4,005		$	(5,305	)	$	(9,300	)	$	(49,049	)


(6)	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income.


(7)	These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).


(8)	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets).


(9)	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02 during the first quarter of 2018, which permitted such release.


(10)	Tax amounts related to the before-tax balances are calculated using a 24% effective rate after the release of previously stranded excess deferred taxes existing as a result of the TCJA; amounts prior to the December 22, 2017 enactment of the TCJA were calculated using a 38% effective rate.

The following table represents amounts (before income tax impacts) included in Accumulated other comprehensive income (in the table above), that have not yet been recognized in net periodic benefit cost as of December 31, 2018 and 2017:

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)



	2018			2017
Net actuarial (loss) gain	$	(435,364	)	$	(448,555	)
Prior service cost	(3,033		)	(4,368		)
Less: amount recognized in regulatory assets	383,170			391,403
Recognized in AOCI	$	(55,227	)	$	(61,520	)

See Note 11 - Pension and Other Postretirement Benefits for more information on the defined benefit pension plans and Note 14 - Derivatives for more information on the FSIRS.

Common Stock

12 Months Ended

Dec. 31, 2018

Pension and Other Postretirement Benefits

Common Stock

Note 7 - Common Stock

On March 29, 2017, the Company filed with the SEC an automatic shelf registration statement on Form S-3 (File No. 333-217018), which became effective upon filing, for the offer and sale of up to $150 million of common stock from time to time in at-the-market offerings under the prospectus included therein and in accordance with the Sales Agency Agreement, dated March 29, 2017, between the Company and BNY Mellon Capital Markets, LLC (the “Equity Shelf Program”). The following table provides the activity in the Equity Shelf Program for the three and twelve months ended December 31, 2018:



	Three Months Ended			Twelve Months Ended
	December 31,			December 31,
	2018	2017		2018		2017
Gross proceeds	—	$	29,999,922	$	85,149,976	$	41,776,795
Less: agent commissions	—	299,999		851,500		417,768
Net proceeds	—	$	29,699,923	$	84,298,476	$	41,359,027

Number of shares sold	—	358,630		1,145,705		505,707
Weighted average price per share	—	$	83.65	$	74.32	$	82.61

As of December 31, 2018, the Company had up to $23,073,229 of common stock available for sale under the program. Net proceeds from the sale of shares of common stock under the Equity Shelf Program are intended for general corporate purposes, including the acquisition of property for the construction, completion, extension, or improvement of pipeline systems and facilities located in and around the communities served by Southwest. Net proceeds during the twelve months ended December 31, 2018 were contributed to, and reflected in the records of, Southwest (as a capital contribution from Southwest Gas Holdings, Inc.).

Aside from the automatic shelf registration, in December 2017, the Company and Southwest jointly filed with the SEC an automatic shelf registration statement (File No. 333-222047), which became effective upon filing and includes a prospectus detailing the Company’s ability to offer and sell, from time to time in amounts at prices and on terms that will be determined at the time of such offering, any combination of common stock, preferred stock, debt securities (which may or may not be guaranteed by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement), guarantees of debt securities issued by Southwest, depository shares, warrants to purchase common stock, preferred stock or depository shares issued by the Company or debt securities issued by the Company or Southwest, units and rights (the “Universal Shelf”). Additionally as part of the Universal Shelf, Southwest may offer and sell, from time to time in amounts at prices and on terms that will be determined at the time of such offering, any combination of debt securities (which may or may not be guaranteed by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement) and guarantees of debt securities issued by the Company or by one or more of its directly or indirectly wholly owned subsidiaries if indicated in the relevant prospectus supplement.

In November 2018, the Company sold, through a prospectus supplement under the Universal Shelf, an aggregate of 3,565,000 shares of common stock, in an underwritten public offering, at $75.50 per share, resulting in proceeds to the Company of $260,073,524, net of $9,083,976 underwriters’ discount. The Company used a portion of the net proceeds from the offering to facilitate, in association with Centuri, the purchase of Linetec, to pay down outstanding borrowings under the Company’s credit facility, and used the remaining amounts for general corporate purposes. Refer to Note 19 - Business Acquisitions regarding the acquisition of Linetec and Note 9 - Short-Term Debt for information on the Company’s credit facility.

During 2018, the Company issued approximately 83,000 shares of common stock through the Restricted Stock/Unit Plan, and Management Incentive Plan.

Additionally during 2018, the Company issued 143,000 shares of common stock through the Dividend Reinvestment and Stock Purchase Plan (“DRSPP”), raising proceeds of approximately $10.6 million.

As of December 31, 2018, there were 4.4 million common shares registered and available for issuance under the provisions of the various stock issuance plans, which does not include the amount of common stock available that is separately disclosed under the Equity Shelf Program above.

Long-Term Debt

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Long-Term Debt

Note 8 - Long-Term Debt

Carrying amounts of long-term debt and related estimated fair values as of December 31, 2018 and December 31, 2017 are disclosed in the following table. Southwest’s revolving credit facility (including commercial paper) and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, as they are repaid quickly (in the case of credit facility borrowings) and have interest rates that reset frequently. These are categorized as Level 1 due to Southwest’s ability to access similar debt arrangements at measurement dates with comparable terms, including variable/market rates. The fair values of Southwest’s debentures, senior notes, and fixed-rate IDRBs were determined utilizing a market-based valuation approach, where fair values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves, credit ratings, and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable. The fair values of debentures and fixed-rate IDRBs are categorized as Level 2. The Centuri secured revolving credit and term loan facility and Centuri other debt obligations are categorized as Level 3, based on significant unobservable inputs to their fair values. Because Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. The fair value hierarchy is described in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.



	December 31, 2018					December 31, 2017
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Southwest Gas Corporation:
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	126,213	$	125,000		$	129,273
Notes, 6.1%, due 2041	125,000			150,728		125,000			158,304
Notes, 3.875%, due 2022	250,000			254,195		250,000			256,163
Notes, 4.875%, due 2043	250,000			268,985		250,000			283,243
Notes, 3.8%, due 2046	300,000			267,030		300,000			302,970
Notes, 3.7%, due 2028	300,000			298,926		—			—
8% Series, due 2026	75,000			93,827		75,000			96,063
Medium-term notes, 7.78% series, due 2022	25,000			27,497		25,000			28,714
Medium-term notes, 7.92% series, due 2027	25,000			30,016		25,000			31,542
Medium-term notes, 6.76% series, due 2027	7,500			8,651		7,500			8,882
Unamortized discount and debt issuance costs	(11,807		)			(9,350		)
	1,470,693					1,173,150
Revolving credit facility and commercial paper	150,000			150,000		150,000			150,000
Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028	50,000			50,000		50,000			50,000
2003 Series A, due 2038	50,000			50,000		50,000			50,000
2008 Series A, due 2038	50,000			50,000		50,000			50,000
2009 Series A, due 2039	50,000			50,000		50,000			50,000
Unamortized discount and debt issuance costs	(2,024		)			(2,119		)
	197,976					197,881
Less: current maturities	—					—
Long-term debt, less current maturities – Southwest Gas Corporation	$	1,818,669				$	1,521,031
Centuri:
Centuri term loan facility	$	255,959		260,135		$	199,578		207,588
Unamortized debt issuance costs	(1,414		)			(1,111		)
	254,545					198,467
Centuri secured revolving credit facility	—			—		56,472			56,525
Centuri other debt obligations	67,104			67,053		47,952			48,183
Less: current maturities	(33,060		)			(25,346		)
Long-term debt, less current maturities – Centuri	$	288,589				$	277,545
Consolidated Southwest Gas Holdings, Inc.:
Southwest Gas Corporation long-term debt	$	1,818,669				$	1,521,031
Centuri long-term debt	321,649					302,891
Less: current maturities	(33,060		)			(25,346		)
Long-term debt, less current maturities – Southwest Gas Holdings, Inc.	$	2,107,258				$	1,798,576

Southwest has a $400 million credit facility which expires in March 2022. Southwest designates $150 million of capacity related to the facility as long-term debt and has designated the remaining $250 million for working capital purposes. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on Southwest’s senior unsecured debt rating. At December 31, 2018, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate. Southwest is also required to pay a commitment fee, of 0.10% per annum, on the unfunded portion of the commitments, which was not significant for the year ended December 31, 2018. At December 31, 2018, $150 million was outstanding on the long-term portion of the credit facility, $50 million of which was in commercial paper (see commercial paper program discussion below). The effective interest rate on the long-term portion of the credit facility was 3.41% at December 31, 2018. Borrowings under the credit facility ranged from none at various times throughout 2018 to a high of $378 million during the first quarter of 2018. With regard to the short-term portion of the credit facility, there was $152 million outstanding at December 31, 2018 and $191 million in borrowings outstanding at December 31, 2017. (See Note 9 – Short-Term Debt).

Southwest has a $50 million commercial paper program. Any issuance under the commercial paper program is supported by Southwest’s current revolving credit facility and, therefore, does not represent additional borrowing capacity. Any borrowing under the commercial paper program will be designated as long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At December 31, 2018, and as noted above, $50 million was outstanding under the commercial paper program.

In March 2018, Southwest issued $300 million in 3.7% Senior Notes at a discount of 0.185%. The notes will mature in March 2028. The proceeds were used to temporarily pay down, in full, the amount then outstanding under the revolving portion of the credit facility and the remainder to repay amounts then outstanding under the commercial paper program.

In November 2018, Centuri, in association with the acquisition of Linetec (refer to Note 19 - Business Acquisitions), amended and restated its senior secured revolving credit and term loan facility, increasing the borrowing capacity from $450 million to $590 million. The line of credit portion of the facility increased to $325 million; amounts borrowed and repaid under the revolving credit facility are available to be re-borrowed. The term loan facility portion increased to $265 million. The $590 million credit and term loan facility expires in November 2023 and continues to be secured by substantially all of Centuri’s assets except those explicitly excluded under the terms of the agreement (including owned real estate and certain construction and transportation equipment). Centuri assets securing the facility at December 31, 2018 totaled $1.1 billion.

Interest rates for Centuri’s $590 million secured revolving credit and term loan facility are calculated at LIBOR, the Canadian Dealer Offered Rate (“CDOR”), or an alternate base rate or Canadian base rate, plus in each case an applicable margin that is determined based on Centuri’s consolidated leverage ratio. The applicable margin ranges from 0.875% to 2.25% for loans bearing interest with reference to LIBOR or CDOR and from 0.00% to 1.25% for loans bearing interest with reference to the alternate base rate or Canadian base rate. Centuri is also required to pay a commitment fee on the unfunded portion of the commitments based on their consolidated leverage ratio. The commitment fee ranges from 0.125% to 0.35% per annum. Borrowings under the secured revolving credit facility ranged from a low of zero during December 2018 to a high of $103 million during July 2018. At December 31, 2018 $256 million in borrowings were outstanding under the combined secured revolving credit and term loan facility.

All amounts outstanding are considered long-term borrowings. The effective interest rate on the secured revolving credit and term loan facility was 4.1% at December 31, 2018.

The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below:



	December 31, 2018		December 31, 2017
2003 Series A	2.61	%	2.44	%
2008 Series A	2.52	%	2.59	%
2009 Series A	2.51	%	2.40	%
Tax-exempt Series A	2.53	%	2.56	%

In Nevada, interest fluctuations due to changing interest rates on Southwest’s 2003 Series A, 2008 Series A, and 2009 Series A variable-rate IDRBs are tracked and recovered from ratepayers through a variable interest expense recovery mechanism.

None of Southwest’s debt instruments have credit triggers or other clauses that result in default if bond ratings are lowered by rating agencies. Interest and fees on certain debt instruments are subject to adjustment depending on Southwest’s bond ratings. Certain debt instruments are subject to a leverage ratio cap and the 6.1% note due 2041 is also subject to a minimum net worth requirement. At December 31, 2018, Southwest was in compliance with all of its covenants. Under the most restrictive of the financial covenants, approximately $2.1 billion in additional debt could be issued while still meeting the leverage ratio requirement. Relating to the minimum net worth requirement, as of December 31, 2018, there is at least $1.2 billion of cushion in equity. No specific dividend restrictions exist under the collective covenants. None of the debt instruments contain material adverse change clauses.

Certain Centuri debt instruments have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2018, Centuri was in compliance with all of its covenants. Under the most restrictive of the covenants, Centuri could issue over $101 million in additional debt and meet the leverage ratio requirement. Centuri has at least $78 million of cushion relating to the minimum fixed charge ratio coverage requirement. Centuri’s revolving credit and term loan facility is secured by underlying assets of the utility infrastructure services segment. Centuri’s covenants limit its ability to provide cash dividends to Southwest Gas Holdings, Inc., its parent. The dividend restriction is equal to a maximum of 60% of its rolling twelve-month consolidated net income.

Estimated maturities of long-term debt for the next five years are (in thousands):



	Southwest		Centuri		Total
2019	$	—	$	33,060	$	33,060
2020	125,000		38,296		163,296
2021	—		33,571		33,571
2022	425,000		35,515		460,515
2023	—		178,614		178,614

Short-Term Debt	12 Months Ended
Short-Term Debt	Dec. 31, 2018
Debt Disclosure [Abstract]
Short-Term Debt	Note 9 - Short-Term Debt In March 2017, Southwest Gas Holdings, Inc. entered into a credit facility with a borrowing capacity of $100 million that expires in March 2022. The Company utilizes this facility for short-term financing needs. Interest rates for this facility are calculated at either LIBOR or the “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. At December 31, 2018, the applicable margin is 1.125% for loans bearing interest with reference to LIBOR and 0.125% for loans bearing interest with reference to the alternative base rate. The Company is also required to pay a commitment fee, of 0.15% per annum, on the unfunded portion of the commitments, which was not significant for the period ended December 31, 2018. Borrowings under the credit facility ranged from none at various times throughout 2018 to a high of $23.5 million during the first quarter of 2018. At December 31, 2018 and 2017, there were no borrowings and $23.5 million outstanding under this facility, respectively, with a weighted average interest rate of 2.65% at December 31, 2017. At December 31, 2018, Southwest Holdings, Inc. was in compliance with all of its credit facility covenants. Interest and fees on the credit facility are subject to adjustment depending on its bond ratings. The credit facility is subject to a leverage ratio cap. No specific dividend restrictions exist under the collective covenants. The credit facility does not contain a material adverse change clause. As discussed in Note 8 - Long-Term Debt, Southwest has a $400 million credit facility that is scheduled to expire in March 2022, of which $250 million has been designated by management for working capital purposes. Southwest had $152 million of short-term borrowings outstanding at December 31, 2018 and $191 million short-term borrowings outstanding at December 31, 2017 with weighted average interest rates of 3.47% and 2.52%, respectively.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 10 - Commitments and Contingencies The Company and Southwest are defendants in miscellaneous legal proceedings. The Company and Southwest are also parties to various regulatory proceedings. The ultimate dispositions of these proceedings are not presently determinable; however, it is the opinion of management that no litigation or regulatory proceeding to which the Company and Southwest are currently subject to will have a material adverse impact on their financial position, results of operations, or cash flows. Southwest maintains liability insurance for various risks associated with the operation of its natural gas pipelines and facilities. In connection with these liability insurance policies, Southwest is responsible for an initial deductible or self-insured retention amount per incident, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year August 2018 to July 2019, these liability insurance policies require Southwest to be responsible for the first $1 million (self-insured retention) of each incident plus the first $4 million in aggregate claims above its self-insured retention in the policy year. Through an assessment process, Southwest may determine that certain costs are likely to be incurred in the future related to specific legal matters. In these circumstances and in accordance with accounting policies, Southwest will make an accrual, as necessary. Centuri maintains liability insurance for various risks associated with its operations. In connection with these liability insurance policies, Centuri is responsible for an initial deductible or self-insured retention amount per occurrence, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year April 2018 to March 2019, Centuri is responsible for the first $300,000 (self-insured retention) per occurrence under these liability insurance policies.

Pension and Other Postretirement Benefits

12 Months Ended

Dec. 31, 2018

Retirement Benefits [Abstract]

Note 11 - Pension and Other Postretirement Benefits

An Employees’ Investment Plan is offered to eligible employees of Southwest through deduction of a percentage of base compensation, subject to IRS limitations. The Employees’ Investment Plan provides for purchases of various mutual fund investments and Company common stock. One-half of amounts deferred by employees are matched, up to a maximum matching contribution of 3.5% of an employee’s annual compensation. The cost of the plan is disclosed below (in thousands):



	2018		2017		2016
Employee Investment Plan cost	$	5,530	$	5,112	$	4,976

Centuri has a separate plan, the cost and liability of which are not significant.

A deferred compensation plan is offered to all officers of Southwest and a separate deferred compensation plan for members of the Company’s Board of Directors. The plans provide the opportunity to defer up to 100% of annual cash compensation. One-half of amounts deferred by officers are matched, up to a maximum matching contribution of 3.5% of an officer’s annual base salary. Upon retirement, payments of compensation deferred, plus interest, are made in equal monthly installments over 10, 15, or 20 years, as elected by the participant. Directors have an additional option to receive such payments over a five-year period. Deferred compensation earns interest at a rate determined each January. The interest rate equals 150% of Moody’s Seasoned Corporate Bond Rate Index.

A noncontributory qualified retirement plan with defined benefits covering substantially all Southwest employees is available in addition to a separate unfunded supplemental executive retirement plan (“SERP”) which is limited to Southwest’s officers. Postretirement benefits other than pensions (“PBOP”) are provided to qualified retirees for health care, dental, and life insurance benefits.

The overfunded or underfunded positions of defined benefit postretirement plans, including pension plans, are recognized in the Consolidated Balance Sheets. Any actuarial gains and losses, prior service costs and transition assets or obligations are recognized in Accumulated other comprehensive income under Stockholders’ equity, net of tax, until they are amortized as a component of net periodic benefit cost.

A regulatory asset has been established for the portion of the total amounts otherwise chargeable to Accumulated other comprehensive income that are expected to be recovered through rates in future periods. Changes in actuarial gains and losses and prior service costs pertaining to the regulatory asset will be recognized as an adjustment to the regulatory asset account as these amounts are amortized and recognized as components of net periodic pension costs each year.

The qualified retirement plan invests the majority of its plan assets in common collective trusts which includes a well-diversified portfolio of domestic and international equity securities and fixed income securities, which are managed by a professional investment manager appointed by Southwest. The investment manager has full discretionary authority to direct the investment of plan assets held in trust within the specific guidelines prescribed by Southwest through the plan’s investment policy statement. In 2016, Southwest adopted a liability driven investment (“LDI”) strategy for part of the portfolio, a form of investing designed to better match the movement in pension plan assets with the impact of interest rate changes and inflation assumption changes on the pension plan liability. The implementation of the LDI strategy will be phased in over time by using a glide path. The glide path is designed to increase the allocation of the plan’s assets to fixed income securities, as the funded status of the plan increases, in order to more closely match the duration of the plan assets to that of the plan liability. Pension plan assets are held in a Master Trust. The pension plan funding policy is in compliance with the federal government’s funding requirements.

Pension costs for these plans are affected by the amount and timing of cash contributions to the plans, the return on plan assets, discount rates, and by employee demographics, including age, compensation, and length of service. Changes made to the provisions of the plans may also impact current and future pension costs. Actuarial formulas are used in the determination of pension costs and are affected by actual plan experience and assumptions about future experience. Key actuarial assumptions include the expected return on plan assets, the discount rate used in determining the projected benefit obligation and pension costs, and the assumed rate of increase in employee compensation. Relatively small changes in these assumptions, particularly the discount rate, may significantly affect pension costs and plan obligations for the qualified retirement plan. In determining the discount rate, management matches the plan’s projected cash flows to a spot-rate yield curve based on highly rated corporate bonds. Changes to the discount rate from year-to-year, if any, are generally made in increments of 25 basis points.

There was a 75 basis points increase in the discount rate between years, as reflected below. The methodology utilized to determine the discount rate was consistent with prior years. The weighted-average rate of compensation increase remained the same (consistent with management’s expectations overall). The asset return assumption (which impacts the following year’s expense) remained unchanged. The rates are presented in the table below:



	December 31, 2018		December 31, 2017
Discount rate	4.50	%	3.75	%
Weighted-average rate of compensation increase	3.25	%	3.25	%
Asset return assumption	7.00	%	7.00	%

Pension expense for 2019 is estimated to be less than that experienced in 2018. Future years’ expense level movements (up or down) will continue to be greatly influenced by long-term interest rates, asset returns, and funding levels.

The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income.



	2018									2017
	Qualified Retirement Plan			SERP			PBOP			Qualified Retirement Plan			SERP			PBOP
(Thousands of dollars)
Change in benefit obligations
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$	1,203,484		$	45,727		$	75,322		$	1,048,353		$	43,311		$	73,865
Service cost	28,555			245			1,473			23,392			309			1,468
Interest cost	44,174			1,658			2,748			46,083			1,883			3,232
Actuarial loss (gain)	(102,919		)	(3,940		)	(6,020		)	133,017			3,334			(71		)
Benefits paid	(57,280		)	(3,087		)	(3,567		)	(47,361		)	(3,110		)	(3,172		)
Benefit obligation at end of year (PBO/PBO/APBO)	1,116,014			40,603			69,956			1,203,484			45,727			75,322
Change in plan assets
Market value of plan assets at beginning of year	871,665			—			54,608			738,962			—			48,113
Actual return on plan assets	(67,771		)	—			(3,061		)	144,064			—			7,742
Employer contributions	44,000			3,087			—			36,000			3,110			—
Benefits paid	(57,280		)	(3,087		)	(4,206		)	(47,361		)	(3,110		)	(1,247		)
Market value of plan assets at end of year	790,614			—			47,341			871,665			—			54,608
Funded status at year end	$	(325,400	)	$	(40,603	)	$	(22,615	)	$	(331,819	)	$	(45,727	)	$	(20,714	)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50		%	4.50		%	4.50		%	3.75		%	3.75		%	3.75		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	N/A			3.25		%	3.25		%	N/A

Estimated funding for the plans above during calendar year 2019 is approximately $55 million, of which $52 million pertains to the retirement plan. Management monitors plan assets and liabilities and could, at its discretion, increase plan funding levels above the minimum in order to achieve a desired funded status and avoid or minimize potential benefit restrictions.

The accumulated benefit obligation for the retirement plan and the SERP is presented below (in thousands):



	December 31, 2018		December 31, 2017
Retirement plan	$	1,024,030	$	1,088,203
SERP	38,793		44,343

Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows (in millions):



	2019		2020		2021		2022		2023		2024-2028
Pension	$	54.0	$	55.0	$	56.0	$	58.0	$	59.0	$	319.0
SERP	3.0		2.9		2.9		2.9		2.8		13.6
PBOP	4.5		4.6		4.6		4.6		4.5		20.9

No assurance can be made that actual funding and benefits paid will match these estimates.

For PBOP measurement purposes, the per capita cost of the covered health care benefits medical rate trend assumption is 6.0%, declining to 4.5%. Fixed contributions are made for health care benefits of employees who retire after 1988, but Southwest pays all covered health care costs for employees who retired prior to 1989. The medical trend rate assumption noted above applies to the benefit obligations of pre-1989 retirees only.

As of January 1, 2018, the Company adopted “Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” The update requires that an employer report the service cost component in the same line item or items as other compensation costs arising from services rendered by the employees during the period. The other components of net benefit cost are required to be presented in the income statement separately from the service cost component and outside a subtotal of income from operations and be appropriately described. The update also allows only the service cost component (and not the other components of periodic benefit costs) to be eligible for capitalization when applicable, making no exception for specialized industries, including rate-regulated industries. This guidance is required to be applied on a retrospective basis for the presentation of the service cost and other components of net benefit cost, and on a prospective basis for the capitalization of only the service cost component of net benefit cost. Amounts capitalized as part of assets prior to the date of adoption were not adjusted through a cumulative effect adjustment. The guidance allows a practical expedient for the retrospective application that permits use of the amounts disclosed for the various components of net benefit cost in the pension and other postretirement benefit plans footnote as the basis for the retrospective application. This is in lieu of determining how much of the various components of net benefit cost were actually reflected in the income statement each period as a result of capitalization of certain costs into assets and their subsequent amortization. The Company and Southwest have elected to utilize the practical expedient.

Therefore, upon adoption of the update to Topic 715, amounts presented in the Company’s and Southwest’s Consolidated Statements of Income for the years ended 2017 and 2016 have been revised as required by the update, as follows (in thousands):



	December 31, 2017								December 31, 2016
	As Reported		Reclassification			Revised			As Reported		Reclassification			Revised
Southwest Gas Holdings, Inc.
Operations and maintenance	$	412,187	$	(19,424	)	$	392,763		$	401,724	$	(19,760	)	$	381,964
Other income (deductions)	13,394		(19,424		)	(6,030		)	9,469		(19,760		)	(10,291		)

Southwest Gas Corporation
Operations and maintenance	$	410,745	$	(19,424	)	$	391,321		$	401,724	$	(19,760	)	$	381,964
Other income (deductions)	13,036		(19,424		)	(6,388		)	8,276		(19,760		)	(11,484		)

Operating income increased by the same amounts that Operations and maintenance expense decreased as reflected in the table above; however, net income was not impacted by this reclassification for either the Company or Southwest.

The service cost component of net periodic benefit costs included in the table below is part of an overhead loading process associated with the cost of labor (refer to discussion above in association with the update to Topic 715). The overhead process ultimately results in allocation of that portion of overall net periodic benefit costs to the same accounts to which productive labor is charged. As a result, service costs become components of various accounts, primarily Operations and maintenance expense, Net utility plant, and Deferred charges and other assets for both the Company and Southwest. Refer to the discussion above regarding the practical expedient elected related to amounts capitalized as part of assets prior to the adoption date.

