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Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 9)

132,639

133,331

(692)

(4,985)

FSIRS realized and unrealized gain, net of tax (Notes 5 and 12)

1,834

Amounts reclassified into net income (Notes 5 and 12)

1,737

Dividends declared Common

(55,087)

Balance at Dec. 31, 2012

1,308,498

$ 47,778

828,777

(50,745)

484,369

(1,681)

Balance, Shares at Dec. 31, 2012

46,148,000

Common stock issuances, Value

11,952

$ 208

11,744

Common stock issuance, Shares

208,000

Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 9)

144,873

145,320

(447)

6,973

Amounts reclassified into net income (Notes 5 and 12)

2,074

Dividends declared Common

(61,975)

Balance at Dec. 31, 2013

$ 1,412,395

$ 47,986

840,521

(41,698)

567,714

(2,128)

Balance, Shares at Dec. 31, 2013

46,356,125

46,356,000

Common stock issuances, Value

$ 11,027

$ 167

10,860

Common stock issuance, Shares

167,000

Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 9)

141,148

(9,913)

Amounts reclassified into net income (Notes 5 and 12)

2,073

Dividends declared Common

(68,715)

Balance at Dec. 31, 2014

$ 1,486,266

$ 48,153

$ 851,381

(50,175)

639,164

(2,257)

$ 20,042

Balance, Shares at Dec. 31, 2014

46,523,184

46,523,000

^[1]

Redeemable noncontrolling interest attributable to acquisition

18,952

Fair value accretion (Note 16)

$ 140,997

141,126

$ (129)

151

(961)

$ (961)

961

Foreign currency exchange translation adj.

$ (637)

$ (22)

[1]

At December 31, 2014, 2.7 million common shares were registered and available for issuance under provisions of the Company's various stock issuance plans. In addition, approximately 36,000 common shares are registered for issuance upon the exercise of options granted under the Stock Incentive Plan (see Note 10).

Consolidated Statements of Equity and Redeemable Noncontrolling Interest (Parenthetical) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 1.46	$ 1.32	$ 1.18
Shares registered and available for issuance	2,700
Common shares registered for issuance upon exercise of options	36	52	125

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Note 1 – Summary of Significant Accounting Policies

Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. 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. Centuri Construction Group Inc. (“Centuri” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems, and industrial construction solutions. Centuri operations are generally conducted under the business names of NPL Construction Co., Link-Line, W.S. Nicholls, and Brigadier.

Basis of Presentation. The Company follows generally accepted accounting principles 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 the natural gas utility operations conforms with U.S. GAAP as applied to 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.

Consolidation. The accompanying financial statements are presented on a consolidated basis and include the accounts of Southwest Gas Corporation and all subsidiaries (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.

In October 2014, the Company, through its subsidiaries, led principally by NPL Construction Co., completed the acquisition of three privately held, affiliated construction businesses for approximately US$221 million. Additional consideration will be paid or accrued through the true-up period. The acquisition extends the construction services operations into Canada and provides additional opportunities for market expansion. Funding for the acquisition was primarily provided by a new $300 million secured revolving credit and term loan facility described in Note 6 – Long-Term Debt. The acquired companies comprise: (i) Link-Line Contractors Ltd., an Ontario corporation (“Link-Line”) that provides construction and maintenance services for the Canadian utility industry, with operations primarily in Ontario, Canada; (ii) W.S. Nicholls Construction, Inc., an Ontario corporation, as well as two additional companies also operating under the name W.S. Nicholls, which together provide industrial construction solutions, fabrication, and civil services to the oil and gas, pulp and paper, and automotive industries, as well as government and private sector customers in British Columbia and Ontario, Canada (collectively “W.S. Nicholls”); and (iii) via asset purchase, the business of Brigadier Pipelines Inc., a Delaware corporation, operating primarily in Pennsylvania as a specialty midstream pipeline contractor (“Brigadier”).

In October 2014, upon completion of the acquisition, the Company restructured its ownership of NPL Construction Co. and Carson Water Company (an inactive wholly owned subsidiary) creating a holding company, a direct subsidiary of Carson Water Company. In January 2015, the holding company was renamed Centuri. In addition, two direct subsidiaries exist under Centuri: Vistus Construction Group Inc. (“Vistus”, U.S. operations) and Lynxus Construction Group Inc. (“Lynxus”, Canadian operations). Three subsidiaries exist under Vistus: NPL Construction Co., Southwest Administrators, and Brigadier Pipelines Inc. Link-Line and W.S. Nicholls are subsidiaries of Lynxus.

Lynxus, including its subsidiaries of Link-Line, W.S. Nicholls, WSN Construction and WSN Industries will be consolidated under the voting interest method of accounting. Brigadier will be consolidated under the voting interest method.

Centuri, through its subsidiaries, holds a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning (“HVAC”) technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC) as a majority-owned subsidiary. Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian construction 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 Company’s investment in Western is not significant in relation to its total assets included in the Consolidated Balance Sheets. At December 31, 2014, Centuri’s investment in Western is $14.7 million and its maximum exposure to loss as a result of its involvement with the entity is estimated at $20.8 million, including obligations under a construction bonding arrangement under which Centuri has guaranteed the performance on certain projects of Western. 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 are deemed worthless.

Centuri, through its subsidiaries, also has a 25% interest in CCI-TBN Toronto, Inc. and a 50% interest in Matheson-Nicholls Joint Venture. Any future changes to the values of these entities will be recorded by Centuri using the equity method. The equity method investment in Western is included in Other Property and Investments in the 2014 Consolidated Balance Sheet.

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 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 includes (millions of dollars):


	2014			2013
Centuri property, equipment, and intangibles	$	405		$	320
Centuri accumulated provision for depreciation and amortization		(187	)		(163	)
Net cash surrender value of COLI policies		99			93
Other property		10			11

Total	$	327		$	261

Restricted Cash. A construction bond that was required to be in place during the completion of one of Centuri’s construction projects is classified in the Consolidated Balance Sheets as restricted cash. The project is expected to be completed within one year. The restricted cash was acquired in conjunction with the acquisition of construction services businesses. See Note 15 – Acquisition of Construction Services Businesses for more information.

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.

Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $23 million in 2014 and $21 million in 2013.

Income Taxes. The Company uses the asset and liability method of accounting for 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, 2014, the Company sustained losses in its foreign jurisdiction and therefore has no undistributed foreign earnings. However, the Company intends to permanently reinvest any future foreign earnings in Canada.

Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. In general, cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. However, cash and cash equivalents at December 31, 2014 also includes two money market fund investments totaling approximately $250,000 which fall within Level 2 (significant other observable inputs) of the fair value hierarchy, due to the asset valuation methods used by money market funds.

During 2013 and 2014, approximately $9.3 million and $8.1 million, respectively, of customer advances, upon contract expiration, were applied as contributions toward utility construction activity and represent a non-cash investing activity. Additionally, in conjunction with the acquisition, the Company acquired several capital leases. During 2014 and after the acquisition date, an additional capital lease obligation was entered into and represents a non-cash financing inflow activity of $204,000. The associated capital lease asset represents a non-cash investing outflow activity of $204,000. In association with the acquisition, cash flows from investing activities includes an $18.9 million non-cash investing outflow due to equity to the noncontrolling interest in a subsidiary to acquire businesses. In addition, a non-cash investing outflow activity of $10.8 million related to an increase in an acquisition consideration payable is included.

Inventories. Inventories are carried at weighted average cost and include natural gas stored underground, liquefied natural gas storage and materials and supplies.

Goodwill. The construction services segment includes Goodwill of $133 million in 2014 ($125 million related to the recent acquisition, which is net of approximately $5 million due to foreign currency exchange translation adjustments between the acquisition date and the end of the year). The December 31, 2014 and 2013 Goodwill amounts shown in the Consolidated Balance Sheets include approximately $8 million of Goodwill recognized when Southwest first acquired NPL Construction Co. and is associated with the construction services segment. Goodwill of $10 million in both 2014 and 2013 is associated with the natural gas operations segment. Goodwill is assessed for impairment annually, as required by U.S. GAAP, or otherwise, if circumstances indicate impairment to the carrying value of goodwill. No impairment was recorded in 2014.

Intangible Assets. Intangible assets 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. Intangible assets have finite lives and are described in Notes 2 and 15.

Accumulated Removal Costs. Approved regulatory practices allow Southwest to include in depreciation expense a component to recover removal costs associated with utility plant retirements. In accordance with the Securities and Exchange Commission’s (“SEC”) position on presentation of these amounts, management has reclassified estimated removal costs from accumulated depreciation to accumulated removal costs within the liabilities section of the balance sheets. The reclassified amounts are presented in the table below (thousands of dollars):


	December 31, 2014		December 31, 2013
Accumulated removal costs	$	304,000	$	279,000

Gas Operating Revenues. Revenues are recorded when customers are billed. Customer billings are based on monthly meter reads and are calculated in accordance with applicable tariffs and state and local laws, regulations, and 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 reading 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.

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

Construction 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. Long-term fixed-price contracts use the percentage-of-completion method of accounting and, therefore, take into account the cost, estimated earnings, and revenue to date on contracts not yet completed. The amount of revenue recognized on fixed-price contracts is based on costs expended to date relative to anticipated final contract costs. 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. In connection with significant changes in estimated costs to complete a large fixed-price contract, construction services results for 2012 reflected a pretax loss of $15 million ($0.20 per share, after tax). The estimated cost changes that resulted in the loss recognized included reductions in projected productivity and higher costs of restoration work. During 2013, profitability on this contract was minimal and as of December 31, 2013, this fixed-price contract was substantially complete. Some unit-price contracts contain caps that if encroached, trigger revenue and loss recognition similar to a fixed-price contract model.

Construction Expenses. The construction expenses classification in the income statement includes payroll expenses, job-related equipment costs, direct construction costs, gains and losses on equipment sales, general and administrative expenses, acquisition and acquisition-related costs, and office-related fixed costs 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 PGA deferrals and recoveries.

Operations and Maintenance Expense. For financial reporting purposes, operations and maintenance expense includes Southwest’s operating and maintenance costs associated with serving utility customers, uncollectible expense, administrative and general salaries and expense, employee benefits expense, 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. Nonutility and construction 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 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.


	2014		2013		2012
(In thousands)
AFUDC:
Debt portion	$	1,228	$	1,260	$	1,129
Equity portion		1,995		2,274		1,943

AFUDC capitalized as part of utility plant	$	3,223	$	3,534	$	3,072

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):


	2014			2013			2012
Change in COLI policies	$	5,300		$	12,400		$	6,600
Interest income		2,602			461			924
Pipe replacement costs		—			(132	)		(2,680	)
Foreign currency transaction gain (loss)		(178	)		—			—
Miscellaneous income and (expense)		(617	)		(429	)		(433	)

Total other income (deductions)	$	7,107		$	12,300		$	4,411

Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). Changes in cash surrender values are directly influenced by the investment portfolio underlying the insurance policies. These life insurance policies on members of management and other key employees are used by Southwest to indemnify itself 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. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work. The replacement program work subject to non-recoverability was substantially completed in 2012.

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, the amounts of which are not material, are included in other income (expense) within net income. 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. In connection with the ownership structure of Centuri and its subsidiary entities, a redeemable noncontrolling interest exists in Lynxus. The Company concluded that this noncontrolling interest meets the definition of a participating security in connection with Accounting Standards Codification (“ASC”) Topic 260, as a result of dividend participation rights of the noncontrolling interest that are different than the underlying proportional ownership interest. Related provisions of ASC 260 would require consideration of the participation right in computing earnings per share. However, as earnings are already attributed to the noncontrolling interest equivalent to the participation right, taking an additional adjustment in computing basic earnings per share would duplicate the economic impact associated with the participating security. Furthermore, as net income attributable to common shareholders of Southwest Gas Corporation has already been adjusted for the attribution of income to the noncontrolling interest, the Company concluded that a diluted earnings per share calculation, assuming exchange of the ownership shares of the noncontrolling interest in Lynxus, for ownership shares in Centuri, would not produce a different result than that which results from computing basic earnings per share. The noncontrolling interest is also redeemable for fair value at specified dates in the future. No adjustment will be made to the Company’s income attributable to common shareholders, in computing earnings per share, to reflect changes in the redemption price, as redemption at fair value is not considered an economic distribution different from other common stockholders. See also Note 15 – Acquisition of Construction Services Businesses and Note 16 – Construction Services Noncontrolling Interests.

Basic earnings per share (“EPS”) are calculated by dividing net income attributable to Southwest Gas Corporation by the weighted-average number of shares outstanding during the period. 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.


	2014		2013		2012
(In thousands)
Average basic shares		46,494		46,318		46,115
Effect of dilutive securities:
Stock options		17		26		42
Performance shares		215		231		254
Restricted stock units		218		183		144

Average diluted shares		46,944		46,758		46,555

Recently Issued Accounting Standards Updates. In May 2014, the Financial Accounting Standards Board (“FASB”) issued the update “Revenue from Contracts with Customers (Topic 606).” The update replaces much of the current guidance regarding revenue recognition including most industry-specific guidance. The core principle of the update is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. An entity will be required to identify the contract with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. In addition to the new revenue recognition requirements, entities will be required to disclose sufficient information to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities may choose between two retrospective transition methods when applying the update. The Company plans to adopt this update, as required, on January 1, 2017 for interim and annual reporting periods. Early adoption is not permitted. The Company is evaluating what impact this standard might have on its consolidated financial statements and disclosures. Additionally, the power and utilities industry as a whole assembled a task force for purposes of considering unique circumstances that relate to our industry and for communicating those industry considerations to the American Institute of Certified Public Accountants. Those undertakings are ongoing.

In August 2014, the FASB issued the update “Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern,” which requires management to asses a company’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Under the update, disclosures are required when conditions give rise to substantial doubt about a company’s ability to continue as a going concern within one year from the financial statement issuance date. The update is effective for the annual period ending after December 15, 2016, and all annual and interim periods thereafter. This update is not expected to have a material impact on the Company’s disclosures.

Subsequent Events. Management of the Company 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.

Reclassifications. Certain reclassifications were made to the prior year’s financial information included in the Consolidated Balance Sheets (goodwill is identified separately from Deferred charges and other assets) in order to present the prior-year information on a basis comparable with the current year’s presentation, with no impact to total assets overall.

Utility Plant and Leases

12 Months Ended

Dec. 31, 2014

Leases [Abstract]

Utility Plant and Leases

Note 2 – Utility Plant and Leases

Net utility plant as of December 31, 2014 and 2013 was as follows (thousands of dollars):


December 31,	2014			2013
Gas plant:
Storage	$	22,531		$	21,282
Transmission		312,300			313,306
Distribution		4,655,640			4,410,598
General		356,072			324,490
Software and software-related intangibles		196,035			168,815
Other		14,021			13,978

		5,556,599			5,252,469
Less: accumulated depreciation		(1,973,098	)		(1,868,504	)
Acquisition adjustments, net		550			730
Construction work in progress		74,332			101,413

Net utility plant	$	3,658,383		$	3,486,108

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


	2014		2013		2012
Depreciation and amortization expense	$	194,360	$	185,283	$	182,612

Included in the figures above is amortization of intangibles of $11.7 million in 2014, $10.3 million in 2013, and $9.3 million in 2012.

Operating Leases and Rentals. In July 2014, the Company purchased for $16.5 million a portion of its corporate headquarters office complex in Las Vegas that it had previously leased. With the completion of the purchase, the lease terminated.

The Company leases certain office and construction equipment. The majority of these leases are short-term and accounted for as operating leases. For the gas 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 rental payments and Centuri’s lease payments that are included in operating expenses (in thousands):


	2014		2013		2012
Southwest Gas	$	5,330	$	8,308	$	7,762
Centuri		30,012		27,118		24,054

Consolidated rental payments/lease expense	$	35,342	$	35,426	$	31,816

The following is a schedule of future minimum lease payments for significant non-cancelable operating leases (with initial or remaining terms in excess of one year) as of December 31, 2014 (thousands of dollars):


Year Ending December 31,
2015	$	5,957
2016		4,236
2017		2,532
2018		1,582
2019		912
Thereafter		831

Total minimum lease payments	$	16,050

Capital Leases. Centuri leases certain construction equipment. These leases are considered capital leases. The amount of capital leases of equipment as of December 31, 2014 and 2013 is as follows (thousands of dollars):


December 31,	2014			2013
Capital leases of equipment	$	5,763		$	—
Less: accumulated amortization		(287	)		—

Net capital leases	$	5,476		$	—

The following is a schedule of future minimum lease payments for non-cancelable capital leases (with initial or remaining terms in excess of one year) as of December 31, 2014 (thousands of dollars):


Year Ending December 31,
2015	$	1,690
2016		1,632
2017		813
2018		591
2019		—
Thereafter		—

		4,726
Less: amount representing interest		(497	)

Total minimum lease payments	$	4,229

Receivables and Related Allowances

12 Months Ended

Dec. 31, 2014

Receivables [Abstract]

Receivables and Related Allowances

Note 3 – 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, 2014, and the percentage of customers in each of the three states.


	December 31, 2014
Gas utility customer accounts receivable balance (in thousands)	$	136,148



	December 31, 2014
Percent of customers by state
Arizona		53	%
Nevada		37	%
California		10	%

Although the Company 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 one month 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, 2011	$	3,182
Additions charged to expense		2,471
Accounts written off, less recoveries		(3,149	)

Balance, December 31, 2012		2,504
Additions charged to expense		3,583
Accounts written off, less recoveries		(4,362	)

Balance, December 31, 2013		1,725
Additions charged to expense		4,146
Accounts written off, less recoveries		(3,616	)

Balance, December 31, 2014	$	2,255

At December 31, 2014, the construction services segment (Centuri) had $142 million in customer accounts receivable. Both the allowance for uncollectibles and write-offs 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

Regulatory Assets and Liabilities

12 Months Ended

Dec. 31, 2014

Regulated Operations [Abstract]

Note 4 – Regulatory Assets and Liabilities

Natural gas operations are 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 Federal Energy Regulatory Commission (“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,	2014			2013
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	390,293		$	249,985
Unrealized net loss on non-trading derivatives (Swaps) (2)		5,425			160
Deferred purchased gas costs (3)		87,556			18,217
Accrued purchased gas costs (4)		2,600			31,500
Unamortized premium on reacquired debt (5)		20,478			19,614
Other (6)		72,132			48,945

		578,484			368,421
Regulatory liabilities:
Accumulated removal costs		(304,000	)		(279,000	)
Unrealized net gain on non-trading derivatives (Swaps) (2)		—			(981	)
Deferred gain on southern Nevada division operations facility (7)		(115	)		(253	)
Unamortized gain on reacquired debt (8)		(10,862	)		(11,398	)
Other (9)		(34,233	)		(26,482	)

Net regulatory assets	$	229,274		$	50,307

(1)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 9).

(2)

The following table details the regulatory assets/(liabilities) offsetting the derivatives (Swaps) at fair value in the balance sheets (thousands of dollars). The actual amounts, when realized at settlement, become a component of purchased gas costs under the Company’s purchased gas adjustment (“PGA”) mechanisms. (See Note 12).