Components of net periodic benefit cost:



	Qualified Retirement Plan									SERP									PBOP
	2018			2017			2016			2018			2017			2016			2018			2017			2016
(Thousands of dollars)
Service cost	$	28,555		$	23,392		$	22,833		$	245		$	309		$	331		$	1,473		$	1,468		$	1,499
Interest cost	44,174			46,083			46,027			1,658			1,883			1,859			2,748			3,232			3,180
Expected return on plan assets	(58,755		)	(55,196		)	(56,558		)	—			—			—			(3,718		)	(3,358		)	(3,149		)
Amortization of prior service cost	—			—			—			—			—			—			1,335			1,335			1,335
Amortization of net actuarial loss	32,115			24,004			25,266			1,502			1,441			1,383			—			—			417
Net periodic benefit cost	$	46,089		$	38,283		$	37,568		$	3,405		$	3,633		$	3,573		$	1,838		$	2,677		$	3,282
Weighted-average assumptions (net benefit cost)
Discount rate	3.75		%	4.50		%	4.50		%	3.75		%	4.50		%	4.50		%	3.75		%	4.50		%	4.50		%
Expected return on plan assets	7.00		%	7.00		%	7.25		%	N/A			N/A			N/A			7.00		%	7.00		%	7.25		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	N/A			N/A			N/A

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income



	2018												2017												2016
	Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP
(Thousands of dollars)
Net actuarial loss (gain) (a)	$	20,426		$	23,607		$	(3,940	)	$	759		$	43,027		$	44,149		$	3,334		$	(4,456	)	$	22,770		$	25,153		$	1,347		$	(3,730	)
Amortization of prior service cost (b)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)
Amortization of net actuarial loss (b)	(33,617		)	(32,115		)	(1,502		)	—			(25,445		)	(24,004		)	(1,441		)	—			(27,066		)	(25,266		)	(1,383		)	(417		)
Regulatory adjustment	8,233			7,657			—			576			(12,340		)	(18,131		)	—			5,791			5,584			102			—			5,482
Recognized in other comprehensive (income) loss	(6,293		)	(851		)	(5,442		)	—			3,907			2,014			1,893			—			(47		)	(11		)	(36		)	—
Net periodic benefit costs recognized in net income	51,332			46,089			3,405			1,838			44,593			38,283			3,633			2,677			44,423			37,568			3,573			3,282
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	45,039		$	45,238		$	(2,037	)	$	1,838		$	48,500		$	40,297		$	5,526		$	2,677		$	44,376		$	37,557		$	3,537		$	3,282

The table above discloses the net gain or loss and prior service cost recognized in other comprehensive income, separated into (a) amounts initially recognized in other comprehensive income, and (b) amounts subsequently recognized as adjustments to other comprehensive income as those amounts are amortized as components of net periodic benefit cost.

See also Note 6 - Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI").

The following table sets forth, by level within the three-level fair value hierarchy, the fair values of the assets of the qualified pension plan and the PBOP as of December 31, 2018 and December 31, 2017. The SERP has no assets.



	December 31, 2018						December 31, 2017
	Qualified Retirement Plan		PBOP		Total		Qualified Retirement Plan		PBOP		Total
Assets at fair value (thousands of dollars):
Level 1 – Quoted prices in active markets for identical financial assets
Mutual funds	$	—	$	25,299	$	25,299	$	—	$	27,020	$	27,020
Total Level 1 Assets (1)	$	—	$	25,299	$	25,299	$	—	$	27,020	$	27,020
Level 2 – Significant other observable inputs
Private commingled equity funds (2)
International	$	309,745	$	8,484	$	318,229	$	340,217	$	10,577	$	350,794
U.S. equity securities	147,693		4,045		151,738		165,937		5,158		171,095
Emerging markets	50,817		1,392		52,209		56,259		1,749		58,008
Private commingled fixed income funds (3)	274,062		7,506		281,568		301,217		9,364		310,581
Pooled funds and mutual funds	5,198		610		5,808		4,676		735		5,411
Government fixed income and mortgage backed securities	163		5		168		172		5		177
Total Level 2 assets (4)	$	787,678	$	22,042	$	809,720	$	868,478	$	27,588	$	896,066
Total Plan assets at fair value	$	787,678	$	47,341	$	835,019	$	868,478	$	54,608	$	923,086
Insurance company general account contracts (5)	2,936		—		2,936		3,187		—		3,187
Total Plan assets	$	790,614	$	47,341	$	837,955	$	871,665	$	54,608	$	926,273


(1)	The Mutual funds category above is a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks, and fixed-income securities. The fund seeks regular income, conservation of principal, and an opportunity for long-term growth of principal and income.

(2)

The private commingled equity funds include common collective trusts that invest in a diversified portfolio of domestic and international securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value (“NAV”), which is the value of securities in the fund less the amount of any liabilities outstanding. Strategies employed by the funds include investment in:


•	International developed countries equities


•	Domestic equities


•	Emerging markets equities

Shares in the private commingled equity funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (Level 2) and are therefore not excluded from the body of the fair value table as a reconciling item.

Two funds are classified as international funds. One invests in international financial markets, primarily those of developed economies in Europe and the Pacific Basin. The fund invests primarily in equity securities issued by foreign corporations, but may invest in other securities perceived as offering attractive investment return opportunities. The other fund provides diversified exposure to global equity markets. The fund seeks to provide long-term capital growth by investing primarily in securities listed on the major developed equity markets of the U.S., Europe, and Asia, as well as within those listed on emerging country equity markets on a tactical basis.

The domestic equities securities funds include a large and medium capitalization fund and a small capitalization fund. The large and medium capitalization fund is designed to track the performance of the large and medium capitalization companies contained in the index, which represents approximately 90% of the market capitalization of the U.S. stock market. The small capitalization fund is designed to provide maximum long-term appreciation through investments that are well diversified by industry.

The emerging markets fund was developed to invest in emerging market equities worldwide. The purposes of the fund’s operations, “emerging market countries,” include every country in the world except the developed markets of the U.S., Canada, Japan, Australia, New Zealand, Hong Kong, and Singapore, and most countries located in Western Europe. Fund investments are made directly in each country or, where direct investment is inefficient or prohibited, through appropriate financial instruments or participation in commingled funds.

(3)

The private commingled fixed income funds consist primarily of fixed income debt securities issued by the U.S. Treasury, government agencies, and fixed income debt securities issued by corporations. The fixed income fund investments may include the use of high yield, international fixed income securities and other instruments, including derivatives, to ensure prudent diversification over a broad spectrum of investments. The changes in the value of the fixed income funds are intended to offset the changes in the pension plan liabilities due to changes in the discount rate.

These funds are shown in the above table at NAV. Shares in the private commingled fixed equity funds may be redeemed given one business day notice. While they are private fixed income funds and reported at NAV, due to the short redemption notice period, the lack of redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (Level 2), and are also not excluded from the body of the fair value table as a reconciling item.


(4)	With the exception of items (2) and (3), which are discussed in detail above, the Level 2 assets consist mainly of pooled funds and mutual funds. These funds are collective short-term funds that invest in Treasury bills and money market funds and are used as a temporary cash repository.


(5)	The insurance company general account contracts are annuity insurance contracts used to pay the pensions of employees who retired prior to 1989. The balance of the account disclosed in the above table is the contract value, which is the result of deposits, withdrawals, and interest credits.

Share-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-Based Compensation

Note 12 - Share-Based Compensation

At December 31, 2018, three share-based compensation plans existed at Southwest: an omnibus incentive plan, a management incentive plan, and a restricted stock/unit plan. All previous grants under the stock option plan expired in 2016. The table below shows total share-based plan compensation expense which was recognized in the Consolidated Statements of Income (in thousands):



	2018		2017		2016
Share-based compensation plan expense, net of related tax benefits	$	4,644	$	6,751	$	7,185
Share-based compensation plan related tax benefits	1,467		4,137		4,404

Under the option plan, options to purchase shares of common stock at a stated exercise price were previously granted to key employees and outside directors. The last option grants were in 2006 and no future grants are currently anticipated. Each option had an exercise price equal to the market price of the Company’s common stock on the date of grant and a maximum term of ten years. The final options were exercised in 2016.

The following tables summarize the stock option plan activity and related information (thousands of options):



	2018		2017		2016
	Number of options	Weighted- average exercise price	Number of options	Weighted- average exercise price	Number of options		Weighted- average exercise price
Outstanding at the beginning of the year	—	N/A	—	N/A	17		$	31.64
Exercised during the year	—	—	—	—	(17	)	31.64
Forfeited or expired during the year	—	—	—	—	—		—
Outstanding and exercisable at year end	—	N/A	—	N/A	—		N/A

The intrinsic value of a stock option is the amount by which the market value of the underlying stock exceeds the exercise price of the option. The aggregate intrinsic value of outstanding and exercisable options, and options that were exercised, are presented in the table below (in thousands):



	2018		2017		2016
Outstanding and exercisable	$	—	$	—	$	—
Exercised	—		—		554



	December 31, 2018		December 31, 2017		December 31, 2016
Market value of Company stock	$	76.50	$	80.48	$	76.62

In 2017, the Board of Directors of the Company and shareholders approved the omnibus incentive plan. The purpose of the omnibus incentive plan is to promote the long-term growth and profitability of the Company by providing directors, employees, and certain other individuals with incentives to increase shareholder value and otherwise contribute to the success of the Company. In addition, the plan will enable the Company to attract, retain, and reward the best available persons for positions of responsibility. The omnibus incentive plan provides for the grant of stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, and other equity-based and cash awards. Employees, directors, and consultants who provide services to the Company or any subsidiary may be eligible under this plan.

Under the management incentive plan, awards were historically granted to encourage key employees of Southwest to remain as employees and to achieve short-term and long-term performance goals. Plan participants were eligible to receive a cash bonus (i.e., short-term incentive) and shares (i.e., long-term incentive). The shares granted vest three years after grant and are then issued as common stock. No new share grants will be made under the management incentive plan as all future incentive share compensation will be granted under the omnibus incentive plan.

Restricted stock/units under the restricted stock/unit plan were issued to attract, motivate, retain, and reward key employees of Southwest with an incentive to attain high levels of individual performance and improved financial performance. The restricted stock/units vest 40% at the end of year one and 30% at the end of years two and three and are issued annually as common stock in accordance with the percentage vested. The restricted stock/unit plan was also established to attract, motivate, and retain experienced and knowledgeable independent directors. Vesting for grants of restricted stock/units to directors occurred immediately upon grant. The issuance of common stock for directors currently occurs when their service on the Board ends. No new grants will be made under the legacy restricted stock/unit plan as all future incentive compensation, including restricted stock, will be granted under programs of the omnibus incentive plan. For grants under the omnibus incentive plan, directors continue to immediately vest in the shares upon grant but are provided the option to defer receipt of equity compensation until they leave the Board of Directors. With regard to management, grants of time-lapse restricted stock vest based on the same percentages indicated above under the legacy program.

Performance-based incentive opportunities under the omnibus plan were granted to all officers of Southwest in the form of performance shares and will be based on, depending on the officer, consolidated earnings per share, utility net income, and utility return on equity, with an adjustment based on relative total shareholder return, in each case, measured over a three-year performance period from January 1, 2018 to December 31, 2020 for the performance shares granted in 2018, and from January 1, 2017 to December 31, 2019 for the performance shares granted in 2017. Southwest recorded $2.1 million and $1.2 million of estimated compensation expense associated with these shares during 2018 and 2017, respectively.

The following table summarizes the activity of the management incentive plan shares and restricted stock/units as of December 31, 2018 (thousands of shares):



	Management Incentive Plan Shares		Weighted- average grant date fair value		Restricted Stock/ Units (1)		Weighted- average grant date fair value
Nonvested/unissued at December 31, 2017	127		$	63.98	305		$	57.41
Granted	—				77		69.16
Dividends	2				7
Forfeited or expired	(1	)	66.86		(1	)	72.39
Vested and issued (2)	(63	)	59.41		(65	)	71.00
Nonvested/unissued at December 31, 2018	65		$	66.51	323		$	56.16


(1)	The number of securities granted includes 34,000 performance shares, which was derived by assuming that target performance will be achieved during the relevant performance period.


(2)	Includes shares for retiree payouts and those converted for taxes.

The weighted average grant date fair value of management incentive plan shares granted in 2017 and 2016 was $85.44 and $59.05, respectively. The weighted average grant date fair value of restricted stock/units granted in 2017 and 2016 was $85.39 and $60.39, respectively.

As of December 31, 2018, total compensation cost related to nonvested management incentive plan shares and restricted stock/units not yet recognized is $4.0 million, which is expected to be recognized over a weighted average period of 1.5 years.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

Note 13 - Income Taxes

On December 22, 2017, the TCJA was enacted. The majority of the provisions of the TCJA are effective for taxable years beginning after December 31, 2017. The TCJA significantly changes the taxation of business entities with specific provisions for regulated public utilities, such as Southwest.

The following are the major provisions (not all-inclusive) of the TCJA’s impact on the Company and Southwest:


•	Reduction of the federal income tax rate from 35% to 21%, effective January 1, 2018.


•	Bonus depreciation considerations for utility property placed-in-service after September 27, 2017.


•	100% bonus depreciation for most non-utility property placed-in-service after September 27, 2017.


•	Interest expense limitations for interest allocable to non-utility businesses. Interest expense allocable to utility businesses will have no limitation.

Under U.S. GAAP, specifically ASC Topic 740 “Income Taxes” (“ASC 740”), the tax effects of changes in tax laws must be recognized in the period in which the law is enacted. Therefore, the TCJA impacted the Company’s and Southwest’s financial statements in the fourth quarter of 2017. ASC 740 also requires deferred tax assets and liabilities to be re-measured at the enacted tax rate expected to apply when temporary differences are to be realized or settled. Thus, at the date of enactment, the Company’s deferred taxes were re-measured using the new federal income tax rate (21%). For regulated entities, the reduction in plant-related deferred tax liabilities is recorded as a regulatory liability to be refunded to customers. For unregulated operations, the change in deferred taxes is recorded as an adjustment to deferred tax expense.

The staff of the SEC recognized the complexity of determining the impact of the TCJA, and on December 22, 2017 issued guidance in Staff Accounting Bulletin 118 (“SAB 118”). SAB 118 provides that to the extent the accounting for certain income tax effects of the TCJA is incomplete, but a company can determine a reasonable estimate for those effects, the company may include in its financial statements the reasonable estimate that it had determined. The reasonable estimate would be reported as a provisional amount in the company’s financial statements during a “measurement period”, not to exceed one year from the date of enactment of the TCJA.

Southwest and the Company included provisional reasonable estimates for the measurement and accounting of the effects of the TCJA, which were reflected in the consolidated financial statements as of and for the year ended December 31, 2017. The Company and Southwest continued to analyze and refine the estimate and classification of all provisional items, during the measurement period, as additional accounting, regulatory, and U.S. Treasury guidance was provided. Adjustments made to the provisional amounts allowed under SAB 118 were identified and recorded as discrete adjustments during the year ended December 31, 2018. The accounting was completed in the fourth quarter of 2018.

Southwest Gas Holdings, Inc.:

The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations (thousands of dollars):



Year ended December 31,	2018		2017		2016
U.S.	$	235,120	$	246,131	$	218,810
Foreign	8,216		12,899		12,713
Total income before income taxes	$	243,336	$	259,030	$	231,523

Income tax expense (benefit) consists of the following (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Current:
Federal	$	(13,476	)	$	(1,316	)	$	541
State	(3,219		)	2,965			5,748
Foreign	2,563			5,203			4,298
	(14,132		)	6,852			10,587
Deferred:
Federal	67,784			58,443			68,270
State	8,901			1,837			140
Foreign	(869		)	(2,044		)	(529		)
	75,816			58,236			67,881
Total income tax expense	$	61,684		$	65,088		$	78,468

Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Deferred federal and state:
Property-related items	$	94,899		$	44,516		$	76,217
Purchased gas cost adjustments	(3,507		)	8,500			361
Employee benefits	(7,334		)	(2,517		)	(1,327		)
Regulatory Adjustments	2,412			14,401			6,322
All other deferred	(10,041		)	(5,935		)	(12,854		)
Total deferred federal and state	76,429			58,965			68,719
Deferred ITC, net	(613		)	(729		)	(838		)
Total deferred income tax expense	$	75,816		$	58,236		$	67,881

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate for 2018, 2017, and 2016 (and the sources of these differences and the effect of each) are summarized as follows:



Year Ended December 31,	2018		2017		2016
U.S. federal statutory income tax rate	21.0	%	35.0	%	35.0	%
Net state taxes	2.9		1.1		1.4
Tax credits	(0.3	)	(0.4	)	(0.4	)
Company owned life insurance	0.1		(1.6	)	(1.2	)
Change in U.S. Federal Income Tax Rate	—		(7.8	)	—
All other differences	1.6		(1.2	)	(0.9	)
Consolidated effective income tax rate	25.3	%	25.1	%	33.9	%

Deferred tax assets and liabilities consist of the following (thousands of dollars):



December 31,	2018			2017
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	105,791		$	98,912
Employee benefits	39,215			31,323
Alternative minimum tax credit	21,603			4,390
Net operating losses and credits	13,125			11,460
Interest rate swap	2,235			3,037
Other	17,215			13,870
Valuation allowance	(1,132		)	(728		)
	198,052			162,264
Deferred tax liabilities:
Property-related items, including accelerated depreciation	678,307			598,371
Regulatory balancing accounts	6,097			6,067
Unamortized ITC	368			981
Debt-related costs	3,110			3,380
Intangibles	7,807			7,656
Other	30,300			21,289
	725,989			637,744
Net noncurrent deferred tax liabilities	$	527,937		$	475,480

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars):



	2018		2017
Unrecognized tax benefits at beginning of year	$	1,430	$	1,231
Gross increases – tax positions in prior period	—		100
Gross decreases – tax positions in prior period	459		—
Gross increases – current period tax positions	—		99
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	971	$	1,430

Southwest Gas Corporation:

The following is a summary of income before taxes for continuing and discontinued operations (refer to Note 1 - Background, Organization, and Summary of Significant Accounting Policies) (thousands of dollars):



Year ended December 31,	2018		2017		2016
Income from continuing operations before income taxes	$	182,833	$	219,953	$	178,007
Income from discontinued operations before income taxes	—		—		53,516
Total income before income taxes	$	182,833	$	219,953	$	231,523

Income tax expense (benefit) consists of the following (thousands of dollars):



Year Ended December 31,	2018			2017		2016
Current:
Federal	$	(17,584	)	$	318	$	(9,695	)
State	(6,783		)	1,420		2,510
	(24,367		)	1,738		(7,185		)
Deferred:
Federal	58,136			60,662		66,037
State	10,222			735		(268		)
	68,358			61,397		65,769
Total income tax expense from continuing operations	43,991			63,135		58,584
Discontinued operations	—			—		19,884
Total income tax expense	$	43,991		$	63,135	$	78,468

Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Deferred federal and state:
Property-related items	$	67,576		$	49,129		$	72,811
Purchased gas cost adjustments	(3,507		)	8,500			361
Employee benefits	2,156			(5,707		)	(139		)
Regulatory Adjustments	2,412			14,401			6,322
All other deferred	334			(4,197		)	(12,748		)
Total deferred federal and state	68,971			62,126			66,607
Deferred ITC, net	(613		)	(729		)	(838		)
Total deferred income tax expense	$	68,358		$	61,397		$	65,769



Year Ended December 31,	2018		2017		2016
U.S. federal statutory income tax rate	21.0	%	35.0	%	35.0	%
Net state taxes	2.1		0.6		0.8
Tax credits	(0.4	)	(0.4	)	(0.5	)
Company owned life insurance	0.3		(1.7	)	(1.5	)
Change in U.S. Federal Income Tax Rate	—		(3.6	)	—
All other differences	1.1		(1.2	)	(0.9	)
Effective income tax rate from continuing operations	24.1	%	28.7	%	32.9	%

Deferred tax assets and liabilities consist of the following (thousands of dollars):



December 31,	2018			2017
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	105,791		$	98,912
Employee benefits	17,337			18,707
Alternative minimum tax credit	21,603			4,390
Net operating losses and credits	4,557			10,070
Interest rate swap	2,235			3,037
Other	9,386			8,820
Valuation allowance	(37		)	(58		)
	160,872			143,878
Deferred tax liabilities:
Property-related items, including accelerated depreciation	614,205			561,493
Regulatory balancing accounts	6,097			6,067
Unamortized ITC	368			981
Debt-related costs	3,110			3,380
Other	27,550			17,200
	651,330			589,121
Net deferred tax liabilities	$	490,458		$	445,243

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars):



	2018		2017
Unrecognized tax benefits at beginning of year	$	1,069	$	903
Gross increases – tax positions in prior period	—		67
Gross decreases – tax positions in prior period	98		—
Gross increases – current period tax positions	—		99
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	971	$	1,069

The Company’s regulated operations accounting for income taxes is impacted by the FASB’s ASC 980 – Regulated Operations. Reductions in accumulated deferred income tax balances due to the reduction in the corporate income tax rates to 21% under the provisions of the TCJA may result in a refund of excess deferred taxes to customers, generally through reductions in future rates. The TCJA includes provisions that stipulate how these excess deferred taxes may be passed back to customers for certain accelerated tax depreciation benefits. Potential refunds of other deferred taxes will be determined in conjunction with appropriate regulatory commissions. As part of the recently concluded Nevada general rate case, excess deferred taxes will begin being refunded to customers starting in January 2019. The December 31, 2018 balance sheets of Southwest and the Company reflect the impact of the TCJA with a recorded regulatory liability of $456 million.

The Company and its subsidiaries file a consolidated federal income tax return in the U.S. and in various states, as well as in Canada. With few exceptions, the Company is no longer subject to U.S. federal, state and local, or Canadian income tax examinations for years before 2014.

The Company and each of its subsidiaries, including Southwest, participate in a tax sharing agreement to establish the method for allocating tax benefits and losses among members of the consolidated group. The consolidated federal income tax is apportioned among the subsidiaries using a separate return method.

At December 31, 2018, the Company has a federal net operating loss carryforward of $62.5 million which may be carried forward indefinitely. The Company also has general business credits of $3.5 million, which begin to expire in 2035. The Company has net capital loss carryforwards of $107,000, which will begin to expire in 2019. At December 31, 2018, the Company has an income tax net operating loss carryforward related to Canadian operations of $7.6 million, which begins to expire in 2034.

Management intends to continue to permanently reinvest any future foreign earnings in Canada.

In assessing whether uncertain tax positions should be recognized in its financial statements, management first determines whether it is more-likely-than-not that a tax position will be sustained upon examination, including resolution of any related appeals or litigation processes, based on the technical merits of the position. In evaluations of whether a tax position has met the more-likely-than-not recognition threshold, management presumes that the position will be examined by the appropriate taxing authority that would have full knowledge of all relevant information. For tax positions that meet the more-likely-than-not recognition threshold, management measures the amount of benefit recognized in the financial statements at the largest amount of benefit that is greater than 50% likely of being realized upon ultimate settlement. Unrecognized tax benefits are recognized in the first financial reporting period in which information becomes available indicating that such benefits will more-likely-than-not be realized. For each reporting period, management applies a consistent methodology to measure unrecognized tax benefits, and all unrecognized tax benefits are reviewed periodically and adjusted as circumstances warrant. Measurement of unrecognized tax benefits is based on management’s assessment of all relevant information, including prior audit experience, the status of audits, conclusions of tax audits, lapsing of applicable statutes of limitation, identification of new issues, and any administrative guidance or developments.

At December 31, 2018, the total amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate was $1 million individually for both the Company and Southwest. No significant increases or decreases in unrecognized tax benefit are expected within the next 12 months.

The Company and Southwest recognize interest expense and income and penalties related to income tax matters in income tax expense. There was no tax-related interest income for 2018, 2017, and 2016.

Derivatives

12 Months Ended

Dec. 31, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivatives

Note 14 - Derivatives

Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts qualify as derivative instruments; however, because the contract price is the prevailing price at the future transaction date, the contract has no determinable fair value. The Swaps’ contract prices are determined at the beginning of each month to reflect that month’s published first of month index price and are recorded at fair value. Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.

The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 25% in the Arizona and California jurisdictions) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from January 2019 through October 2020. Under such contracts, Southwest pays the counterparty a fixed rate and receives from the counterparty a floating rate per MMBtu (“dekatherm”) of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):

The following table sets forth the gains and (losses) recognized on Southwest’s Swaps (derivatives) for the years ended December 31, 2018, 2017, and 2016 and their location in the Consolidated Statements of Income:



Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	2018				2017				2016
Swaps	Net cost of gas sold	$	(2,113	)		$	(11,572	)		$	5,006
Swaps	Net cost of gas sold	2,113			*	11,572			*	(5,006		)	*
Total		$	—			$	—			$	—

* Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.

No gains (losses) were recognized in net income or other comprehensive income during the periods presented for derivatives designated as cash flow hedging instruments. Previously, Southwest entered into two forward-starting interest rate swaps (“FSIRS”), both of which were designated cash flow hedges, to partially hedge the risk of interest rate variability during the period leading up to the planned issuance of debt. The first FSIRS terminated in December 2010, and the second, in March 2012. Losses on both FSIRS are being amortized over ten-year periods from Accumulated other comprehensive income (loss) into interest expense.

The following table sets forth the fair values of the Swaps and their location in the Consolidated Balance Sheets of Southwest and the Company (thousands of dollars).

Fair values of derivatives not designated as hedging instruments:



December 31, 2018
Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Prepaid and other current assets	$	243	$	(99	)	$	144
Swaps	Other current liabilities	1,595		(3,347		)	(1,752		)
Swaps	Other deferred credits	141		(251		)	(110		)
Total		$	1,979	$	(3,697	)	$	(1,718	)

December 31, 2017
Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	11	$	(4,468	)	$	(4,457	)
Swaps	Prepaid and other current assets	19		(1,342		)	(1,323		)
Total		$	30	$	(5,810	)	$	(5,780	)

The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). Master netting arrangements exist with each counterparty that provide for the net settlement (in the settlement month) of all contracts through a single payment. As applicable, management has elected to reflect the net amounts in its balance sheets. No outstanding collateral associated with the Swaps existed during any period presented in the above table.

Pursuant to regulatory deferral accounting treatment for rate-regulated entities, unrealized gains and losses in fair value of the Swaps are recorded as a regulatory asset and/or liability. When the Swaps mature, any prior positions held are reversed and the settled position is recorded as an increase or decrease of purchased gas under the related PGA mechanism in determining deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income.

The following table presents the amounts paid to and received from counterparties for settlements of matured Swaps.



	Year ended December 31, 2018		Year ended December 31, 2017		Year ended December 31, 2016
(Thousands of dollars)
Paid to counterparties	$	6,781	$	3,100	$	5,583
Received from counterparties	$	606	$	1,685	$	726

The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Consolidated Balance Sheets (thousands of dollars).



December 31, 2018 Instrument	Balance Sheet Location	Net Total
Swaps	Other current liabilities	$	(144	)
Swaps	Prepaid and other current assets	1,752
Swaps	Deferred charges and other assets	110

December 31, 2017 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaid and other current assets	$	4,457
Swaps	Deferred charges and other assets	1,323

Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at December 31, 2018 and December 31, 2017 using futures settlement prices, published by the CME Group, for the delivery of natural gas at Henry Hub adjusted by the prices of basis future settlements, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs are observable in the marketplace throughout the full term of the Swaps, but have been credit-risk adjusted with no significant impact to the overall fair value measurement.

The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, financial assets and liabilities that were accounted for at fair value:

Level 2 – Significant other observable inputs



	December 31, 2018			December 31, 2017
(Thousands of dollars)
Assets at fair value:
Prepaid and other current assets – Swaps	$	144		$	—
Deferred charges and other assets – Swaps	—			—
Liabilities at fair value:
Other current liabilities – Swaps	(1,752		)	(4,457		)
Other deferred credits – Swaps	(110		)	(1,323		)
Net Assets (Liabilities)	$	(1,718	)	$	(5,780	)

No financial assets or liabilities associated with the Swaps, which were accounted for at fair value, fell within Level 1 or Level 3 of the fair value hierarchy.

Segment Information

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

Segment Information

Note 15 - Segment Information

The Company’s operating segments are determined based on the nature of their activities. The natural gas operations segment is engaged in the business of purchasing, distributing, and transporting natural gas. Revenues are generated from the distribution and transportation of natural gas. The utility infrastructure services segment is primarily engaged in the business of providing utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems, and providing industrial construction solutions. Although our utility infrastructure services operations are geographically dispersed, they are aggregated and reported as a single segment as each reporting unit has similar economic characteristics. Over 99% of the total Company’s long-lived assets are in the U.S.

The accounting policies of the reported segments are the same as those described within Note 1 - Background, Organization, and Summary of Significant Accounting Policies. Centuri accounts for the services provided to Southwest at contractual prices at contract inception. Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below (in thousands).



	December 31, 2018		December 31, 2017
Accounts receivable for Centuri services	$	18,830	$	12,987

The following table presents the amount of revenues for both segments by geographic area (thousands of dollars):



	December 31, 2018		December 31, 2017		December 31, 2016
Revenues (a)
United States	$	2,664,670	$	2,345,134	$	2,256,600
Canada	215,343		203,658		203,890
Total	$	2,880,013	$	2,548,792	$	2,460,490


(a)	Revenues are attributed to countries based on the location of customers.

The Company has two reportable segments: natural gas operations and utility infrastructure services. Southwest has a single reportable segment that is referred to herein as the natural gas operations segment of the Company. In order to reconcile to net income as disclosed in the Consolidated Statements of Income, an Other column is included associated with impacts related to corporate and administrative activities related to Southwest Gas Holdings, Inc. The financial information pertaining to the natural gas operations and utility infrastructure services segments for each of the three years in the period ended December 31, 2018 is as follows (thousands of dollars):



2018	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,357,728	$	1,386,371	$	—		$	2,744,099
Intersegment sales	—		135,914		—			135,914
Total	$	1,357,728	$	1,522,285	$	—		$	2,880,013
Interest revenue	$	6,020	$	88	$	—		$	6,108
Interest expense	$	81,740	$	14,190	$	741		$	96,671
Depreciation and amortization	$	191,816	$	57,396	$	—		$	249,212
Income tax expense	$	43,991	$	18,420	$	(727	)	$	61,684
Segment net income	$	138,842	$	44,977	$	(1,542	)	$	182,277
Segment assets	$	6,141,584	$	1,215,573	$	572		$	7,357,729
Capital expenditures	$	682,869	$	83,045	$	—		$	765,914

2017	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,302,308	$	1,149,325	$	—		$	2,451,633
Intersegment sales	—		97,159		—			97,159
Total	$	1,302,308	$	1,246,484	$	—		$	2,548,792
Interest revenue	$	2,784	$	3	$	—		$	2,787
Interest expense	$	69,733	$	7,986	$	345		$	78,064
Depreciation and amortization	$	201,922	$	49,029	$	—		$	250,951
Income tax expense	$	63,135	$	2,390	$	(437	)	$	65,088
Segment net income	$	156,818	$	38,360	$	(1,337	)	$	193,841
Segment assets	$	5,482,669	$	752,496	$	1,901		$	6,237,066
Capital expenditures	$	560,448	$	63,201	$	—		$	623,649

2016	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,321,412	$	1,040,957	$	—		$	2,362,369
Intersegment sales	—		98,121		—			98,121
Total	$	1,321,412	$	1,139,078	$	—		$	2,460,490
Interest revenue	$	1,848	$	1	$	—		$	1,849
Interest expense	$	66,997	$	6,663	$	—		$	73,660
Depreciation and amortization	$	233,463	$	55,669	$	—		$	289,132
Income tax expense	$	58,584	$	19,884	$	—		$	78,468
Segment net income	$	119,423	$	32,618	$	—		$	152,041
Segment assets	$	5,001,756	$	579,370	$	—		$	5,581,126
Capital expenditures	$	457,120	$	72,411	$	—		$	529,531

Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Quarterly Financial Data (Unaudited)

Note 16 - Quarterly Financial Data (Unaudited)



	Quarter Ended
	March 31		June 30		September 30			December 31
(Thousands of dollars, except per share amounts)
2018
Southwest Gas Holdings, Inc.:
Operating revenues	$	754,330	$	670,883	$	668,146		$	786,654
Operating income (4)	129,560		53,338		39,681			134,854
Net income	78,294		21,551		12,331			131,160
Net income attributable to Southwest Gas Holdings, Inc.	79,091		21,551		12,331			69,304
Basic earnings per common share (1)	1.63		0.44		0.25			1.36
Diluted earnings per common share (1)	1.63		0.44		0.25			1.36

Southwest Gas Corporation:
Operating revenues	$	494,313	$	275,679	$	217,523		$	370,213
Operating income (4)	141,173		24,675		3			115,962
Net income (loss)	90,349		2,622		(13,670		)	59,541

2017
Southwest Gas Holdings, Inc.:
Operating revenues	$	654,737	$	560,469	$	593,153		$	740,433
Operating income (4)	124,347		48,265		34,988			135,524
Net income	69,005		18,121		10,420			96,396
Net income attributable to Southwest Gas Holdings, Inc.	69,308		17,864		10,204			96,465
Basic earnings per common share (1)	1.46		0.38		0.21			2.00
Diluted earnings per common share (1)	1.45		0.37		0.21			2.00

Southwest Gas Corporation:
Operating revenues	$	462,602	$	260,162	$	213,059		$	366,485
Operating income (4)	135,922		32,346		9,921			117,885
Net income (loss)	76,938		9,522		(4,024		)	74,382



	Quarter Ended
	March 31			June 30		September 30			December 31
2016
Southwest Gas Holdings, Inc.: (2)
Operating revenues	$	731,248		$	547,748	$	539,969		$	641,525
Operating income (4)	139,036			33,057		20,478			122,903
Net income (loss)	75,355			9,099		2,907			65,694
Net income attributable to Southwest Gas Holdings, Inc.	75,446			8,943		2,472			65,180
Basic earnings per common share (1)	1.59			0.19		0.05			1.37
Diluted earnings per common share (1)	1.58			0.19		0.05			1.36

Southwest Gas Corporation: (2) (3)
Operating revenues
Continuing operations	$	525,100		$	255,648	$	200,179		$	340,485
Discontinued operations – utility infrastructure services	206,148			292,100		339,790			301,040
Total	$	731,248		$	547,748	$	539,969		$	641,525
Operating income (loss) (4)
Continuing operations	$	140,885		$	20,210	$	(5,292	)	$	100,685
Discontinued operations – utility infrastructure services	(1,849		)	12,847		25,770			22,218
Total	$	139,036		$	33,057	$	20,478		$	122,903
Net income (loss)
Continuing operations	$	77,583		$	2,358	$	(12,405	)	$	51,887
Discontinued operations – utility infrastructure services	(2,137		)	6,585		14,877			13,293
Total	$	75,446		$	8,943	$	2,472		$	65,180


(1)	The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares.


(2)	Refer to Notes 1 and 18. Effective 2017, Southwest Gas Holdings, Inc. (“Company) is the successor equity issuer to Southwest Gas Corporation (“Southwest”). Both Southwest and Centuri became subsidiaries of the Company.


(3)	Periods prior to 2017 depict Centuri amounts as discontinued operations of Southwest.

(4)

Periods prior to 2018 depict revised operating income for the reclassification of non-service components of net periodic benefit costs, associated with pensions and other post-retirement benefits, out of the operations and maintenance line item of both the Company’s and Southwest’s Condensed Consolidated Statements of Income due to the adoption of ASU 2017-07. The resultant figures for Operating income, but not Net income, were increased accordingly to reflect reclassification. See Note 11 - Pension and Other Postretirement Benefits for further information relating to the adoption of this update.

The demand for natural gas is seasonal, and it is the opinion of management that comparisons of earnings for interim periods do not reliably reflect overall trends and changes in operations. Also, the timing of general rate relief can have a significant impact on earnings for interim periods.

Utility Infrastructure Services Noncontrolling Interests

12 Months Ended

Dec. 31, 2018

Noncontrolling Interest [Abstract]

Utility Infrastructure Services Noncontrolling Interests

Note 17 - Utility Infrastructure Services Noncontrolling Interests

In conjunction with the acquisition of the Canadian utility infrastructure services businesses in October 2014, the previous owners of the acquired companies retained a 3.4% equity interest in Centuri, which, subject to an eligibility timeline, would have been redeemable (in its entirety) at the election of the noncontrolling parties beginning in July 2022. In August 2017, in advance of when otherwise eligible, the parties agreed to a redemption. Southwest Gas Holdings, Inc. paid $23 million to the previous owners, thereby acquiring the remaining 3.4% equity interest in Centuri. Accordingly, Centuri is now a wholly owned subsidiary of the Company.

In connection with the acquisition of Linetec in November 2018, the previous owner retained a 20% equity interest in Linetec, subject to certain rights based on the passage of time or upon the occurrence of certain triggering events. Effective January 2022, the Company has the right, but not the obligation, to purchase at fair value (subject to a floor) a portion of the interest held by the noncontrolling party, and in incremental amounts each year thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of the interest retained by the noncontrolling party is subject to the election beginning in 2024. If the Company does not exercise its rights at each or any of the specified intervals, the noncontrolling party has the ability, but not the obligation, to exit their investment retained by requiring Centuri to purchase a similar portion of their interest up to the maximum cumulative amounts specified and at each interval discussed above. The outstanding noncontrolling interest is not subject to minimum purchase provisions and following the eligibility dates for the elections, they do not expire. The redemption price represents the greater of fair value of the ownership interest to be redeemed on the redemption date or a floor amount under the terms of the agreement.

The Company has determined that this noncontrolling interest is a redeemable noncontrolling interest and, in accordance with SEC guidance, is classified as mezzanine equity (temporary equity) in the Company’s Consolidated Balance Sheets. As of November 30, 2018, the redeemable noncontrolling interest was reported at its estimated fair value of $81.7 million. The fair value was estimated using a market approach that utilizes certain financial metrics from guideline public companies of similar industry and operating characteristics; however, no significant change in the fair value occurred through December 31, 2018. Changes in the value of the redeemable noncontrolling interest will be recognized as they occur and the carrying value will be adjusted accordingly at each quarterly reporting date. Generally, adjustments to the redemption value are expected to impact retained earnings, but not net income.

The following depicts changes to the balance of the redeemable noncontrolling interest between the indicated periods.



	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2016	$	22,590
Net Income (loss) attributable to redeemable noncontrolling interest	248
Foreign currency exchange translation adjustment	11
Centuri distribution to redeemable noncontrolling interest	(204		)
Adjustment to redemption value	355
Redemption of Centuri shares from noncontrolling parties	(23,000		)
Balance, December 31, 2017	—
Redeemable noncontrolling interest acquired	81,659
Net income attributable to redeemable noncontrolling interest	172
Balance, December 31, 2018	$	81,831

Centuri also holds a 95% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC) as a majority-owned subsidiary. The interest is immaterial to the consolidated financial statements, but is identified as the Noncontrolling interest within Total equity on the Company’s Consolidated Balance Sheets.

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation

Note 18 - Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation

In association with the January 2017 holding company reorganization, no substantive change occurred with regard to the Company’s business segments on the whole. Centuri operations remain part of continuing operations of the controlled group of companies, and financial information related to Centuri continues to be included in the consolidated financial statements of the Company. While Centuri has since expanded its footprint with the Linetec and Neuco acquisitions (See Note 19 - Business Acquisitions), its core business has remained consistent.

As part of the holding company reorganization, however, Centuri is no longer a subsidiary of Southwest; whereas historically, Centuri had been a direct subsidiary of Southwest. To give effect to this change, the consolidated financial statements related to Southwest, which are separately included in this report, depict Centuri-related amounts as discontinued operations for periods prior to January 2017.

Due to the discontinued operations accounting reflection, the following table presents the major income statement components of discontinued operations – utility infrastructure services reported in the Consolidated Income Statements of Southwest for the period prior to the beginning of 2017:

Results of Utility Infrastructure Services



	Year Ended December 31,
	2016
(Thousands of dollars)
Utility infrastructure services revenues	$	1,139,078
Operating expenses:
Utility infrastructure services expenses	1,024,423
Depreciation and amortization	55,669
Operating income	58,986
Other income (deductions)	1,193
Net interest deductions	6,663
Income before income taxes	53,516
Income tax expense	19,884
Net income	33,632
Net income attributable to noncontrolling interests	1,014
Discontinued operations – utility infrastructure services – net income	$	32,618

Business Acquisitions

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

Business Acquisitions

Note 19 - Business Acquisitions

As indicated in Note 1 - Background, Organization, and Summary of Significant Accounting Policies, on November 30, 2018, the Company, through its subsidiaries, led principally by Centuri, completed the acquisition of an 80% interest in a privately held utility infrastructure services business, Linetec Services, LLC (“Linetec”) for approximately $326.6 million, with the remaining 20% retained by the seller. The above figure includes unremitted amounts as of the closing date as follows: $30 million associated with the subsequent collection of Linetec unbilled customer receivable balances recorded at their estimated realizable values as of the acquisition date, $29.6 million of seller liabilities related to future payments to certain Linetec employees, and $16.9 million representing consideration held back for estimated purchase price and working capital adjustments to be paid during 2019. Additionally, $24.1 million of consideration transferred related to future estimated tax payments related to a mutual 338(h)(10) election under U.S. Treasury regulations (deemed asset sale/purchase under those tax regulations). In December 2018, $25 million of the $29.6 million referred to above was paid to certain Linetec employees.

The acquisition will extend the utility services operations in the southeastern region of the U.S. and provide additional opportunities for expansion of the amount of work Centuri performs for electric utilities. Funding for the acquisition was provided by a portion of net proceeds from the Company’s equity offering in November 2018 and from Centuri’s $590 million secured revolving credit and term loan facility, as amended, described below and in Note 7 - Common Stock and Note 8 - Long-Term Debt.

The Company is currently performing a detailed valuation analysis of the assets and liabilities of the acquired company, which was substantially completed during the fourth quarter of 2018. Certain payments were estimated as of the acquisition date and will be adjusted when finally paid in 2019. The necessary analysis will consider acquired intangibles including customer relationships, trade names, and customer contracts. Based on preliminary results, a substantial majority of the purchase price will be allocated to goodwill and other finite-lived intangible assets.

Assets acquired and liabilities assumed in the transaction were recorded, generally, at their acquisition date fair values. Transaction costs associated with the acquisition were expensed as incurred. The Company’s allocation of the purchase price was based on an evaluation of the appropriate fair values and represented management’s best estimate based on available data (including market data, data regarding customers of the acquired businesses, terms of acquisition-related agreements, analysis of historical and projected results, and other types of data). The analysis included consideration of types of intangibles that were acquired, including customer relationships, trade names, and customer contracts. The final purchase accounting has not yet been completed. Further refinement is expected to occur, including potential changes to income taxes and intangibles, as well as additional consideration payments held back.

The preliminary estimated fair values of assets acquired and liabilities assumed as of November 30, 2018, are as follows (in millions of dollars):



Cash and cash equivalents	$	3.9
Accounts receivable	32.8
Revenue earned on contracts in progress in excess of billings	21.6
Prepaid expenses and other current assets	1.1
Property and equipment	89.4
Intangible assets	89.3
Goodwill	188.5
Total assets acquired	426.6

Accounts payable	8.0
Accrued liabilities	6.9
Deferred compensation and related accrued taxes	3.4
Redeemable noncontrolling interest	81.7
Total liabilities assumed and noncontrolling interest	100.0
Net assets acquired	$	326.6

Goodwill consists of the value associated with the assembled workforce, consolidation of operations, and the estimated economic value attributable to future opportunities related to the transaction. As the business of Linetec was deemed an asset purchase for tax purposes, the $188.5 million of tax-basis goodwill is expected to be deductible for tax purposes. As of the acquisition date, other intangible assets totaled $89.3 million which will be amortized over a weighted-average life of 19 years. Of the $89.3 million of intangible assets, $79 million is attributable to customer relationships with an assigned life of 20 years, $10 million is attributable to a trade name with a 15-year useful life, and $300,000 is attributable to customer contracts with a useful life of one year. The intangible assets other than goodwill are included in Other property and investments in the Company’s Consolidated Balance Sheets.

The unaudited pro forma consolidated financial information for fiscal 2018 and fiscal 2017 (assuming the acquisition of Linetec occurred as of the beginning fiscal 2017) is as follows (in thousands of dollars, except per share amounts):



	Year Ended December 31,
	2018		2017
Total operating revenues	$	3,037,209	$	2,626,721
Net income attributable to Southwest Gas Holdings, Inc.	$	187,642	$	192,368
Basic earnings per share	$	3.80	$	4.01
Diluted earnings per share	$	3.79	$	4.01

Acquisition costs of $6.9 million that were incurred during 2018, and included in Utility infrastructure expenses in the Consolidated Statements of Income, were excluded from the 2018 unaudited pro forma consolidated financial information shown above and included in the 2017 amounts. No material nonrecurring pro forma adjustments directly attributable to the business combination were included in the unaudited pro forma consolidated financial information.

The pro forma financial information includes assumptions and adjustments made to incorporate various items including, but not limited to, additional interest expense and depreciation and amortization expense, and tax effects, as appropriate. The pro forma financial information has been prepared for comparative purposes only, and is not intended to be indicative of what the Company’s results would have been had the acquisition occurred at the beginning of the periods presented or of the results which may occur in the future, for a number of reasons. These reasons include, but are not limited to, differences between the assumptions used to prepare the pro forma information, potential cost savings from operating efficiencies, and the impact of incremental costs incurred in integrating the businesses.

Actual results from Linetec operations, excluding transaction costs incurred by Centuri, included in the Consolidated Statements of Income since the date of acquisition are as follows (in thousands of dollars):



	Year ended December 31, 2018
Utility infrastructure services revenues	$	14,119
Net income attributable to Southwest Gas Holdings, Inc.	690

In November 2017, the Company, through its subsidiaries, led principally by Centuri, completed the acquisition of Neuco, a privately held utility infrastructure services business, for approximately $99 million, less assumed debt. The acquisition extended the utility infrastructure services to the Northeastern region of the U.S., and provided additional opportunities for expansion. Funding for the acquisition was primarily provided by Centuri’s revolving credit and term loan facilities, described in Note 8 - Long-Term Debt.

Assets acquired and liabilities assumed in the transaction were recorded, generally, at their acquisition date fair values. Transaction costs associated with the acquisition were expensed as incurred. The Company’s allocation of the purchase price was based on an evaluation of the appropriate fair values and represented management’s best estimate based on available data (including market data, data regarding customers of the acquired businesses, terms of acquisition-related agreements, analysis of historical and projected results, and other types of data). The analysis included consideration of types of intangibles that were acquired, including non-competition agreements, customer relationships, trade names, and work backlog. The final purchase accounting has been completed.

The fair values of assets acquired and liabilities assumed as of November 1, 2017, the acquisition date, were as follows (in millions of dollars):



Cash and cash equivalents	$	0.8
Contracts receivable	18.3
Other receivables	5.4
Property, plant and equipment	15.1
Prepaid expenses and deposits	1.6
Intangible assets	44.8
Goodwill	32.2
Total assets acquired	118.2
Current liabilities	(18.6		)
Other long-term liabilities	(0.3		)
Net assets acquired	$	99.3

The allocation of the purchase price of Neuco was accounted for in accordance with applicable accounting guidance. Goodwill, which is generally not deductible for tax purposes, consists of the value associated with the assembled workforce and consolidation of operations. However, as the business of Neuco was acquired via asset purchase for tax purposes, the approximately $32 million of tax-basis goodwill is expected to be deductible for tax purposes. Following a one-year post-acquisition measurement period, the values were adjusted as reflected in the table above, with no significant overall impact to the Company’s consolidated balance sheets.

The unaudited pro forma consolidated financial information for fiscal 2017 is as follows (in thousands of dollars, except per share amounts):



	Year Ended December 31,
	2017
Total operating revenues	$	2,639,452
Net income attributable to Southwest Gas Holdings, Inc.	$	203,245
Basic earnings per share	$	4.24
Diluted earnings per share	$	4.24

The Company incurred and expensed acquisition costs of $2.6 million related to the acquisition. No acquisition-related costs were incurred during 2018.

Background, Organization and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of Operations

Basis of Presentation

Consolidation

Fair Value Measurements

Level 1 – quoted prices (unadjusted) in active markets for identical assets or liabilities that a company has the ability to access at the measurement date.

Level 2 – inputs other than quoted prices included within Level 1 that are observable for similar assets or liabilities, either directly or indirectly.

Net Utility Plant

Intangible Assets

Cash and Cash Equivalents

Income Taxes

Deferred Purchased Gas Costs

Prepaids and Other Current Assets

Goodwill

Other Current Liabilities

Accumulated Removal Costs

Revenue Recognition

Operations and Maintenance Expense

Depreciation and Amortization

Allowance for Funds Used During Construction ("AFUDC")

Other Income (Deductions)

Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the Consolidated Statements of Income (thousands of dollars):



	2018			2017			2016
Southwest Gas Corporation – natural gas operations segment:
Increase (decrease) in COLI policies	$	(3,200	)	$	10,300		$	7,400
Interest income	6,020			2,784			1,848
Equity AFUDC	3,627			2,296			2,289
Non-service post-retirement benefit cost	(21,059		)	(19,424		)	(19,760		)
Miscellaneous income and (expense)	(2,628		)	(2,344		)	(3,261		)
Southwest Gas Corporation – total other income (deductions)	(17,240		)	(6,388		)	(11,484		)
Utility infrastructure services segment:
Interest income	88			3			1
Foreign transaction gain (loss)	(222		)	(754		)	(22		)
Equity in earnings of unconsolidated investment – Western	531			1,052			69
Miscellaneous income and (expense)	(635		)	44			1,145
Centuri – total other income (deductions)	(238		)	345			1,193
Corporate and administrative	52			13			—
Consolidated Southwest Gas Holdings, Inc. - total other income (deductions)	$	(17,426	)	$	(6,030	)	$	(10,291	)

Foreign Currency Translation

Earnings Per Share

Recently Accounting Standards Updates

Recent Accounting Standards Updates.

Accounting pronouncements adopted in 2018:

Accounting pronouncements that will be effective in 2019:


•	A lease liability for the obligation to make lease payments, measured on a discounted basis; and


•	A right-of-use asset for the right to use, or control the use of, a specified asset for the lease term.

•


•	To elect to adopt the practical expedient to exclude all easements in place prior to January 1, 2019 from treatment under Topic 842. However, the Company and Southwest will evaluate any new easements entered into after the effective date of the standard to determine if the arrangements should be accounted for as leases.


•	To make an accounting policy election by asset class to include both the lease and non-lease components (as defined in the guidance) as a single component.


•	To make an accounting policy election to not apply Topic 842 to short-term leases, as permitted.


•	To not elect to use hindsight in determining the lease term and in assessing impairment of right-of-use assets.

Recently issued accounting pronouncements that will be effective after 2019:

Subsequent Events

Background, Organization and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Other Property and Investments

Other property and investments on Southwest’s and the Company’s Consolidated Balance Sheets includes (thousands of dollars):



	2018			2017
Net cash surrender value of COLI policies	$	114,405		$	117,341
Other property	1,741			1,773
Total Southwest Gas Corporation	116,146			119,114
Centuri property, equipment, and intangibles	792,191			554,730
Centuri accumulated provision for depreciation and amortization	(298,939		)	(258,906		)
Other property	14,153			13,242
Total Southwest Gas Holdings, Inc.	$	623,551		$	428,180

Summary of Intangible Assets

These intangible assets are included in Other property and investments on the Company’s Consolidated Balance Sheets. Centuri has $159.8 million and $80.7 million of intangible assets at December 31, 2018 and 2017, respectively, as detailed in the following table (thousands of dollars):



December 31, 2018	Gross Carrying Amount		Accumulated Amortization			Net Carrying Amount
Customer relationships	$	152,533	$	(11,716	)	$	140,817
Trade names and trademarks	23,013		(5,234		)	17,779
Customer contracts backlog	270		(3		)	267
Noncompete agreements	2,022		(1,064		)	958
Total	$	177,838	$	(18,017	)	$	159,821

December 31, 2017
Customer relationships	$	76,254	$	(6,743	)	$	69,511
Trade names and trademarks	13,754		(4,080		)	9,674
Noncompete agreements	2,060		(543		)	1,517
Total	$	92,068	$	(11,366	)	$	80,702

Schedule of Estimated Future Amortization of Intangible Assets

The estimated future amortization of the intangible assets for the next five years and thereafter is as follows (in thousands):



2019	$	10,622
2020	10,634
2021	10,214
2022	10,127
2023	10,127
Thereafter	108,097
Total	$	159,821

Schedule of Goodwill



	Natural Gas Operations		Utility Infrastructure Services			Total Company
(In thousands of dollars)
Balance, December 31, 2016	$	10,095	$	129,888		$	139,983
Goodwill from Neuco acquisition	—		32,028			32,028
Foreign currency translation adjustment	—		7,303			7,303
Balance, December 31, 2017	10,095		169,219			179,314
Additional goodwill from Neuco acquisition	—		182			182
Goodwill from Linetec acquisition	—		188,494			188,494
Foreign currency translation adjustment	—		(8,945		)	(8,945		)
Balance, December 31, 2018	$	10,095	$	348,950		$	359,045

Schedule of Capitalized and Debt Portion of AFUDC

Utility plant construction costs, including AFUDC, are recovered in authorized rates through depreciation when completed projects are placed into operation, and general rate relief is requested and granted.