Instrument	Balance Sheet Location	2014		2013
Swaps	Deferred charges and other assets	$	363	$	4
Swaps	Prepaids and other current assets		5,062		156
Swaps	Other current liabilities		—		(801	)
Swaps	Other deferred credits		—		(180	)

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

(4)	Included in Prepaids 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)

Other regulatory assets including deferred costs associated with rate cases, regulatory studies, and state mandated public purpose programs (including low income and conservation programs), as well as margin and interest-tracking accounts, amounts associated with accrued absence time, and deferred post-retirement benefits other than pensions. Recovery periods vary.

(7)	Balance was originally being amortized over a four-year period beginning in the fourth quarter of 2009. As a result of the most recent Nevada general rate case, the amortization period was extended through October 2015.

(8)	Included in Other deferred credits on the Consolidated Balance Sheet. Amortized over life of debt instruments.

(9)	Other regulatory liabilities includes amounts associated with income tax and gross-up.

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

12 Months Ended

Dec. 31, 2014

Text Block [Abstract]

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

Note 5 – 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 Company’s Consolidated Balance Sheets and Consolidated Statements of Equity and Redeemable Noncontrolling Interest.

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


	2014									2013									2012
	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
(Thousands of dollars)
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(173,646	)	$	65,985		$	(107,661	)	$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
Amortization of prior service cost		355			(135	)		220			355			(135	)		220			—			—			—
Amortization of transition obligation		—			—			—			—			—			—			867			(329	)		538
Amortization of net actuarial (gain)/loss		23,656			(8,989	)		14,667			34,177			(12,987	)		21,190			25,597			(9,727	)		15,870
Prior service cost		(6,661	)		2,531			(4,130	)		—			—			—			(2,423	)		921			(1,502	)
Regulatory adjustment		140,308			(53,317	)		86,991			(123,630	)		46,979			(76,651	)		42,771			(16,253	)		26,518

Pension plans other comprehensive income (loss)		(15,988	)		6,075			(9,913	)		11,247			(4,274	)		6,973			(8,041	)		3,056			(4,985	)
FSIRS (designated hedging activities):
Unrealized/realized gain (loss)		—			—			—			—			—			—			2,959			(1,125	)		1,834
Amounts reclassified into net income		3,345			(1,272	)		2,073			3,345			(1,271	)		2,074			2,801			(1,064	)		1,737

FSIRS other comprehensive income (loss)		3,345			(1,272	)		2,073			3,345			(1,271	)		2,074			5,760			(2,189	)		3,571
Foreign currency translation adjustments:
Translation adjustments		(659	)		—			(659	)		—			—			—			—			—			—

Foreign currency other comprehensive income (loss)		(659	)		—			(659	)		—			—			—			—			—			—

Total other comprehensive income (loss)	$	(13,302	)	$	4,803		$	(8,499	)	$	14,592		$	(5,545	)	$	9,047		$	(2,281	)	$	867		$	(1,414	)

(1)

Tax amounts are calculated using a 38% rate. 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, as repatriation of earnings is not anticipated.

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	$	33,000
SERP net actuarial loss		1,300
PBOP net actuarial loss		300
PBOP prior service cost		1,300

Approximately $2.1 million of previously realized losses (net of tax) related to the forward-starting interest rate swaps (“FSIRS”), included in AOCI at December 31, 2014, 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 (Note 9)									FSIRS (Note 12)									Foreign Currency Items
	Before- Tax			Tax (Expense) Benefit			After- Tax			Before- Tax			Tax (Expense) Benefit			After- Tax			Before- Tax			Tax (Expense) Benefit		After- Tax			AOCI
Beginning Balance AOCI December 31, 2013	$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	—		$	—	$	—		$	(41,698	)

Net actuarial gain/(loss)		(173,646	)		65,985			(107,661	)		—			—			—			—			—		—			(107,661	)
Translation adjustments		—			—			—			—			—			—			(659	)		—		(659	)		(659	)

Other comprehensive income before reclassi fications		(173,646	)		65,985			(107,661	)		—			—			—			(659	)		—		(659	)		(108,320	)
FSIRS amounts reclassified from AOCI (1)		—			—			—			3,345			(1,272	)		2,073			—			—		—			2,073
Amortization of prior service cost (2)		355			(135	)		220			—			—			—			—			—		—			220
Amortization of net actuarial loss (2)		23,656			(8,989	)		14,667			—			—			—			—			—		—			14,667
Prior service cost		(6,661	)		2,531			(4,130	)		—			—			—			—			—		—			(4,130	)
Regulatory adjustment (3)		140,308			(53,317	)		86,991			—			—			—			—			—		—			86,991

Net current period other comprehensive income (loss)		(15,988	)		6,075			(9,913	)		3,345			(1,272	)		2,073			(659	)		—		(659	)		(8,499	)
Less: Translation adjustment attributable to redeemable noncontrolling interest		—			—			—			—			—			—			(22	)		—		(22	)		(22	)

Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation		(15,988	)		6,075			(9,913	)		3,345			(1,272	)		2,073			(637	)		—		(637	)		(8,477	)

Ending Balance AOCI December 31, 2014	$	(57,211	)	$	21,740		$	(35,471	)	$	(22,688	)	$	8,621		$	(14,067	)	$	(637	)	$	—	$	(637	)	$	(50,175	)

(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 9 – 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).

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, 2014 and 2013:

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)


	2014			2013
Net actuarial (loss) gain	$	(439,131	)	$	(289,141	)
Prior service cost		(8,373	)		(2,067	)
Less: amount recognized in regulatory assets		390,293			249,985

Recognized in AOCI	$	(57,211	)	$	(41,223	)

See Note 9 – Pension and Other Postretirement Benefits for more information on the defined benefit pension plans and Note 12 – Derivatives and Fair Value Measurements for more information on the FSIRS.

Long-Term Debt

12 Months Ended

Dec. 31, 2014

Debt Disclosure [Abstract]

Long-Term Debt

Note 6 – Long-Term Debt

Carrying amounts of the Company’s long-term debt and their related estimated fair values as of December 31, 2014 and December 31, 2013 are disclosed in the following table. The fair values of the 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. They are categorized as Level 1 (quoted prices for identical financial instruments) within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, due to the Company’s ability to access similar debt arrangements at measurement dates with comparable terms, including variable rates. The fair values of debentures, senior notes, and fixed-rate IDRBs were determined utilizing a market-based valuation approach, where fair market 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 market values of debentures and fixed-rate IDRBs are categorized as Level 2 (observable market inputs based on market prices of similar securities). The Centuri secured revolving credit and term loan facility and Centuri other debt obligations (not actively traded) are categorized as Level 3, based on significant unobservable inputs to their fair values. Since 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 utilizes 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.


December 31,	2014					2013
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	133,403	$	125,000		$	130,953
Notes, 6.1%, due 2041		125,000			157,290		125,000			141,873
Notes, 3.875%, due 2022		250,000			262,030		250,000			252,485
Notes, 4.875%, due 2043		250,000			280,903		250,000			257,280
8% Series, due 2026		75,000			102,296		75,000			96,263
Medium-term notes, 7.59% series, due 2017		25,000			27,573		25,000			28,741
Medium-term notes, 7.78% series, due 2022		25,000			31,144		25,000			30,586
Medium-term notes, 7.92% series, due 2027		25,000			33,695		25,000			31,497
Medium-term notes, 6.76% series, due 2027		7,500			9,156		7,500			8,468
Unamortized discount		(5,223	)				(5,560	)

		902,277					901,940

Revolving credit facility and commercial paper		150,000			150,000		10,000			10,000

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
Fixed-rate bonds:
5.25% 2003 Series D, due 2038		20,000			20,277		20,000			20,150
5.25% 2004 Series A, due 2034		—			—		65,000			64,522
5.00% 2004 Series B, due 2033		31,200			31,223		31,200			30,284
4.85% 2005 Series A, due 2035		100,000			100,071		100,000			95,192
4.75% 2006 Series A, due 2036		24,855			25,399		24,855			22,974
Unamortized discount		(1,943	)				(2,776	)

		374,112					438,279

Centuri secured revolving credit and term loan facility		199,267			200,341		—			—
Centuri other debt obligations		31,128			31,127		42,213			42,119

		1,656,784					1,392,432
Less: current maturities		(19,192	)				(11,105	)

Long-term debt, less current maturities	$	1,637,592				$	1,381,327

In March 2014, Southwest amended its $300 million credit and commercial paper facility. The facility was previously scheduled to expire in March 2017, but was extended to March 2019. Southwest will continue to use $150 million of the facility as long-term debt and the remaining $150 million for working capital purposes. In addition to extending the credit facility, among other amendments, the applicable margins and unused commitment fees were reduced and the Pricing Level definitions were modified. 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 the Company’s senior unsecured debt rating. At December 31, 2014, 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. At December 31, 2014, $150 million was outstanding on the long-term portion of the credit facility, including $50 million in commercial paper (see commercial paper program discussion below). The effective interest rate on the long-term portion of the credit facility was 1.21% at December 31, 2014. Borrowings under the credit facility ranged from none during the first and second quarters of 2014 to a high of $165 million during November and December 2014. With regard to the short-term portion of the credit facility, there was $5 million outstanding at December 31, 2014 and no borrowings outstanding at December 31, 2013. (See Note 7 – Short-Term Debt).

Also in March 2014, the Company amended the note purchase agreement associated with its 6.1% $125 million notes (“Notes”). The amendment modifies the Permitted Lien category, thereby permitting liens securing indebtedness not to exceed 10% of total capitalization as of the end of any quarter. This provision in the agreement prohibits liens on any property securing other indebtedness under bank facilities unless the Notes are secured in a similar manner. The provision was amended to clarify that it only applies to bank facilities of Southwest Gas Corporation.

The Company has a $50 million commercial paper program. Any issuance under the commercial paper program is supported by the Company’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, 2014, and as noted above, $50 million was outstanding on the commercial paper program. The effective interest rate on the commercial paper program was 0.65% at December 31, 2014.

In October 2014, construction services subsidiaries of the Company entered into a $300 million secured revolving credit and term loan facility. The facility is scheduled to expire in October 2019 and replaces the previous $75 million credit facility, which was scheduled to expire in June 2015. The facility is secured by substantially all of Centuri’s assets except ones explicitly excluded under the terms of the agreement including owned real estate and certain certificated vehicles. Centuri assets securing the facility at December 31, 2014 totaled $477 million.

Interest rates for Centuri’s $300 million secured facility are calculated at the 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 1.00% 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 the consolidated leverage ratio. The commitment fee ranges from 0.15% to 0.40% per annum. Borrowings under the credit facility ranged from a low of $199 million during December 2014 to a high of $232 million during October 2014. All amounts outstanding are considered long-term borrowings. The outstanding amount on the facility at December 31, 2014 includes term loans in the amount of $142 million. The effective interest rate on the facility was 3.31% at December 31, 2014.

In November 2014, Southwest’s $65 million 2004 5.25% Series A fixed-rate IDRBs (originally due in 2034) were redeemed at par plus accrued interest.

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


	December 31, 2014			December 31, 2013
2003 Series A		0.85	%		1.43	%
2008 Series A		0.90	%		1.41	%
2009 Series A		0.89	%		1.01	%
Tax-exempt Series A		0.84	%		1.07	%

In Nevada, interest fluctuations due to changing interest rates on the 2003 Series A, 2008 Series A, and 2009 Series A variable-rate IDRBs are tracked and recovered from ratepayers through an interest balancing account.

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


2015	$	19,192
2016		20,991
2017		43,899
2018		16,985
2019		304,328

No debt instruments have credit triggers or other clauses that result in default if Company bond ratings are lowered by rating agencies. Certain Company debt instruments contain securities ratings covenants that, if set in motion, would increase financing costs. Certain debt instruments also have leverage ratio caps and minimum net worth requirements. At December 31, 2014, the Company is in compliance with all of its covenants. Under the most restrictive of the covenants, the Company could issue approximately $1.9 billion in additional debt and meet the leverage ratio requirement. The Company has at least $900 million of cushion in equity relating to the minimum net worth requirement.

Certain Centuri debt instruments have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2014, Centuri is in compliance with all of its covenants. Under the most restrictive of the covenants, Centuri could issue approximately $88 million in additional debt and meet the leverage ratio requirement. Centuri has at least $35 million of cushion in equity relating to the minimum fixed charge ratio coverage requirement.

Short-Term Debt	12 Months Ended
Short-Term Debt	Dec. 31, 2014
Debt Disclosure [Abstract]
Short-Term Debt	Note 7 – Short-Term Debt As discussed in Note 6, Southwest has a $300 million credit facility that is scheduled to expire in March 2019, of which $150 million has been designated by management for working capital purposes. The Company had $5 million in short-term borrowings outstanding at December 31, 2014 and no short-term borrowings outstanding at December 31, 2013. The effective interest rate on the short-term portion of the credit facility was 1.13% at December 31, 2014.

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 8 – Commitments and Contingencies The Company is a defendant in miscellaneous legal proceedings. The Company is also a party 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 is currently subject will have a material adverse impact on its financial position or results of operations. The Company maintains liability insurance for various risks associated with the operation of its natural gas pipelines and facilities. In connection with these liability insurance policies, the Company 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 2014 to July 2015, these liability insurance policies require Southwest to be responsible for the first $1 million dollars (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, the Company 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, the Company will make an accrual.

Pension and Other Postretirement Benefits

12 Months Ended

Dec. 31, 2014

Compensation and Retirement Disclosure [Abstract]

Pension and Other Postretirement Benefits

Note 9 – Pension and Other Postretirement Benefits

Southwest has an Employees’ Investment Plan that provides for purchases of various mutual fund investments and Company common stock by eligible Southwest employees through deduction of a percentage of base compensation, subject to IRS limitations. Southwest matches one-half of amounts deferred by employees, up to a maximum matching contribution of 3.5% of an employee’s annual compensation. Centuri has a separate plan, the cost and liability of which are not significant. The cost of the Southwest plan is listed below (in thousands):


	2014		2013		2012
Employee Investment Plan cost	$	4,816	$	4,850	$	4,707

Southwest has a deferred compensation plan for all officers and a separate deferred compensation plan for members of the Board of Directors. The plans provide the opportunity to defer up to 100% of annual cash compensation. Southwest matches one-half of amounts deferred by officers, 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.

Southwest has a noncontributory qualified retirement plan with defined benefits covering substantially all employees and a separate unfunded supplemental retirement plan (“SERP”) which is limited to officers. Southwest also provides postretirement benefits other than pensions (“PBOP”) to its qualified retirees for health care, dental, and life insurance benefits.

The Company recognizes the overfunded or underfunded positions of defined benefit postretirement plans, including pension plans, in its 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.

In accordance with regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities, the Company has established a regulatory asset 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.

Investment objectives and strategies for the qualified retirement plan are developed and approved by the Pension Plan Investment Committee of the Board of Directors of the Company. They are designed to enhance capital, maintain minimum liquidity required for retirement plan operations and effectively manage pension assets.

A target portfolio of investments in the qualified retirement plan is developed by the Pension Plan Investment Committee and is reevaluated periodically. Asset return assumptions are determined by evaluating performance expectations of the target portfolio. Projected benefit obligations are estimated using actuarial assumptions and Company benefit policy. A target mix of assets is then determined based on acceptable risk versus estimated returns in order to fund the benefit obligation. At December 31, 2014, the percentage ranges of the target portfolio are:


Type of Investment	Percentage Range
Equity securities		59 to 71
Debt securities		31 to 37
Other		up to 5

The Company’s 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.

U.S. GAAP states that the assumed discount rate should reflect the rate at which the pension benefits could be effectively settled. In making this estimate, in addition to rates implicit in current prices of annuity contracts that could be used to settle the liabilities, employers may look to rates of return on high-quality fixed-income investments available on December 31 of each year and expected to be available during the period to maturity of the pension benefits. In determining the discount rate, the Company 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.

Due to a lower interest rate environment for high-quality fixed income investments, the Company reduced the discount rate at December 31, 2014 from 2013. The methodology utilized to determine the discount rate was consistent with prior years. The weighted-average rate of compensation increase was also lowered (consistent with management’s expectations overall). The asset return assumption (which impacts the following year’s expense) was not changed. The rates are presented in the table below:


	December 31, 2014			December 31, 2013
Discount rate		4.25	%		5.00	%
Weighted-average rate of compensation increase		2.75	%		3.25	%
Asset return assumption		7.75	%		7.75	%

A landmark change to a new actuarial mortality table by the Society of Actuaries, which takes into account longer life spans for plan participants and forms a new basis for retirement plan obligations in the U.S., will significantly increase the expense level for 2015. It also negatively impacts the funded status of the plan at the end of 2014. Pension expense for 2015 is estimated to increase by $10 million compared to 2014 because of the extended mortality assumption and lower discount rate. 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 Statements of Income.


	2014									2013
	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)	$	886,714		$	36,143		$	58,020		$	902,812		$	37,373		$	59,704
Service cost		21,360			292			1,101			23,056			373			1,220
Interest cost		43,440			1,745			2,829			37,607			1,535			2,482
Plan amendments		—			—			6,661			—			—			—
Actuarial loss (gain)		144,606			5,459			4,567			(44,768	)		(661	)		(4,073	)
Benefits paid		(35,880	)		(2,463	)		(976	)		(31,993	)		(2,477	)		(1,313	)

Benefit obligation at end of year (PBO/PBO/APBO)		1,060,240			41,176			72,202			886,714			36,143			58,020

Change in plan assets
Market value of plan assets at beginning of year		719,944			—			42,314			609,750			—			35,250
Actual return on plan assets		34,732			—			2,859			96,187			—			7,319
Employer contributions		36,000			2,463			—			46,000			2,477			169
Benefits paid		(35,880	)		(2,463	)		(281	)		(31,993	)		(2,477	)		(424	)

Market value of plan assets at end of year		754,796			—			44,892			719,944			—			42,314

Funded status at year end	$	(305,444	)	$	(41,176	)	$	(27,310	)	$	(166,770	)	$	(36,143	)	$	(15,706	)

Weighted-average assumptions (benefit obligation)
Discount rate		4.25	%		4.25	%		4.25	%		5.00	%		5.00	%		5.00	%
Weighted-average rate of compensation increase		2.75	%		2.75	%		N/A			3.25	%		3.25	%		N/A

Estimated funding for the plans above during calendar year 2015 is approximately $39 million of which $36 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, 2014		December 31, 2013
Retirement plan	$	886,215	$	794,919
SERP		39,125		33,894

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


	2015	2016	2017	2018		2019		2020-2024
Pension	$ 39.1	$41.3	$43.1		$45.3		$47.5		$272.7
PBOP	3.3	3.6	3.8		4.0		4.1		20.4
SERP	2.6	2.6	2.6		2.6		2.6		13.0

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% declining to 5%. The Company makes fixed contributions for health care benefits of employees who retire after 1988, but 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.

Components of net periodic benefit cost


	Qualified Retirement Plan									SERP									PBOP
	2014			2013			2012			2014			2013			2012			2014			2013			2012
(Thousands of dollars)
Service cost	$	21,360		$	23,056		$	20,319		$	292		$	373		$	274		$	1,101		$	1,220		$	977
Interest cost		43,440			37,607			38,266			1,745			1,535			1,629			2,829			2,482			2,547
Expected return on plan assets		(53,342	)		(49,840	)		(45,780	)		—			—			—			(3,264	)		(2,824	)		(2,404	)
Amortization of prior service cost		—			—			—			—			—			—			355			355			—
Amortization of transition obligation		—			—			—			—			—			—			—			—			867
Amortization of net actuarial loss		22,873			32,261			23,883			783			971			683			—			945			1,031

Net periodic benefit cost	$	34,331		$	43,084		$	36,688		$	2,820		$	2,879		$	2,586		$	1,021		$	2,178		$	3,018

Weighted-average assumptions (net benefit cost)
Discount rate		5.00	%		4.25	%		5.00	%		5.00	%		4.25	%		5.00	%		5.00	%		4.25	%		5.00	%
Expected return on plan assets		7.75	%		8.00	%		8.00	%		7.75	%		8.00	%		8.00	%		7.75	%		8.00	%		8.00	%
Weighted-average rate of compensation increase		3.25	%		2.75	%		3.00	%		3.25	%		2.75	%		3.00	%		N/A			N/A			N/A

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


	2014												2013												2012
	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)	$	173,646		$	163,215		$	5,460		$	4,971		$	(100,345	)	$	(91,115	)	$	(662	)	$	(8,568	)	$	74,853		$	70,016		$	4,111		$	726
Amortization of prior service cost (b)		(355	)		—			—			(355	)		(355	)		—			—			(355	)		—			—			—			—
Amortization of transition obligation (b)		—			—			—			—			—			—			—			—			(867	)		—			—			(867	)
Amortization of net actuarial loss (b)		(23,656	)		(22,872	)		(784	)		—			(34,177	)		(32,261	)		(971	)		(945	)		(25,597	)		(23,883	)		(683	)		(1,031	)
Prior service cost		6,661			—			—			6,661			—			—			—			—			2,423			—			—			2,423
Regulatory adjustment		(140,308	)		(129,031	)		—			(11,277	)		123,630			113,762			—			9,868			(42,771	)		(41,520	)		—			(1,251	)

Recognized in other comprehensive
(income) loss		15,988			11,312			4,676			—			(11,247	)		(9,614	)		(1,633	)		—			8,041			4,613			3,428			—
Net periodic benefit costs recognized in net income		38,172			34,331			2,820			1,021			48,141			43,084			2,879			2,178			42,292			36,688			2,586			3,018

Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	54,160		$	45,643		$	7,496		$	1,021		$	36,894		$	33,470		$	1,246		$	2,178		$	50,333		$	41,301		$	6,014			$3,018

The table above discloses the net gain or loss, prior service cost, and transition amount 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 5 – Other Comprehensive Income and Accumulated Other Comprehensive Income (“AOCI”).

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 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 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, 2014 and December 31, 2013. The SERP has no assets. There were no transfers between Levels 1 and 2 during 2014.


	December 31, 2014						December 31, 2013
	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
Common stock
Agriculture	$	6,661	$	198	$	6,859	$	8,224	$	244	$	8,468
Capital equipment		2,222		66		2,288		3,891		115		4,006
Chemicals/materials		5,233		155		5,388		8,228		244		8,472
Consumer goods		41,731		1,238		42,969		54,329		1,611		55,940
Energy and mining		18,502		549		19,051		36,126		1,071		37,197
Finance/insurance		20,685		613		21,298		37,643		1,116		38,759
Healthcare		37,846		1,122		38,968		40,426		1,199		41,625
Information technology		25,881		767		26,648		24,636		731		25,367
Services		28,846		855		29,701		31,212		926		32,138
Telecommunications/internet/media		18,498		549		19,047		24,270		720		24,990
Other		10,958		325		11,283		16,455		488		16,943
Real estate investment trusts		5,713		169		5,882		5,779		171		5,950
Mutual funds		86,159		24,567		110,726		76,764		22,495		99,259
Government fixed income securities		44,694		1,325		46,019		34,495		1,023		35,518
Preferred securities		568		17		585		—		—		—

Total Level 1 Assets (1)	$	354,197	$	32,515	$	386,712	$	402,478	$	32,154	$	434,632

Level 2 – Significant other observable inputs
Commercial paper	$	—	$	—	$	—	$	1,411	$	42	$	1,453
Government fixed income and mortgage backed securities		48,312		1,433		49,745		49,298		1,462		50,760
Corporate fixed income securities
Asset-backed and mortgage-backed		27,071		803		27,874		20,697		614		21,311
Banking		23,289		691		23,980		19,004		564		19,568
Insurance		6,182		183		6,365		6,481		192		6,673
Utilities		4,232		126		4,358		5,278		156		5,434
Other		23,120		686		23,806		19,649		582		20,231
Pooled funds and mutual funds		11,968		984		12,952		8,111		1,150		9,261
State and local obligations		1,499		44		1,543		1,370		41		1,411

Total Level 2 assets (2)	$	145,673	$	4,950	$	150,623	$	131,299	$	4,803	$	136,102

Level 3 – Significant unobservable inputs
Commingled equity funds	$	259,235	$	7,687	$	266,922	$	189,452	$	5,618	$	195,070

Total Level 3 assets (3)	$	259,235	$	7,687	$	266,922	$	189,452	$	5,618	$	195,070

Total Plan assets at fair value	$	759,105	$	45,152	$	804,257	$	723,229	$	42,575	$	765,804
Insurance company general account contracts (4)		4,003		—		4,003		4,296		—		4,296

Total Plan assets (5)	$	763,108	$	45,152	$	808,260	$	727,525	$	42,575	$	770,100

(1)	Common stock, Real Estate Investment Trusts, Mutual funds, and U.S. Government securities listed or regularly traded on a national securities exchange are valued at quoted market prices as of the last business day of the calendar year.

The Mutual funds category above is an intermediate-term bond fund whose manager employs multiple concurrent strategies and takes only moderate risk in each, thereby reducing the risk of poor performance arising from any single source, and a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks and fixed-income securities. Strategies utilized by the bond fund include duration management, yield curve or maturity structuring, sector rotation, and all bottom-up techniques including in-house credit and quantitative research. Strategies employed by the balanced fund include pursuit of regular income, conservation of principal, and an opportunity for long-term growth of principal and income.

(2)	The fair value of investments in debt securities with remaining maturities of one year or more is determined by dealers who make markets in such securities or by an independent pricing service, which considers yield or price of bonds of comparable quality, coupon, maturity, and type.

The pooled funds and mutual funds are two collective short-term funds that invest in Treasury bills and money market funds. These funds are used as a temporary cash repository for the pension plan’s various investment managers.

(3)	Assets not considered Level 1 or Level 2 are valued using assumptions based on the best information available under the circumstances, such as investment manager pricing.

The commingled equity funds include private equity funds that invest in domestic and international securities (predominately Level 1 assets) regularly traded on securities exchanges. These funds are shown in the above table at net asset value. Investment strategies employed by the funds include:

•

Domestic large capitalization value equities

•

International developed countries value and growth equities

•

Emerging markets equities

•

International small capitalization equities

The terms and conditions under which shares in the commingled equity funds may be redeemed vary among the funds; the notice required ranges from one day to 30 days prior to the valuation date (month end). One of the commingled equity funds requires the payment of a minimal impact fee to be applied to redemptions and subscriptions of $5 million or greater; the relative fee diminishes the greater the transaction. Other such funds may impose fees to recover direct costs incurred by the fund at redemption, but are indeterminable prior to redemption.

(4)	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.

(5)

The assets in the above table exceed the market value of plan assets shown in the funded status table by $8,572,000 (qualified retirement plan – $8,312,000, PBOP – $260,000) and $7,842,000 (qualified retirement plan – $7,581,000, PBOP – $261,000) for 2014 and 2013, respectively, which includes a payable for securities purchased, partially offset by receivables for interest, dividends, and securities sold.

Fair Value Measurements Using Significant Unobservable Inputs (Level 3)


	Commingled Equity Funds
(Thousands of dollars):
Balance, December 31, 2012	$	127,467
Actual return on plan assets:
Relating to assets still held at the reporting date		21,903
Relating to assets sold during the period		—
Purchases		45,700
Sales		—
Settlements		—
Transfers in and/or out of Level 3		—

Balance, December 31, 2013		195,070
Actual return on plan assets:
Relating to assets still held at the reporting date		(6,932	)
Relating to assets sold during the period		2,069
Purchases		82,615
Sales		(5,900	)
Settlements		—
Transfers in and/or out of Level 3		—

Balance, December 31, 2014	$	266,922

Stock-Based Compensation

12 Months Ended

Dec. 31, 2014

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

Stock-Based Compensation

Note 10 – Stock-Based Compensation

At December 31, 2014, the Company had three stock-based compensation plans: a stock option plan, a performance share stock plan which includes a cash award, and a restricted stock/unit plan. The table below shows total stock-based plan compensation expense, including the cash award, which was recognized in the consolidated statements of income (in thousands):


	2014		2013		2012
Stock-based compensation plan expense, net of related tax benefits	$	8,130	$	8,012	$	7,396
Stock-based compensation plan related tax benefits		4,983		4,910		4,533

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

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


	2014					2013					2012
	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		52		$	27.57		125		$	28.13		177		$	27.28
Exercised during the year		(16	)		24.31		(72	)		28.44		(52	)		25.25
Forfeited or expired during the year		—			—		(1	)		33.07		—			—

Outstanding and exercisable at year end		36		$	28.97		52		$	27.57		125		$	28.13

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):


	2014		2013		2012
Outstanding and exercisable	$	1,194	$	1,473	$	1,788
Exercised		451		1,402		928


	December 31, 2014		December 31, 2013		December 31, 2012
Market value of Southwest Gas stock	$	61.81	$	55.91	$	42.41

The weighted-average remaining contractual life for outstanding options was one year for 2014. All outstanding options are fully vested and exercisable. The following table summarizes information about stock options outstanding at December 31, 2014 (thousands of options):


	Options Outstanding and Exercisable
Range of Exercise Price	Number outstanding		Weighted-average remaining contractual life		Weighted-average exercise price
$25.00 to $26.10		17		0.5 Years	$	25.71
$29.08 to $33.07		19		1.5 Years	$	31.83

The Company received $379,000 in cash from the exercise of options during 2014 and a corresponding tax benefit of $167,000 which was recorded in additional paid-in capital.

Under the performance share stock plan, the Company may issue performance shares to encourage key employees to remain in its employment and to achieve short-term and long-term performance goals. Plan participants are eligible to receive a cash bonus (i.e., short-term incentive) and performance shares (i.e., long-term incentive). The performance shares vest three years after grant (and are subject to a final adjustment as determined by the Board of Directors) and are then issued as common stock.

The Company awards restricted stock/units under the restricted stock/unit plan to attract, motivate, retain, and reward key employees with an incentive to attain high levels of individual performance and improved financial performance of the Company. 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 occurs immediately upon grant. The issuance of common stock for directors occurs when their service on the Board ends.

The following table summarizes the activity of the performance share stock and restricted stock/unit plans as of December 31, 2014 (thousands of shares):


	Performance Shares			Weighted- average grant date fair value		Restricted Stock/ Units			Weighted- average grant date fair value
Nonvested/unissued at beginning of year		323		$	39.16		245		$	38.00
Granted		77			53.73		83			53.73
Dividends		7					7
Forfeited or expired		—			—		—			—
Vested and issued*		(136	)		36.23		(78	)		40.87

Nonvested/unissued at December 31, 2014		271		$	43.71		257		$	41.22

*	Includes shares for retiree payouts and those converted for taxes.

The average grant date fair value of performance shares and restricted stock/units granted in 2013 and 2012 was $44.83 and $41.34, respectively.

As of December 31, 2014, total compensation cost related to nonvested performance shares and restricted stock/units not yet recognized is $2.9 million.

Income Taxes

12 Months Ended

Dec. 31, 2014

Income Tax Disclosure [Abstract]

Income Taxes

Note 11 – Income Taxes

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


Year ended December 31,	2014			2013		2012
U.S.	$	221,471		$	222,815	$	207,915
Foreign		(1,950	)		—		—

Total income before income taxes	$	219,521		$	222,815	$	207,915

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


Year Ended December 31,	2014			2013		2012
Current:
Federal	$	1,739		$	3,549	$	2,296
State		5,073			5,107		5,744
Foreign		2,193			—		—

		9,005			8,656		8,040

Deferred:
Federal		71,439			67,414		65,551
State		614			1,872		1,685
Foreign		(2,685	)		—		—

		69,368			69,286		67,236

Total income tax expense	$	78,373		$	77,942	$	75,276

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


Year Ended December 31,	2014			2013			2012
Deferred federal and state:
Property-related items	$	52,814		$	62,737		$	64,249
Purchased gas cost adjustments		15,049			16,189			1,755
Employee benefits		109			(2,769	)		564
All other deferred		2,257			(6,010	)		1,529

Total deferred federal and state		70,229			70,147			68,097
Deferred ITC, net		(861	)		(861	)		(861	)

Total deferred income tax expense	$	69,368		$	69,286		$	67,236

A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate for 2012, 2013, and 2014 (and the sources of these differences and the effect of each) are summarized as follows:


Year Ended December 31,	2014			2013			2012
U.S. federal statutory income tax rate		35.0	%		35.0	%		35.0	%
Net state taxes		1.9			2.4			2.6
Property-related items		0.1			0.1			0.2
Tax credits		(0.5	)		(0.4	)		(0.4	)
Company owned life insurance		(1.0	)		(2.1	)		(1.3	)
All other differences		0.2			—			0.1

Consolidated effective income tax rate		35.7	%		35.0	%		36.2	%

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


December 31,	2014			2013
Deferred tax assets:
Deferred income taxes for future amortization of ITC	$	2,146		$	2,679
Employee benefits		31,557			25,591
Alternative minimum tax credit		20,172			19,739
Net operating losses and credits		9,719			15,113
Interest rate swap		8,622			9,893
Other		25,872			22,334
Valuation allowance		(253	)		(200	)

		97,835			95,149

Deferred tax liabilities:
Property-related items, including accelerated depreciation		736,810			694,024
Regulatory balancing accounts		33,736			18,688
Property-related items previously flowed through		28			836
Unamortized ITC		3,410			4,271
Debt-related costs		5,066			4,713
Intangibles		12,792			—
Other		27,572			15,898

		819,414			738,430

Net deferred tax liabilities		721,579			643,281

Current		(2,109	)		(31,130	)
Noncurrent		723,688			674,411

Net deferred tax liabilities	$	721,579		$	643,281

The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, various states, and in Canada. The Company is currently under examination in Canada for years 2011 and 2012. With few exceptions, the Company is no longer subject to United States federal, state and local, or Canadian income tax examinations for years before 2010.

At December 31, 2014, the Company has a U.S. federal net operating loss carryforward of $28 million which expires in 2031. The Company also has federal general business credits of $711,000, which begin to expire in 2031. The Company also has U.S. federal net capital loss carryforwards of $494,000, which begin to expire in 2016. At December 31, 2014, the Company has an income tax net operating loss carryforward related to Canadian operations of $1.1 million which expires in 2034.

As of December 31, 2014, the Company sustained losses in its foreign jurisdiction and therefore has no undistributed foreign earnings. However, the Company intends to permanently reinvest any future foreign earnings in Canada.

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


	2014		2013
Unrecognized tax benefits at beginning of year	$	—	$	—
Gross increases-tax positions in prior period		305		—
Gross decreases-tax positions in prior period		—		—
Gross increases-current period tax positions		—		—
Gross decreases-current period tax positions		—		—
Settlements		—		—
Lapse in statute of limitations		—		—

Unrecognized tax benefits at end of year	$	305	$	—

In assessing whether uncertain tax positions should be recognized in its financial statements, Southwest 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 evaluating whether a tax position has met the more-likely-than-not recognition threshold, Southwest 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, Southwest measures the amount of benefit recognized in the financial statements at the largest amount of benefit that is greater than 50 percent likely of being realized upon ultimate settlement. Southwest recognizes unrecognized tax benefits 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. Southwest’s measurement of its 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 limitations, identification of new issues, and any administrative guidance or developments

The total amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate was $0 at December 31, 2014. No significant increases or decreases in unrecognized tax benefit are expected within the next 12 months.

The Company recognizes interest expense and income and penalties related to income tax matters in income tax expense. Tax-related interest income included in income tax expense in the consolidated statements of income is shown in the table below (in thousands):


	2014		2013		2012
Tax-related interest income	$	—	$	—	$	24

Final and Proposed Income Tax Regulations. In September 2013, the United States Department of the Treasury and the Internal Revenue Service (“IRS”) issued final and proposed regulations for the tax treatment of tangible property. The final regulations include standards for determining whether and when a taxpayer must capitalize costs incurred in acquiring, maintaining, or improving tangible property. The final regulations are generally effective for tax years beginning on or after January 1, 2014, and were eligible for adoption in earlier years under certain circumstances. Proposed regulations were also released that revise the rules for dispositions of tangible property and general asset accounts. The Company expects the IRS to issue natural gas industry guidance which will facilitate its analysis regarding the regulations’ impact on natural gas distribution networks. Based upon preliminary analysis of the final and proposed regulations, and in anticipation of specific guidance for the natural gas industry, the Company expects the regulations could result in a modest acceleration of tax deductibility and the deferral of tax payments.

Derivatives and Fair Value Measurements

12 Months Ended

Dec. 31, 2014

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Derivatives and Fair Value Measurements

Note 12 – Derivatives and Fair Value Measurements

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 have no significant market value. The Swaps are recorded at fair value.

In late 2013, the Company suspended further swaps and fixed-price purchases pursuant to the Volatility Mitigation Program (“VMP”) for its Nevada service territories. The decision did not impact previously executed purchase arrangements. Agreements, under the Nevada VMP program, made prior to the suspension will terminate following the March 2015 delivery month. The Company, along with its regulators, will continue to evaluate this strategy in light of prevailing or anticipated changing market conditions.

The fixed-price contracts and Swaps are utilized by Southwest under its ongoing volatility mitigation programs to effectively fix the price on a portion (for the 2014/2015 heating season, up to 25%, depending on the jurisdiction) 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 2015 through March 2016. 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):


	December 31, 2014		December 31, 2013
Contract notional amounts		5,105		13,571

Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.

The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the years ended December 31, 2014, 2013, and 2012 and their location in the Consolidated Statements of Income (thousands of dollars):

Gains (losses) recognized in income for derivatives not designated as hedging instruments:

(Thousands of dollars)


Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	2014			2013			2012
Swaps	Net cost of gas sold	$	(2,363	)	$	976		$	(4,854	)
Swaps	Net cost of gas sold		2,363	*		(976	)*		4,854	*

Total		$	—		$	—		$	—

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

In January 2010, 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 fixed-rate debt to replace maturing debt. The first FSIRS terminated in December 2010. The second FSIRS terminated in March 2012. At settlement of the second FSIRS, Southwest paid an aggregate $21.8 million to the counterparties. No gain or loss (ineffective portion) was recognized in income for either FSIRS during any period, including the period presented in the following table.

Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:


	Year Ended December 31, 2014		Year Ended December 31, 2013		Year Ended December 31, 2012
(Thousands of dollars)
Amount of gain/(loss) realized/unrealized on FSIRS recognized in other comprehensive income on derivative	$	—	$	—	$	2,959

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

Fair values of derivatives not designated as hedging instruments:


December 31, 2014 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Other current liabilities	$	—	$	(5,062	)	$	(5,062	)
Swaps	Other deferred credits		—		(363	)		(363	)

Total		$	—	$	(5,425	)	$	(5,425	)


December 31, 2013 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	257	$	(77	)	$	180
Swaps	Prepaids and other current assets		1,054		(253	)		801
Swaps	Other current liabilities		126		(282	)		(156	)
Swaps	Other deferred credits		7		(11	)		(4	)

Total		$	1,444	$	(623	)	$	821

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

Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its 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 shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.