	2018			2017			2016
(In thousands)
AFUDC:
Debt portion	$	3,264		$	1,666		$	1,175
Equity portion	3,627			2,296			2,289
AFUDC capitalized as part of utility plant	$	6,891		$	3,962		$	3,464
AFUDC rate	5.85		%	5.95		%	7.35		%

Schedule of Other Income (Deductions)

The following table provides the composition of significant items included in Other income (deductions) on the Consolidated Statements of Income (thousands of dollars):



	2018			2017			2016
Southwest Gas Corporation – natural gas operations segment:
Increase (decrease) in COLI policies	$	(3,200	)	$	10,300		$	7,400
Interest income	6,020			2,784			1,848
Equity AFUDC	3,627			2,296			2,289
Non-service post-retirement benefit cost	(21,059		)	(19,424		)	(19,760		)
Miscellaneous income and (expense)	(2,628		)	(2,344		)	(3,261		)
Southwest Gas Corporation – total other income (deductions)	(17,240		)	(6,388		)	(11,484		)
Utility infrastructure services segment:
Interest income	88			3			1
Foreign transaction gain (loss)	(222		)	(754		)	(22		)
Equity in earnings of unconsolidated investment – Western	531			1,052			69
Miscellaneous income and (expense)	(635		)	44			1,145
Centuri – total other income (deductions)	(238		)	345			1,193
Corporate and administrative	52			13			—
Consolidated Southwest Gas Holdings, Inc. - total other income (deductions)	$	(17,426	)	$	(6,030	)	$	(10,291	)

Schedule of Earnings Per Share, Basic and Diluted

A reconciliation of the denominator used in the Basic and Diluted EPS calculations is shown in the following table.



	2018	2017	2016
(In thousands)
Average basic shares	49,419	47,965	47,469
Effect of dilutive securities:
Stock options	—	—	1
Management Incentive Plan shares	25	8	124
Restricted stock units (1)	32	18	220
Average diluted shares	49,476	47,991	47,814


(1)	The number of securities granted for 2018 and 2017 includes 23,000 and 7,000 performance shares, respectively, the total of which was derived by assuming that target performance will be achieved during the relevant performance period.

Utility Plant and Leases (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Net Utility Plant

Net utility plant as of December 31, 2018 and 2017 was as follows (thousands of dollars):



December 31,	2018			2017
Gas plant:
Storage	$	26,825		$	25,019
Transmission	386,159			363,396
Distribution	6,049,380			5,600,769
General	416,643			396,252
Software and software-related intangibles	241,158			230,030
Other	14,074			14,178
	7,134,239			6,629,644
Less: accumulated depreciation and amortization	(2,234,029		)	(2,231,242		)
Construction work in progress	193,028			125,248
Net utility plant	$	5,093,238		$	4,523,650

Schedule of Depreciation and Amortization Expense

Depreciation and amortization expense on gas plant, including intangibles, was as follows (thousands of dollars):



	2018		2017		2016
Depreciation and amortization expense	$	185,719	$	187,075	$	214,037

Schedule of Rental Payments for Operating Leases

The table below presents Southwest’s and Centuri’s rental and lease payments that are included in operating expenses (in thousands):



	2018		2017		2016
Southwest Gas Corporation	$	4,556	$	4,926	$	4,357
Centuri	59,491		62,310		53,956
Consolidated rental payments/lease expense	$	64,047	$	67,236	$	58,313

Schedule of Future Minimum Lease Payments for Operating Leases

The following is a schedule of future minimum lease payments for operating leases (with initial or remaining terms in excess of one year) as of December 31, 2018 (thousands of dollars):



	Southwest		Centuri		Consolidated Total
2019	$	898	$	10,053	$	10,951
2020	363		7,656		8,019
2021	299		5,760		6,059
2022	163		5,163		5,326
2023	79		3,681		3,760
Thereafter	177		10,511		10,688
Total minimum lease payments	$	1,979	$	42,824	$	44,803

Revenue (Tables)

12 Months Ended

Dec. 31, 2018

Revenue from Contract with Customer [Abstract]

Summary of Revenue Disaggregated by Service Type and Contract Type



	December 31,
(Thousands of dollars)	2018		2017		2016
Residential	$	887,220	$	857,204	$	895,330
Small commercial	255,083		243,513		251,092
Large commercial	53,192		52,379		53,582
Industrial/other	23,489		22,026		19,753
Transportation	86,990		87,759		87,106
Revenue from contracts with customers	1,305,974		1,262,881		1,306,863
Alternative revenue program revenues (deferrals)	45,979		35,347		12,530
Other revenues (a)	5,775		4,080		2,019
Total Gas operating revenues	$	1,357,728	$	1,302,308	$	1,321,412


(a)	Includes various other revenues, and during the first six months of 2018, included $12.5 million as a reserve against revenue associated with a tax reform savings adjustment. During the third quarter of 2018, amounts previously recognized were reclassified to the various categories of revenue from contracts with customers when incorporated in tariff rates.

The following tables display Centuri’s revenue from contracts with customers disaggregated by service type and contract type:



(Thousands of dollars)	December 31,
	2018		2017		2016
Service Types:
Gas infrastructure services	$	1,123,682	$	891,139	$	914,970
Electric power infrastructure services	32,629		18,114		27,915
Other	365,974		337,231		196,193
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078



(Thousands of dollars)	December 31,
	2018		2017		2016
Contract Types:
Master services agreement	$	1,102,412	$	885,513	$	852,472
Bid contract	419,873		360,971		286,606
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078

Unit priced contracts	$	1,258,419	$	968,856	$	886,919
Fixed priced contracts	117,298		127,497		95,494
Time and materials contracts	146,568		150,131		156,665
Total Utility infrastructure services revenues	$	1,522,285	$	1,246,484	$	1,139,078

Summary of Information about Receivables, Revenue Earned on Contracts in Progress in Excess of Billings, Which are Included Within Accounts Receivable, Net of Allowances, and Amounts Billed in Excess of Revenue Earned on Contracts



(Thousands of dollars)	December 31, 2018		December 31, 2017
Contracts receivable, net	$	186,249	$	221,859
Revenue earned on contracts in progress in excess of billings	87,520		5,768
Amounts billed in excess of revenue earned on contracts	4,211		9,602

Schedule of Construction Services Contracts Receivable

services contracts receivable consists of the following:



(Thousands of dollars)	December 31, 2018			December 31, 2017
Billed on completed contracts and contracts in progress	$	184,100		$	219,298
Other receivables	2,588			2,622
Contracts receivable, gross	186,688			221,920
Allowance for doubtful accounts	(439		)	(61		)
Contracts receivable, net	$	186,249		$	221,859

Receivables and Related Allowances (Tables)

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Schedule of Accounts Receivable

The table below contains information about the gas utility customer accounts receivable balance (net of allowance) at December 31, 2018 and 2017, and the percentage of customers in each of the three states, which was consistent with the prior year.



	December 31, 2018		December 31, 2017
Gas utility customer accounts receivable balance (in thousands)	$	138,149	$	119,444

Schedule of Percent of Customers by State



	December 31, 2018
Percent of customers by state
Arizona	53	%
Nevada	37	%
California	10	%

Schedule of Allowance for Uncollectibles

Activity in the allowance account for uncollectibles is summarized as follows (thousands of dollars):



	Allowance for Uncollectibles
Balance, December 31, 2015	$	2,270
Additions charged to expense	3,264
Accounts written off, less recoveries	(3,010		)
Balance, December 31, 2016	2,524
Additions charged to expense	2,310
Accounts written off, less recoveries	(2,723		)
Balance, December 31, 2017	2,111
Additions charged to expense	2,959
Accounts written off, less recoveries	(2,902		)
Balance, December 31, 2018	$	2,168

Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Regulatory Assets

The following table represents existing regulatory assets and liabilities (thousands of dollars):



December 31,	2018			2017
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	383,170		$	391,403
Unrealized net loss on non-trading derivatives (Swaps) (2)	1,862			5,780
Deferred purchased gas costs (3)	4,928			14,581
Accrued purchased gas costs (4)	29,000			17,000
Unamortized premium on reacquired debt (5)	19,599			20,913
Accrued absence time (8)	14,126			13,870
Margin & interest-tracking (9)	88,290			42,354
Other (10)	32,616			25,997
	573,591			531,898
Regulatory liabilities:
Deferred purchased gas costs (3)	(79,762		)	(6,841		)
Accumulated removal costs	(383,000		)	(315,000		)
Unrealized net gain on non-trading derivatives (Swaps) (2)	(144		)	—
Unamortized gain on reacquired debt (6)	(8,717		)	(9,253		)
Regulatory excess deferred taxes and gross-up (7)	(458,834		)	(433,908		)
Other (10)	(19,911		)	(33,184		)
Net regulatory liabilities	$	(376,777	)	$	(266,288	)


(1)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(2)


(3)	Balance recovered or refunded on an ongoing basis with interest.


(4)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.


(5)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.


(6)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(7)


(8)	Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.


(9)	Margin tracking/decoupling mechanisms are alternative revenue programs and revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) are recognized as revenue so long as recovery is expected to take place within 24 months. Included in Prepaid and other current assets on the Consolidated Balance Sheets.

(10)



Other Regulatory Assets	2018		2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	6,253	$	4,832
Infrastructure replacement programs and similar (b) (e)	12,486		9,627
Environmental compliance programs (c) (e)	5,046		9,702
Other (d)	8,831		1,836
	$	32,616	$	25,997


a)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. See Prepaid and other current assets in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.


b)	Included in Deferred charges and other assets on the Consolidated Balance Sheets.


c)	In 2018, approximately $4.5 million included in Prepaid and other current assets and $596,000 included in Deferred charges and other assets on the Consolidated Balance Sheets. In2017, $9.2 million included in Prepaid and other current assets and $527,000 included in Deferred charges and other assets on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.



Other Regulatory Liabilities	2018			2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	(8,598	)	$	(10,213	)
Margin, interest- and property tax-tracking accounts (b) (e)	(7,273		)	(9,505		)
Environmental compliance programs (e) (f)	—			(8,574		)
Regulatory accounts for differences related to pension funding (c)	(3,221		)	(3,178		)
Other (d) (e)	(819		)	(1,714		)
	$	(19,911	)	$	(33,184	)


a)	Included in Other current liabilities on the Consolidated Balance Sheets.


b)	In 2018, $(539,000) included in Other current liabilities and $(6.7) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, $(6.6) million included in Other current liabilities and $(2.9) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


c)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


d)	In 2018, $(810,000) included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, approximately $(1.7) million included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.


f)	In 2018, included in Prepaid and other current assets on the Consolidated Balance Sheets. In 2017, included in Other current liabilities on the Consolidated Balance Sheets.



Other Regulatory Assets	2018		2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	6,253	$	4,832
Infrastructure replacement programs and similar (b) (e)	12,486		9,627
Environmental compliance programs (c) (e)	5,046		9,702
Other (d)	8,831		1,836
	$	32,616	$	25,997


a)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. See Prepaid and other current assets in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.


b)	Included in Deferred charges and other assets on the Consolidated Balance Sheets.


c)	In 2018, approximately $4.5 million included in Prepaid and other current assets and $596,000 included in Deferred charges and other assets on the Consolidated Balance Sheets. In2017, $9.2 million included in Prepaid and other current assets and $527,000 included in Deferred charges and other assets on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.

Schedule of Regulatory Liabilities



Other Regulatory Liabilities	2018			2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	(8,598	)	$	(10,213	)
Margin, interest- and property tax-tracking accounts (b) (e)	(7,273		)	(9,505		)
Environmental compliance programs (e) (f)	—			(8,574		)
Regulatory accounts for differences related to pension funding (c)	(3,221		)	(3,178		)
Other (d) (e)	(819		)	(1,714		)
	$	(19,911	)	$	(33,184	)


a)	Included in Other current liabilities on the Consolidated Balance Sheets.


b)	In 2018, $(539,000) included in Other current liabilities and $(6.7) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, $(6.6) million included in Other current liabilities and $(2.9) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


c)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


d)	In 2018, $(810,000) included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, approximately $(1.7) million included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.


f)	In 2018, included in Prepaid and other current assets on the Consolidated Balance Sheets. In 2017, included in Other current liabilities on the Consolidated Balance Sheets.

The following table represents existing regulatory assets and liabilities (thousands of dollars):



December 31,	2018			2017
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	383,170		$	391,403
Unrealized net loss on non-trading derivatives (Swaps) (2)	1,862			5,780
Deferred purchased gas costs (3)	4,928			14,581
Accrued purchased gas costs (4)	29,000			17,000
Unamortized premium on reacquired debt (5)	19,599			20,913
Accrued absence time (8)	14,126			13,870
Margin & interest-tracking (9)	88,290			42,354
Other (10)	32,616			25,997
	573,591			531,898
Regulatory liabilities:
Deferred purchased gas costs (3)	(79,762		)	(6,841		)
Accumulated removal costs	(383,000		)	(315,000		)
Unrealized net gain on non-trading derivatives (Swaps) (2)	(144		)	—
Unamortized gain on reacquired debt (6)	(8,717		)	(9,253		)
Regulatory excess deferred taxes and gross-up (7)	(458,834		)	(433,908		)
Other (10)	(19,911		)	(33,184		)
Net regulatory liabilities	$	(376,777	)	$	(266,288	)


(1)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 11 - Pension and Other Postretirement Benefits).

(2)


(3)	Balance recovered or refunded on an ongoing basis with interest.


(4)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis.


(5)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments.


(6)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. Amortized over life of debt instruments.

(7)


(8)	Regulatory recovery occurs on a one-year lag basis through the labor loading process. Included in Prepaid and other current assets on the Consolidated Balance Sheets.


(9)	Margin tracking/decoupling mechanisms are alternative revenue programs and revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) are recognized as revenue so long as recovery is expected to take place within 24 months. Included in Prepaid and other current assets on the Consolidated Balance Sheets.

(10)



Other Regulatory Assets	2018		2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	6,253	$	4,832
Infrastructure replacement programs and similar (b) (e)	12,486		9,627
Environmental compliance programs (c) (e)	5,046		9,702
Other (d)	8,831		1,836
	$	32,616	$	25,997


a)	Included in Prepaid and other current assets on the Consolidated Balance Sheets. See Prepaid and other current assets in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.


b)	Included in Deferred charges and other assets on the Consolidated Balance Sheets.


c)	In 2018, approximately $4.5 million included in Prepaid and other current assets and $596,000 included in Deferred charges and other assets on the Consolidated Balance Sheets. In2017, $9.2 million included in Prepaid and other current assets and $527,000 included in Deferred charges and other assets on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.



Other Regulatory Liabilities	2018			2017
State mandated public purpose programs (including low income and conservation programs) (a) (e)	$	(8,598	)	$	(10,213	)
Margin, interest- and property tax-tracking accounts (b) (e)	(7,273		)	(9,505		)
Environmental compliance programs (e) (f)	—			(8,574		)
Regulatory accounts for differences related to pension funding (c)	(3,221		)	(3,178		)
Other (d) (e)	(819		)	(1,714		)
	$	(19,911	)	$	(33,184	)


a)	Included in Other current liabilities on the Consolidated Balance Sheets.


b)	In 2018, $(539,000) included in Other current liabilities and $(6.7) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, $(6.6) million included in Other current liabilities and $(2.9) million included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


c)	Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


d)	In 2018, $(810,000) included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. In 2017, approximately $(1.7) million included in Other current liabilities and $(9,000) included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.


e)	Balance recovered or refunded on an ongoing basis, generally with interest.


f)	In 2018, included in Prepaid and other current assets on the Consolidated Balance Sheets. In 2017, included in Other current liabilities on the Consolidated Balance Sheets.

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Estimated Amounts Amortized from Accumulated Other Comprehensive Income or Regulatory Assets into Net Periodic Benefit Cost



(Thousands of dollars)	2018									2017									2016
	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(20,426	)	$	4,902		$	(15,524	)	$	(43,027	)	$	10,326		$	(32,701	)	$	(22,770	)	$	8,652		$	(14,118	)
Amortization of prior service cost	1,335			(320		)	1,015			1,335			(507		)	828			1,335			(507		)	828
Amortization of net actuarial (gain)/loss	33,617			(8,068		)	25,549			25,445			(9,669		)	15,776			27,066			(10,285		)	16,781
Regulatory adjustment	(8,233		)	1,976			(6,257		)	12,340			250			12,590			(5,584		)	2,122			(3,462		)
Pension plans other comprehensive income (loss)	6,293			(1,510		)	4,783			(3,907		)	400			(3,507		)	47			(18		)	29
FSIRS (designated hedging activities):
Amounts reclassified into net income	3,345			(804		)	2,541			3,344			(1,271		)	2,073			3,345			(1,270		)	2,075
FSIRS other comprehensive income (loss)	3,345			(804		)	2,541			3,344			(1,271		)	2,073			3,345			(1,270		)	2,075
Total other comprehensive income (loss) – Southwest Gas Corporation	9,638			(2,314		)	7,324			(563		)	(871		)	(1,434		)	3,392			(1,288		)	2,104
Foreign currency translation adjustments:
Translation adjustments	(3,010		)	—			(3,010		)	1,771			—			1,771			161			—			161
Foreign currency other comprehensive income (loss)	(3,010		)	—			(3,010		)	1,771			—			1,771			161			—			161
Total other comprehensive income (loss) – Southwest Gas Holdings, Inc.	$	6,628		$	(2,314	)	$	4,314		$	1,208		$	(871	)	$	337		$	3,553		$	(1,288	)	$	2,265

The estimated amounts that will be amortized from accumulated other comprehensive income or regulatory assets into net periodic benefit cost over the next year are summarized below (in thousands):



Retirement plan net actuarial loss	$	22,000
SERP net actuarial loss	1,000
PBOP prior service cost	1,300

Rollforward of Accumulated Other Comprehensive Income

The following table represents a rollforward of AOCI, presented on Southwest’s Consolidated Balance Sheets:

AOCI—Rollforward

(Thousands of dollars)



	Defined Benefit Plans									FSIRS
	Before-Tax			Tax (Expense) Benefit (10)			After- Tax			Before- Tax			Tax (Expense) Benefit (10)			After- Tax			Other			AOCI
Beginning Balance AOCI December 31, 2017	$	(61,520	)	$	22,293		$	(39,227	)	$	(12,655	)	$	4,809		$	(7,846	)	$	—		$	(47,073	)
Net actuarial gain/(loss)	(20,426		)	4,902			(15,524		)	—			—			—			—			(15,524		)
Other comprehensive income before reclassifications	(20,426		)	4,902			(15,524		)	—			—			—			—			(15,524		)
FSIRS amounts reclassified from AOCI (6)	—			—			—			3,345			(804		)	2,541			—			2,541
Amortization of prior service cost (7)	1,335			(320		)	1,015			—			—			—			—			1,015
Amortization of net actuarial loss (7)	33,617			(8,068		)	25,549			—			—			—			—			25,549
Regulatory adjustment (8)	(8,233		)	1,976			(6,257		)	—			—			—			—			(6,257		)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation	6,293			(1,510		)	4,783			3,345			(804		)	2,541			—			7,324
Reclassification of excess deferred taxes (9)	—			—			—			—			—			—			(9,300		)	(9,300		)
Ending Balance AOCI December 31, 2018	$	(55,227	)	$	20,783		$	(34,444	)	$	(9,310	)	$	4,005		$	(5,305	)	$	(9,300	)	$	(49,049	)


(6)	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income.


(7)	These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).


(8)	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets).


(9)	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02 during the first quarter of 2018, which permitted such release.


(10)	Tax amounts related to the before-tax balances are calculated using a 24% effective rate after the release of previously stranded excess deferred taxes existing as a result of the TCJA; amounts prior to the December 22, 2017 enactment of the TCJA were calculated using a 38% effective rate.

The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity:

AOCI—Rollforward

(Thousands of dollars)



	Defined Benefit Plans									FSIRS									Foreign Currency Items
	Before-Tax			Tax (Expense) Benefit (5)			After-Tax			Before- Tax			Tax (Expense) Benefit (5)			After-Tax			Before- Tax			Tax (Expense) Benefit		After-Tax			Other			AOCI
Beginning Balance AOCI December 31, 2017	$	(61,520	)	$	22,293		$	(39,227	)	$	(12,655	)	$	4,809		$	(7,846	)	$	(609	)	$	—	$	(609	)	$	—		$	(47,682	)
Net actuarial gain/(loss)	(20,426		)	4,902			(15,524		)	—			—			—			—			—		—			—			(15,524		)
Translation adjustments	—			—			—			—			—			—			(3,010		)	—		(3,010		)	—			(3,010		)
Other comprehensive income before reclassifications	(20,426		)	4,902			(15,524		)	—			—			—			(3,010		)	—		(3,010		)	—			(18,534		)
FSIRS amounts reclassified from AOCI (1)	—			—			—			3,345			(804		)	2,541			—			—		—			—			2,541
Amortization of prior service cost (2)	1,335			(320		)	1,015			—			—			—			—			—		—			—			1,015
Amortization of net actuarial loss (2)	33,617			(8,068		)	25,549			—			—			—			—			—		—			—			25,549
Regulatory adjustment (3)	(8,233		)	1,976			(6,257		)	—			—			—			—			—		—			—			(6,257		)
Net current period other comprehensive income (loss) attributable to Southwest Gas Holdings, Inc.	6,293			(1,510		)	4,783			3,345			(804		)	2,541			(3,010		)	—		(3,010		)	—			4,314
Reclassification of excess deferred taxes (4)	—			—			—			—			—			—			—			—		—			(9,300		)	(9,300		)
Ending Balance AOCI December 31, 2018	$	(55,227	)	$	20,783		$	(34,444	)	$	(9,310	)	$	4,005		$	(5,305	)	$	(3,619	)	$	—	$	(3,619	)	$	(9,300	)	$	(52,668	)


(1)	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income.


(2)	These AOCI components are included in the computation of net periodic benefit cost (see Note 11 - Pension and Other Postretirement Benefits for additional details).


(3)	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets).


(4)	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02 during the first quarter of 2018, which permitted such release.


(5)	Tax amounts related to the before-tax balance are calculated using a 24% effective rate after the release of previously stranded excess deferred taxes existing as a result of the TCJA; amounts prior to the December 22, 2017 enactment of the TCJA were calculated using a 38% effective rate.

Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)



	2018			2017
Net actuarial (loss) gain	$	(435,364	)	$	(448,555	)
Prior service cost	(3,033		)	(4,368		)
Less: amount recognized in regulatory assets	383,170			391,403
Recognized in AOCI	$	(55,227	)	$	(61,520	)

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income



	2018												2017												2016
	Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP
(Thousands of dollars)
Net actuarial loss (gain) (a)	$	20,426		$	23,607		$	(3,940	)	$	759		$	43,027		$	44,149		$	3,334		$	(4,456	)	$	22,770		$	25,153		$	1,347		$	(3,730	)
Amortization of prior service cost (b)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)
Amortization of net actuarial loss (b)	(33,617		)	(32,115		)	(1,502		)	—			(25,445		)	(24,004		)	(1,441		)	—			(27,066		)	(25,266		)	(1,383		)	(417		)
Regulatory adjustment	8,233			7,657			—			576			(12,340		)	(18,131		)	—			5,791			5,584			102			—			5,482
Recognized in other comprehensive (income) loss	(6,293		)	(851		)	(5,442		)	—			3,907			2,014			1,893			—			(47		)	(11		)	(36		)	—
Net periodic benefit costs recognized in net income	51,332			46,089			3,405			1,838			44,593			38,283			3,633			2,677			44,423			37,568			3,573			3,282
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	45,039		$	45,238		$	(2,037	)	$	1,838		$	48,500		$	40,297		$	5,526		$	2,677		$	44,376		$	37,557		$	3,537		$	3,282

Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of Common Stock Activity

The following table provides the activity in the Equity Shelf Program for the three and twelve months ended December 31, 2018:



	Three Months Ended			Twelve Months Ended
	December 31,			December 31,
	2018	2017		2018		2017
Gross proceeds	—	$	29,999,922	$	85,149,976	$	41,776,795
Less: agent commissions	—	299,999		851,500		417,768
Net proceeds	—	$	29,699,923	$	84,298,476	$	41,359,027

Number of shares sold	—	358,630		1,145,705		505,707
Weighted average price per share	—	$	83.65	$	74.32	$	82.61

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt

The Centuri secured revolving credit and term loan facility and Centuri other debt obligations are categorized as Level 3, based on significant unobservable inputs to their fair values. Because Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. The fair value hierarchy is described in Note 1 - Background, Organization, and Summary of Significant Accounting Policies.