	Year ended December 31, 2014		Year ended December 31, 2013		Year ended December 31, 2012
(Thousands of dollars)
Paid to counterparties	$	829	$	3,148	$	14,843

Received from counterparties	$	4,713	$	915	$	634

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


December 31, 2014 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	5,062
Swaps	Deferred charges and other assets		363

December 31, 2013 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(180	)
Swaps	Other current liabilities		(801	)
Swaps	Prepaids and other current assets		156
Swaps	Deferred charges and other assets		4

Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at December 31, 2014 and 2013 using New York Mercantile Exchange (“NYMEX”) futures settlement prices for delivery of natural gas at Henry Hub adjusted by the price of NYMEX ClearPort basis Swaps, 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 (inputs, other than quoted prices, for similar assets or liabilities) 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 measure.

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, the Company’s financial assets and liabilities that were accounted for at fair value (see Note 9 – Pension and Other Post Retirement Benefits for definitions of the levels of the fair value hierarchy):

Level 2 - Significant other observable inputs


	December 31, 2014			December 31, 2013
(Thousands of dollars)
Assets at fair value:
Prepaids and other current assets - Swaps	$	—		$	801
Deferred charges and other assets - Swaps		—			180
Liabilities at fair value:
Other current liabilities - Swaps		(5,062	)		(156	)
Other deferred credits - Swaps		(363	)		(4	)

Net Assets (Liabilities)	$	(5,425	)	$	821

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, 2014

Segment Reporting [Abstract]

Segment Information

Note 13 – Segment Information

Company 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 construction 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.

The accounting policies of the reported segments are the same as those described within Note 1 – Summary of Significant Accounting Policies. Centuri accounts for the services provided to Southwest at contractual (market) 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, 2014		December 31, 2013
Accounts receivable for Centuri services	$	9,169	$	10,787

The following table presents the amount of revenues and long-lived assets by geographic area (thousands of dollars):


	December 31, 2014		December 31, 2013
Revenues (a)
United States	$	2,069,513	$	1,950,782
Canada		52,194		—

Total	$	2,121,707	$	1,950,782

Long-lived assets
United States	$	6,021,476		5,674,449
Canada		14,475		—

Total	$	6,035,951	$	5,674,449

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

The financial information pertaining to the natural gas operations and construction services segments for each of the three years in the period ended December 31, 2014 is as follows (thousands of dollars):


2014	Gas Operations		Construction Services		Adjustments (a)			Total
Revenues from unaffiliated customers	$	1,382,087	$	647,432				$	2,029,519
Intersegment sales		—		92,188					92,188

Total	$	1,382,087	$	739,620				$	2,121,707

Interest revenue	$	2,596	$	6				$	2,602

Interest expense	$	68,299	$	3,770				$	72,069

Depreciation and amortization	$	204,144	$	48,883				$	253,027

Income tax expense	$	63,597	$	14,776				$	78,373

Segment net income	$	116,872	$	24,254				$	141,126

Segment assets	$	4,657,709	$	567,405	$	(10,599	)	$	5,214,515

Capital expenditures	$	350,025	$	46,873				$	396,898

2013	Gas Operations		Construction Services		Adjustments (b)			Total
Revenues from unaffiliated customers	$	1,300,154	$	562,475				$	1,862,629
Intersegment sales		—		88,153					88,153

Total	$	1,300,154	$	650,628				$	1,950,782

Interest revenue	$	456	$	5				$	461

Interest expense	$	62,555	$	1,145				$	63,700

Depreciation and amortization	$	193,848	$	42,969				$	236,817

Income tax expense	$	65,377	$	12,565				$	77,942

Segment net income	$	124,169	$	21,151				$	145,320

Segment assets	$	4,272,029	$	293,811	$	(666	)	$	4,565,174

Capital expenditures	$	314,578	$	49,698				$	364,276


2012	Gas Operations		Construction Services		Adjustments			Total
Revenues from unaffiliated customers	$	1,321,728	$	522,676				$	1,844,404
Intersegment sales		—		83,374					83,374

Total	$	1,321,728	$	606,050				$	1,927,778

Interest revenue	$	915	$	9				$	924

Interest expense	$	66,957	$	1,063				$	68,020

Depreciation and amortization	$	186,035	$	37,387				$	223,422

Income tax expense	$	64,973	$	10,303				$	75,276

Segment net income	$	116,619	$	16,712				$	133,331

Segment assets	$	4,204,948	$	283,109				$	4,488,057

Capital expenditures	$	308,951	$	86,761				$	395,712

(a)

Construction services segment assets include two liabilities that were netted against gas operations segment assets during consolidation in 2014. They are: Income taxes payable of $3.3 million, netted against income taxes receivable, net and deferred income taxes of $1.4 million, netted against deferred income taxes, net. Construction services segment assets exclude a long-term deferred tax benefit of $1.4 million, which was netted against gas operations segment deferred income taxes and investment tax credits, net during consolidation. Gas operations segment assets include a deferred income tax liability of $4.5 million, which was netted against a construction services segment asset for deferred income taxes, net during consolidation.

(b)	Construction services segment assets include income taxes payable of $666,000 in 2013, which was netted against gas operations segment income taxes receivable, net during consolidation.

Quarterly Financial Information Disclosure [Abstract]

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

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+ Details

Quarterly Financial Data

12 Months Ended

Dec. 31, 2014

Quarterly Financial Data

Note 14 – Quarterly Financial Data (Unaudited)


	Quarter Ended
	March 31		June 30			September 30			December 31
(Thousands of dollars, except per share amounts)
2014
Operating revenues	$	608,396	$	453,153		$	432,475		$	627,683
Operating income		127,065		26,755			18,290			112,373
Net income		70,697		9,627			1,927			58,897
Net income attributable to Southwest Gas Corporation		70,783		9,627			1,970			58,746
Basic earnings per common share*		1.52		0.21			0.04			1.26
Diluted earnings per common share*		1.51		0.21			0.04			1.25

2013
Operating revenues	$	613,505	$	411,574		$	387,346		$	538,357
Operating income		138,394		28,908			6,141			100,772
Net income (loss)		80,674		10,067			(3,057	)		57,189
Net income (loss) attributable to Southwest Gas Corporation		80,773		10,108			(2,864	)		57,303
Basic earnings (loss) per common share*		1.75		0.22			(0.06	)		1.24
Diluted earnings (loss) per common share*		1.73		0.22			(0.06	)		1.22

2012
Operating revenues	$	657,645	$	409,768		$	371,799		$	488,566
Operating income		134,623		15,380			6,310			115,211
Net income (loss)		78,835		(3,888	)		(4,414	)		62,106
Net income (loss) attributable to Southwest Gas Corporation		78,919		(3,676	)		(4,305	)		62,393
Basic earnings (loss) per common share*		1.71		(0.08	)		(0.09	)		1.35
Diluted earnings (loss) per common share*		1.70		(0.08	)		(0.09	)		1.34

*	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 outstanding.

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 the operations of the Company. Also, the timing of general rate relief can have a significant impact on earnings for interim periods. See Management’s Discussion and Analysis for additional discussion of operating results. Additionally, see Note 15 – Acquisition of Construction Services Businesses below regarding an acquisition in the last quarter of 2014 and see Note-16 – Construction Services Noncontrolling Interests regarding allocation of earnings.

Acquisition of Construction Services Businesses

12 Months Ended

Dec. 31, 2014

Business Combinations [Abstract]

Acquisition of Construction Services Businesses

Note 15 – Acquisition of Construction Services Businesses

As indicated in Note 1 – Summary of Significant Accounting Policies, under Consolidation, the Company, through its subsidiaries, completed the acquisition of three privately held, affiliated construction businesses for $221 million. Previous owners of the acquired companies retained an approximate 10% interest in the Canadian subsidiaries of Centuri.

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 the impacts of differences between Accounting Standards for Private Enterprises in Canada and U. S. GAAP applicable to public companies, as well as consideration of types of intangibles that were acquired, including non-competition agreements, customer relationships, trade names, and work backlog. The final purchase accounting has not yet been completed. Further refinement is expected to occur, including changes to income taxes and intangibles. However, no material changes are expected. The preliminary estimated fair values of assets acquired and liabilities assumed as of October 1, 2014, are as follows (in millions of dollars):


Cash, cash equivalents, and restricted cash	$	3
Contracts receivable and other receivables		62
Property, plant and equipment		17
Other assets		17
Intangible assets		52
Goodwill		130

Total assets acquired		281
Current liabilities		39
Deferred income tax—long-term		17
Other long-term liabilities		4

Net assets acquired	$	221

Acquired contracts receivable and other receivables are expected to be collected.

The preliminary allocation of the purchase price of Link-Line, W.S. Nicholls, and Brigadier was accounted for in accordance with the 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. The business of Brigadier was acquired via asset purchase. Therefore, the $4.9 million of tax-basis goodwill assigned to Brigadier is expected to be deductible for tax purposes. All other goodwill associated with the acquisition is not deductible for tax purposes. At December 31, 2014, the balance of goodwill associated with the acquisition was $125 million after consideration of changes in foreign currency adjustments. At December 31, 2014, other intangible assets totaled $48.2 million (after foreign currency adjustments of $1.9 million and approximately $1.5 million accumulated amortization). Intangible assets (as of December 2014) consist of $500,000 in non-competition agreements (net of approximately $40,000 of accumulated amortization, with a 5-year weighted-average useful life), $36.5 million in customer relationships (net of approximately $550,000 accumulated amortization, with useful lives ranging from 12 to 21 years), $10 million in trade names (net of approximately $250,000 accumulated amortization, with useful lives ranging from 4 to 20 years), and $1.2 million in work backlog (net of approximately $700,000 accumulated amortization, with an 8-month useful life). The intangible assets other than goodwill are included in Other property and investments in the Consolidated Balance Sheets. The estimated future amortization of the intangible assets acquired in the acquisition for the next five years is as follows (in thousands):


2015	$	4,615
2016		3,371
2017		3,371
2018		3,123
2019		2,370

In connection with the acquisition, previous owners retained certain ownership rights, specifically, an approximate 10% stock ownership interest in the associated Canadian businesses. However, while the actual ownership interest was approximately 10% of Lynxus (Canadian operations), the terms of the underlying equity agreements include dividend participation rights equal to 3.4% of dividends declared at the level of Centuri. Additionally, these same agreements include, among other terms, the ability of the prior owners to exit their investment retained by requiring Centuri to purchase a portion of their interest (in Lynxus) commencing October 2016 and in incremental amounts each anniversary date thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election after September 2021. Furthermore, the equity agreements include an exchange feature such that the noncontrolling ownership interest retained by the parties, in the Canadian subsidiaries, may be convertible into shares equivalent to a 3.4% interest in Centuri (redeemable noncontrolling interest). In consideration of these circumstances and the underlying agreements, it was deemed appropriate to allocate earnings to the redeemable noncontrolling interest at an amount equivalent to approximately 3.4% of total Centuri earnings (rather than based on the actual current proportional ownership). Through this earnings allocation process, approximately 96.6% of Centuri earnings are attributable to the Company. This earnings allocation takes place before the redeemable noncontrolling interest balance, included on the balance sheet, is adjusted based on its redemption value. Adjustments to redemption value through December 31, 2014 impacted retained earnings but not current net income. See also Earnings Per Share included within Note 1, and discussion below regarding adjustment to fair value, as well as Note 16 – Construction Services Noncontrolling Interests for more information.

The unaudited pro forma consolidated financial information for fiscal 2014 and fiscal 2013 (assuming the acquisition of Link-Line, W.S. Nicholls, and Brigadier occurred as of the beginning fiscal 2013) is as follows (in thousands of dollars, except per share amounts):


	Year Ended December 31,
	2014		2013
Total operating revenues	$	2,295,318	$	2,203,272
Net income attributable to Southwest Gas Corporation	$	149,588	$	143,424
Basic earnings per share	$	3.22	$	3.10
Diluted earnings per share	$	3.19	$	3.07

Acquisition costs of $5 million that were incurred during 2014, and included in construction expenses in the Consolidated Statements of Income, were excluded from the 2014 unaudited pro forma consolidated financial information shown above and included in the 2013 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 intercompany eliminations 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 Link-Line, W.S. Nicholls, and Brigadier operations included in the Consolidated Statements of Income since the date of acquisition are as follows (in thousands of dollars):


	Year ended December 31, 2014
Construction revenues	$	54,264
Net income attributable to Link-Line, W.S. Nicholls, and Brigadier	$	1,859

Construction Services Noncontrolling Interests

The entire disclosure for a business combination (or series of individually immaterial business combinations) completed during the period, including background, timing, and recognized assets and liabilities. The disclosure may include leverage buyout transactions (as applicable).

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+ Details

Construction Services Noncontrolling Interests

12 Months Ended

Dec. 31, 2014

Noncontrolling Interest [Abstract]

Note 16 – Construction Services Noncontrolling Interests

As discussed in Note 15 – Acquisition of Construction Services Businesses, at the close of the acquisition, previous owners of the acquired companies retained an approximate 10% equity interest in the Canadian businesses that were acquired. The agreement, associated with the approximate 10% indirect equity interest of the sellers, provides special dividend rights which entitle the sellers, as holders, to dividends equal to 3.4% of dividends paid at the level of Centuri and subject to certain conditions, such interests may become exchangeable for a 3.4% equity interest in Centuri. Additionally, the previous owners may exit their investment retained by requiring Centuri to purchase a portion of their interest (in Lynxus) commencing October 2016 and in incremental amounts each anniversary date thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election after September 2021.

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 Consolidated Balance Sheets. The redeemable noncontrolling interest is reported at $20 million, the estimated redemption value as of December 31, 2014. Based on the fair value model employed, the estimated redemption value of the redeemable noncontrolling interest increased by approximately $961,000 during the fourth quarter of 2014. 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. Any adjustment to the redemption value impacts retained earnings, but does not impact net income.


	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2013	$	—
Redeemable noncontrolling interest related to acquisition		18,952
Net Income (loss) attributable to redeemable noncontrolling interest		151
Foreign currency exchange translation adjustment		(22	)
Adjustment to redemption value		961

Balance, December 31, 2014	$	20,042

The redemption value of the redeemable noncontrolling interest was determined using a Monte Carlo simulation method. First, a market approach was utilized to determine a construction services enterprise value as of the acquisition date. Potential guideline publicly-traded companies were identified by using a selection criteria, including actively traded equities, their financial solvency, and other factors. Once the guideline companies were determined, enterprise value was calculated using a weighted approach of projected earnings before interest expense and taxes (“EBIT”) and earnings before interest expense, taxes, and depreciation and amortization expense (“EBITDA”). After an estimated fair value was determined, a Monte Carlo simulation was used to assign a value to the noncontrolling interest of the sellers. Other assumptions used in this analysis included dividends, probability of events, and a discount due to lack of control (the sellers do not influence operations).

Centuri also holds a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning (“HVAC”) technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC) as a majority-owned subsidiary. The interest is immaterial to the consolidated financial statements.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Nature of Operations

Basis of Presentation

Consolidation

Net Utility Plant

Other Property and Investments

Other Property and Investments. Other property and investments includes (millions of dollars):


	2014			2013
Centuri property, equipment, and intangibles	$	405		$	320
Centuri accumulated provision for depreciation and amortization		(187	)		(163	)
Net cash surrender value of COLI policies		99			93
Other property		10			11

Total	$	327		$	261

Restricted Cash

Deferred Purchased Gas Costs

Prepaids and Other Current Assets

Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $23 million in 2014 and $21 million in 2013.

Income Taxes

Cash and Cash Equivalents

Inventories

Inventories. Inventories are carried at weighted average cost and include natural gas stored underground, liquefied natural gas storage and materials and supplies.

Goodwill

Intangible Assets

Accumulated Removal Costs


	December 31, 2014		December 31, 2013
Accumulated removal costs	$	304,000	$	279,000

Gas Operating Revenues

Construction Revenues

Construction Expenses

Net Cost of Gas Sold

Operations and Maintenance Expense

Depreciation and Amortization

Allowance for Funds Used During Construction ("AFUDC")


	2014		2013		2012
(In thousands)
AFUDC:
Debt portion	$	1,228	$	1,260	$	1,129
Equity portion		1,995		2,274		1,943

AFUDC capitalized as part of utility plant	$	3,223	$	3,534	$	3,072

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):


	2014			2013			2012
Change in COLI policies	$	5,300		$	12,400		$	6,600
Interest income		2,602			461			924
Pipe replacement costs		—			(132	)		(2,680	)
Foreign currency transaction gain (loss)		(178	)		—			—
Miscellaneous income and (expense)		(617	)		(429	)		(433	)

Total other income (deductions)	$	7,107		$	12,300		$	4,411

Foreign Currency Translation

Earnings Per Share


	2014		2013		2012
(In thousands)
Average basic shares		46,494		46,318		46,115
Effect of dilutive securities:
Stock options		17		26		42
Performance shares		215		231		254
Restricted stock units		218		183		144

Average diluted shares		46,944		46,758		46,555

Recently Issued Accounting Standards Updates

Subsequent Events

Reclassifications

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Schedule of Other Property and Investments

Other Property and Investments. Other property and investments includes (millions of dollars):


	2014			2013
Centuri property, equipment, and intangibles	$	405		$	320
Centuri accumulated provision for depreciation and amortization		(187	)		(163	)
Net cash surrender value of COLI policies		99			93
Other property		10			11

Total	$	327		$	261

Schedule of Accumulated Removal Costs

The reclassified amounts are presented in the table below (thousands of dollars):


	December 31, 2014		December 31, 2013
Accumulated removal costs	$	304,000	$	279,000

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.


	2014		2013		2012
(In thousands)
AFUDC:
Debt portion	$	1,228	$	1,260	$	1,129
Equity portion		1,995		2,274		1,943

AFUDC capitalized as part of utility plant	$	3,223	$	3,534	$	3,072

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):


	2014			2013			2012
Change in COLI policies	$	5,300		$	12,400		$	6,600
Interest income		2,602			461			924
Pipe replacement costs		—			(132	)		(2,680	)
Foreign currency transaction gain (loss)		(178	)		—			—
Miscellaneous income and (expense)		(617	)		(429	)		(433	)

Total other income (deductions)	$	7,107		$	12,300		$	4,411

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.


	2014		2013		2012
(In thousands)
Average basic shares		46,494		46,318		46,115
Effect of dilutive securities:
Stock options		17		26		42
Performance shares		215		231		254
Restricted stock units		218		183		144

Average diluted shares		46,944		46,758		46,555

Utility Plant and Leases (Tables)

12 Months Ended

Dec. 31, 2014

Leases [Abstract]

Schedule of Net Utility Plant

Net utility plant as of December 31, 2014 and 2013 was as follows (thousands of dollars):


December 31,	2014			2013
Gas plant:
Storage	$	22,531		$	21,282
Transmission		312,300			313,306
Distribution		4,655,640			4,410,598
General		356,072			324,490
Software and software-related intangibles		196,035			168,815
Other		14,021			13,978

		5,556,599			5,252,469
Less: accumulated depreciation		(1,973,098	)		(1,868,504	)
Acquisition adjustments, net		550			730
Construction work in progress		74,332			101,413

Net utility plant	$	3,658,383		$	3,486,108

Schedule of Depreciation and Amortization Expense

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


	2014		2013		2012
Depreciation and amortization expense	$	194,360	$	185,283	$	182,612

Schedule of Rental Payments for Operating Leases

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


	2014		2013		2012
Southwest Gas	$	5,330	$	8,308	$	7,762
Centuri		30,012		27,118		24,054

Consolidated rental payments/lease expense	$	35,342	$	35,426	$	31,816

Schedule of Future Minimum Lease Payments for Operating Leases


Year Ending December 31,
2015	$	5,957
2016		4,236
2017		2,532
2018		1,582
2019		912
Thereafter		831

Total minimum lease payments	$	16,050

Schedule of Capital Leases of Equipment

The amount of capital leases of equipment as of December 31, 2014 and 2013 is as follows (thousands of dollars):


December 31,	2014			2013
Capital leases of equipment	$	5,763		$	—
Less: accumulated amortization		(287	)		—

Net capital leases	$	5,476		$	—

Schedule of Future Minimum Lease Payments for Capital Leases

The following is a schedule of future minimum lease payments for non-cancelable capital leases (with initial or remaining terms in excess of one year) as of December 31, 2014 (thousands of dollars):


Year Ending December 31,
2015	$	1,690
2016		1,632
2017		813
2018		591
2019		—
Thereafter		—

		4,726
Less: amount representing interest		(497	)

Total minimum lease payments	$	4,229

Receivables and Related Allowances (Tables)

12 Months Ended

Dec. 31, 2014

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, 2014,


	December 31, 2014
Gas utility customer accounts receivable balance (in thousands)	$	136,148

Schedule of Percent of Customers by State


The table below contains information about the percentage of customers in each of the three states at December 31, 2014.
	December 31, 2014
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, 2011	$	3,182
Additions charged to expense		2,471
Accounts written off, less recoveries		(3,149	)

Balance, December 31, 2012		2,504
Additions charged to expense		3,583
Accounts written off, less recoveries		(4,362	)

Balance, December 31, 2013		1,725
Additions charged to expense		4,146
Accounts written off, less recoveries		(3,616	)

Balance, December 31, 2014	$	2,255

Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2014

Regulated Operations [Abstract]

Schedule of Regulatory Assets and Liabilities

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


December 31,	2014			2013
Regulatory assets:
Accrued pension and other postretirement benefit costs (1)	$	390,293		$	249,985
Unrealized net loss on non-trading derivatives (Swaps) (2)		5,425			160
Deferred purchased gas costs (3)		87,556			18,217
Accrued purchased gas costs (4)		2,600			31,500
Unamortized premium on reacquired debt (5)		20,478			19,614
Other (6)		72,132			48,945

		578,484			368,421
Regulatory liabilities:
Accumulated removal costs		(304,000	)		(279,000	)
Unrealized net gain on non-trading derivatives (Swaps) (2)		—			(981	)
Deferred gain on southern Nevada division operations facility (7)		(115	)		(253	)
Unamortized gain on reacquired debt (8)		(10,862	)		(11,398	)
Other (9)		(34,233	)		(26,482	)

Net regulatory assets	$	229,274		$	50,307

(1)	Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 9).

(2)


Instrument	Balance Sheet Location	2014		2013
Swaps	Deferred charges and other assets	$	363	$	4
Swaps	Prepaids and other current assets		5,062		156
Swaps	Other current liabilities		—		(801	)
Swaps	Other deferred credits		—		(180	)

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

(4)	Included in Prepaids 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)

(7)	Balance was originally being amortized over a four-year period beginning in the fourth quarter of 2009. As a result of the most recent Nevada general rate case, the amortization period was extended through October 2015.

(8)	Included in Other deferred credits on the Consolidated Balance Sheet. Amortized over life of debt instruments.

(9)	Other regulatory liabilities includes amounts associated with income tax and gross-up.

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

12 Months Ended

Dec. 31, 2014

Text Block [Abstract]


	2014									2013									2012
	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
(Thousands of dollars)
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(173,646	)	$	65,985		$	(107,661	)	$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
Amortization of prior service cost		355			(135	)		220			355			(135	)		220			—			—			—
Amortization of transition obligation		—			—			—			—			—			—			867			(329	)		538
Amortization of net actuarial (gain)/loss		23,656			(8,989	)		14,667			34,177			(12,987	)		21,190			25,597			(9,727	)		15,870
Prior service cost		(6,661	)		2,531			(4,130	)		—			—			—			(2,423	)		921			(1,502	)
Regulatory adjustment		140,308			(53,317	)		86,991			(123,630	)		46,979			(76,651	)		42,771			(16,253	)		26,518

Pension plans other comprehensive income (loss)		(15,988	)		6,075			(9,913	)		11,247			(4,274	)		6,973			(8,041	)		3,056			(4,985	)
FSIRS (designated hedging activities):
Unrealized/realized gain (loss)		—			—			—			—			—			—			2,959			(1,125	)		1,834
Amounts reclassified into net income		3,345			(1,272	)		2,073			3,345			(1,271	)		2,074			2,801			(1,064	)		1,737

FSIRS other comprehensive income (loss)		3,345			(1,272	)		2,073			3,345			(1,271	)		2,074			5,760			(2,189	)		3,571
Foreign currency translation adjustments:
Translation adjustments		(659	)		—			(659	)		—			—			—			—			—			—

Foreign currency other comprehensive income (loss)		(659	)		—			(659	)		—			—			—			—			—			—

Total other comprehensive income (loss)	$	(13,302	)	$	4,803		$	(8,499	)	$	14,592		$	(5,545	)	$	9,047		$	(2,281	)	$	867		$	(1,414	)

(1)

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

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	$	33,000
SERP net actuarial loss		1,300
PBOP net actuarial loss		300
PBOP prior service cost		1,300

Rollforward of Accumulated Other Comprehensive Income

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 (Note 9)									FSIRS (Note 12)									Foreign Currency Items
	Before- Tax			Tax (Expense) Benefit			After- Tax			Before- Tax			Tax (Expense) Benefit			After- Tax			Before- Tax			Tax (Expense) Benefit		After- Tax			AOCI
Beginning Balance AOCI December 31, 2013	$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	—		$	—	$	—		$	(41,698	)

Net actuarial gain/(loss)		(173,646	)		65,985			(107,661	)		—			—			—			—			—		—			(107,661	)
Translation adjustments		—			—			—			—			—			—			(659	)		—		(659	)		(659	)

Other comprehensive income before reclassi fications		(173,646	)		65,985			(107,661	)		—			—			—			(659	)		—		(659	)		(108,320	)
FSIRS amounts reclassified from AOCI (1)		—			—			—			3,345			(1,272	)		2,073			—			—		—			2,073
Amortization of prior service cost (2)		355			(135	)		220			—			—			—			—			—		—			220
Amortization of net actuarial loss (2)		23,656			(8,989	)		14,667			—			—			—			—			—		—			14,667
Prior service cost		(6,661	)		2,531			(4,130	)		—			—			—			—			—		—			(4,130	)
Regulatory adjustment (3)		140,308			(53,317	)		86,991			—			—			—			—			—		—			86,991

Net current period other comprehensive income (loss)		(15,988	)		6,075			(9,913	)		3,345			(1,272	)		2,073			(659	)		—		(659	)		(8,499	)
Less: Translation adjustment attributable to redeemable noncontrolling interest		—			—			—			—			—			—			(22	)		—		(22	)		(22	)

Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation		(15,988	)		6,075			(9,913	)		3,345			(1,272	)		2,073			(637	)		—		(637	)		(8,477	)

Ending Balance AOCI December 31, 2014	$	(57,211	)	$	21,740		$	(35,471	)	$	(22,688	)	$	8,621		$	(14,067	)	$	(637	)	$	—	$	(637	)	$	(50,175	)

(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 9 – 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).

Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)


	2014			2013
Net actuarial (loss) gain	$	(439,131	)	$	(289,141	)
Prior service cost		(8,373	)		(2,067	)
Less: amount recognized in regulatory assets		390,293			249,985

Recognized in AOCI	$	(57,211	)	$	(41,223	)

Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2014

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 (not actively traded) are categorized as Level 3, based on significant unobservable inputs to their fair values. Since 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 utilizes 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.


December 31,	2014					2013
	Carrying Amount			Market Value		Carrying Amount			Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	133,403	$	125,000		$	130,953
Notes, 6.1%, due 2041		125,000			157,290		125,000			141,873
Notes, 3.875%, due 2022		250,000			262,030		250,000			252,485
Notes, 4.875%, due 2043		250,000			280,903		250,000			257,280
8% Series, due 2026		75,000			102,296		75,000			96,263
Medium-term notes, 7.59% series, due 2017		25,000			27,573		25,000			28,741
Medium-term notes, 7.78% series, due 2022		25,000			31,144		25,000			30,586
Medium-term notes, 7.92% series, due 2027		25,000			33,695		25,000			31,497
Medium-term notes, 6.76% series, due 2027		7,500			9,156		7,500			8,468
Unamortized discount		(5,223	)				(5,560	)

		902,277					901,940

Revolving credit facility and commercial paper		150,000			150,000		10,000			10,000

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
Fixed-rate bonds:
5.25% 2003 Series D, due 2038		20,000			20,277		20,000			20,150
5.25% 2004 Series A, due 2034		—			—		65,000			64,522
5.00% 2004 Series B, due 2033		31,200			31,223		31,200			30,284
4.85% 2005 Series A, due 2035		100,000			100,071		100,000			95,192
4.75% 2006 Series A, due 2036		24,855			25,399		24,855			22,974
Unamortized discount		(1,943	)				(2,776	)

		374,112					438,279

Centuri secured revolving credit and term loan facility		199,267			200,341		—			—
Centuri other debt obligations		31,128			31,127		42,213			42,119

		1,656,784					1,392,432
Less: current maturities		(19,192	)				(11,105	)

Long-term debt, less current maturities	$	1,637,592				$	1,381,327

Summary of Effective Interest Rates on Variable-Rate IDRBs

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


	December 31, 2014			December 31, 2013
2003 Series A		0.85	%		1.43	%
2008 Series A		0.90	%		1.41	%
2009 Series A		0.89	%		1.01	%
Tax-exempt Series A		0.84	%		1.07	%

Estimated Maturities of Long-Term Debt

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


2015	$	19,192
2016		20,991
2017		43,899
2018		16,985
2019		304,328

Compensation and Retirement Disclosure [Abstract]

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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

+ Details

Pension and Other Postretirement Benefits (Tables)

12 Months Ended

Dec. 31, 2014

Cost of Retirement Plan

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


	2014		2013		2012
Employee Investment Plan cost	$	4,816	$	4,850	$	4,707

Schedule of Allocation of Plan Assets

At December 31, 2014, the percentage ranges of the target portfolio are:


Type of Investment	Percentage Range
Equity securities		59 to 71
Debt securities		31 to 37
Other		up to 5

Schedule of Assumptions Used

The rates are presented in the table below:


	December 31, 2014			December 31, 2013
Discount rate		4.25	%		5.00	%
Weighted-average rate of compensation increase		2.75	%		3.25	%
Asset return assumption		7.75	%		7.75	%

Schedule of Amounts Recognized in Balance Sheet and Income Statement

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


	2014									2013
	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)	$	886,714		$	36,143		$	58,020		$	902,812		$	37,373		$	59,704
Service cost		21,360			292			1,101			23,056			373			1,220
Interest cost		43,440			1,745			2,829			37,607			1,535			2,482
Plan amendments		—			—			6,661			—			—			—
Actuarial loss (gain)		144,606			5,459			4,567			(44,768	)		(661	)		(4,073	)
Benefits paid		(35,880	)		(2,463	)		(976	)		(31,993	)		(2,477	)		(1,313	)

Benefit obligation at end of year (PBO/PBO/APBO)		1,060,240			41,176			72,202			886,714			36,143			58,020

Change in plan assets
Market value of plan assets at beginning of year		719,944			—			42,314			609,750			—			35,250
Actual return on plan assets		34,732			—			2,859			96,187			—			7,319
Employer contributions		36,000			2,463			—			46,000			2,477			169
Benefits paid		(35,880	)		(2,463	)		(281	)		(31,993	)		(2,477	)		(424	)

Market value of plan assets at end of year		754,796			—			44,892			719,944			—			42,314

Funded status at year end	$	(305,444	)	$	(41,176	)	$	(27,310	)	$	(166,770	)	$	(36,143	)	$	(15,706	)

Weighted-average assumptions (benefit obligation)
Discount rate		4.25	%		4.25	%		4.25	%		5.00	%		5.00	%		5.00	%
Weighted-average rate of compensation increase		2.75	%		2.75	%		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, 2014		December 31, 2013
Retirement plan	$	886,215	$	794,919
SERP		39,125		33,894

Schedule of Expected Benefit Payments

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


	2015	2016	2017	2018		2019		2020-2024
Pension	$ 39.1	$41.3	$43.1		$45.3		$47.5		$272.7
PBOP	3.3	3.6	3.8		4.0		4.1		20.4
SERP	2.6	2.6	2.6		2.6		2.6		13.0

Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions

Components of net periodic benefit cost


	Qualified Retirement Plan									SERP									PBOP
	2014			2013			2012			2014			2013			2012			2014			2013			2012
(Thousands of dollars)
Service cost	$	21,360		$	23,056		$	20,319		$	292		$	373		$	274		$	1,101		$	1,220		$	977
Interest cost		43,440			37,607			38,266			1,745			1,535			1,629			2,829			2,482			2,547
Expected return on plan assets		(53,342	)		(49,840	)		(45,780	)		—			—			—			(3,264	)		(2,824	)		(2,404	)
Amortization of prior service cost		—			—			—			—			—			—			355			355			—
Amortization of transition obligation		—			—			—			—			—			—			—			—			867
Amortization of net actuarial loss		22,873			32,261			23,883			783			971			683			—			945			1,031

Net periodic benefit cost	$	34,331		$	43,084		$	36,688		$	2,820		$	2,879		$	2,586		$	1,021		$	2,178		$	3,018

Weighted-average assumptions (net benefit cost)
Discount rate		5.00	%		4.25	%		5.00	%		5.00	%		4.25	%		5.00	%		5.00	%		4.25	%		5.00	%
Expected return on plan assets		7.75	%		8.00	%		8.00	%		7.75	%		8.00	%		8.00	%		7.75	%		8.00	%		8.00	%
Weighted-average rate of compensation increase		3.25	%		2.75	%		3.00	%		3.25	%		2.75	%		3.00	%		N/A			N/A			N/A

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

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


	2014												2013												2012
	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)	$	173,646		$	163,215		$	5,460		$	4,971		$	(100,345	)	$	(91,115	)	$	(662	)	$	(8,568	)	$	74,853		$	70,016		$	4,111		$	726
Amortization of prior service cost (b)		(355	)		—			—			(355	)		(355	)		—			—			(355	)		—			—			—			—
Amortization of transition obligation (b)		—			—			—			—			—			—			—			—			(867	)		—			—			(867	)
Amortization of net actuarial loss (b)		(23,656	)		(22,872	)		(784	)		—			(34,177	)		(32,261	)		(971	)		(945	)		(25,597	)		(23,883	)		(683	)		(1,031	)
Prior service cost		6,661			—			—			6,661			—			—			—			—			2,423			—			—			2,423
Regulatory adjustment		(140,308	)		(129,031	)		—			(11,277	)		123,630			113,762			—			9,868			(42,771	)		(41,520	)		—			(1,251	)

Recognized in other comprehensive
(income) loss		15,988			11,312			4,676			—			(11,247	)		(9,614	)		(1,633	)		—			8,041			4,613			3,428			—
Net periodic benefit costs recognized in net income		38,172			34,331			2,820			1,021			48,141			43,084			2,879			2,178			42,292			36,688			2,586			3,018

Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$	54,160		$	45,643		$	7,496		$	1,021		$	36,894		$	33,470		$	1,246		$	2,178		$	50,333		$	41,301		$	6,014			$3,018

Schedule of Fair Value of Plan Assets


	December 31, 2014						December 31, 2013
	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
Common stock
Agriculture	$	6,661	$	198	$	6,859	$	8,224	$	244	$	8,468
Capital equipment		2,222		66		2,288		3,891		115		4,006
Chemicals/materials		5,233		155		5,388		8,228		244		8,472
Consumer goods		41,731		1,238		42,969		54,329		1,611		55,940
Energy and mining		18,502		549		19,051		36,126		1,071		37,197
Finance/insurance		20,685		613		21,298		37,643		1,116		38,759
Healthcare		37,846		1,122		38,968		40,426		1,199		41,625
Information technology		25,881		767		26,648		24,636		731		25,367
Services		28,846		855		29,701		31,212		926		32,138
Telecommunications/internet/media		18,498		549		19,047		24,270		720		24,990
Other		10,958		325		11,283		16,455		488		16,943
Real estate investment trusts		5,713		169		5,882		5,779		171		5,950
Mutual funds		86,159		24,567		110,726		76,764		22,495		99,259
Government fixed income securities		44,694		1,325		46,019		34,495		1,023		35,518
Preferred securities		568		17		585		—		—		—

Total Level 1 Assets (1)	$	354,197	$	32,515	$	386,712	$	402,478	$	32,154	$	434,632

Level 2 – Significant other observable inputs
Commercial paper	$	—	$	—	$	—	$	1,411	$	42	$	1,453
Government fixed income and mortgage backed securities		48,312		1,433		49,745		49,298		1,462		50,760
Corporate fixed income securities
Asset-backed and mortgage-backed		27,071		803		27,874		20,697		614		21,311
Banking		23,289		691		23,980		19,004		564		19,568
Insurance		6,182		183		6,365		6,481		192		6,673
Utilities		4,232		126		4,358		5,278		156		5,434
Other		23,120		686		23,806		19,649		582		20,231
Pooled funds and mutual funds		11,968		984		12,952		8,111		1,150		9,261
State and local obligations		1,499		44		1,543		1,370		41		1,411

Total Level 2 assets (2)	$	145,673	$	4,950	$	150,623	$	131,299	$	4,803	$	136,102

Level 3 – Significant unobservable inputs
Commingled equity funds	$	259,235	$	7,687	$	266,922	$	189,452	$	5,618	$	195,070

Total Level 3 assets (3)	$	259,235	$	7,687	$	266,922	$	189,452	$	5,618	$	195,070

Total Plan assets at fair value	$	759,105	$	45,152	$	804,257	$	723,229	$	42,575	$	765,804
Insurance company general account contracts (4)		4,003		—		4,003		4,296		—		4,296

Total Plan assets (5)	$	763,108	$	45,152	$	808,260	$	727,525	$	42,575	$	770,100

(1)	Common stock, Real Estate Investment Trusts, Mutual funds, and U.S. Government securities listed or regularly traded on a national securities exchange are valued at quoted market prices as of the last business day of the calendar year.

(2)	The fair value of investments in debt securities with remaining maturities of one year or more is determined by dealers who make markets in such securities or by an independent pricing service, which considers yield or price of bonds of comparable quality, coupon, maturity, and type.

(3)	Assets not considered Level 1 or Level 2 are valued using assumptions based on the best information available under the circumstances, such as investment manager pricing.