	December 31, 2018					December 31, 2017
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Southwest Gas Corporation:
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	126,213	$	125,000		$	129,273
Notes, 6.1%, due 2041	125,000			150,728		125,000			158,304
Notes, 3.875%, due 2022	250,000			254,195		250,000			256,163
Notes, 4.875%, due 2043	250,000			268,985		250,000			283,243
Notes, 3.8%, due 2046	300,000			267,030		300,000			302,970
Notes, 3.7%, due 2028	300,000			298,926		—			—
8% Series, due 2026	75,000			93,827		75,000			96,063
Medium-term notes, 7.78% series, due 2022	25,000			27,497		25,000			28,714
Medium-term notes, 7.92% series, due 2027	25,000			30,016		25,000			31,542
Medium-term notes, 6.76% series, due 2027	7,500			8,651		7,500			8,882
Unamortized discount and debt issuance costs	(11,807		)			(9,350		)
	1,470,693					1,173,150
Revolving credit facility and commercial paper	150,000			150,000		150,000			150,000
Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028	50,000			50,000		50,000			50,000
2003 Series A, due 2038	50,000			50,000		50,000			50,000
2008 Series A, due 2038	50,000			50,000		50,000			50,000
2009 Series A, due 2039	50,000			50,000		50,000			50,000
Unamortized discount and debt issuance costs	(2,024		)			(2,119		)
	197,976					197,881
Less: current maturities	—					—
Long-term debt, less current maturities – Southwest Gas Corporation	$	1,818,669				$	1,521,031
Centuri:
Centuri term loan facility	$	255,959		260,135		$	199,578		207,588
Unamortized debt issuance costs	(1,414		)			(1,111		)
	254,545					198,467
Centuri secured revolving credit facility	—			—		56,472			56,525
Centuri other debt obligations	67,104			67,053		47,952			48,183
Less: current maturities	(33,060		)			(25,346		)
Long-term debt, less current maturities – Centuri	$	288,589				$	277,545
Consolidated Southwest Gas Holdings, Inc.:
Southwest Gas Corporation long-term debt	$	1,818,669				$	1,521,031
Centuri long-term debt	321,649					302,891
Less: current maturities	(33,060		)			(25,346		)
Long-term debt, less current maturities – Southwest Gas Holdings, Inc.	$	2,107,258				$	1,798,576

Summary of Effective Interest Rates on Variable-Rate IDRBs

The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below:



	December 31, 2018		December 31, 2017
2003 Series A	2.61	%	2.44	%
2008 Series A	2.52	%	2.59	%
2009 Series A	2.51	%	2.40	%
Tax-exempt Series A	2.53	%	2.56	%

Estimated Maturities of Long-Term Debt

Estimated maturities of long-term debt for the next five years are (in thousands):



	Southwest		Centuri		Total
2019	$	—	$	33,060	$	33,060
2020	125,000		38,296		163,296
2021	—		33,571		33,571
2022	425,000		35,515		460,515
2023	—		178,614		178,614

Schedule of Assumptions Used

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21506-112644

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21521-112644

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21538-112644

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Pension and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2018

Retirement Benefits [Abstract]

Cost of Retirement Plan

The cost of the plan is disclosed below (in thousands):



	2018		2017		2016
Employee Investment Plan cost	$	5,530	$	5,112	$	4,976

The rates are presented in the table below:



	December 31, 2018		December 31, 2017
Discount rate	4.50	%	3.75	%
Weighted-average rate of compensation increase	3.25	%	3.25	%
Asset return assumption	7.00	%	7.00	%

Schedule of Defined Benefit Plans Disclosures

The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income.



	2018									2017
	Qualified Retirement Plan			SERP			PBOP			Qualified Retirement Plan			SERP			PBOP
(Thousands of dollars)
Change in benefit obligations
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$	1,203,484		$	45,727		$	75,322		$	1,048,353		$	43,311		$	73,865
Service cost	28,555			245			1,473			23,392			309			1,468
Interest cost	44,174			1,658			2,748			46,083			1,883			3,232
Actuarial loss (gain)	(102,919		)	(3,940		)	(6,020		)	133,017			3,334			(71		)
Benefits paid	(57,280		)	(3,087		)	(3,567		)	(47,361		)	(3,110		)	(3,172		)
Benefit obligation at end of year (PBO/PBO/APBO)	1,116,014			40,603			69,956			1,203,484			45,727			75,322
Change in plan assets
Market value of plan assets at beginning of year	871,665			—			54,608			738,962			—			48,113
Actual return on plan assets	(67,771		)	—			(3,061		)	144,064			—			7,742
Employer contributions	44,000			3,087			—			36,000			3,110			—
Benefits paid	(57,280		)	(3,087		)	(4,206		)	(47,361		)	(3,110		)	(1,247		)
Market value of plan assets at end of year	790,614			—			47,341			871,665			—			54,608
Funded status at year end	$	(325,400	)	$	(40,603	)	$	(22,615	)	$	(331,819	)	$	(45,727	)	$	(20,714	)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50		%	4.50		%	4.50		%	3.75		%	3.75		%	3.75		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	N/A			3.25		%	3.25		%	N/A

Schedule of Accumulated Benefit Obligation

The accumulated benefit obligation for the retirement plan and the SERP is presented below (in thousands):



	December 31, 2018		December 31, 2017
Retirement plan	$	1,024,030	$	1,088,203
SERP	38,793		44,343

Schedule of Expected Benefit Payments

Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows (in millions):



	2019		2020		2021		2022		2023		2024-2028
Pension	$	54.0	$	55.0	$	56.0	$	58.0	$	59.0	$	319.0
SERP	3.0		2.9		2.9		2.9		2.8		13.6
PBOP	4.5		4.6		4.6		4.6		4.5		20.9

Schedule of New Accounting Pronouncements



	December 31, 2017								December 31, 2016
	As Reported		Reclassification			Revised			As Reported		Reclassification			Revised
Southwest Gas Holdings, Inc.
Operations and maintenance	$	412,187	$	(19,424	)	$	392,763		$	401,724	$	(19,760	)	$	381,964
Other income (deductions)	13,394		(19,424		)	(6,030		)	9,469		(19,760		)	(10,291		)

Southwest Gas Corporation
Operations and maintenance	$	410,745	$	(19,424	)	$	391,321		$	401,724	$	(19,760	)	$	381,964
Other income (deductions)	13,036		(19,424		)	(6,388		)	8,276		(19,760		)	(11,484		)

Schedule of Net Benefit Cost

Components of net periodic benefit cost:



	Qualified Retirement Plan									SERP									PBOP
	2018			2017			2016			2018			2017			2016			2018			2017			2016
(Thousands of dollars)
Service cost	$	28,555		$	23,392		$	22,833		$	245		$	309		$	331		$	1,473		$	1,468		$	1,499
Interest cost	44,174			46,083			46,027			1,658			1,883			1,859			2,748			3,232			3,180
Expected return on plan assets	(58,755		)	(55,196		)	(56,558		)	—			—			—			(3,718		)	(3,358		)	(3,149		)
Amortization of prior service cost	—			—			—			—			—			—			1,335			1,335			1,335
Amortization of net actuarial loss	32,115			24,004			25,266			1,502			1,441			1,383			—			—			417
Net periodic benefit cost	$	46,089		$	38,283		$	37,568		$	3,405		$	3,633		$	3,573		$	1,838		$	2,677		$	3,282
Weighted-average assumptions (net benefit cost)
Discount rate	3.75		%	4.50		%	4.50		%	3.75		%	4.50		%	4.50		%	3.75		%	4.50		%	4.50		%
Expected return on plan assets	7.00		%	7.00		%	7.25		%	N/A			N/A			N/A			7.00		%	7.00		%	7.25		%
Weighted-average rate of compensation increase	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	3.25		%	N/A			N/A			N/A

Schedule of Amounts Recognized in Other Comprehensive Income

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)



	2018			2017
Net actuarial (loss) gain	$	(435,364	)	$	(448,555	)
Prior service cost	(3,033		)	(4,368		)
Less: amount recognized in regulatory assets	383,170			391,403
Recognized in AOCI	$	(55,227	)	$	(61,520	)

Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income



	2018												2017												2016
	Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP			Total			Qualified Retirement Plan			SERP			PBOP
(Thousands of dollars)
Net actuarial loss (gain) (a)	$	20,426		$	23,607		$	(3,940	)	$	759		$	43,027		$	44,149		$	3,334		$	(4,456	)	$	22,770		$	25,153		$	1,347		$	(3,730	)
Amortization of prior service cost (b)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)	(1,335		)	—			—			(1,335		)
Amortization of net actuarial loss (b)	(33,617		)	(32,115		)	(1,502		)	—			(25,445		)	(24,004		)	(1,441		)	—			(27,066		)	(25,266		)	(1,383		)	(417		)
Regulatory adjustment	8,233			7,657			—			576			(12,340		)	(18,131		)	—			5,791			5,584			102			—			5,482
Recognized in other comprehensive (income) loss	(6,293		)	(851		)	(5,442		)	—			3,907			2,014			1,893			—			(47		)	(11		)	(36		)	—
Net periodic benefit costs recognized in net income	51,332			46,089			3,405			1,838			44,593			38,283			3,633			2,677			44,423			37,568			3,573			3,282
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	45,039		$	45,238		$	(2,037	)	$	1,838		$	48,500		$	40,297		$	5,526		$	2,677		$	44,376		$	37,557		$	3,537		$	3,282

Schedule of Fair Value of Plan Assets



	December 31, 2018						December 31, 2017
	Qualified Retirement Plan		PBOP		Total		Qualified Retirement Plan		PBOP		Total
Assets at fair value (thousands of dollars):
Level 1 – Quoted prices in active markets for identical financial assets
Mutual funds	$	—	$	25,299	$	25,299	$	—	$	27,020	$	27,020
Total Level 1 Assets (1)	$	—	$	25,299	$	25,299	$	—	$	27,020	$	27,020
Level 2 – Significant other observable inputs
Private commingled equity funds (2)
International	$	309,745	$	8,484	$	318,229	$	340,217	$	10,577	$	350,794
U.S. equity securities	147,693		4,045		151,738		165,937		5,158		171,095
Emerging markets	50,817		1,392		52,209		56,259		1,749		58,008
Private commingled fixed income funds (3)	274,062		7,506		281,568		301,217		9,364		310,581
Pooled funds and mutual funds	5,198		610		5,808		4,676		735		5,411
Government fixed income and mortgage backed securities	163		5		168		172		5		177
Total Level 2 assets (4)	$	787,678	$	22,042	$	809,720	$	868,478	$	27,588	$	896,066
Total Plan assets at fair value	$	787,678	$	47,341	$	835,019	$	868,478	$	54,608	$	923,086
Insurance company general account contracts (5)	2,936		—		2,936		3,187		—		3,187
Total Plan assets	$	790,614	$	47,341	$	837,955	$	871,665	$	54,608	$	926,273


(1)	The Mutual funds category above is a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks, and fixed-income securities. The fund seeks regular income, conservation of principal, and an opportunity for long-term growth of principal and income.

(2)


•	International developed countries equities


•	Domestic equities


•	Emerging markets equities

(3)


(4)	With the exception of items (2) and (3), which are discussed in detail above, the Level 2 assets consist mainly of pooled funds and mutual funds. These funds are collective short-term funds that invest in Treasury bills and money market funds and are used as a temporary cash repository.


(5)	The insurance company general account contracts are annuity insurance contracts used to pay the pensions of employees who retired prior to 1989. The balance of the account disclosed in the above table is the contract value, which is the result of deposits, withdrawals, and interest credits.

Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Share-Based Plan Compensation Expense, Including Cash Award

The table below shows total share-based plan compensation expense which was recognized in the Consolidated Statements of Income (in thousands):



	2018		2017		2016
Share-based compensation plan expense, net of related tax benefits	$	4,644	$	6,751	$	7,185
Share-based compensation plan related tax benefits	1,467		4,137		4,404

Schedule of Stock Options Activity

The following tables summarize the stock option plan activity and related information (thousands of options):



	2018		2017		2016
	Number of options	Weighted- average exercise price	Number of options	Weighted- average exercise price	Number of options		Weighted- average exercise price
Outstanding at the beginning of the year	—	N/A	—	N/A	17		$	31.64
Exercised during the year	—	—	—	—	(17	)	31.64
Forfeited or expired during the year	—	—	—	—	—		—
Outstanding and exercisable at year end	—	N/A	—	N/A	—		N/A



	2018		2017		2016
Outstanding and exercisable	$	—	$	—	$	—
Exercised	—		—		554

Summary of Market Prices of Common Stock



	December 31, 2018		December 31, 2017		December 31, 2016
Market value of Company stock	$	76.50	$	80.48	$	76.62

Schedule of Nonvested Performance and Restricted Stock Unit Plans

The following table summarizes the activity of the management incentive plan shares and restricted stock/units as of December 31, 2018 (thousands of shares):



	Management Incentive Plan Shares		Weighted- average grant date fair value		Restricted Stock/ Units (1)		Weighted- average grant date fair value
Nonvested/unissued at December 31, 2017	127		$	63.98	305		$	57.41
Granted	—				77		69.16
Dividends	2				7
Forfeited or expired	(1	)	66.86		(1	)	72.39
Vested and issued (2)	(63	)	59.41		(65	)	71.00
Nonvested/unissued at December 31, 2018	65		$	66.51	323		$	56.16


(1)	The number of securities granted includes 34,000 performance shares, which was derived by assuming that target performance will be achieved during the relevant performance period.


(2)	Includes shares for retiree payouts and those converted for taxes.

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations

The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations (thousands of dollars):



Year ended December 31,	2018		2017		2016
U.S.	$	235,120	$	246,131	$	218,810
Foreign	8,216		12,899		12,713
Total income before income taxes	$	243,336	$	259,030	$	231,523

Summary of Income Tax Expense (Benefit)

Income tax expense (benefit) consists of the following (thousands of dollars):



Year Ended December 31,	2018			2017		2016
Current:
Federal	$	(17,584	)	$	318	$	(9,695	)
State	(6,783		)	1,420		2,510
	(24,367		)	1,738		(7,185		)
Deferred:
Federal	58,136			60,662		66,037
State	10,222			735		(268		)
	68,358			61,397		65,769
Total income tax expense from continuing operations	43,991			63,135		58,584
Discontinued operations	—			—		19,884
Total income tax expense	$	43,991		$	63,135	$	78,468

Income tax expense (benefit) consists of the following (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Current:
Federal	$	(13,476	)	$	(1,316	)	$	541
State	(3,219		)	2,965			5,748
Foreign	2,563			5,203			4,298
	(14,132		)	6,852			10,587
Deferred:
Federal	67,784			58,443			68,270
State	8,901			1,837			140
Foreign	(869		)	(2,044		)	(529		)
	75,816			58,236			67,881
Total income tax expense	$	61,684		$	65,088		$	78,468

Significant Components of Deferred Income Tax Expense (Benefit)

Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Deferred federal and state:
Property-related items	$	67,576		$	49,129		$	72,811
Purchased gas cost adjustments	(3,507		)	8,500			361
Employee benefits	2,156			(5,707		)	(139		)
Regulatory Adjustments	2,412			14,401			6,322
All other deferred	334			(4,197		)	(12,748		)
Total deferred federal and state	68,971			62,126			66,607
Deferred ITC, net	(613		)	(729		)	(838		)
Total deferred income tax expense	$	68,358		$	61,397		$	65,769

Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars):



Year Ended December 31,	2018			2017			2016
Deferred federal and state:
Property-related items	$	94,899		$	44,516		$	76,217
Purchased gas cost adjustments	(3,507		)	8,500			361
Employee benefits	(7,334		)	(2,517		)	(1,327		)
Regulatory Adjustments	2,412			14,401			6,322
All other deferred	(10,041		)	(5,935		)	(12,854		)
Total deferred federal and state	76,429			58,965			68,719
Deferred ITC, net	(613		)	(729		)	(838		)
Total deferred income tax expense	$	75,816		$	58,236		$	67,881

Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate



Year Ended December 31,	2018		2017		2016
U.S. federal statutory income tax rate	21.0	%	35.0	%	35.0	%
Net state taxes	2.1		0.6		0.8
Tax credits	(0.4	)	(0.4	)	(0.5	)
Company owned life insurance	0.3		(1.7	)	(1.5	)
Change in U.S. Federal Income Tax Rate	—		(3.6	)	—
All other differences	1.1		(1.2	)	(0.9	)
Effective income tax rate from continuing operations	24.1	%	28.7	%	32.9	%



Year Ended December 31,	2018		2017		2016
U.S. federal statutory income tax rate	21.0	%	35.0	%	35.0	%
Net state taxes	2.9		1.1		1.4
Tax credits	(0.3	)	(0.4	)	(0.4	)
Company owned life insurance	0.1		(1.6	)	(1.2	)
Change in U.S. Federal Income Tax Rate	—		(7.8	)	—
All other differences	1.6		(1.2	)	(0.9	)
Consolidated effective income tax rate	25.3	%	25.1	%	33.9	%

Deferred Tax Assets and Liabilities

Deferred tax assets and liabilities consist of the following (thousands of dollars):



December 31,	2018			2017
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	105,791		$	98,912
Employee benefits	17,337			18,707
Alternative minimum tax credit	21,603			4,390
Net operating losses and credits	4,557			10,070
Interest rate swap	2,235			3,037
Other	9,386			8,820
Valuation allowance	(37		)	(58		)
	160,872			143,878
Deferred tax liabilities:
Property-related items, including accelerated depreciation	614,205			561,493
Regulatory balancing accounts	6,097			6,067
Unamortized ITC	368			981
Debt-related costs	3,110			3,380
Other	27,550			17,200
	651,330			589,121
Net deferred tax liabilities	$	490,458		$	445,243

Deferred tax assets and liabilities consist of the following (thousands of dollars):



December 31,	2018			2017
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$	105,791		$	98,912
Employee benefits	39,215			31,323
Alternative minimum tax credit	21,603			4,390
Net operating losses and credits	13,125			11,460
Interest rate swap	2,235			3,037
Other	17,215			13,870
Valuation allowance	(1,132		)	(728		)
	198,052			162,264
Deferred tax liabilities:
Property-related items, including accelerated depreciation	678,307			598,371
Regulatory balancing accounts	6,097			6,067
Unamortized ITC	368			981
Debt-related costs	3,110			3,380
Intangibles	7,807			7,656
Other	30,300			21,289
	725,989			637,744
Net noncurrent deferred tax liabilities	$	527,937		$	475,480

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars):



	2018		2017
Unrecognized tax benefits at beginning of year	$	1,069	$	903
Gross increases – tax positions in prior period	—		67
Gross decreases – tax positions in prior period	98		—
Gross increases – current period tax positions	—		99
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	971	$	1,069

A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars):



	2018		2017
Unrecognized tax benefits at beginning of year	$	1,430	$	1,231
Gross increases – tax positions in prior period	—		100
Gross decreases – tax positions in prior period	459		—
Gross increases – current period tax positions	—		99
Gross decreases – current period tax positions	—		—
Settlements	—		—
Lapse in statute of limitations	—		—
Unrecognized tax benefits at end of year	$	971	$	1,430

Summary of Income Before Taxes for Continuing and Discontinued Operations



Year ended December 31,	2018		2017		2016
Income from continuing operations before income taxes	$	182,833	$	219,953	$	178,007
Income from discontinued operations before income taxes	—		—		53,516
Total income before income taxes	$	182,833	$	219,953	$	231,523

Derivatives (Tables)

12 Months Ended

Dec. 31, 2018

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Notional Amounts under Swaps Contracts

The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):

Amount of Gain or Losses Recognized in Income on Derivatives



Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	2018				2017				2016
Swaps	Net cost of gas sold	$	(2,113	)		$	(11,572	)		$	5,006
Swaps	Net cost of gas sold	2,113			*	11,572			*	(5,006		)	*
Total		$	—			$	—			$	—

* Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.

Fair Values of Swaps in Consolidated Balance Sheets

The following table sets forth the fair values of the Swaps and their location in the Consolidated Balance Sheets of Southwest and the Company (thousands of dollars).

Fair values of derivatives not designated as hedging instruments:



December 31, 2018
Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Prepaid and other current assets	$	243	$	(99	)	$	144
Swaps	Other current liabilities	1,595		(3,347		)	(1,752		)
Swaps	Other deferred credits	141		(251		)	(110		)
Total		$	1,979	$	(3,697	)	$	(1,718	)

December 31, 2017
Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	11	$	(4,468	)	$	(4,457	)
Swaps	Prepaid and other current assets	19		(1,342		)	(1,323		)
Total		$	30	$	(5,810	)	$	(5,780	)

Paid to and Received from Counterparties for Settlements of Matured Swaps

The following table presents the amounts paid to and received from counterparties for settlements of matured Swaps.



	Year ended December 31, 2018		Year ended December 31, 2017		Year ended December 31, 2016
(Thousands of dollars)
Paid to counterparties	$	6,781	$	3,100	$	5,583
Received from counterparties	$	606	$	1,685	$	726

Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets

The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Consolidated Balance Sheets (thousands of dollars).



December 31, 2018 Instrument	Balance Sheet Location	Net Total
Swaps	Other current liabilities	$	(144	)
Swaps	Prepaid and other current assets	1,752
Swaps	Deferred charges and other assets	110

December 31, 2017 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaid and other current assets	$	4,457
Swaps	Deferred charges and other assets	1,323

Summary of Financial Assets and Liabilities at Fair Value

Level 2 – Significant other observable inputs



	December 31, 2018			December 31, 2017
(Thousands of dollars)
Assets at fair value:
Prepaid and other current assets – Swaps	$	144		$	—
Deferred charges and other assets – Swaps	—			—
Liabilities at fair value:
Other current liabilities – Swaps	(1,752		)	(4,457		)
Other deferred credits – Swaps	(110		)	(1,323		)
Net Assets (Liabilities)	$	(1,718	)	$	(5,780	)

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

Accounts Receivable for Services

Segment Information (Tables)

12 Months Ended

Dec. 31, 2018

Segment Reporting [Abstract]

Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below (in thousands).



	December 31, 2018		December 31, 2017
Accounts receivable for Centuri services	$	18,830	$	12,987

Schedule of Revenues by Geographic Area

The following table presents the amount of revenues for both segments by geographic area (thousands of dollars):



	December 31, 2018		December 31, 2017		December 31, 2016
Revenues (a)
United States	$	2,664,670	$	2,345,134	$	2,256,600
Canada	215,343		203,658		203,890
Total	$	2,880,013	$	2,548,792	$	2,460,490


(a)	Revenues are attributed to countries based on the location of customers.

Schedule of Segment Reporting Information

The financial information pertaining to the natural gas operations and utility infrastructure services segments for each of the three years in the period ended December 31, 2018 is as follows (thousands of dollars):



2018	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,357,728	$	1,386,371	$	—		$	2,744,099
Intersegment sales	—		135,914		—			135,914
Total	$	1,357,728	$	1,522,285	$	—		$	2,880,013
Interest revenue	$	6,020	$	88	$	—		$	6,108
Interest expense	$	81,740	$	14,190	$	741		$	96,671
Depreciation and amortization	$	191,816	$	57,396	$	—		$	249,212
Income tax expense	$	43,991	$	18,420	$	(727	)	$	61,684
Segment net income	$	138,842	$	44,977	$	(1,542	)	$	182,277
Segment assets	$	6,141,584	$	1,215,573	$	572		$	7,357,729
Capital expenditures	$	682,869	$	83,045	$	—		$	765,914

2017	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,302,308	$	1,149,325	$	—		$	2,451,633
Intersegment sales	—		97,159		—			97,159
Total	$	1,302,308	$	1,246,484	$	—		$	2,548,792
Interest revenue	$	2,784	$	3	$	—		$	2,787
Interest expense	$	69,733	$	7,986	$	345		$	78,064
Depreciation and amortization	$	201,922	$	49,029	$	—		$	250,951
Income tax expense	$	63,135	$	2,390	$	(437	)	$	65,088
Segment net income	$	156,818	$	38,360	$	(1,337	)	$	193,841
Segment assets	$	5,482,669	$	752,496	$	1,901		$	6,237,066
Capital expenditures	$	560,448	$	63,201	$	—		$	623,649

2016	Gas Operations		Utility Infrastructure Services		Other			Total
Revenues from unaffiliated customers	$	1,321,412	$	1,040,957	$	—		$	2,362,369
Intersegment sales	—		98,121		—			98,121
Total	$	1,321,412	$	1,139,078	$	—		$	2,460,490
Interest revenue	$	1,848	$	1	$	—		$	1,849
Interest expense	$	66,997	$	6,663	$	—		$	73,660
Depreciation and amortization	$	233,463	$	55,669	$	—		$	289,132
Income tax expense	$	58,584	$	19,884	$	—		$	78,468
Segment net income	$	119,423	$	32,618	$	—		$	152,041
Segment assets	$	5,001,756	$	579,370	$	—		$	5,581,126
Capital expenditures	$	457,120	$	72,411	$	—		$	529,531

Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Quarterly Financial Information



	Quarter Ended
	March 31		June 30		September 30			December 31
(Thousands of dollars, except per share amounts)
2018
Southwest Gas Holdings, Inc.:
Operating revenues	$	754,330	$	670,883	$	668,146		$	786,654
Operating income (4)	129,560		53,338		39,681			134,854
Net income	78,294		21,551		12,331			131,160
Net income attributable to Southwest Gas Holdings, Inc.	79,091		21,551		12,331			69,304
Basic earnings per common share (1)	1.63		0.44		0.25			1.36
Diluted earnings per common share (1)	1.63		0.44		0.25			1.36

Southwest Gas Corporation:
Operating revenues	$	494,313	$	275,679	$	217,523		$	370,213
Operating income (4)	141,173		24,675		3			115,962
Net income (loss)	90,349		2,622		(13,670		)	59,541

2017
Southwest Gas Holdings, Inc.:
Operating revenues	$	654,737	$	560,469	$	593,153		$	740,433
Operating income (4)	124,347		48,265		34,988			135,524
Net income	69,005		18,121		10,420			96,396
Net income attributable to Southwest Gas Holdings, Inc.	69,308		17,864		10,204			96,465
Basic earnings per common share (1)	1.46		0.38		0.21			2.00
Diluted earnings per common share (1)	1.45		0.37		0.21			2.00

Southwest Gas Corporation:
Operating revenues	$	462,602	$	260,162	$	213,059		$	366,485
Operating income (4)	135,922		32,346		9,921			117,885
Net income (loss)	76,938		9,522		(4,024		)	74,382



	Quarter Ended
	March 31			June 30		September 30			December 31
2016
Southwest Gas Holdings, Inc.: (2)
Operating revenues	$	731,248		$	547,748	$	539,969		$	641,525
Operating income (4)	139,036			33,057		20,478			122,903
Net income (loss)	75,355			9,099		2,907			65,694
Net income attributable to Southwest Gas Holdings, Inc.	75,446			8,943		2,472			65,180
Basic earnings per common share (1)	1.59			0.19		0.05			1.37
Diluted earnings per common share (1)	1.58			0.19		0.05			1.36

Southwest Gas Corporation: (2) (3)
Operating revenues
Continuing operations	$	525,100		$	255,648	$	200,179		$	340,485
Discontinued operations – utility infrastructure services	206,148			292,100		339,790			301,040
Total	$	731,248		$	547,748	$	539,969		$	641,525
Operating income (loss) (4)
Continuing operations	$	140,885		$	20,210	$	(5,292	)	$	100,685
Discontinued operations – utility infrastructure services	(1,849		)	12,847		25,770			22,218
Total	$	139,036		$	33,057	$	20,478		$	122,903
Net income (loss)
Continuing operations	$	77,583		$	2,358	$	(12,405	)	$	51,887
Discontinued operations – utility infrastructure services	(2,137		)	6,585		14,877			13,293
Total	$	75,446		$	8,943	$	2,472		$	65,180


(1)	The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares.