•

Domestic large capitalization value equities

•

International developed countries value and growth equities

•

Emerging markets equities

•

International small capitalization equities

(4)	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.

(5)

Schedule of Effect of Significant Unobservable Inputs

Fair Value Measurements Using Significant Unobservable Inputs (Level 3)


	Commingled Equity Funds
(Thousands of dollars):
Balance, December 31, 2012	$	127,467
Actual return on plan assets:
Relating to assets still held at the reporting date		21,903
Relating to assets sold during the period		—
Purchases		45,700
Sales		—
Settlements		—
Transfers in and/or out of Level 3		—

Balance, December 31, 2013		195,070
Actual return on plan assets:
Relating to assets still held at the reporting date		(6,932	)
Relating to assets sold during the period		2,069
Purchases		82,615
Sales		(5,900	)
Settlements		—
Transfers in and/or out of Level 3		—

Balance, December 31, 2014	$	266,922

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2014

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

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

The table below shows total stock-based plan compensation expense, including the cash award, which was recognized in the consolidated statements of income (in thousands):


	2014		2013		2012
Stock-based compensation plan expense, net of related tax benefits	$	8,130	$	8,012	$	7,396
Stock-based compensation plan related tax benefits		4,983		4,910		4,533

Schedule of Stock Options Activity

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


	2014					2013					2012
	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		52		$	27.57		125		$	28.13		177		$	27.28
Exercised during the year		(16	)		24.31		(72	)		28.44		(52	)		25.25
Forfeited or expired during the year		—			—		(1	)		33.07		—			—

Outstanding and exercisable at year end		36		$	28.97		52		$	27.57		125		$	28.13

Schedule of Aggregate Intrinsic Value of Outstanding and Exercisable Options

The aggregate intrinsic value of outstanding and exercisable options, and options that were exercised, are presented in the table below (in thousands):


	2014		2013		2012
Outstanding and exercisable	$	1,194	$	1,473	$	1,788
Exercised		451		1,402		928

Summary of Market Prices of Common Stock


	December 31, 2014		December 31, 2013		December 31, 2012
Market value of Southwest Gas stock	$	61.81	$	55.91	$	42.41

Schedule of Stock Options Outstanding and Exercisable

The following table summarizes information about stock options outstanding at December 31, 2014 (thousands of options):


	Options Outstanding and Exercisable
Range of Exercise Price	Number outstanding		Weighted-average remaining contractual life		Weighted-average exercise price
$25.00 to $26.10		17		0.5 Years	$	25.71
$29.08 to $33.07		19		1.5 Years	$	31.83

Schedule of Nonvested Performance and Restricted Stock Unit Plans

The following table summarizes the activity of the performance share stock and restricted stock/unit plans as of December 31, 2014 (thousands of shares):


	Performance Shares			Weighted- average grant date fair value		Restricted Stock/ Units			Weighted- average grant date fair value
Nonvested/unissued at beginning of year		323		$	39.16		245		$	38.00
Granted		77			53.73		83			53.73
Dividends		7					7
Forfeited or expired		—			—		—			—
Vested and issued*		(136	)		36.23		(78	)		40.87

Nonvested/unissued at December 31, 2014		271		$	43.71		257		$	41.22

*	Includes shares for retiree payouts and those converted for taxes.

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2014

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,	2014			2013		2012
U.S.	$	221,471		$	222,815	$	207,915
Foreign		(1,950	)		—		—

Total income before income taxes	$	219,521		$	222,815	$	207,915

Summary of Income Tax Expense (Benefit)

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


Year Ended December 31,	2014			2013		2012
Current:
Federal	$	1,739		$	3,549	$	2,296
State		5,073			5,107		5,744
Foreign		2,193			—		—

		9,005			8,656		8,040

Deferred:
Federal		71,439			67,414		65,551
State		614			1,872		1,685
Foreign		(2,685	)		—		—

		69,368			69,286		67,236

Total income tax expense	$	78,373		$	77,942	$	75,276

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,	2014			2013			2012
Deferred federal and state:
Property-related items	$	52,814		$	62,737		$	64,249
Purchased gas cost adjustments		15,049			16,189			1,755
Employee benefits		109			(2,769	)		564
All other deferred		2,257			(6,010	)		1,529

Total deferred federal and state		70,229			70,147			68,097
Deferred ITC, net		(861	)		(861	)		(861	)

Total deferred income tax expense	$	69,368		$	69,286		$	67,236

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


Year Ended December 31,	2014			2013			2012
U.S. federal statutory income tax rate		35.0	%		35.0	%		35.0	%
Net state taxes		1.9			2.4			2.6
Property-related items		0.1			0.1			0.2
Tax credits		(0.5	)		(0.4	)		(0.4	)
Company owned life insurance		(1.0	)		(2.1	)		(1.3	)
All other differences		0.2			—			0.1

Consolidated effective income tax rate		35.7	%		35.0	%		36.2	%

Deferred Tax Assets and Liabilities

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


December 31,	2014			2013
Deferred tax assets:
Deferred income taxes for future amortization of ITC	$	2,146		$	2,679
Employee benefits		31,557			25,591
Alternative minimum tax credit		20,172			19,739
Net operating losses and credits		9,719			15,113
Interest rate swap		8,622			9,893
Other		25,872			22,334
Valuation allowance		(253	)		(200	)

		97,835			95,149

Deferred tax liabilities:
Property-related items, including accelerated depreciation		736,810			694,024
Regulatory balancing accounts		33,736			18,688
Property-related items previously flowed through		28			836
Unamortized ITC		3,410			4,271
Debt-related costs		5,066			4,713
Intangibles		12,792			—
Other		27,572			15,898

		819,414			738,430

Net deferred tax liabilities		721,579			643,281

Current		(2,109	)		(31,130	)
Noncurrent		723,688			674,411

Net deferred tax liabilities	$	721,579		$	643,281

Reconciliation of Unrecognized Tax Benefits

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


	2014		2013
Unrecognized tax benefits at beginning of year	$	—	$	—
Gross increases-tax positions in prior period		305		—
Gross decreases-tax positions in prior period		—		—
Gross increases-current period tax positions		—		—
Gross decreases-current period tax positions		—		—
Settlements		—		—
Lapse in statute of limitations		—		—

Unrecognized tax benefits at end of year	$	305	$	—

Summary of Tax-Related Interest Income

Tax-related interest income included in income tax expense in the consolidated statements of income is shown in the table below (in thousands):


	2014		2013		2012
Tax-related interest income	$	—	$	—	$	24

Derivatives and Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2014

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):


	December 31, 2014		December 31, 2013
Contract notional amounts		5,105		13,571

Amount of Gain or Losses Recognized in Income on Derivatives

Gains (losses) recognized in income for derivatives not designated as hedging instruments:

(Thousands of dollars)


Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	2014			2013			2012
Swaps	Net cost of gas sold	$	(2,363	)	$	976		$	(4,854	)
Swaps	Net cost of gas sold		2,363	*		(976	)*		4,854	*

Total		$	—		$	—		$	—

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

Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments

Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:


	Year Ended December 31, 2014		Year Ended December 31, 2013		Year Ended December 31, 2012
(Thousands of dollars)
Amount of gain/(loss) realized/unrealized on FSIRS recognized in other comprehensive income on derivative	$	—	$	—	$	2,959

Fair Values of Swaps in Consolidated Balance Sheets

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

Fair values of derivatives not designated as hedging instruments:


December 31, 2014 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Other current liabilities	$	—	$	(5,062	)	$	(5,062	)
Swaps	Other deferred credits		—		(363	)		(363	)

Total		$	—	$	(5,425	)	$	(5,425	)


December 31, 2013 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	257	$	(77	)	$	180
Swaps	Prepaids and other current assets		1,054		(253	)		801
Swaps	Other current liabilities		126		(282	)		(156	)
Swaps	Other deferred credits		7		(11	)		(4	)

Total		$	1,444	$	(623	)	$	821

Paid to and Received from Counterparties for Settlements of Matured Swaps

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


	Year ended December 31, 2014		Year ended December 31, 2013		Year ended December 31, 2012
(Thousands of dollars)
Paid to counterparties	$	829	$	3,148	$	14,843

Received from counterparties	$	4,713	$	915	$	634

Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in 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, 2014 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	5,062
Swaps	Deferred charges and other assets		363

December 31, 2013 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(180	)
Swaps	Other current liabilities		(801	)
Swaps	Prepaids and other current assets		156
Swaps	Deferred charges and other assets		4

Significant Other Observable Inputs

Level 2 - Significant other observable inputs


	December 31, 2014			December 31, 2013
(Thousands of dollars)
Assets at fair value:
Prepaids and other current assets - Swaps	$	—		$	801
Deferred charges and other assets - Swaps		—			180
Liabilities at fair value:
Other current liabilities - Swaps		(5,062	)		(156	)
Other deferred credits - Swaps		(363	)		(4	)

Net Assets (Liabilities)	$	(5,425	)	$	821

Segment Information (Tables)

12 Months Ended

Dec. 31, 2014

Segment Reporting [Abstract]

Accounts Receivable for Services

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


	December 31, 2014		December 31, 2013
Accounts receivable for Centuri services	$	9,169	$	10,787

Schedule of Revenues and Long-Lived Assets by Geographic Area

The following table presents the amount of revenues and long-lived assets by geographic area (thousands of dollars):


	December 31, 2014		December 31, 2013
Revenues (a)
United States	$	2,069,513	$	1,950,782
Canada		52,194		—

Total	$	2,121,707	$	1,950,782

Long-lived assets
United States	$	6,021,476		5,674,449
Canada		14,475		—

Total	$	6,035,951	$	5,674,449

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

Schedule of Segment Reporting Information


2014	Gas Operations		Construction Services		Adjustments (a)			Total
Revenues from unaffiliated customers	$	1,382,087	$	647,432				$	2,029,519
Intersegment sales		—		92,188					92,188

Total	$	1,382,087	$	739,620				$	2,121,707

Interest revenue	$	2,596	$	6				$	2,602

Interest expense	$	68,299	$	3,770				$	72,069

Depreciation and amortization	$	204,144	$	48,883				$	253,027

Income tax expense	$	63,597	$	14,776				$	78,373

Segment net income	$	116,872	$	24,254				$	141,126

Segment assets	$	4,657,709	$	567,405	$	(10,599	)	$	5,214,515

Capital expenditures	$	350,025	$	46,873				$	396,898

2013	Gas Operations		Construction Services		Adjustments (b)			Total
Revenues from unaffiliated customers	$	1,300,154	$	562,475				$	1,862,629
Intersegment sales		—		88,153					88,153

Total	$	1,300,154	$	650,628				$	1,950,782

Interest revenue	$	456	$	5				$	461

Interest expense	$	62,555	$	1,145				$	63,700

Depreciation and amortization	$	193,848	$	42,969				$	236,817

Income tax expense	$	65,377	$	12,565				$	77,942

Segment net income	$	124,169	$	21,151				$	145,320

Segment assets	$	4,272,029	$	293,811	$	(666	)	$	4,565,174

Capital expenditures	$	314,578	$	49,698				$	364,276


2012	Gas Operations		Construction Services		Adjustments			Total
Revenues from unaffiliated customers	$	1,321,728	$	522,676				$	1,844,404
Intersegment sales		—		83,374					83,374

Total	$	1,321,728	$	606,050				$	1,927,778

Interest revenue	$	915	$	9				$	924

Interest expense	$	66,957	$	1,063				$	68,020

Depreciation and amortization	$	186,035	$	37,387				$	223,422

Income tax expense	$	64,973	$	10,303				$	75,276

Segment net income	$	116,619	$	16,712				$	133,331

Segment assets	$	4,204,948	$	283,109				$	4,488,057

Capital expenditures	$	308,951	$	86,761				$	395,712

(a)

(b)	Construction services segment assets include income taxes payable of $666,000 in 2013, which was netted against gas operations segment income taxes receivable, net during consolidation.

Quarterly Financial Data (Tables)

12 Months Ended

Dec. 31, 2014

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)
2014
Operating revenues	$	608,396	$	453,153		$	432,475		$	627,683
Operating income		127,065		26,755			18,290			112,373
Net income		70,697		9,627			1,927			58,897
Net income attributable to Southwest Gas Corporation		70,783		9,627			1,970			58,746
Basic earnings per common share*		1.52		0.21			0.04			1.26
Diluted earnings per common share*		1.51		0.21			0.04			1.25

2013
Operating revenues	$	613,505	$	411,574		$	387,346		$	538,357
Operating income		138,394		28,908			6,141			100,772
Net income (loss)		80,674		10,067			(3,057	)		57,189
Net income (loss) attributable to Southwest Gas Corporation		80,773		10,108			(2,864	)		57,303
Basic earnings (loss) per common share*		1.75		0.22			(0.06	)		1.24
Diluted earnings (loss) per common share*		1.73		0.22			(0.06	)		1.22

2012
Operating revenues	$	657,645	$	409,768		$	371,799		$	488,566
Operating income		134,623		15,380			6,310			115,211
Net income (loss)		78,835		(3,888	)		(4,414	)		62,106
Net income (loss) attributable to Southwest Gas Corporation		78,919		(3,676	)		(4,305	)		62,393
Basic earnings (loss) per common share*		1.71		(0.08	)		(0.09	)		1.35
Diluted earnings (loss) per common share*		1.70		(0.08	)		(0.09	)		1.34

*	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 outstanding.

Acquisition of Construction Services Businesses (Tables)

12 Months Ended

Dec. 31, 2014

Business Combinations [Abstract]

Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed

The preliminary estimated fair values of assets acquired and liabilities assumed as of October 1, 2014, are as follows (in millions of dollars):


Cash, cash equivalents, and restricted cash	$	3
Contracts receivable and other receivables		62
Property, plant and equipment		17
Other assets		17
Intangible assets		52
Goodwill		130

Total assets acquired		281
Current liabilities		39
Deferred income tax—long-term		17
Other long-term liabilities		4

Net assets acquired	$	221

Schedule of Estimated Future Amortization of Intangible Assets Acquired

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


2015	$	4,615
2016		3,371
2017		3,371
2018		3,123
2019		2,370

Schedule of Unaudited Pro Forma Consolidated Financial Information


	Year Ended December 31,
	2014		2013
Total operating revenues	$	2,295,318	$	2,203,272
Net income attributable to Southwest Gas Corporation	$	149,588	$	143,424
Basic earnings per share	$	3.22	$	3.10
Diluted earnings per share	$	3.19	$	3.07

Schedule of Actual Results Included In Consolidated Statements of Income

Actual results from Link-Line, W.S. Nicholls, and Brigadier operations included in the Consolidated Statements of Income since the date of acquisition are as follows (in thousands of dollars):


	Year ended December 31, 2014
Construction revenues	$	54,264
Net income attributable to Link-Line, W.S. Nicholls, and Brigadier	$	1,859

Construction Services Noncontrolling Interests (Tables)

12 Months Ended

Dec. 31, 2014

Noncontrolling Interest [Abstract]

Summary of Redeemable Noncontrolling Interest


	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2013	$	—
Redeemable noncontrolling interest related to acquisition		18,952
Net Income (loss) attributable to redeemable noncontrolling interest		151
Foreign currency exchange translation adjustment		(22	)
Adjustment to redemption value		961

Balance, December 31, 2014	$	20,042

Summary of Significant Accounting Policies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Oct. 31, 2014 USD ($) Business	Dec. 31, 2014 USD ($) Business Segment	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($) $ / shares
Significant Accounting Policies [Line Items]
Number of segments \| Segment		2
Number of business acquisitions \| Business	3	3
Purchase agreements including debt assumed	$ 221,000,000	$ 190,497,000
Non-cash construction advances - non-cash investing item		8,100,000	$ 9,300,000
Non-cash investing outflow activity		10,800,000
Goodwill		143,160,000	17,810,000
Goodwill impairment charges		0
Recognized pretax losses, after tax earnings per share impacts \| $ / shares				$ 0.20
Secured Revolving Credit and Term Loan Facility [Member]
Significant Accounting Policies [Line Items]
Secured revolving credit and term loan facility	$ 300,000,000
Construction Services [Member]
Significant Accounting Policies [Line Items]
Goodwill		133,000,000
Recognized pretax profit/losses				$ 15,000,000
Gas Operations [Member]
Significant Accounting Policies [Line Items]
Goodwill		10,000,000	10,000,000
Recent Acquisitions [Member] \| Construction Services [Member]
Significant Accounting Policies [Line Items]
Goodwill recognized by acquisition		125,000,000
Reduction of goodwill due to foreign currency exchange translation adjustments		5,000,000
NPL Construction Co. [Member] \| Construction Services [Member]
Significant Accounting Policies [Line Items]
Goodwill		8,000,000	8,000,000
Non-controlling Interest [Member]
Significant Accounting Policies [Line Items]
Non-cash investing outflow activity		18,900,000
Pooled funds and mutual funds [Member] \| Level 2 - Significant other observable inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments		250,000
Capital Lease Obligation [Member]
Significant Accounting Policies [Line Items]
Non-cash financing inflow activity		204,000
Capital Lease Asset [Member]
Significant Accounting Policies [Line Items]
Non-cash investing outflow activity		$ 204,000
Intellichoice Energy, LLC [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		65.00%
W.S. Nicholls Western Construction LTD. [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		50.00%
Investment amount		$ 14,700,000
Exposure to loss as a result of its investment		$ 20,800,000
Matheson-Nicholls Joint Venture [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		50.00%
CCI-TBN Toronto, Inc [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		25.00%
Gas Pipe Inventory and Operating Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe inventory and operating supplies		$ 23,000,000	$ 21,000,000

- Details

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Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Net cash surrender value of COLI policies	$ 99,000	$ 93,000
Other property	10,000	11,000
Total	326,743	260,871
Centuri Construction Group [Member]
Investment [Line Items]
Centuri property, equipment, and intangibles	405,000	320,000
Centuri accumulated provision for depreciation and amortization	$ (187,000)	$ (163,000)

Summary of Significant Accounting Policies - Schedule of Accumulated Removal Costs (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Accumulated Removal Costs [Abstract]
Accumulated removal costs	$ 304,000	$ 279,000

Summary of Significant Accounting Policies - Schedule of Capitalized and Debt Portion of AFUDC (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Debt portion	$ 1,228	$ 1,260	$ 1,129
Equity portion	1,995	2,274	1,943
AFUDC capitalized as part of utility plant	$ 3,223	$ 3,534	$ 3,072

Summary of Significant Accounting Policies - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other Income and Expenses [Abstract]
Change in COLI policies	$ 5,300	$ 12,400	$ 6,600
Interest income	2,602	461	924
Pipe replacement costs		(132)	(2,680)
Foreign currency transaction gain (loss)	(178)
Miscellaneous income and (expense)	(617)	(429)	(433)
Total other income (deductions)	$ 7,107	$ 12,300	$ 4,411

Summary of Significant Accounting Policies - Schedule of Earnings Per Share, Basic and Diluted (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Average basic shares	46,494	46,318	46,115
Stock options	17	26	42
Performance shares	215	231	254
Restricted stock units	218	183	144
Average diluted shares	46,944	46,758	46,555