(2)	Refer to Notes 1 and 18. Effective 2017, Southwest Gas Holdings, Inc. (“Company) is the successor equity issuer to Southwest Gas Corporation (“Southwest”). Both Southwest and Centuri became subsidiaries of the Company.


(3)	Periods prior to 2017 depict Centuri amounts as discontinued operations of Southwest.

(4)

Utility Infrastructure Services Noncontrolling Interests (Tables)

12 Months Ended

Dec. 31, 2018

Noncontrolling Interest [Abstract]

Summary of Redeemable Noncontrolling Interest

The following depicts changes to the balance of the redeemable noncontrolling interest between the indicated periods.



	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2016	$	22,590
Net Income (loss) attributable to redeemable noncontrolling interest	248
Foreign currency exchange translation adjustment	11
Centuri distribution to redeemable noncontrolling interest	(204		)
Adjustment to redemption value	355
Redemption of Centuri shares from noncontrolling parties	(23,000		)
Balance, December 31, 2017	—
Redeemable noncontrolling interest acquired	81,659
Net income attributable to redeemable noncontrolling interest	172
Balance, December 31, 2018	$	81,831

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation (Tables)

12 Months Ended

Dec. 31, 2018

Restructuring and Related Activities [Abstract]

Summary of Discontinued Operations - Construction Services in Condensed Consolidated Balance Sheet and Condensed Consolidated Income Statements

Results of Utility Infrastructure Services



	Year Ended December 31,
	2016
(Thousands of dollars)
Utility infrastructure services revenues	$	1,139,078
Operating expenses:
Utility infrastructure services expenses	1,024,423
Depreciation and amortization	55,669
Operating income	58,986
Other income (deductions)	1,193
Net interest deductions	6,663
Income before income taxes	53,516
Income tax expense	19,884
Net income	33,632
Net income attributable to noncontrolling interests	1,014
Discontinued operations – utility infrastructure services – net income	$	32,618

Summary of Fair Values of Assets Acquired and Liabilities Assumed

Tabular disclosure of information related to a disposal group. Includes, but is not limited to, a discontinued operation, disposal classified as held-for-sale or disposed of by means other than sale or disposal of an individually significant component.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=109226691&loc=SL51724579-110230

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109222650&loc=d3e1474-107760

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5D
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721677-107760

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109222160&loc=SL51721533-107759

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721659-107760

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4B
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721665-107760

Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2941-110230

Reference 8: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5C
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721675-107760

Reference 9: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721683-107760

Reference 10: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109222650&loc=d3e1361-107760

Reference 11: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5A
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721671-107760

Reference 12: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721673-107760

Reference 13: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 4A
-URI http://asc.fasb.org/extlink&oid=109222650&loc=SL51721663-107760

+ Details

Business Acquisitions (Tables)

12 Months Ended

Dec. 31, 2018

Business Combinations [Abstract]

The fair values of assets acquired and liabilities assumed as of November 1, 2017, the acquisition date, were as follows (in millions of dollars):



Cash and cash equivalents	$	0.8
Contracts receivable	18.3
Other receivables	5.4
Property, plant and equipment	15.1
Prepaid expenses and deposits	1.6
Intangible assets	44.8
Goodwill	32.2
Total assets acquired	118.2
Current liabilities	(18.6		)
Other long-term liabilities	(0.3		)
Net assets acquired	$	99.3

The preliminary estimated fair values of assets acquired and liabilities assumed as of November 30, 2018, are as follows (in millions of dollars):



Cash and cash equivalents	$	3.9
Accounts receivable	32.8
Revenue earned on contracts in progress in excess of billings	21.6
Prepaid expenses and other current assets	1.1
Property and equipment	89.4
Intangible assets	89.3
Goodwill	188.5
Total assets acquired	426.6

Accounts payable	8.0
Accrued liabilities	6.9
Deferred compensation and related accrued taxes	3.4
Redeemable noncontrolling interest	81.7
Total liabilities assumed and noncontrolling interest	100.0
Net assets acquired	$	326.6

Schedule of Pro Forma Consolidated Financial Information



	Year Ended December 31,
	2018		2017
Total operating revenues	$	3,037,209	$	2,626,721
Net income attributable to Southwest Gas Holdings, Inc.	$	187,642	$	192,368
Basic earnings per share	$	3.80	$	4.01
Diluted earnings per share	$	3.79	$	4.01

The unaudited pro forma consolidated financial information for fiscal 2017 is as follows (in thousands of dollars, except per share amounts):



	Year Ended December 31,
	2017
Total operating revenues	$	2,639,452
Net income attributable to Southwest Gas Holdings, Inc.	$	203,245
Basic earnings per share	$	4.24
Diluted earnings per share	$	4.24

Schedule of Statements of Income Operations



	Year ended December 31, 2018
Utility infrastructure services revenues	$	14,119
Net income attributable to Southwest Gas Holdings, Inc.	690

Background, Organization and Summary of Significant Accounting Policies - Narrative (Detail) - USD ($)

1 Months Ended

3 Months Ended

12 Months Ended

Nov. 30, 2018

Jun. 30, 2018

Mar. 31, 2018

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Aug. 31, 2017

Jul. 31, 2017

Percentage of equity interest acquired

3.40%

Amortization expenses

$ 7,600,000

$ 4,100,000

$ 3,200,000

Cumulative foreign earnings

25,000,000

Goodwill impairment charges

Reclassification from AOCI to retained earnings resulting from TCJA

^[1]

$ 9,300,000

Centuri

Proceeds from contributed capital

$ 1,000,000

Intangible assets

$ 159,821,000

80,702,000

Centuri | W.S. Nicholls Western Construction LTD

Equity method investment amount

VIR ownership percentage

50.00%

$ 11,200,000

12,700,000

Dividends received

1,000,000

Exposure to loss as a result of its investment

44,900,000

Earnings from equity method investment

500,000

Southwest Gas Corporation

Reclassification from AOCI to retained earnings resulting from TCJA

Dividends received

$ 12,461,000

$ 9,300,000

Money Market Funds

Money market fund investments

59,900,000

22,200,000

Money Market Funds | Southwest Gas Corporation

Money market fund investments

18,000,000

20,800,000

Gas Pipe Materials and Supplies | Prepaids and other current assets

Gas pipe materials and operating supplies

56,000,000

33,000,000

Prepaid and other current assets

$ 74,000,000

$ 40,000,000

Centuri

Ownership percentage by parent

96.60%

Percentage of equity interest acquired

3.40%

IntelliChoice Energy LLC | Centuri

Ownership percentage by parent

65.00%

95.00%

Linetec | Centuri

Percentage of voting interests acquired

80.00%

Business combination consideration

$ 326,600,000

$ 75,600,000

[1]	Release of excess deferred taxes accumulated prior to December 22, 2017 (date of enactment of the TCJA), as a result of the adoption of ASU 2018-02, which permitted such release. See Note 1 - Background, Organization, and Summary of Significant Accounting Policies.

Background, Organization and Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Other property	$ 14,153	$ 13,242
Total	623,551	428,180
Southwest Gas Corporation
Investment [Line Items]
Net cash surrender value of COLI policies	114,405	117,341
Other property	1,741	1,773
Total	116,146	119,114
Centuri
Investment [Line Items]
Centuri property, equipment, and intangibles	792,191	554,730
Centuri accumulated provision for depreciation and amortization	$ (298,939)	$ (258,906)

Background, Organization and Summary of Significant Accounting Policies - Schedule of Goodwill (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Goodwill, Beginning balance	$ 179,314	$ 139,983
Foreign currency translation adjustment	(8,945)	7,303
Goodwill, Ending balance	359,045	179,314
Neuco
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	182	32,028
Linetec
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	188,494
Natural Gas Operations
Goodwill [Roll Forward]
Goodwill, Beginning balance	10,095	10,095
Foreign currency translation adjustment	0	0
Goodwill, Ending balance	10,095	10,095
Natural Gas Operations \| Neuco
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	0	0
Natural Gas Operations \| Linetec
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	0
Utility Infrastructure Services
Goodwill [Roll Forward]
Goodwill, Beginning balance	169,219	129,888
Foreign currency translation adjustment	(8,945)	7,303
Goodwill, Ending balance	348,950	169,219
Utility Infrastructure Services \| Neuco
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	182	$ 32,028
Utility Infrastructure Services \| Linetec
Goodwill [Roll Forward]
Goodwill from Neuco acquisition	$ 188,494

Background, Organization and Summary of Significant Accounting Policies - Summary of Intangible Assets (Detail) - Centuri - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 177,838	$ 92,068
Accumulated Amortization	(18,017)	(11,366)
Net Carrying Amount	159,821	80,702
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	152,533	76,254
Accumulated Amortization	(11,716)	(6,743)
Net Carrying Amount	140,817	69,511
Trade names and trademarks
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	23,013	13,754
Accumulated Amortization	(5,234)	(4,080)
Net Carrying Amount	17,779	9,674
Customer contracts backlog
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	270
Accumulated Amortization	(3)
Net Carrying Amount	267
Noncompete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	2,022	2,060
Accumulated Amortization	(1,064)	(543)
Net Carrying Amount	$ 958	$ 1,517

Background, Organization and Summary of Significant Accounting Policies - Schedule of Estimated Future Amortization of Intangible Assets (Detail) - Centuri - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
2019	$ 10,622
2020	10,634
2021	10,214
2022	10,127
2023	10,127
Thereafter	108,097
Net Carrying Amount	$ 159,821	$ 80,702

Background, Organization and Summary of Significant Accounting Policies - Schedule of Capitalized and Debt Portion of AFUDC (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Debt portion	$ 3,264	$ 1,666	$ 1,175
Equity portion	3,627	2,296	2,289
AFUDC capitalized as part of utility plant	$ 6,891	$ 3,962	$ 3,464
AFUDC rate	5.85%	5.95%	7.35%

Background, Organization and Summary of Significant Accounting Policies - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other income (deductions)
Interest income	$ 6,108	$ 2,787	$ 1,849
Equity AFUDC	3,627	2,296	2,289
Total other income (deductions)	(17,426)	(6,030)	(10,291)
Southwest Gas Corporation
Other income (deductions)
Total other income (deductions)	(17,240)	(6,388)	(11,484)
Corporate and administrative
Other income (deductions)
Total other income (deductions)	52	13	0
Gas Operations
Other income (deductions)
Interest income	6,020	2,784	1,848
Gas Operations \| Operating Segments \| Southwest Gas Corporation
Other income (deductions)
Increase (decrease) in COLI policies	(3,200)	10,300	7,400
Interest income	6,020	2,784	1,848
Equity AFUDC	3,627	2,296	2,289
Non-service post-retirement benefit cost	(21,059)	(19,424)	(19,760)
Miscellaneous income and (expense)	(2,628)	(2,344)	(3,261)
Total other income (deductions)	(17,240)	(6,388)	(11,484)
Utility Infrastructure Services
Other income (deductions)
Interest income	88	3	1
Utility Infrastructure Services \| Operating Segments \| Centuri
Other income (deductions)
Interest income	88	3	1
Foreign transaction gain (loss)	(222)	(754)	(22)
Equity in earnings of unconsolidated investment – Western	531	1,052	69
Miscellaneous income and (expense)	(635)	44	1,145
Total other income (deductions)	$ (238)	$ 345	$ 1,193

Background, Organization and Summary of Significant Accounting Policies - Schedule of Earnings Per Share, Basic and Diluted (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Average basic shares (in shares)	49,419,000	47,965,000	47,469,000
Effect of dilutive securities:
Stock options (in shares)	0	0	1,000
Management Incentive Plan shares (in shares)	25,000	8,000	124,000
Restricted stock units (in shares)	32,000	18,000	220,000
Average diluted shares (in shares)	49,476,000	47,991,000	47,814,000
Number of performance share units granted	23,000	7,000

Utility Plant and Leases - Schedule of Net Utility Plant (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 7,134,239	$ 6,629,644
Less: accumulated depreciation	(2,234,029)	(2,231,242)
Construction work in progress	193,028	125,248
Net utility plant	5,093,238	4,523,650
Software and software-related intangibles
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	241,158	230,030
Storage
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	26,825	25,019
Transmission
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	386,159	363,396
Distribution
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	6,049,380	5,600,769
General
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	416,643	396,252
Other
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 14,074	$ 14,178

Utility Plant and Leases - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage	2.70%	3.00%	3.00%
Amortization of intangibles	$ 7.6	$ 4.1	$ 3.2
Gas, Transmission and Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage	3.00%	3.00%	3.00%
Gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of intangibles	$ 13.6	$ 14.3	$ 14.8

Utility Plant and Leases - Schedule of Depreciation and Amortization Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 249,212	$ 250,951	$ 289,132
Gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 185,719	$ 187,075	$ 214,037

Utility Plant and Leases - Schedule of Rental Payments for Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 64,047	$ 67,236	$ 58,313
Southwest Gas Corporation
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	4,556	4,926	4,357
Centuri
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 59,491	$ 62,310	$ 53,956

Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Operating Leases (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
2019	$ 10,951
2020	8,019
2021	6,059
2022	5,326
2023	3,760
Thereafter	10,688
Total minimum lease payments	44,803
Natural Gas Operations
Operating Leased Assets [Line Items]
2019	898
2020	363
2021	299
2022	163
2023	79
Thereafter	177
Total minimum lease payments	1,979
Centuri
Operating Leased Assets [Line Items]
2019	10,053
2020	7,656
2021	5,760
2022	5,163
2023	3,681
Thereafter	10,511
Total minimum lease payments	$ 42,824

Revenue - Narrative (Detail) $ in Thousands	12 Months Ended
Revenue - Narrative (Detail) $ in Thousands	Dec. 31, 2018 USD ($) Segment Type Contract Customer	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of customers \| Customer	2,000,000
Number of customers party to contracts with rate components subject to negotiation (less than) \| Customer	24
Revenue earned on contracts in progress in excess of billings	$ 87,520	$ 5,768
Southwest Gas Corporation
Segment Reporting Information [Line Items]
Number of types of services provided to customers \| Type	2
Centuri
Segment Reporting Information [Line Items]
Number of types of services provided to customers \| Type	3
Number of types of agreements with customers \| Type	2
Increases due to amounts billed to customers in excess of revenue earned during the period	$ 30,000
Change in contract liability, revenue recognized	$ 9,600
Number of contracts with original duration more than one year \| Contract	15
Gas Operations
Segment Reporting Information [Line Items]
Number of operating segment \| Segment	1
Accounting Standards Update 2014-09 \| Difference between Revenue Guidance in Effect before and after Topic 606 \| Centuri
Segment Reporting Information [Line Items]
Revenue earned on contracts in progress in excess of billings	$ 50,600

Revenue - Schedule of Gas Operating Revenue (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		$ 1,305,974	$ 1,262,881	$ 1,306,863
Alternative revenue program revenues (deferrals)		45,979	35,347	12,530
Other revenues		5,775	4,080	2,019
Total Gas operating revenues		1,357,728	1,302,308	1,321,412
Tax Reform Savings Revenue Adjustments
Disaggregation of Revenue [Line Items]
Other revenues	$ 12,500
Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		887,220	857,204	895,330
Small commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		255,083	243,513	251,092
Large commercial
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		53,192	52,379	53,582
Industrial/other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		23,489	22,026	19,753
Transportation
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers		$ 86,990	$ 87,759	$ 87,106

Revenue - Summary of Revenue Disaggregated by Service Type, and Contract Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 1,305,974	$ 1,262,881	$ 1,306,863
Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,522,285	1,246,484	1,139,078
Master services agreement \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,102,412	885,513	852,472
Bid contract \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	419,873	360,971	286,606
Unit priced contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,258,419	968,856	886,919
Fixed priced contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	117,298	127,497	95,494
Time and materials contracts \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	146,568	150,131	156,665
Gas infrastructure services \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,123,682	891,139	914,970
Electric power infrastructure services \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	32,629	18,114	27,915
Other \| Centuri
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 365,974	$ 337,231	$ 196,193

Revenue - Summary of Information about Receivables (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contracts receivable, net	$ 186,249	$ 221,859
Revenue earned on contracts in progress in excess of billings	87,520	5,768
Amounts billed in excess of revenue earned on contracts	$ 4,211	$ 9,602

Revenue - Revenue Performance Obligation (Details) - Centuri - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 $ in Millions	Dec. 31, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Transaction price allocated to unsatisfied performance obligations of contracts	$ 38.6
Revenue, remaining performance obligation, expected timing of satisfaction, period	4 years

Revenue - Schedule of Utility Infrastructure Services Contracts Receivable (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Billed on completed contracts and contracts in progress	$ 184,100	$ 219,298
Other receivables	2,588	2,622
Contracts receivable, gross	186,688	221,920
Allowance for doubtful accounts	(439)	(61)
Contracts receivable, net	$ 186,249	$ 221,859

Receivables and Related Allowances - Schedule of Accounts Receivable (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Gas utility customer accounts receivable balance (in thousands)	$ 138,149	$ 119,444

Receivables and Related Allowances - Schedule of Percent of Customers by State (Detail)	12 Months Ended
	Dec. 31, 2018
Arizona
Receivables And Related Allowances [Line Items]
Percent of customers by state	53.00%
Nevada
Receivables And Related Allowances [Line Items]
Percent of customers by state	37.00%
California
Receivables And Related Allowances [Line Items]
Percent of customers by state	10.00%

Receivables and Related Allowances - Schedule of Allowance for Uncollectibles (Detail) - Allowance for Uncollectibles - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning Balance	$ 2,111	$ 2,524	$ 2,270
Additions charged to expense	2,959	2,310	3,264
Accounts written off, less recoveries	(2,902)	(2,723)	(3,010)
Ending Balance	$ 2,168	$ 2,111	$ 2,524

Receivables and Related Allowances - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 413,926	$ 347,375
Centuri
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 273,800

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	$ 573,591	$ 531,898
Net regulatory liabilities	(376,777)	(266,288)
Deferred purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(79,762)	(6,841)
Accumulated removal costs
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(383,000)	(315,000)
Unrealized net gain on non-trading derivatives (Swaps)
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(144)	0
Unamortized gain on reacquired debt
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(8,717)	(9,253)
Regulatory excess deferred taxes and gross -up
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(458,834)	(433,908)
Other regulatory liabilities
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(19,911)	(33,184)
Gross-up related to contributions in aid of construction
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(2,900)
Accrued pension and other postretirement benefit costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	383,170	391,403
Unrealized net loss on non-trading derivatives (Swaps)
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	1,862	5,780
Deferred purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	4,928	14,581
Accrued purchased gas costs
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	29,000	17,000
Unamortized premium on reacquired debt
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	19,599	20,913
Accrued absence time
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	14,126	13,870
Margin & interest-tracking
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	88,290	42,354
Other regulatory assets
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	$ 32,616	$ 25,997

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Regulatory Assets [Line Items]
Regulatory assets	$ 573,591	$ 531,898
State mandated public purpose programs (including low income and conservation programs)
Regulatory Assets [Line Items]
Regulatory assets	6,253	4,832
Infrastructure replacement programs and similar
Regulatory Assets [Line Items]
Regulatory assets	12,486	9,627
Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	5,046	9,702
Other
Regulatory Assets [Line Items]
Regulatory assets	8,831	1,836
Other regulatory assets
Regulatory Assets [Line Items]
Regulatory assets	32,616	25,997
Self-insurance cost
Regulatory Assets [Line Items]
Regulatory assets	6,000
Prepaids and other current assets \| Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	4,500	9,200
Prepaids and other current assets \| Other
Regulatory Assets [Line Items]
Regulatory assets	197	531
Deferred charges and other assets \| Environmental compliance programs
Regulatory Assets [Line Items]
Regulatory assets	596	527
Deferred charges and other assets \| Other
Regulatory Assets [Line Items]
Regulatory assets	$ 8,600	$ 1,300

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
State mandated public purpose programs (including low income and conservation programs)
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (8,598)	$ (10,213)
Margin and interest-tracking accounts
Regulatory Liabilities [Line Items]
Regulatory liabilities	(7,273)	(9,505)
Margin and interest-tracking accounts \| Other current liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	(539)	(6,600)
Margin and interest-tracking accounts \| Other deferred credits and other long-term liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	(6,700)	(2,900)
Environmental compliance programs
Regulatory Liabilities [Line Items]
Regulatory liabilities	0	(8,574)
Regulatory accounts for differences related to pension funding
Regulatory Liabilities [Line Items]
Regulatory liabilities	(3,221)	(3,178)
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities	(819)	(1,714)
Other \| Other current liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	(810)	(1,700)
Other \| Other deferred credits and other long-term liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	(9)	(9)
Other regulatory liabilities
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (19,911)	$ (33,184)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to OCI (Detail) - USD ($) $ in Thousands	12 Months Ended				24 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 21, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax		$ (18,534)
Other comprehensive income (loss), before tax		6,628	$ 1,208	$ 3,553
Other comprehensive income (loss), tax		(2,314)	(871)	(1,288)
Total other comprehensive income, net of tax		$ 4,314	$ 337	$ 2,265
Effective income tax rate	24.00%	25.30%	25.10%	33.90%	38.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax		$ (15,524)
Other comprehensive income (loss), before tax		9,638	$ (563)	$ 3,392
Other comprehensive income (loss), tax		(2,314)	(871)	(1,288)
Total other comprehensive income, net of tax		$ 7,324	$ (1,434)	$ 2,104
Effective income tax rate	24.00%	24.10%	28.70%	32.90%	38.00%
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax		$ (20,426)
Other comprehensive income (loss) before reclassifications, tax		4,902
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)
Other comprehensive income (loss), before tax		6,293	$ (3,907)	$ 47
Other comprehensive income (loss), tax		(1,510)	400	(18)
Total other comprehensive income, net of tax		4,783	(3,507)	29
Defined Benefit Plans \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax		(20,426)
Other comprehensive income (loss) before reclassifications, tax		4,902
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)
Other comprehensive income (loss), before tax		6,293
Other comprehensive income (loss), tax		(1,510)
Total other comprehensive income, net of tax		4,783
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax		1,335	1,335	1,335
Reclassification from AOCI, tax		(320)	(507)	(507)
Reclassification from AOCI, net of tax		1,015	828	828
Other comprehensive income (loss), before tax		1,335
Other comprehensive income (loss), tax		(320)
Total other comprehensive income, net of tax		1,015
Amortization of prior service cost \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax		1,335
Other comprehensive income (loss), tax		(320)
Total other comprehensive income, net of tax		1,015
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax		(20,426)	(43,027)	(22,770)
Other comprehensive income (loss) before reclassifications, tax		4,902	10,326	8,652
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)	(32,701)	(14,118)
Reclassification from AOCI, before Tax		33,617	25,445	27,066
Reclassification from AOCI, tax		(8,068)	(9,669)	(10,285)
Reclassification from AOCI, net of tax		25,549	15,776	16,781
Other comprehensive income (loss), before tax		33,617
Other comprehensive income (loss), tax		(8,068)
Total other comprehensive income, net of tax		25,549
Net actuarial gain/(loss) \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax		33,617
Other comprehensive income (loss), tax		(8,068)
Total other comprehensive income, net of tax		25,549
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax		(8,233)	12,340	(5,584)
Reclassification from AOCI, tax		1,976	250	2,122
Reclassification from AOCI, net of tax		(6,257)	12,590	(3,462)
Other comprehensive income (loss), before tax		(8,233)
Other comprehensive income (loss), tax		1,976
Total other comprehensive income, net of tax		(6,257)
Regulatory adjustment \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax		(8,233)
Other comprehensive income (loss), tax		1,976
Total other comprehensive income, net of tax		(6,257)
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax		3,345	3,344	3,345
Reclassification from AOCI, tax		(804)	(1,271)	(1,270)
Reclassification from AOCI, net of tax		2,541	2,073	2,075
Other comprehensive income (loss), before tax		3,345	3,344	3,345
Other comprehensive income (loss), tax		(804)	(1,271)	(1,270)
Total other comprehensive income, net of tax		2,541	2,073	2,075
FSIRS \| Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax		3,345
Reclassification from AOCI, tax		(804)
Reclassification from AOCI, net of tax		2,541
Other comprehensive income (loss), before tax		3,345
Other comprehensive income (loss), tax		(804)
Total other comprehensive income, net of tax		2,541
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax		(3,010)	1,771	161
Other comprehensive income (loss) before reclassifications, tax		0	0	0
Other comprehensive income (loss) before reclassifications, net of tax		$ (3,010)	$ 1,771	$ 161

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended				24 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 21, 2017
Equity [Abstract]
Effective income tax rate	24.00%	25.30%	25.10%	33.90%	38.00%
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.5	$ 2.5

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Schedule of Estimated Amounts Amortized from Accumulated Other Comprehensive Income or Regulatory Assets into Net Periodic Benefit Cost (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	$ 22,000
SERP
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	1,000
PBOP
Defined Benefit Plan Disclosure [Line Items]
PBOP prior service cost	$ 1,300