Utility Plant and Leases - Schedule of Net Utility Plant (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 5,556,599	$ 5,252,469
Less: accumulated depreciation	(1,973,098)	(1,868,504)
Acquisition adjustments, net	550	730
Construction work in progress	74,332	101,413
Net utility plant	3,658,383	3,486,108
Storage [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	22,531	21,282
Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	312,300	313,306
Distribution [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	4,655,640	4,410,598
General [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	356,072	324,490
Software and software-related intangibles [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	196,035	168,815
Other [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 14,021	$ 13,978

Utility Plant and Leases - Schedule of Depreciation and Amortization Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 253,027	$ 236,817	$ 223,422
Gas plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 194,360	$ 185,283	$ 182,612

Utility Plant and Leases - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Jul. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Regulated Operations [Abstract]
Amortization of intangibles		$ 11.7	$ 10.3	$ 9.3
Purchase of portion of corporate headquarters office complex	$ 16.5

Utility Plant and Leases - Schedule of Rental Payments for Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 35,342	$ 35,426	$ 31,816
Southwest Gas [Member]
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	5,330	8,308	7,762
Centuri Construction Group [Member]
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 30,012	$ 27,118	$ 24,054

Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Operating Leases (Detail) $ in Thousands	Dec. 31, 2014 USD ($)
Leases [Abstract]
2015	$ 5,957
2016	4,236
2017	2,532
2018	1,582
2019	912
Thereafter	831
Total minimum lease payments	$ 16,050

Utility Plant and Leases - Schedule of Capital Leases of Equipment (Detail) $ in Thousands	Dec. 31, 2014 USD ($)
Leases [Abstract]
Capital leases of equipment	$ 5,763
Less: accumulated amortization	(287)
Net capital leases	$ 5,476

Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Capital Leases (Detail) $ in Thousands	Dec. 31, 2014 USD ($)
Leases [Abstract]
2015	$ 1,690
2016	1,632
2017	813
2018	591
2019	0
Thereafter	0
Future minimum lease payments due	4,726
Less: amount representing interest	(497)
Total minimum lease payments	$ 4,229

Receivables and Related Allowances - Schedule of Accounts Receivable (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2014 USD ($)
Receivables [Abstract]
Gas utility customer accounts receivable balance	$ 136,148

Receivables and Related Allowances - Schedule of Percent of Customers by State (Detail)	12 Months Ended
	Dec. 31, 2014
Arizona [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	53.00%
Nevada [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	37.00%
California [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	10.00%

Receivables and Related Allowances - Schedule of Allowance for Uncollectibles (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Receivables [Abstract]
Beginning Balance	$ 1,725	$ 2,504	$ 3,182
Additions charged to expense	4,146	3,583	2,471
Accounts written off, less recoveries	(3,616)	(4,362)	(3,149)
Ending Balance	$ 2,255	$ 1,725	$ 2,504

Receivables and Related Allowances - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 281,824	$ 219,469
Centuri Construction Group [Member]
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 142,000

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	$ 578,484	$ 368,421
Net regulatory assets	229,274	50,307
Accrued pension and other postretirement benefit costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	390,293	249,985
Unrealized net loss on non-trading derivatives (Swaps) [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	5,425	160
Deferred purchased gas costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	87,556	18,217
Accrued purchased gas costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	2,600	31,500
Unamortized premium on reacquired debt [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	20,478	19,614
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	72,132	48,945
Accumulated removal costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(304,000)	(279,000)
Unrealized net gain on non-trading derivatives (Swaps) [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total		(981)
Deferred gain on southern Nevada division operations facility [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(115)	(253)
Unamortized gain on reacquired debt [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(10,862)	(11,398)
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	$ (34,233)	$ (26,482)

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Parenthetical) (Detail) - Swaps [Member] - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Deferred charges and other assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	$ 363	$ 4
Prepaids and other current assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	$ 5,062	156
Other current liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value		(801)
Other deferred credits [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value		$ (180)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount	$ (173,646)	$ 100,345	$ (74,853)
Amortization of prior service cost, Before-Tax Amount	355	355
Amortization of transition obligation, Before-Tax Amount			867
Amortization of net actuarial (gain)/loss, Before-Tax Amount	23,656	34,177	25,597
Prior service cost, Before-Tax Amount	(6,661)		(2,423)
Regulatory adjustment, Before-Tax Amount	140,308	(123,630)	42,771
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	(15,988)	11,247	(8,041)
Unrealized/realized gain (loss), Before-Tax Amount			2,959
Amounts reclassified into net income, Before-Tax Amount	3,345	3,345	2,801
FSIRS other comprehensive income (loss), Before-Tax Amount	3,345	3,345	5,760
Translation adjustments, Before-Tax Amount	(659)
Foreign currency other comprehensive income (loss), Before-Tax Amount	(659)
Total other comprehensive income (loss), Before-Tax Amount	(13,302)	14,592	(2,281)
Net actuarial gain/(loss), Tax (Expense) or Benefit	65,985	(38,131)	28,444
Amortization of prior service cost, Tax (Expense) or Benefit	(135)	(135)
Amortization of transition obligation, Tax (Expense) or Benefit			(329)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(8,989)	(12,987)	(9,727)
Prior service cost, Tax (Expense) or Benefit	2,531		921
Regulatory adjustment, Tax (Expense) or Benefit	(53,317)	46,979	(16,253)
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	6,075	(4,274)	3,056
Unrealized/realized gain (loss), Tax (Expense) or Benefit			(1,125)
Amounts reclassified into net income, Tax (Expense) or Benefit	(1,272)	(1,271)	(1,064)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(1,272)	(1,271)	(2,189)
Translation adjustments, Tax (Expense) or Benefit	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	4,803	(5,545)	867
Net actuarial gain/(loss), Net-of-Tax Amount	(107,661)	62,214	(46,409)
Amortization of prior service cost, Net-of-Tax Amount	220	220
Amortization of transition obligation, Net-of-Tax Amount			538
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	14,667	21,190	15,870
Prior service cost, Net-of-Tax Amount	(4,130)		(1,502)
Regulatory adjustment, Net-of-Tax Amount	86,991	(76,651)	26,518
Net defined benefit pension plans	(9,913)	6,973	(4,985)
Unrealized/realized loss, Net-of-Tax Amount			1,834
Amounts reclassified into net income (Notes 5 and 12)	2,073	2,074	1,737
FSIRS other comprehensive income (loss), Net-of-Tax Amount	2,073	2,074	3,571
Translation adjustments, Net-of-Tax Amount	(659)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(659)
Total other comprehensive income (loss), net of tax	$ (8,499)	$ 9,047	$ (1,414)

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2014 USD ($)
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Additional Information (Detail) $ in Millions	Dec. 31, 2014 USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

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	12 Months Ended
	Dec. 31, 2014 USD ($)
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	$ 33,000
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	1,300
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	300
PBOP prior service cost	$ 1,300

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Before-Tax	$ (41,223)
Net actuarial gain/(loss), Defined Benefit Plans Before-Tax	(173,646)	$ 100,345	$ (74,853)
Translation adjustments, Defined Benefit Plans Before-Tax	0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(1,272)	(1,271)	(1,064)
Amortization of prior service costs, Defined Benefit Plans Before-Tax	355	355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	23,656	34,177	25,597
Prior service cost, Defined Benefit Plans Before-Tax	(6,661)		(2,423)
Regulatory adjustment, Defined Benefit Plans Before-Tax	140,308	(123,630)	42,771
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	(15,988)	11,247	(8,041)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Before-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax	(15,988)
Ending Balance AOCI December 31, 2014, Defined Benefit Plans Before-Tax	(57,211)	(41,223)
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Tax (Expense) Benefit	15,665
Net actuarial gain/(loss), Tax Expense or Benefit	65,985	(38,131)	28,444
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit	0
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit	(135)	(135)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(8,989)	(12,987)	(9,727)
Prior service cost, Defined Benefit Plans Tax (Expense) Benefit	2,531		921
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	(53,317)	46,979	(16,253)
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	6,075	(4,274)	3,056
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Tax (Expense) Benefit	6,075
Ending Balance AOCI December 31, 2014 Defined Benefit Plans Tax (Expense) Benefit	21,740	15,665
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans After-Tax	(25,558)
Net actuarial gain/(loss), Defined Benefit Plans After-Tax	(107,661)	62,214	(46,409)
Translation adjustments, Defined Benefit Plans After-Tax	0
Amortization of prior service costs, Defined Benefit Plans After-Tax	220	220
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	14,667	21,190	15,870
Prior service cost	(4,130)		(1,502)
Regulatory adjustment, Defined Benefit Plans After-Tax	86,991
Net defined benefit pension plans	(9,913)	6,973	(4,985)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans After-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax	(9,913)
Ending Balance AOCI December 31, 2014, Defined Benefit Plans After-Tax	(35,471)	(25,558)
Beginning Balance AOCI December 31, 2013, FSIRS - Before-Tax	(26,033)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	3,345	3,345	2,801
FSIRS other comprehensive income (loss), Before-Tax Amount	3,345	3,345	5,760
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Before-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax	3,345
Ending Balance AOCI December 31, 2014, FSIRS - Before-Tax	(22,688)	(26,033)
Beginning Balance AOCI December 31, 2013, FSIRS - Tax (Expense) Benefit	9,893
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(1,272)	(1,271)	(2,189)
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit	(1,272)
Ending Balance AOCI September 30, 2014, FSIRS - Tax (Expense) Benefit	8,621	9,893
Beginning Balance AOCI December 31, 2013, FSIRS - After-Tax	(16,140)
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	2,073	2,074	1,737
Net forward-starting interest rate swaps	2,073	2,074	3,571
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - After-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax	2,073
Ending Balance AOCI December 31, 2014, FSIRS - After-Tax	(14,067)	(16,140)
Net actuarial gain/(loss), Foreign Currency Items Before-Tax	0
Translation adjustments, Foreign Currency Items Before-Tax	(659)
Other comprehensive income before reclassifications, Foreign Currency Items Before-Tax	(659)
FSIRS amounts reclassified from AOCI, Foreign Currency Items Before-Tax	0
Amortization of prior service cost, Foreign Currency Items Before-Tax	0
Amortization of net actuarial loss, Foreign Currency Items Before-Tax	0
Prior service cost, Foreign Currency Items Before-Tax	0
Regulatory adjustment, Foreign Currency Items Before-Tax	0
Net current period other comprehensive income (loss), Foreign Currency Items Before-Tax	(659)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax	(22)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax	(659)
Ending Balance AOCI December 31, 2014, Foreign Currency Items Before-Tax	(637)
Beginning Balance AOCI December 31, 2013,Foreign Currency items - Tax (Expense) Benefit	0
Net actuarial gain/(loss),Foreign Currency items - Tax (Expense) Benefit	0
Translation adjustments,Foreign Currency items - Tax (Expense) Benefit	0	0	0
Other comprehensive income before reclassifications,Foreign Currency items - Tax (Expense) Benefit	0
FSIRS amounts reclassified from AOCI ,Foreign Currency items - Tax (Expense) Benefit	0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit	0
Amortization of net actuarial loss,Foreign Currency items - Tax (Expense) Benefit	0
Prior service cost ,Foreign Currency items - Tax (Expense) Benefit	0
Regulatory adjustment,Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss),Foreign Currency items - Tax (Expense) Benefit	0
Less: Translation adjustment attributable to redeemable noncontrolling interest ,Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation , Foreign Currency items - Tax (Expense) Benefit	0	0	0
Ending Balance AOCI December 31, 2014,Foreign Currency items - Tax (Expense) Benefit	0	0
Net actuarial gain/(loss), Foreign Currency items - After-Tax	0
Translation adjustments, Foreign Currency items - After-Tax	(659)
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	(659)
FSIRS amounts reclassified from AOCI, Foreign Currency items - After-Tax	0
Amortization of prior service cost, Foreign Currency items - After-Tax	0
Amortization of net actuarial loss, Foreign Currency items - After-Tax	0
Prior service cost, Foreign Currency items - After-Tax	0
Regulatory adjustment, Foreign Currency items - After-Tax	0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax	(659)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax	(22)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax	(659)
Ending Balance AOCI December 31, 2014, Foreign Currency items - After-Tax	(637)
Beginning Balance AOCI December 31, 2013, AOCI	(41,698)
Net actuarial gain/(loss), AOCI	(107,661)	62,214	(46,409)
Translation adjustments, AOCI	(659)
Amounts reclassified into net income (Notes 5 and 12)	2,073	2,074	1,737
Amortization of prior service cost, AOCI	220	220
Amortization of net actuarial loss	14,667	21,190	$ 15,870
Prior service cost, AOCI	(4,130)
Regulatory adjustment, AOCI	86,991
Net current period other comprehensive income (loss)	(8,499)
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	(22)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	(8,477)
Ending Balance AOCI December 31, 2014, AOCI	(50,175)	$ (41,698)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit	0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(1,272)
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax	(173,646)
Prior service cost, FSIRS - Tax (Expense) Benefit	0
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit	0
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit	65,985
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax	(107,661)
Translation adjustments, FSIRS - Before-Tax	0
Other comprehensive income before reclassifications, FSIRS - Before-Tax	0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	3,345
Prior service cost, FSIRS - Before-Tax	0
Translation adjustments, FSIRS - After-Tax	0
Other comprehensive income before reclassifications, FSIRS - After-Tax	0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	2,073
Prior service cost, FSIRS - After-Tax	0
Other comprehensive income before reclassifications, AOCI	(108,320)
Amounts reclassified into net income (Notes 5 and 12)	$ 2,073

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Prior service cost	$ 6,661		$ 2,423
Less: amount recognized in regulatory assets	578,484	$ 368,421
Recognized in AOCI	(57,211)	(41,223)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(439,131)	(289,141)
Prior service cost	(8,373)	(2,067)
Less: amount recognized in regulatory assets	390,293	249,985
Recognized in AOCI	$ (57,211)	$ (41,223)

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Less: current maturities	$ (19,192)		$ (11,105)
Long-term debt, less current maturities	1,637,592		1,381,327
Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable		$ 125,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Total long-term debt	1,656,784		1,392,432
Less: current maturities	(19,192)		(11,105)
Long-term debt, less current maturities	1,637,592		1,381,327
Total long-term debt	1,656,784		1,392,432
Carrying Amount [Member] \| Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	199,267
Carrying Amount [Member] \| Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	31,128		42,213
Carrying Amount [Member] \| Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	374,112		438,279
Carrying Amount [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount	(5,223)		(5,560)
Notes payable	902,277		901,940
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable	75,000		75,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	7,500		7,500
Carrying Amount [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility	150,000		10,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Unamortized discount	(1,943)		(2,776)
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	20,000		20,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Unsecured debt			65,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured debt	31,200		31,200
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000		100,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855		24,855
Market Value [Member] \| Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	200,341
Market Value [Member] \| Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	31,127		42,119
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	133,403		130,953
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	157,290		141,873
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	262,030		252,485
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	280,903		257,280
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	102,296		96,263
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,573		28,741
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,144		30,586
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	33,695		31,497
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,156		8,468
Market Value [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	150,000		10,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000		50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000		50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000		50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000		50,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	20,277		20,150
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value			64,522
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	31,223		30,284
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	100,071		95,192
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	$ 25,399		$ 22,974

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Nov. 30, 2014	Mar. 31, 2014
Notes, 4.45%, due 2020 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	Dec. 01, 2020	Dec. 01, 2020
Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate				6.10%
Notes, 6.1%, due 2041 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	Feb. 15, 2041	Feb. 15, 2041
Notes, 3.875%, due 2022 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%	3.875%
Debt instrument due date	Apr. 01, 2022	Apr. 01, 2022
Notes, 4.875%, due 2043 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.875%	4.875%
Debt instrument due date	Oct. 01, 2043	Oct. 01, 2043
8% Series, due 2026 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 01, 2026	Aug. 01, 2026
Medium-term notes, 7.59% series, due 2017 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	Jan. 17, 2017	Jan. 17, 2017
Medium-term notes, 7.78% series, due 2022 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	Feb. 03, 2022	Feb. 03, 2022
Medium-term notes, 7.92% series, due 2027 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	Jun. 04, 2027	Jun. 04, 2027
Medium-term notes, 6.76% series, due 2027 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	Sep. 24, 2027	Sep. 24, 2027
Tax-exempt Series A, due 2028 [Member] \| Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2028	Dec. 01, 2028
2003 Series A, due 2038 [Member] \| Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
2008 Series A, due 2038 [Member] \| Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
2009 Series A, due 2039 [Member] \| Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2039	Dec. 01, 2039
5.25% 2003 Series D, due 2038 [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			5.25%
5.25% 2004 Series A, due 2034 [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Jul. 01, 2034	Jul. 01, 2034
5.00% 2004 Series B, due 2033 [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	Dec. 01, 2033	Dec. 01, 2033
4.85% 2005 Series A, due 2035 [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	Oct. 01, 2035	Oct. 01, 2035
4.75% 2006 Series A, due 2036 [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	Sep. 01, 2036	Sep. 01, 2036

Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended			3 Months Ended		12 Months Ended
	Dec. 31, 2014	Nov. 30, 2014	Oct. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Line of credit designated for working capital purposes	$ 150,000,000					$ 150,000,000
Minimum current borrowing usage				$ 0	$ 0
Maximum current borrowing usage	165,000,000	$ 165,000,000
Redemption of long term debt						$ 139,155,000	$ 137,013,000	$ 427,043,000
Debt instrument covenant description						Certain Company debt instruments contain securities ratings covenants that, if set in motion, would increase financing costs. Certain debt instruments also have leverage ratio caps and minimum net worth requirements. At December 31, 2014, the Company is in compliance with all of its covenants.
Issuance of additional debt						$ 1,900,000,000
Minimum net worth						$ 900,000,000
Previous Revolving credit facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			$ 75,000,000
Credit facility expiration date			Jun. 30, 2015
Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			$ 300,000,000
Credit facility expiration date			Oct. 31, 2019
Effective interest rate						3.31%
Minimum current borrowing usage	199,000,000
Maximum current borrowing usage			$ 232,000,000
Term loans	142,000,000					$ 142,000,000
Secured Revolving Credit and Term Loan Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Commitment fee			0.40%
Secured Revolving Credit and Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Commitment fee			0.15%
Short-term credit facility [Member]
Debt Instrument [Line Items]
Credit facility outstanding	5,000,000					$ 5,000,000	$ 0
Centuri Construction Group [Member]
Debt Instrument [Line Items]
Debt secured by assets			$ 477,000,000
Debt instrument covenant description						Certain Centuri debt instruments have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2014, Centuri is in compliance with all of its covenants.
Issuance of additional debt						$ 88,000,000
Equity cushion relating to minimum fixed charge ratio coverage requirement						$ 35,000,000
LIBOR [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin			2.25%
LIBOR [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin			1.00%
CDOR [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin			2.25%
CDOR [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin			1.00%
Alternate Base Rate or Canadian Base Rate [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin			1.25%
Alternate Base Rate or Canadian Base Rate [Member] \| Secured Revolving Credit and Term Loan Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Applicable margin			0.00%
5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.25%
Redemption of long term debt		$ 65,000,000
Debt instrument original due date						Jul. 01, 2034
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit designated as long term debt					150,000,000
Line of credit designated for working capital purposes					150,000,000
Credit facility maximum borrowing capacity					$ 300,000,000
Credit facility expiration date						Mar. 01, 2019
Long-term portion of the credit facility	150,000,000					$ 150,000,000
Effective interest rate						1.21%
Southwest Gas Credit Facility [Member] \| Before Amendment [Member]
Debt Instrument [Line Items]
Credit facility expiration date						Mar. 01, 2017
Southwest Gas Credit Facility [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin						1.00%
Southwest Gas Credit Facility [Member] \| Alternative base rate [Member]
Debt Instrument [Line Items]
Applicable margin						0.00%
Commercial paper program [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	50,000,000					$ 50,000,000
Credit facility outstanding	$ 50,000,000					$ 50,000,000
Commercial paper program [Member] \| Short-term credit facility [Member]
Debt Instrument [Line Items]
Effective interest rate						0.65%
Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate					6.10%
Notes payable					$ 125,000,000
Notes, 6.1%, due 2041 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Percentage limit on liens securing indebtedness					10.00%