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - AOCI Rollforward (Details) - USD ($) $ in Thousands	12 Months Ended				24 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 21, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable To Noncontrolling Interest, Other	$ (9,300)	$ (9,300)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		1,812,403	$ 1,661,273	$ 1,592,325	$ 1,592,325
Beginning balance, attributable to parent		1,814,768
Other comprehensive income (loss) before reclassifications, net of tax		(18,534)
Other comprehensive income (loss), before tax		6,628	1,208	3,553
Other comprehensive income (loss), tax		(2,314)	(871)	(1,288)
Total other comprehensive income, net of tax		4,314	337	2,265
Accumulated Other Comprehensive Income, Reclassification Of Excess Deferred Taxes		(9,300)
Ending balance, attributable to parent	2,252,042	2,252,042	1,814,768
Ending balance	$ 2,251,590	$ 2,251,590	$ 1,812,403	$ 1,661,273
Effective income tax rate	24.00%	25.30%	25.10%	33.90%	38.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax		$ (61,520)
Beginning balance, tax		22,293
Beginning balance		(39,227)
Other comprehensive income (loss) before reclassifications, before tax		(20,426)
Other comprehensive income (loss) before reclassifications, tax		4,902
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)
Other comprehensive income (loss), before tax		6,293	$ (3,907)	$ 47
Other comprehensive income (loss), tax		(1,510)	400	(18)
Total other comprehensive income, net of tax		4,783	(3,507)	29
Ending balance, before tax	$ (55,227)	(55,227)	(61,520)
Ending balance, tax	20,783	20,783	22,293
Ending balance	(34,444)	(34,444)	(39,227)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax		1,335	1,335	1,335
Reclassification from AOCI, tax		(320)	(507)	(507)
Reclassification from AOCI, net of tax		1,015	828	828
Other comprehensive income (loss), before tax		1,335
Other comprehensive income (loss), tax		(320)
Total other comprehensive income, net of tax		1,015
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax		(20,426)	(43,027)	(22,770)
Other comprehensive income (loss) before reclassifications, tax		4,902	10,326	8,652
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)	(32,701)	(14,118)
Reclassification from AOCI, before Tax		33,617	25,445	27,066
Reclassification from AOCI, tax		(8,068)	(9,669)	(10,285)
Reclassification from AOCI, net of tax		25,549	15,776	16,781
Other comprehensive income (loss), before tax		33,617
Other comprehensive income (loss), tax		(8,068)
Total other comprehensive income, net of tax		25,549
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax		(8,233)	12,340	(5,584)
Reclassification from AOCI, tax		1,976	250	2,122
Reclassification from AOCI, net of tax		(6,257)	12,590	(3,462)
Other comprehensive income (loss), before tax		(8,233)
Other comprehensive income (loss), tax		1,976
Total other comprehensive income, net of tax		(6,257)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax		(12,655)
Beginning balance, tax		4,809
Beginning balance		(7,846)
Reclassification from AOCI, before Tax		3,345	3,344	3,345
Reclassification from AOCI, tax		(804)	(1,271)	(1,270)
Reclassification from AOCI, net of tax		2,541	2,073	2,075
Other comprehensive income (loss), before tax		3,345	3,344	3,345
Other comprehensive income (loss), tax		(804)	(1,271)	(1,270)
Total other comprehensive income, net of tax		2,541	2,073	2,075
Ending balance, before tax	(9,310)	(9,310)	(12,655)
Ending balance, tax	4,005	4,005	4,809
Ending balance	(5,305)	(5,305)	(7,846)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax		(609)
Beginning balance		(609)
Other comprehensive income (loss) before reclassifications, before tax		(3,010)
Other comprehensive income (loss) before reclassifications, net of tax		(3,010)
Ending balance, before tax	(3,619)	(3,619)	(609)
Ending balance	(3,619)	(3,619)	(609)
AOCI Including Portion Attributable to Noncontrolling Interest
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, net of tax		4,314
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(47,682)	(48,008)	(50,268)	$ (50,268)
Ending balance	(52,668)	(52,668)	(47,682)	(48,008)
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable To Noncontrolling Interest, Other	(9,300)	(9,300)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		1,609,999	1,667,880	1,593,186	$ 1,593,186
Beginning balance, attributable to parent		1,609,999
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)
Other comprehensive income (loss), before tax		9,638	(563)	3,392
Other comprehensive income (loss), tax		(2,314)	(871)	(1,288)
Total other comprehensive income, net of tax		7,324	(1,434)	2,104
Accumulated Other Comprehensive Income, Reclassification Of Excess Deferred Taxes		(9,300)
Ending balance, attributable to parent	1,782,460	1,782,460	1,609,999
Ending balance	$ 1,782,460	$ 1,782,460	$ 1,609,999	$ 1,667,880
Effective income tax rate	24.00%	24.10%	28.70%	32.90%	38.00%
Southwest Gas Corporation \| Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax		$ (61,520)
Beginning balance, attributable to parent, tax		22,293
Beginning balance, attributable to parent		(39,227)
Other comprehensive income (loss) before reclassifications, before tax		(20,426)
Other comprehensive income (loss) before reclassifications, tax		4,902
Other comprehensive income (loss) before reclassifications, net of tax		(15,524)
Other comprehensive income (loss), before tax		6,293
Other comprehensive income (loss), tax		(1,510)
Total other comprehensive income, net of tax		4,783
Ending balance, attributable to parent, before tax	$ (55,227)	(55,227)	$ (61,520)
Ending balance, attributable to parent, tax	20,783	20,783	22,293
Ending balance, attributable to parent	(34,444)	(34,444)	(39,227)
Southwest Gas Corporation \| Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax		1,335
Other comprehensive income (loss), tax		(320)
Total other comprehensive income, net of tax		1,015
Southwest Gas Corporation \| Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax		33,617
Other comprehensive income (loss), tax		(8,068)
Total other comprehensive income, net of tax		25,549
Southwest Gas Corporation \| Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax		(8,233)
Other comprehensive income (loss), tax		1,976
Total other comprehensive income, net of tax		(6,257)
Southwest Gas Corporation \| FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax		(12,655)
Beginning balance, attributable to parent, tax		4,809
Beginning balance, attributable to parent		(7,846)
Reclassification from AOCI, before Tax		3,345
Reclassification from AOCI, tax		(804)
Reclassification from AOCI, net of tax		2,541
Other comprehensive income (loss), before tax		3,345
Other comprehensive income (loss), tax		(804)
Total other comprehensive income, net of tax		2,541
Ending balance, attributable to parent, before tax	(9,310)	(9,310)	(12,655)
Ending balance, attributable to parent, tax	4,005	4,005	4,809
Ending balance, attributable to parent	(5,305)	(5,305)	(7,846)
Southwest Gas Corporation \| AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(47,073)	(45,639)	$ (47,743)	$ (47,743)
Beginning balance, attributable to parent		(47,073)
Ending balance, attributable to parent	(49,049)	(49,049)	(47,073)
Ending balance	$ (49,049)	$ (49,049)	$ (47,073)	$ (45,639)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Net actuarial (loss) gain	$ (435,364)	$ (448,555)
Prior service cost	(3,033)	(4,368)
Less: amount recognized in regulatory assets	383,170	391,403
Recognized in AOCI	$ (55,227)	$ (61,520)

Common Stock (Detail) - USD ($)		1 Months Ended	3 Months Ended		12 Months Ended
Common Stock (Detail) - USD ($)	Mar. 29, 2017	Nov. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Net proceeds					$ 354,402,000	$ 41,155,000	$ 472,000
Number of common stock sold through public offering (in shares)		3,565,000
Sale of stock price (in USD per hare)		$ 75.50
Proceeds from sale of common stock		$ 260,073,524
Underwriters discount		$ 9,083,976
Common stock issuances					$ 358,083,000	52,817,000	$ 6,780,000
Shares registered and available for issuance			4,400,000		4,400,000
Restricted Stock/Unit Plan and Management Incentive Plan
Class of Stock [Line Items]
Common stock issued (in shares)					83,000
Dividend Reinvestment and Stock Purchase Plan
Class of Stock [Line Items]
Common stock issued (in shares)					143,000
Common stock issuances					$ 10,600,000
Equity Shelf Program
Class of Stock [Line Items]
Maximum value of common stock issuance	$ 150,000,000
Gross proceeds			$ 0	$ 29,999,922	85,149,976	41,776,795
Less: agent commissions			0	299,999	851,500	417,768
Net proceeds			$ 0	$ 29,699,923	$ 84,298,476	$ 41,359,027
Number of shares sold (in shares)			0	358,630	1,145,705	505,707
Weighted average price per share (USD per share)			$ 0.00	$ 83.65	$ 74.32	$ 82.61
Common stock available for sale under the program			$ 23,073,229		$ 23,073,229

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: current maturities	$ (33,060)		$ (25,346)
Long-term debt, less current maturities	2,107,258		1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(33,060)		(25,346)
Long-term debt, less current maturities	2,107,258		1,798,576
Southwest Gas Corporation
Debt Instrument [Line Items]
Long-term debt, less current maturities	1,818,669		1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt, less current maturities	1,818,669		$ 1,521,031
Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	$ 300,000
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate		3.70%
Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.45%		4.45%
Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.10%		6.10%
Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.875%		3.875%
Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	4.875%		4.875%
Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.80%		3.80%
Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	3.70%		3.70%
Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	8.00%		8.00%
Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.78%		7.78%
Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	7.92%		7.92%
Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Long-term Debt, Current and Noncurrent Abstract
Debt instrument interest rate	6.76%		6.76%
Carrying Amount
Debt Instrument [Line Items]
Less: current maturities	$ (33,060)		$ (25,346)
Long-term debt, less current maturities	2,107,258		1,798,576
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(33,060)		(25,346)
Long-term debt, less current maturities	2,107,258		1,798,576
Carrying Amount \| Southwest Gas Corporation
Debt Instrument [Line Items]
Less: current maturities	0		0
Long-term debt, less current maturities	1,818,669		1,521,031
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	1,818,669		1,521,031
Less: current maturities	0		0
Long-term debt, less current maturities	1,818,669		1,521,031
Carrying Amount \| Centuri
Debt Instrument [Line Items]
Less: current maturities	(33,060)		(25,346)
Long-term debt, less current maturities	288,589		277,545
Unamortized debt issuance costs	(1,414)		(1,111)
Long-term Debt, Current and Noncurrent Abstract
Long-term debt	321,649		302,891
Less: current maturities	(33,060)		(25,346)
Long-term debt, less current maturities	288,589		277,545
Centuri term loan/secured revolving credit facility	254,545		198,467
Carrying Amount \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	0		56,472
Carrying Amount \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility	150,000		150,000
Carrying Amount \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	255,959		199,578
Carrying Amount \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	67,104		47,952
Carrying Amount \| Debentures \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(11,807)		(9,350)
Notes payable	1,470,693		1,173,150
Carrying Amount \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	300,000		300,000
Carrying Amount \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable			0
Carrying Amount \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	75,000		75,000
Carrying Amount \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable	7,500		7,500
Carrying Amount \| IDRBs \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(2,024)		(2,119)
Unsecured debt	197,976		197,881
Carrying Amount \| IDRBs \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount \| IDRBs \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Market Value \| Centuri secured revolving credit facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	0		56,525
Market Value \| Revolving credit facility and commercial paper \| Southwest Gas Corporation
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000		150,000
Market Value \| Centuri term loan facility \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri term loan/secured revolving credit facility	260,135		207,588
Market Value \| Centuri other debt obligations \| Centuri
Long-term Debt, Current and Noncurrent Abstract
Centuri other debt obligations	67,053		48,183
Market Value \| Debentures \| Notes, 4.45%, due 2020 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	126,213		129,273
Market Value \| Debentures \| Notes, 6.1%, due 2041 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	150,728		158,304
Market Value \| Debentures \| Notes, 3.875%, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	254,195		256,163
Market Value \| Debentures \| Notes, 4.875%, due 2043 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	268,985		283,243
Market Value \| Debentures \| Notes, 3.8%, due 2046 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	267,030		302,970
Market Value \| Debentures \| Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	298,926		0
Market Value \| Debentures \| 8% Series, due 2026 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	93,827		96,063
Market Value \| Debentures \| Medium-term notes, 7.78% series, due 2022 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,497		28,714
Market Value \| Debentures \| Medium-term notes, 7.92% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	30,016		31,542
Market Value \| Debentures \| Medium-term notes, 6.76% series, due 2027 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,651		8,882
Market Value \| IDRBs \| Tax-exempt Series A, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value \| IDRBs \| 2003 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value \| IDRBs \| 2008 Series A, due 2038 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value \| IDRBs \| 2009 Series A, due 2039 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 50,000		$ 50,000

Long-Term Debt - Narrative (Detail) - USD ($)	3 Months Ended	12 Months Ended
Long-Term Debt - Narrative (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Short term borrowings outstanding		$ 152,000,000			$ 214,500,000
Issuance of additional debt		2,100,000,000
Equity cushion relating to minimum net worth requirement		$ 1,200,000,000
Percentage of maximum dividend payout restriction		60.00%
Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 400,000,000
Line of credit designated for working capital purposes		250,000,000
Short term borrowings outstanding		152,000,000			191,000,000
Centuri
Debt Instrument [Line Items]
Issuance of additional debt		101,000,000
Equity cushion relating to minimum fixed charge ratio coverage requirement		78,000,000
Southwest Gas Credit Facility \| Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		400,000,000
Line of credit designated as long term debt		150,000,000
Line of credit designated for working capital purposes		250,000,000
Long-term debt		$ 150,000,000
Effective interest rates		3.41%
Maximum current borrowing usage	$ 378,000,000
Short term borrowings outstanding		$ 152,000,000			$ 191,000,000
Commitment fee percentage		0.10%
Southwest Gas Credit Facility \| LIBOR \| Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin		1.00%
Southwest Gas Credit Facility \| Alternative Base Rate \| Southwest Gas Corporation
Debt Instrument [Line Items]
Applicable margin		0.00%
Commercial Paper Program \| Southwest Gas Corporation
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 50,000,000
Borrowings outstanding under facility		50,000,000
Secured Revolving Credit Facility and Term Loan \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			$ 590,000,000	$ 450,000,000
Borrowings outstanding under facility		$ 256,000,000
Effective interest rates		4.10%
Maximum current borrowing usage		$ 103,000,000
Minimum current borrowing usage		0
Debt secured by assets		$ 1,100,000,000
Secured Revolving Credit Facility and Term Loan \| Centuri \| Minimum
Debt Instrument [Line Items]
Commitment fee percentage		0.125%
Secured Revolving Credit Facility and Term Loan \| Centuri \| Maximum
Debt Instrument [Line Items]
Commitment fee percentage		0.35%
Secured Revolving Credit Facility and Term Loan \| Canadian Dealer Offered Rate \| Centuri
Debt Instrument [Line Items]
Applicable margin		0.875%
Secured Revolving Credit Facility and Term Loan \| Canadian Dealer Offered Rate \| Centuri \| Maximum
Debt Instrument [Line Items]
Applicable margin		2.25%
Secured Revolving Credit Facility and Term Loan \| Alternative Base Rate \| Centuri \| Minimum
Debt Instrument [Line Items]
Applicable margin		0.00%
Secured Revolving Credit Facility and Term Loan \| Alternative Base Rate \| Centuri \| Maximum
Debt Instrument [Line Items]
Applicable margin		1.25%
Centuri term loan facility \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			265,000,000
Line of Credit \| Centuri
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			$ 325,000,000
Notes, 3.7%, due 2028 \| Southwest Gas Corporation
Debt Instrument [Line Items]
Notes payable		$ 300,000,000
Debt instrument interest rate	3.70%
Debt instrument discount rate	0.185%

Long-Term Debt - Summary of Effective Interest Rates on Variable-Rate IDRBs (Detail)	Dec. 31, 2018	Dec. 31, 2017
2003 Series A, due 2038
Debt Instrument [Line Items]
Effective interest rates	2.61%	2.44%
2008 Series A, due 2038
Debt Instrument [Line Items]
Effective interest rates	2.52%	2.59%
2009 Series A, due 2039
Debt Instrument [Line Items]
Effective interest rates	2.51%	2.40%
Tax-exempt Series A, due 2028
Debt Instrument [Line Items]
Effective interest rates	2.53%	2.56%

Long-Term Debt - Estimated Maturities of Long-Term Debt (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Debt Instrument [Line Items]
2019	$ 33,060
2020	163,296
2021	33,571
2022	460,515
2023	178,614
Southwest Gas Corporation
Debt Instrument [Line Items]
2019	0
2020	125,000
2021	0
2022	425,000
2023	0
Centuri
Debt Instrument [Line Items]
2019	33,060
2020	38,296
2021	33,571
2022	35,515
2023	$ 178,614

Short-Term Debt (Detail) - USD ($)	3 Months Ended	12 Months Ended
Short-Term Debt (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Short-term Debt [Line Items]
Short term borrowings outstanding		$ 152,000,000	$ 214,500,000
Southwest Gas Corporation
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity		400,000,000
Short term borrowings outstanding		152,000,000	$ 191,000,000
Line of credit designated for working capital purposes		$ 250,000,000
Short-term borrowings, weighted average interest rate		3.47%	2.52%
Credit Facility $100 Million Expires March 2022
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity				$ 100,000,000
Commitment fee percentage		0.15%
Maximum current borrowing usage	$ 23,500,000
Short term borrowings outstanding		$ 0	$ 23,500,000
Short-term borrowings, weighted average interest rate			2.65%
Credit Facility $100 Million Expires March 2022 \| Alternative Base Rate
Short-term Debt [Line Items]
Applicable margin		0.125%
Credit Facility $100 Million Expires March 2022 \| LIBOR
Short-term Debt [Line Items]
Applicable margin		1.125%

Commitments and Contingencies (Detail)	Dec. 31, 2018 USD ($)
Southwest Gas Corporation
Loss Contingencies [Line Items]
Self-insured retention amount associated with general liability claims	$ 1,000,000
Additional self-insured retention amount of general liability	4,000,000
Centuri
Loss Contingencies [Line Items]
Self-insured retention amount associated with general liability claims	$ 300,000

Pension and Other Postretirement Benefits - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Maximum contribution of employer	3.50%
Deferment of annual cash compensation	100.00%
Matching contribution of employer	3.50%
Period of additional option to receive payment	5 years
Deferred compensation, percentage of multiple interest rate	150.00%
Increments of changes to the discount rate, basis points	75.00%	25.00%
Future estimated funding amount	$ 55
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Future estimated funding amount	$ 52
Maximum \| PBOP
Defined Benefit Plan Disclosure [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption	6.00%
Minimum \| PBOP
Defined Benefit Plan Disclosure [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption	4.50%

Pension and Other Postretirement Benefits - Cost of Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Employee Investment Plan cost	$ 5,530	$ 5,112	$ 4,976

Pension and Other Postretirement Benefits - Schedule of Assumptions Used (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Discount rate	4.50%	3.75%
Weighted-average rate of compensation increase	3.25%	3.25%
Asset return assumption	7.00%	7.00%

Pension and Other Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet and Income Statement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets
Market value of plan assets at beginning of year	$ 926,273
Market value of plan assets at end of year	837,955	$ 926,273
Qualified Retirement Plan
Change in benefit obligations
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	1,203,484	1,048,353
Service cost	28,555	23,392	$ 22,833
Interest cost	44,174	46,083	46,027
Actuarial loss (gain)	(102,919)	133,017
Benefits paid	(57,280)	(47,361)
Benefit obligation at end of year (PBO/PBO/APBO)	1,116,014	1,203,484	1,048,353
Change in plan assets
Market value of plan assets at beginning of year	871,665	738,962
Actual return on plan assets	(67,771)	144,064
Employer contributions	44,000	36,000
Benefits paid	(57,280)	(47,361)
Market value of plan assets at end of year	790,614	871,665	738,962
Funded status at year end	$ (325,400)	$ (331,819)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	3.75%
Weighted-average rate of compensation increase	3.25%	3.25%
SERP
Change in benefit obligations
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 45,727	$ 43,311
Service cost	245	309	331
Interest cost	1,658	1,883	1,859
Actuarial loss (gain)	(3,940)	3,334
Benefits paid	(3,087)	(3,110)
Benefit obligation at end of year (PBO/PBO/APBO)	40,603	45,727	43,311
Change in plan assets
Market value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	3,087	3,110
Benefits paid	(3,087)	(3,110)
Market value of plan assets at end of year	0	0	0
Funded status at year end	$ (40,603)	$ (45,727)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	3.75%
Weighted-average rate of compensation increase	3.25%	3.25%
PBOP
Change in benefit obligations
Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 75,322	$ 73,865
Service cost	1,473	1,468	1,499
Interest cost	2,748	3,232	3,180
Actuarial loss (gain)	(6,020)	(71)
Benefits paid	(3,567)	(3,172)
Benefit obligation at end of year (PBO/PBO/APBO)	69,956	75,322	73,865
Change in plan assets
Market value of plan assets at beginning of year	54,608	48,113
Actual return on plan assets	(3,061)	7,742
Employer contributions	0	0
Benefits paid	(4,206)	(1,247)
Market value of plan assets at end of year	47,341	54,608	$ 48,113
Funded status at year end	$ (22,615)	$ (20,714)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	3.75%

Pension and Other Postretirement Benefits - Schedule of Accumulated Benefit Obligation (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 1,024,030	$ 1,088,203
SERP
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 38,793	$ 44,343

Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail) $ in Millions	Dec. 31, 2018 USD ($)
Pension
Defined Benefit Plan Disclosure [Line Items]
2019	$ 54.0
2020	55.0
2021	56.0
2022	58.0
2023	59.0
2024-2028	319.0
SERP
Defined Benefit Plan Disclosure [Line Items]
2019	3.0
2020	2.9
2021	2.9
2022	2.9
2023	2.8
2024-2028	13.6
PBOP
Defined Benefit Plan Disclosure [Line Items]
2019	4.5
2020	4.6
2021	4.6
2022	4.6
2023	4.5
2024-2028	$ 20.9

Pension and Other Postretirement Benefits - Schedule of New Accounting Pronouncements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance	$ 406,393	$ 392,763	$ 381,964
Other income (deductions)	(17,426)	(6,030)	(10,291)
Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance	404,813	391,321	381,964
Other income (deductions)	$ (17,240)	(6,388)	(11,484)
As Reported
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		412,187	401,724
Other income (deductions)		13,394	9,469
As Reported \| Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		410,745	401,724
Other income (deductions)		13,036	8,276
Accounting Standards Update 2015-04 \| Reclassification
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		(19,424)	(19,760)
Other income (deductions)		(19,424)	(19,760)
Accounting Standards Update 2015-04 \| Reclassification \| Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		(19,424)	(19,760)
Other income (deductions)		$ (19,424)	$ (19,760)

Pension and Other Postretirement Benefits - Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 51,332	$ 44,593	$ 44,423
Weighted-average assumptions (net benefit cost)
Discount rate	4.50%	3.75%
Expected return on plan assets	7.00%	7.00%
Weighted-average rate of compensation increase	3.25%	3.25%
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 28,555	$ 23,392	22,833
Interest cost	44,174	46,083	46,027
Expected return on plan assets	(58,755)	(55,196)	(56,558)
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	32,115	24,004	25,266
Net periodic benefit cost	$ 46,089	$ 38,283	$ 37,568
Weighted-average assumptions (net benefit cost)
Discount rate	3.75%	4.50%	4.50%
Expected return on plan assets	7.00%	7.00%	7.25%
Weighted-average rate of compensation increase	3.25%	3.25%	3.25%
SERP
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 245	$ 309	$ 331
Interest cost	1,658	1,883	1,859
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	1,502	1,441	1,383
Net periodic benefit cost	$ 3,405	$ 3,633	$ 3,573
Weighted-average assumptions (net benefit cost)
Discount rate	3.75%	4.50%	4.50%
Weighted-average rate of compensation increase	3.25%	3.25%	3.25%
PBOP
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,473	$ 1,468	$ 1,499
Interest cost	2,748	3,232	3,180
Expected return on plan assets	(3,718)	(3,358)	(3,149)
Amortization of prior service cost	1,335	1,335	1,335
Amortization of net actuarial loss	0	0	417
Net periodic benefit cost	$ 1,838	$ 2,677	$ 3,282
Weighted-average assumptions (net benefit cost)
Discount rate	3.75%	4.50%	4.50%
Expected return on plan assets	7.00%	7.00%	7.25%

Pension and Other Postretirement Benefits - Schedule of Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 20,426	$ 43,027	$ 22,770
Amortization of prior service cost	(1,335)	(1,335)	(1,335)
Amortization of net actuarial loss	(33,617)	(25,445)	(27,066)
Regulatory adjustment	8,233	(12,340)	5,584
Recognized in other comprehensive (income) loss	(6,293)	3,907	(47)
Net periodic benefit costs recognized in net income	51,332	44,593	44,423
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	45,039	48,500	44,376
Qualified Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	23,607	44,149	25,153
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(32,115)	(24,004)	(25,266)
Regulatory adjustment	7,657	(18,131)	102
Recognized in other comprehensive (income) loss	(851)	2,014	(11)
Net periodic benefit costs recognized in net income	46,089	38,283	37,568
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	45,238	40,297	37,557
SERP
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	(3,940)	3,334	1,347
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(1,502)	(1,441)	(1,383)
Regulatory adjustment	0	0	0
Recognized in other comprehensive (income) loss	(5,442)	1,893	(36)
Net periodic benefit costs recognized in net income	3,405	3,633	3,573
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	(2,037)	5,526	3,537
PBOP
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	759	(4,456)	(3,730)
Amortization of prior service cost	(1,335)	(1,335)	(1,335)
Amortization of net actuarial loss	0	0	(417)
Regulatory adjustment	576	5,791	5,482
Recognized in other comprehensive (income) loss	0	0	0
Net periodic benefit costs recognized in net income	1,838	2,677	3,282
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$ 1,838	$ 2,677	$ 3,282

Share-Based Compensation - Narrative (Detail) $ / shares in Units, $ in Thousands	2 Months Ended	12 Months Ended
	Feb. 28, 2019 shares	Dec. 31, 2018 USD ($) plan $ / shares shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans \| plan		3
Award vesting period				10 years
Compensation expense \| $		$ 6,111	$ 10,888	$ 5,456
Compensation cost related to nonvested management incentive plan shares, performance shares, and restricted stock/units not yet recognized \| $		$ 4,000
Compensation cost not yet recognized, period for recognition		1 year 6 months
Management Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		3 years
Number of shares, granted (in shares)		0
Weighted average grant date fair value (in USD per share) \| $ / shares			$ 85.44	$ 59.05
Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)		77,000
Weighted average grant date fair value (in USD per share) \| $ / shares		$ 69.16	$ 85.39	$ 60.39
Restricted Stock/Unit Plan \| Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)	0
Restricted Stock/Unit Plan \| Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares, granted (in shares)		34,000
Performance measurement period		3 years
Compensation expense \| $		$ 2,100	$ 1,200
Vesting Tranche One \| Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage		40.00%
Vesting Tranche Two \| Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage		30.00%
Vesting Tranche Three \| Restricted Stock/Unit Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting percentage		30.00%

Share-Based Compensation - Schedule of Share-Based Plan Compensation Expense, Including Cash Award (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based compensation plan expense, net of related tax benefits	$ 4,644	$ 6,751	$ 7,185
Share-based compensation plan related tax benefits	$ 1,467	$ 4,137	$ 4,404

Share-Based Compensation - Schedule of Stock Options Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of options
Outstanding at the beginning of the year (in shares)	0	0	17
Exercised during the year (in shares)	0	0	(17)
Forfeited or expired during the year (in shares)	0	0	0
Outstanding and exercisable at year end (in shares)	0	0	0
Weighted- average exercise price
Outstanding at the beginning of the year (in USD per share)			$ 31.64
Exercised during the year (in USD per share)	$ 0.00	$ 0.00	31.64
Forfeited or expired during the year (in USD per share)	$ 0.00	$ 0.00	$ 0.00
Outstanding and exercisable	$ 0	$ 0	$ 0
Exercised	$ 0	$ 0	$ 554

Share-Based Compensation - Summary of Market Prices of Common Stock (Detail) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Market value of Company stock	$ 76.5	$ 80.48	$ 76.62

Share-Based Compensation - Schedule of Nonvested Performance and Restricted Stock Unit Plans (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Management Incentive Plan
Number of nonvested/unissued shares
Nonvested/unissued at beginning of year (in shares)	127,000
Granted (in shares)	0
Dividends (in shares)	2,000
Forfeited or expired (in shares)	(1,000)
Vested and issued (in shares)	(63,000)
Nonvested/unissued at year end (in shares)	65,000	127,000
Weighted- average grant date fair value
Beginning balance (in USD per share)	$ 63.98
Granted (in USD per share)		$ 85.44	$ 59.05
Forfeited or expired (in USD per share)	66.86
Vested and issued (in USD per share)	59.41
Ending balance (in USD per share)	$ 66.51	$ 63.98
Restricted Stock/Unit Plan
Number of nonvested/unissued shares
Nonvested/unissued at beginning of year (in shares)	305,000
Granted (in shares)	77,000
Dividends (in shares)	7,000
Forfeited or expired (in shares)	(1,000)
Vested and issued (in shares)	(65,000)
Nonvested/unissued at year end (in shares)	323,000	305,000
Weighted- average grant date fair value
Beginning balance (in USD per share)	$ 57.41
Granted (in USD per share)	69.16	$ 85.39	$ 60.39
Forfeited or expired (in USD per share)	72.39
Vested and issued (in USD per share)	71.00
Ending balance (in USD per share)	$ 56.16	$ 57.41
Restricted Stock/Unit Plan \| Performance Shares
Number of nonvested/unissued shares
Granted (in shares)	34,000

Income Taxes - Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
U.S.	$ 235,120	$ 246,131	$ 218,810
Foreign	8,216	12,899	12,713
Total income before income taxes	$ 243,336	$ 259,030	$ 231,523

Income Taxes - Summary of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ (13,476)	$ (1,316)	$ 541
State	(3,219)	2,965	5,748
Foreign	2,563	5,203	4,298
Total current income tax expense	(14,132)	6,852	10,587
Deferred:
Federal	67,784	58,443	68,270
State	8,901	1,837	140
Foreign	(869)	(2,044)	(529)
Total deferred income tax expense	75,816	58,236	67,881
Total income tax expense	61,684	65,088	78,468
Southwest Gas Corporation
Current:
Federal	(17,584)	318	(9,695)
State	(6,783)	1,420	2,510
Total current income tax expense	(24,367)	1,738	(7,185)
Deferred:
Federal	58,136	60,662	66,037
State	10,222	735	(268)
Total deferred income tax expense	68,358	61,397	65,769
Total income tax expense	43,991	63,135	58,584
Discontinued operations	0	0	19,884
Total income tax expense	$ 43,991	$ 63,135	$ 78,468

Income Taxes - Significant Components of Deferred Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred federal and state:
Total deferred federal and state	$ 76,429	$ 58,965	$ 68,719
Deferred ITC, net	(613)	(729)	(838)
Total deferred income tax expense	75,816	58,236	67,881
Property-related items
Deferred federal and state:
Total deferred federal and state	94,899	44,516	76,217
Purchased gas cost adjustments
Deferred federal and state:
Total deferred federal and state	(3,507)	8,500	361
Employee benefits
Deferred federal and state:
Total deferred federal and state	(7,334)	(2,517)	(1,327)
Regulatory Adjustments
Deferred federal and state:
Total deferred federal and state	2,412	14,401	6,322
All other deferred
Deferred federal and state:
Total deferred federal and state	(10,041)	(5,935)	(12,854)
Southwest Gas Corporation
Deferred federal and state:
Total deferred federal and state	68,971	62,126	66,607
Deferred ITC, net	(613)	(729)	(838)
Total deferred income tax expense	68,358	61,397	65,769
Southwest Gas Corporation \| Property-related items
Deferred federal and state:
Total deferred federal and state	67,576	49,129	72,811
Southwest Gas Corporation \| Purchased gas cost adjustments
Deferred federal and state:
Total deferred federal and state	(3,507)	8,500	361
Southwest Gas Corporation \| Employee benefits
Deferred federal and state:
Total deferred federal and state	2,156	(5,707)	(139)
Southwest Gas Corporation \| Regulatory Adjustments
Deferred federal and state:
Total deferred federal and state	2,412	14,401	6,322
Southwest Gas Corporation \| All other deferred
Deferred federal and state:
Total deferred federal and state	$ 334	$ (4,197)	$ (12,748)

Income Taxes - Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate (Detail)	12 Months Ended				24 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 21, 2017
Income Taxes [Line Items]
U.S. federal statutory income tax rate		21.00%	35.00%	35.00%
Net state taxes		2.90%	1.10%	1.40%
Tax credits		(0.30%)	(0.40%)	(0.40%)
Company owned life insurance		0.10%	(1.60%)	(1.20%)
Change in U.S. Federal Income Tax Rate		0.00%	(7.80%)	0.00%
All other differences		1.60%	(1.20%)	(0.90%)
Consolidated effective income tax rate	24.00%	25.30%	25.10%	33.90%	38.00%
Southwest Gas Corporation
Income Taxes [Line Items]
U.S. federal statutory income tax rate		21.00%	35.00%	35.00%
Net state taxes		2.10%	0.60%	0.80%
Tax credits		(0.40%)	(0.40%)	(0.50%)
Company owned life insurance		0.30%	(1.70%)	(1.50%)
Change in U.S. Federal Income Tax Rate		0.00%	(3.60%)	0.00%
All other differences		1.10%	(1.20%)	(0.90%)
Consolidated effective income tax rate	24.00%	24.10%	28.70%	32.90%	38.00%

Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	$ 105,791	$ 98,912
Employee benefits	39,215	31,323
Alternative minimum tax credit	21,603	4,390
Net operating losses and credits	13,125	11,460
Interest rate swap	2,235	3,037
Other	17,215	13,870
Valuation allowance	(1,132)	(728)
Deferred tax assets, total	198,052	162,264
Deferred tax liabilities:
Property-related items, including accelerated depreciation	678,307	598,371
Regulatory balancing accounts	6,097	6,067
Unamortized ITC	368	981
Debt-related costs	3,110	3,380
Intangibles	7,807	7,656
Other	30,300	21,289
Deferred tax liabilities, total	725,989	637,744
Net deferred tax liabilities	527,937	475,480
Southwest Gas Corporation
Deferred tax assets:
Deferred income taxes for future amortization of ITC and excess deferred taxes	105,791	98,912
Employee benefits	17,337	18,707
Alternative minimum tax credit	21,603	4,390
Net operating losses and credits	4,557	10,070
Interest rate swap	2,235	3,037
Other	9,386	8,820
Valuation allowance	(37)	(58)
Deferred tax assets, total	160,872	143,878
Deferred tax liabilities:
Property-related items, including accelerated depreciation	614,205	561,493
Regulatory balancing accounts	6,097	6,067
Unamortized ITC	368	981
Debt-related costs	3,110	3,380
Other	27,550	17,200
Deferred tax liabilities, total	651,330	589,121
Net deferred tax liabilities	$ 490,458	$ 445,243

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of year	$ 1,430	$ 1,231
Gross increases – tax positions in prior period	0	100
Gross decreases – tax positions in prior period	459	0
Gross increases – current period tax positions	0	99
Gross decreases – current period tax positions	0	0
Settlements	0	0
Lapse in statute of limitations	0	0
Unrecognized tax benefits at end of year	971	1,430
Southwest Gas Corporation
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits at beginning of year	1,069	903
Gross increases – tax positions in prior period	0	67
Gross decreases – tax positions in prior period	98	0
Gross increases – current period tax positions	0	99
Gross decreases – current period tax positions	0	0
Settlements	0	0
Lapse in statute of limitations	0	0
Unrecognized tax benefits at end of year	$ 971	$ 1,069

Income Taxes - Summary of Income Before Taxes for Continuing and Discontinued Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income from continuing operations before income taxes	$ 243,336	$ 259,030	$ 231,523
Southwest Gas Corporation
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Income from continuing operations before income taxes	182,833	219,953	178,007
Income from discontinued operations before income taxes	0	0	53,516
Total income before income taxes	$ 182,833	$ 219,953	$ 231,523

Amount before tax of income (loss) from a discontinued operation. Includes, but is not limited to, the income (loss) from operations during the phase-out period, gain (loss) on disposal, gain (loss) for reversal of write-down (write-down) to fair value, less cost to sell, and adjustments to a prior period gain (loss) on disposal.

Movement In Redeemable Noncontrolling Interest [Roll Forward]

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Regulatory liability related to TCJA	$ 456,000,000
Tax credit carryforward	3,500,000
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	1,000,000
Capital Loss Carryforward
Income Tax Disclosure [Line Items]
Tax credit carryforward	107,000
Canada
Income Tax Disclosure [Line Items]
Operating loss carryforward	7,600,000
Southwest Gas Corporation
Income Tax Disclosure [Line Items]
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	1,000,000
Significant increases or decreases in unrecognized tax benefit within the next 12 months	0
Tax-related interest income	0	$ 0	$ 0
Federal
Income Tax Disclosure [Line Items]
Operating loss carryforward	$ 62,500,000

Derivatives - Narrative (Detail) - USD ($)	12 Months Ended
Derivatives - Narrative (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 2,541,000	$ 2,073,000	$ 2,075,000
Derivative instrument loss at settlement amortization period	10 years
Cash Flow Hedging
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 0
Arizona
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%
California
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%

Derivatives - Notional Amounts under Swaps Contracts (Detail) - MMBTU MMBTU in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Contract notional amounts	13,387	10,929

Derivatives - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0
Net cost of gas sold
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	(2,113)	(11,572)	5,006
Regulatory Deferral Accounting Treatment \| Net cost of gas sold
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 2,113	$ 11,572	$ (5,006)

Derivatives - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset Derivatives	$ 1,979	$ 30
Liability Derivatives	(3,697)	(5,810)
Net Total	(1,718)	(5,780)
Swaps \| Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Asset Derivatives	243	19
Liability Derivatives	(99)	(1,342)
Net Total	144	(1,323)
Swaps \| Other current liabilities
Derivatives, Fair Value [Line Items]
Asset Derivatives	1,595
Liability Derivatives	(3,347)
Net Total	(1,752)
Swaps \| Other deferred credits
Derivatives, Fair Value [Line Items]
Asset Derivatives	141
Liability Derivatives	(251)
Net Total	$ (110)
Swaps \| Deferred charges and other assets
Derivatives, Fair Value [Line Items]
Asset Derivatives		11
Liability Derivatives		(4,468)
Net Total		$ (4,457)

Derivatives - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 6,781	$ 3,100	$ 5,583
Received from counterparties	$ 606	$ 1,685	$ 726

Derivatives - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other current liabilities
Derivatives, Fair Value [Line Items]
Regulatory assets/(liabilities) offsetting the derivatives at fair value	$ (144)
Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Regulatory assets/(liabilities) offsetting the derivatives at fair value	1,752	$ 4,457
Deferred charges and other assets
Derivatives, Fair Value [Line Items]
Regulatory assets/(liabilities) offsetting the derivatives at fair value	$ 110	$ 1,323

Derivatives - Summary of Financial Assets and Liabilities at Fair Value (Detail) - Level 2 – Significant other observable inputs - Swaps - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (1,718)	$ (5,780)
Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Derivative Assets	144	0
Deferred charges and other assets
Derivatives, Fair Value [Line Items]
Derivative Assets	0	0
Other current liabilities
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(1,752)	(4,457)
Other deferred credits
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (110)	$ (1,323)

Segment Information - Narrative (Detail)	12 Months Ended
Segment Information - Narrative (Detail)	Dec. 31, 2018 Segment
Segment Reporting Information [Line Items]
Number of reportable segments	2
Net Assets, Geographic Area \| Geographic Concentration Risk \| United States \| Minimum
Segment Reporting Information [Line Items]
Long-lived assets	99.00%

Segment Information - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Centuri
Segment Reporting Information [Line Items]
Accounts receivable for Centuri services	$ 18,830	$ 12,987

Segment Information - Schedule of Revenues by Geographic Area (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Revenues	$ 786,654	$ 668,146	$ 670,883	$ 754,330	$ 740,433	$ 593,153	$ 560,469	$ 654,737	$ 641,525	$ 539,969	$ 547,748	$ 731,248	$ 2,880,013	$ 2,548,792	$ 2,460,490
United States
Segment Reporting Information [Line Items]
Total Revenues													2,664,670	2,345,134	2,256,600
Canada
Segment Reporting Information [Line Items]
Total Revenues													$ 215,343	$ 203,658	$ 203,890

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues													$ 2,880,013	$ 2,548,792	$ 2,460,490
Interest revenue													6,108	2,787	1,849
Interest expense													96,671	78,064	73,660
Depreciation and amortization													249,212	250,951	289,132
Income tax expense													61,684	65,088	78,468
Segment net income	$ 69,304	$ 12,331	$ 21,551	$ 79,091	$ 96,465	$ 10,204	$ 17,864	$ 69,308	$ 65,180	$ 2,472	$ 8,943	$ 75,446	182,277	193,841	152,041
Segment assets	7,357,729				6,237,066				5,581,126				7,357,729	6,237,066	5,581,126
Capital expenditures													765,914	623,649	529,531
Intersegment sales
Segment Reporting Information [Line Items]
Revenues													135,914	97,159	98,121
Operating Segments
Segment Reporting Information [Line Items]
Revenues													2,744,099	2,451,633	2,362,369
Other
Segment Reporting Information [Line Items]
Revenues													0	0	0
Interest revenue													0	0	0
Interest expense													741	345	0
Depreciation and amortization													0	0	0
Income tax expense													(727)	(437)	0
Segment net income													(1,542)	(1,337)	0
Segment assets	572				1,901				0				572	1,901	0
Capital expenditures													0	0	0
Gas Operations
Segment Reporting Information [Line Items]
Revenues													1,357,728	1,302,308	1,321,412
Interest revenue													6,020	2,784	1,848
Interest expense													81,740	69,733	66,997
Depreciation and amortization													191,816	201,922	233,463
Income tax expense													43,991	63,135	58,584
Segment net income													138,842	156,818	119,423
Segment assets	6,141,584				5,482,669				5,001,756				6,141,584	5,482,669	5,001,756
Capital expenditures													682,869	560,448	457,120
Gas Operations \| Intersegment sales
Segment Reporting Information [Line Items]
Revenues													0	0	0
Gas Operations \| Operating Segments
Segment Reporting Information [Line Items]
Revenues													1,357,728	1,302,308	1,321,412
Utility Infrastructure Services
Segment Reporting Information [Line Items]
Revenues													1,522,285	1,246,484	1,139,078
Interest revenue													88	3	1
Interest expense													14,190	7,986	6,663
Depreciation and amortization													57,396	49,029	55,669
Income tax expense													18,420	2,390	19,884
Segment net income													44,977	38,360	32,618
Segment assets	$ 1,215,573				$ 752,496				$ 579,370				1,215,573	752,496	579,370
Capital expenditures													83,045	63,201	72,411
Utility Infrastructure Services \| Intersegment sales
Segment Reporting Information [Line Items]
Revenues													135,914	97,159	98,121
Utility Infrastructure Services \| Operating Segments
Segment Reporting Information [Line Items]
Revenues													$ 1,386,371	$ 1,149,325	$ 1,040,957

Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues	$ 786,654	$ 668,146	$ 670,883	$ 754,330	$ 740,433	$ 593,153	$ 560,469	$ 654,737	$ 641,525	$ 539,969	$ 547,748	$ 731,248	$ 2,880,013	$ 2,548,792	$ 2,460,490
Operating income	134,854	39,681	53,338	129,560	135,524	34,988	48,265	124,347	122,903	20,478	33,057	139,036	357,433	343,124	315,474
Net income (loss)	131,160	12,331	21,551	78,294	96,396	10,420	18,121	69,005	65,694	2,907	9,099	75,355	181,652	193,942	153,055
Net income attributable to Southwest Gas Holdings, Inc.	$ 69,304	$ 12,331	$ 21,551	$ 79,091	$ 96,465	$ 10,204	$ 17,864	$ 69,308	$ 65,180	$ 2,472	$ 8,943	$ 75,446	$ 182,277	$ 193,841	$ 152,041
Basic earnings per common share (in USD per share)	$ 1.36	$ 0.25	$ 0.44	$ 1.63	$ 2.00	$ 0.21	$ 0.38	$ 1.46	$ 1.37	$ 0.05	$ 0.19	$ 1.59	$ 3.69	$ 4.04	$ 3.20
Diluted earnings per common share (in USD per share)	$ 1.36	$ 0.25	$ 0.44	$ 1.63	$ 2.00	$ 0.21	$ 0.37	$ 1.45	$ 1.36	$ 0.05	$ 0.19	$ 1.58	$ 3.68	$ 4.04	$ 3.18
Southwest Gas Corporation
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues	$ 370,213	$ 217,523	$ 275,679	$ 494,313	$ 366,485	$ 213,059	$ 260,162	$ 462,602	$ 641,525	$ 539,969	$ 547,748	$ 731,248
Operating income	115,962	3	24,675	141,173	117,885	9,921	32,346	135,922	122,903	20,478	33,057	139,036	$ 281,813	$ 296,074	$ 256,488
Net income (loss)	$ 59,541	$ (13,670)	$ 2,622	$ 90,349	$ 74,382	$ (4,024)	$ 9,522	$ 76,938					138,842	156,818	153,055
Net income attributable to Southwest Gas Holdings, Inc.									65,180	2,472	8,943	75,446	$ 138,842	$ 156,818	$ 152,041
Southwest Gas Corporation \| Continuing Operations
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues									340,485	200,179	255,648	525,100
Operating income									100,685	(5,292)	20,210	140,885
Net income attributable to Southwest Gas Holdings, Inc.									51,887	(12,405)	2,358	77,583
Southwest Gas Corporation \| Discontinued Operations
Schedule Of Quarterly Financial Information [Line Items]
Operating revenues									301,040	339,790	292,100	206,148
Operating income									22,218	25,770	12,847	(1,849)
Net income attributable to Southwest Gas Holdings, Inc.									$ 13,293	$ 14,877	$ 6,585	$ (2,137)

Utility Infrastructure Services Noncontrolling Interests - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended
	Aug. 31, 2017	Dec. 31, 2018	Nov. 30, 2018	Jul. 31, 2018	Mar. 31, 2018	Jul. 31, 2017
Noncontrolling Interest [Line Items]
Purchase price of the previous owners' equity interest in Centuri	$ 23.0
Percentage of equity interest acquired	3.40%
Percentage of interest retained by noncontrolling party subject to election		100.00%
Redeemable noncontrolling interest, fair value			$ 81.7
Centuri
Noncontrolling Interest [Line Items]
Ownership percentage by noncontrolling owners				3.40%
Percentage of equity interest acquired	3.40%
Ownership percentage by parent						96.60%
Linetec
Noncontrolling Interest [Line Items]
Ownership percentage by noncontrolling owners			20.00%
IntelliChoice Energy LLC
Noncontrolling Interest [Line Items]
Ownership percentage by noncontrolling owners			35.00%
Centuri \| IntelliChoice Energy LLC
Noncontrolling Interest [Line Items]
Ownership percentage by parent		95.00%			65.00%

Utility Infrastructure Services Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) - USD ($)
$ in Thousands

12 Months Ended

Dec. 31, 2018

Dec. 31, 2017

Dec. 31, 2016

Balance, December 31, 2016

$ 0

$ 22,590

Net Income (loss) attributable to redeemable noncontrolling interest

172

248

Centuri distribution to redeemable noncontrolling interest

$ 5

(204)

(439)

Adjustment to redemption value

355

Redemption of Centuri shares from noncontrolling parties

(23,000)

Redeemable noncontrolling interest acquired

^[1]

81,659

Balance, December 31, 2017

$ 81,831

$ 0

$ 22,590

[1]	The Company, through its subsidiary, Centuri, completed the acquisition of a privately held utility infrastructure services business. Refer to Note 19 - Business Acquisitions.

Reorganization Impacts - Discontinued Operations Solely Related to Southwest Gas Corporation (Detail) - Southwest Gas Corporation - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Income before income taxes	$ 0	$ 0	$ 53,516
Income tax expense	0	0	19,884
Discontinued operations - utility infrastructure services, Income	0	0	33,632
Net income attributable to noncontrolling interests	0	0	1,014
Discontinued operations – utility infrastructure services – net income	$ 0	$ 0	32,618
Utility Infrastructure Services
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Utility infrastructure services revenues			1,139,078
Operating expenses:
Utility infrastructure services expenses			1,024,423
Depreciation and amortization			55,669
Operating income			58,986
Other income (deductions)			1,193
Net interest deductions			6,663
Income before income taxes			53,516
Income tax expense			19,884
Discontinued operations - utility infrastructure services, Income			33,632
Net income attributable to noncontrolling interests			1,014
Discontinued operations – utility infrastructure services – net income			$ 32,618

Business Acquisitions - Narrative (Detail) - USD ($)		1 Months Ended			12 Months Ended
Business Acquisitions - Narrative (Detail) - USD ($)	Nov. 30, 2018	Dec. 31, 2018	Nov. 30, 2018	Nov. 30, 2017	Dec. 31, 2018	Oct. 31, 2018	Dec. 31, 2017	Nov. 01, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 359,045,000			$ 359,045,000		$ 179,314,000		$ 139,983,000
Acquisition costs					0
Linetec
Business Acquisition [Line Items]
Goodwill	$ 188,500,000		$ 188,500,000
Net consideration of acquisition less assumed debt	326,600,000		326,600,000	$ 99,000,000
Acquired intangible assets	$ 89,300,000		89,300,000
Acquired intangible assets, weighted average useful life	19 years
Acquisition costs					$ 6,900,000
Linetec \| Customer relationships
Business Acquisition [Line Items]
Acquired intangible assets	$ 79,000,000		79,000,000
Acquired intangible assets, weighted average useful life	20 years
Linetec \| Trade Names
Business Acquisition [Line Items]
Acquired intangible assets	$ 10,000,000		10,000,000
Acquired intangible assets, weighted average useful life	15 years
Linetec \| Customer Contracts
Business Acquisition [Line Items]
Acquired intangible assets	$ 300,000		$ 300,000
Acquired intangible assets, weighted average useful life	1 year
Neuco
Business Acquisition [Line Items]
Goodwill								$ 32,200,000
Net consideration of acquisition less assumed debt								99,300,000
Acquired intangible assets								44,800,000
Acquisition costs				$ 2,600,000
Goodwill expected tax deductible amount								$ 32,000,000
Centuri \| Linetec
Business Acquisition [Line Items]
Percentage of voting interests acquired	80.00%		80.00%
Business combination consideration	$ 326,600,000		$ 75,600,000
Subsequent collection of acquiree's unbilled receivable	30,000,000		30,000,000
Acquiree's liabilities related to future payments to employees	29,600,000		$ 29,600,000
Consideration held back for estimated purchase price and working capital adjustment to be paid	16,900,000
Consideration transferred related to future estimated tax payments	$ 24,100,000
Payment to acquiree's employees		$ 25,000,000
Linetec
Business Acquisition [Line Items]
Ownership percentage by noncontrolling owners	20.00%		20.00%
Secured Revolving Credit Facility and Term Loan \| Centuri
Business Acquisition [Line Items]
Credit facility maximum borrowing capacity	$ 590,000,000		$ 590,000,000			$ 450,000,000

Business Acquisitions - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Nov. 30, 2018	Dec. 31, 2017	Nov. 30, 2017	Nov. 01, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Goodwill	$ 359,045		$ 179,314			$ 139,983
Neuco
Business Acquisition [Line Items]
Cash and cash equivalents					$ 800
Accounts receivable					18,300
Other receivables					5,400
Prepaid expenses and other current assets					1,600
Property and equipment					15,100
Intangible assets					44,800
Goodwill					32,200
Total assets acquired					118,200
Current liabilities					(18,600)
Other long-term liabilities					(300)
Net assets acquired					$ 99,300
Linetec
Business Acquisition [Line Items]
Cash and cash equivalents		$ 3,900
Accounts receivable		32,800
Revenue earned on contracts in progress in excess of billings		21,600
Prepaid expenses and other current assets		1,100
Property and equipment		89,400
Intangible assets		89,300
Goodwill		188,500
Total assets acquired		426,600
Accounts payable		8,000
Accrued liabilities		6,900
Deferred compensation and related accrued taxes		3,400
Redeemable noncontrolling interest		81,700
Total liabilities assumed and noncontrolling interest		100,000
Net assets acquired		$ 326,600		$ 99,000

Business Acquisitions - Schedule of Pro Forma Consolidated Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Neuco
Business Acquisition [Line Items]
Total operating revenues		$ 2,639,452
Net income attributable to Southwest Gas Holdings, Inc.		$ 203,245
Basic earnings per share (in USD per Share)		$ 4.24
Diluted earnings per share (in USD per Share)		$ 4.24
Linetec
Business Acquisition [Line Items]
Total operating revenues	$ 3,037,209	$ 2,626,721
Net income attributable to Southwest Gas Holdings, Inc.	$ 187,642	$ 192,368
Basic earnings per share (in USD per Share)	$ 3.80	$ 4.01
Diluted earnings per share (in USD per Share)	$ 3.79	$ 4.01

Business Acquisitions - Schedule of Statements of Income Operations (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Income Statements, Captions [Line Items]
Utility infrastructure services revenues													$ 1,522,285	$ 1,246,484	$ 1,139,078
Net income attributable to Southwest Gas Holdings, Inc.	$ 69,304	$ 12,331	$ 21,551	$ 79,091	$ 96,465	$ 10,204	$ 17,864	$ 69,308	$ 65,180	$ 2,472	$ 8,943	$ 75,446	182,277	$ 193,841	$ 152,041
Linetec
Condensed Income Statements, Captions [Line Items]
Utility infrastructure services revenues													14,119
Net income attributable to Southwest Gas Holdings, Inc.													$ 690

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.