Long-Term Debt - Summary of Effective Interest Rates on Variable-Rate IDRBs (Detail)	Dec. 31, 2014	Dec. 31, 2013
2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Effective interest rates	0.85%	1.43%
2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Effective interest rates	0.90%	1.41%
2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Effective interest rates	0.89%	1.01%
Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Effective interest rates	0.84%	1.07%

Long-Term Debt - Estimated Maturities of Long-Term Debt (Detail) $ in Thousands	Dec. 31, 2014 USD ($)
Debt Disclosure [Abstract]
2015	$ 19,192
2016	20,991
2017	43,899
2018	16,985
2019	$ 304,328

Short-Term Debt - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Working capital purposes	$ 150,000,000
Short-term borrowings outstanding on the credit facility	$ 5,000,000	$ 0
Effective interest rate on short-term borrowings	1.13%
March 2019 [Member]
Short-term Debt [Line Items]
Current borrowing capacity	$ 300,000,000
Short-term debt expiration date	Mar. 01, 2019

Commitments and Contingencies - Additional Information (Detail) $ in Millions	Dec. 31, 2014 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Self-insured retention amount associated with general liability claims	$ 1
Additional self-insured retention amount of general liability	$ 4

Pension and Other Postretirement Benefits - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2014 USD ($) BasisPoint
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Maximum contribution of employer	3.50%
Deferment of annual cash compensation	100.00%
Matching contribution of employer	3.50%
Deferred compensation payments on retirement	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.
Additional Deferred Compensation Payment Options, period of payment, years	5 years
Deferred compensation, percentage of multiple interest rate	150.00%
Increments of changes to the discount rate, basis points \| BasisPoint	25
Estimated increase in pension expense	$ 10
Future estimated funding amount	39
Future estimated funding amount of retirement plan	$ 36
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption	6.00%
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption	5.00%

Pension and Other Postretirement Benefits - Cost of Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Employee Investment Plan cost	$ 4,816	$ 4,850	$ 4,707

Pension and Other Postretirement Benefits - Schedule of Allocation of Plan Assets (Detail)	12 Months Ended
	Dec. 31, 2014
Equity securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Investment percentage range, minimum	59.00%
Investment percentage range, maximum	71.00%
Debt securities [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Investment percentage range, minimum	31.00%
Investment percentage range, maximum	37.00%
Other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Investment percentage range, maximum	5.00%

Pension and Other Postretirement Benefits - Schedule of Assumptions Used (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Discount rate	4.25%	5.00%
Weighted-average rate of compensation increase	2.75%	3.25%
Asset return assumption	7.75%	7.75%

Pension and Other Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet and Income Statement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	$ 173,646	$ (100,345)	$ 74,853
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	886,714	902,812
Service cost	21,360	23,056	20,319
Interest cost	43,440	37,607	38,266
Benefit obligation at end of year (PBO/PBO/APBO)	1,060,240	886,714	902,812
Market value of plan assets, beginning balance	719,944	609,750
Actual return on plan assets	34,732	96,187
Employer contributions	36,000	46,000
Market value of plan assets, ending balance	754,796	719,944	609,750
Funded status at year end	(305,444)	(166,770)
Qualified Retirement Plan [Member] \| Change in benefit obligations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	144,606	(44,768)
Benefits paid	(35,880)	(31,993)
Qualified Retirement Plan [Member] \| Change in plan assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Benefits paid	$ (35,880)	$ (31,993)
Qualified Retirement Plan [Member] \| Benefit obligation [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	4.25%	5.00%
Weighted-average rate of compensation increase	2.75%	3.25%
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 36,143	$ 37,373
Service cost	292	373	274
Interest cost	1,745	1,535	1,629
Benefit obligation at end of year (PBO/PBO/APBO)	41,176	36,143	37,373
Employer contributions	2,463	2,477
Funded status at year end	(41,176)	(36,143)
SERP [Member] \| Change in benefit obligations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	5,459	(661)
Benefits paid	(2,463)	(2,477)
SERP [Member] \| Change in plan assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Benefits paid	$ (2,463)	$ (2,477)
SERP [Member] \| Benefit obligation [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	4.25%	5.00%
Weighted-average rate of compensation increase	2.75%	3.25%
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 58,020	$ 59,704
Service cost	1,101	1,220	977
Interest cost	2,829	2,482	2,547
Plan amendments	6,661
Benefit obligation at end of year (PBO/PBO/APBO)	72,202	58,020	59,704
Market value of plan assets, beginning balance	42,314	35,250
Actual return on plan assets	2,859	7,319
Employer contributions		169
Market value of plan assets, ending balance	44,892	42,314	$ 35,250
Funded status at year end	(27,310)	(15,706)
PBOP [Member] \| Change in benefit obligations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	4,567	(4,073)
Benefits paid	(976)	(1,313)
PBOP [Member] \| Change in plan assets [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Benefits paid	$ (281)	$ (424)
PBOP [Member] \| Benefit obligation [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	4.25%	5.00%

Pension and Other Postretirement Benefits - Schedule of Accumulated Benefit Obligation (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation	$ 886,215	$ 794,919
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Accumulated benefit obligation	$ 39,125	$ 33,894

Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail) $ in Millions	Dec. 31, 2014 USD ($)
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
2015	$ 39.1
2016	41.3
2017	43.1
2018	45.3
2019	47.5
2020-2024	272.7
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
2015	3.3
2016	3.6
2017	3.8
2018	4.0
2019	4.1
2020-2024	20.4
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
2015	2.6
2016	2.6
2017	2.6
2018	2.6
2019	2.6
2020-2024	$ 13.0

Pension and Other Postretirement Benefits - Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net periodic benefit cost	$ 38,172	$ 48,141	$ 42,292
Discount rate	4.25%	5.00%
Expected return on plan assets	7.75%	7.75%
Weighted-average rate of compensation increase	2.75%	3.25%
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 21,360	$ 23,056	20,319
Interest cost	43,440	37,607	38,266
Expected return on plan assets	(53,342)	(49,840)	(45,780)
Amortization of net actuarial loss	22,873	32,261	23,883
Net periodic benefit cost	$ 34,331	$ 43,084	$ 36,688
Qualified Retirement Plan [Member] \| Net benefit cost [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	5.00%	4.25%	5.00%
Expected return on plan assets	7.75%	8.00%	8.00%
Weighted-average rate of compensation increase	3.25%	2.75%	3.00%
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 292	$ 373	$ 274
Interest cost	1,745	1,535	1,629
Amortization of net actuarial loss	783	971	683
Net periodic benefit cost	$ 2,820	$ 2,879	$ 2,586
SERP [Member] \| Net benefit cost [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	5.00%	4.25%	5.00%
Expected return on plan assets	7.75%	8.00%	8.00%
Weighted-average rate of compensation increase	3.25%	2.75%	3.00%
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 1,101	$ 1,220	$ 977
Interest cost	2,829	2,482	2,547
Expected return on plan assets	(3,264)	(2,824)	(2,404)
Amortization of prior service cost	355	355
Amortization of transition obligation			867
Amortization of net actuarial loss		945	1,031
Net periodic benefit cost	$ 1,021	$ 2,178	$ 3,018
PBOP [Member] \| Net benefit cost [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	5.00%	4.25%	5.00%
Expected return on plan assets	7.75%	8.00%	8.00%

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, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	$ 173,646	$ (100,345)	$ 74,853
Amortization of prior service cost	(355)	(355)
Amortization of transition obligation			(867)
Amortization of net actuarial loss	(23,656)	(34,177)	(25,597)
Prior service cost	6,661		2,423
Regulatory adjustment	(140,308)	123,630	(42,771)
Recognized in other comprehensive (income) loss	15,988	(11,247)	8,041
Net periodic benefit costs recognized in net income	38,172	48,141	42,292
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	54,160	36,894	50,333
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	163,215	(91,115)	70,016
Amortization of net actuarial loss	(22,872)	(32,261)	(23,883)
Regulatory adjustment	(129,031)	113,762	(41,520)
Recognized in other comprehensive (income) loss	11,312	(9,614)	4,613
Net periodic benefit costs recognized in net income	34,331	43,084	36,688
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	45,643	33,470	41,301
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	5,460	(662)	4,111
Amortization of net actuarial loss	(784)	(971)	(683)
Recognized in other comprehensive (income) loss	4,676	(1,633)	3,428
Net periodic benefit costs recognized in net income	2,820	2,879	2,586
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	7,496	1,246	6,014
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	4,971	(8,568)	726
Amortization of prior service cost	(355)	(355)
Amortization of transition obligation			(867)
Amortization of net actuarial loss		(945)	(1,031)
Prior service cost	6,661		2,423
Regulatory adjustment	(11,277)	9,868	(1,251)
Net periodic benefit costs recognized in net income	1,021	2,178	3,018
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$ 1,021	$ 2,178	$ 3,018

Pension and Other Postretirement Benefits - Schedule of Fair Value of Plan Assets (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Amounts not included in funded status table	$ 8,572,000	$ 7,842,000
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Amounts not included in funded status table	8,312,000	7,581,000
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Amounts not included in funded status table	$ 260,000	$ 261,000
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Redemption notice period	1 day
Impact fee on redemptions and subscriptions of $5 million or more	$ 5,000,000
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Redemption notice period		30 days

Pension and Other Postretirement Benefits - Schedule of Effect of Significant Unobservable Inputs (Detail) - Commingled equity funds [Member] - Level 3 - Significant unobservable inputs [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Beginning balance	$ 195,070	$ 127,467
Actual return on plan assets, Relating to assets still held/Sold during the period	(6,932)	21,903
Purchases	82,615	45,700
Sales	(5,900)
Settlements	0	0
Transfers in and/or out of Level 3	0	0
Ending balance	266,922	$ 195,070
Realized [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Actual return on plan assets, Relating to assets still held/Sold during the period	$ 2,069

Stock-Based Compensation - Schedule of Stock-Based Plan Compensation Expense, Including Cash Award (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation plan expense, net of related tax benefits	$ 8,130	$ 8,012	$ 7,396
Stock-based compensation plan related tax benefits	$ 4,983	$ 4,910	$ 4,533

Stock-Based Compensation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 $ / shares	Dec. 31, 2012 $ / shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options, maximum term, years	10 years
Weighted-average remaining contractual life of outstanding options, years	1 year
Cash received from exercise of stock options	$ 379,000
Tax benefit realized from exercise of stock options	$ 167,000
Percentage of restricted stock/units vested during first year	0.40
Percentage of restricted stock/units vested during second and third years	0.30
Average grant date fair value of performance shares and restricted stock/units granted \| $ / shares		$ 44.83	$ 41.34
Total compensation cost related to nonvested performance shares and restricted stock/units not yet recognized	$ 2,900,000

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options, Outstanding at the beginning of the year	52	125	177
Number of options, Exercised during the year	(16)	(72)	(52)
Number of options, Forfeited or expired during the year		(1)
Number of options, Outstanding and exercisable at year end	36	52	125
Weighted-average exercise price, Outstanding at the beginning of the year	$ 27.57	$ 28.13	$ 27.28
Weighted-average exercise price, Exercised during the year	24.31	28.44	25.25
Weighted-average exercise price, Forfeited or expired during the year		33.07
Weighted-average exercise price, Outstanding and exercisable at year end	$ 28.97	$ 27.57	$ 28.13

Stock-Based Compensation - Schedule of Aggregate Intrinsic Value of Outstanding and Exercisable Options (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Outstanding and exercisable	$ 1,194	$ 1,473	$ 1,788
Exercised	$ 451	$ 1,402	$ 928

Stock-Based Compensation - Summary of Market Prices of Common Stock (Detail) - USD ($)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Market value of Southwest Gas stock	$ 61.81	$ 55.91	$ 42.41

Stock-Based Compensation - Schedule of Stock Options Outstanding and Exercisable (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2014 $ / shares shares
$25.0 to $26.10 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower limit	$ 25.00
Range of Exercise Price, Upper limit	$ 26.10
Number outstanding \| shares	17
Weighted-remaining average contractual life, years	6 months
Weighted-average exercise price	$ 25.71
$29.08 to $33.07 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Exercise Price, Lower limit	29.08
Range of Exercise Price, Upper limit	$ 33.07
Number outstanding \| shares	19
Weighted-remaining average contractual life, years	1 year 6 months
Weighted-average exercise price	$ 31.83

Stock-Based Compensation - Schedule of Nonvested Performance and Restricted Stock Unit Plans (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value, Granted		$ 44.83	$ 41.34
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested/unissued, Number of Units, at beginning of year	323
Nonvested, Number of Units, Granted	77
Nonvested, Number of Units, Dividends	7
Nonvested, Number of Units, Forfeited or expired	0
Nonvested, Number of Units, Vested and issued	(136)
Nonvested/unissued, Number of Units, Ending balance	271	323
Weighted-average grant date fair value, Beginning balance	$ 39.16
Weighted-average grant date fair value, Granted	53.73
Weighted-average grant date fair value, Dividends
Weighted-average grant date fair value, Forfeited or expired	0
Weighted-average grant date fair value, Vested and issued	36.23
Weighted-average grant date fair value, Ending balance	$ 43.71	$ 39.16
Restricted Stock/Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested/unissued, Number of Units, at beginning of year	245
Nonvested, Number of Units, Granted	83
Nonvested, Number of Units, Dividends	7
Nonvested, Number of Units, Forfeited or expired	0
Nonvested, Number of Units, Vested and issued	(78)
Nonvested/unissued, Number of Units, Ending balance	257	245
Weighted-average grant date fair value, Beginning balance	$ 38.00
Weighted-average grant date fair value, Granted	53.73
Weighted-average grant date fair value, Dividends
Weighted-average grant date fair value, Forfeited or expired	0
Weighted-average grant date fair value, Vested and issued	40.87
Weighted-average grant date fair value, Ending balance	$ 41.22	$ 38.00

Income Taxes - Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Line Items]
Total income before income taxes	$ 219,521	$ 222,815	$ 207,915
Foreign [Member]
Income Tax [Line Items]
Total income before income taxes	(1,950)
United States [Member]
Income Tax [Line Items]
Total income before income taxes	$ 221,471	$ 222,815	$ 207,915

Income Taxes - Summary of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$ 1,739	$ 3,549	$ 2,296
State	5,073	5,107	5,744
Foreign	2,193
Income tax expense (benefit), Current	9,005	8,656	8,040
Deferred:
Federal	71,439	67,414	65,551
State	614	1,872	1,685
Foreign	(2,685)
Total deferred income tax expense	69,368	69,286	67,236
Total income tax expense	$ 78,373	$ 77,942	$ 75,276

Income Taxes - Significant Components of Deferred Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred federal and state:
Total deferred federal and state	$ 70,229	$ 70,147	$ 68,097
Deferred ITC, net	(861)	(861)	(861)
Total deferred income tax expense	69,368	69,286	67,236
Property-related items [Member]
Deferred federal and state:
Total deferred federal and state	52,814	62,737	64,249
Purchased gas cost adjustments [Member]
Deferred federal and state:
Total deferred federal and state	15,049	16,189	1,755
Employee benefits [Member]
Deferred federal and state:
Total deferred federal and state	109	(2,769)	564
All other deferred [Member]
Deferred federal and state:
Total deferred federal and state	$ 2,257	$ (6,010)	$ 1,529

Income Taxes - Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	35.00%	35.00%	35.00%
Net state taxes	1.90%	2.40%	2.60%
Property-related items	0.10%	0.10%	0.20%
Tax credits	(0.50%)	(0.40%)	(0.40%)
Company owned life insurance	(1.00%)	(2.10%)	(1.30%)
All other differences	0.20%		0.10%
Consolidated effective income tax rate	35.70%	35.00%	36.20%

Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Deferred income taxes for future amortization of ITC	$ 2,146	$ 2,679
Employee benefits	31,557	25,591
Alternative minimum tax credit	20,172	19,739
Net operating losses and credits	9,719	15,113
Interest rate swap	8,622	9,893
Other	25,872	22,334
Valuation allowance	(253)	(200)
Deferred tax assets, total	97,835	95,149
Deferred tax liabilities:
Property-related items, including accelerated depreciation	736,810	694,024
Regulatory balancing accounts	33,736	18,688
Property-related items previously flowed through	28	836
Unamortized ITC	3,410	4,271
Debt-related costs	5,066	4,713
Intangibles	12,792
Other	27,572	15,898
Deferred tax liabilities, total	819,414	738,430
Net deferred tax liabilities	721,579	643,281
Current	(2,109)	(31,130)
Noncurrent	723,688	674,411
Net deferred tax liabilities	$ 721,579	$ 643,281

Income Taxes - Additional Information (Detail)	12 Months Ended
Income Taxes - Additional Information (Detail)	Dec. 31, 2014 USD ($)
Income Tax Disclosure [Line Items]
Federal general business credit carryforward	$ 711,000
Federal net operating loss carryforward, expiration year	2031
Federal general business credits, expiration year	2031
Net operating loss carryforward expiration year	2034
Undistributed foreign earnings	$ 0
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	0
Significant increases or decreases in unrecognized tax benefit within the next 12 months	$ 0
Minimum [Member]
Income Tax Disclosure [Line Items]
Amount of benefit being recognized in financial statements upon ultimate settlement, Percentage	50.00%
Canada [Member]
Income Tax Disclosure [Line Items]
Federal net operating loss carryforward	$ 1,100,000
Canada [Member] \| Tax Year 2011 [Member]
Income Tax Disclosure [Line Items]
Tax examination, year	2011
Canada [Member] \| Tax Year 2012 [Member]
Income Tax Disclosure [Line Items]
Tax examination, year	2012
United States [Member]
Income Tax Disclosure [Line Items]
Federal net operating loss carryforward	$ 28,000,000
Federal net capital loss carryforward	$ 494,000

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of year	$ 0
Gross increases-tax positions in prior period	305
Gross decreases-tax positions in prior period	0	$ 0
Gross increases-current period tax positions	0	0
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	$ 305	$ 0

Income Taxes - Summary of Tax-Related Interest Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax-related interest income			$ 24

Derivatives and Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%
Gain or loss (ineffective portion) recognized in income	$ 0
Senior Notes, 3.875%, Due 2022 [Member]
Derivative [Line Items]
Agreement settlement paid	$ 21,800,000
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Jan. 01, 2015
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Mar. 31, 2016

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU MMBTU in Thousands	Dec. 31, 2014	Dec. 31, 2013
Offsetting [Abstract]
Contract notional amounts	5,105	13,571

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - Cash flow hedging [Member] - Net cost of gas sold [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (2,363)	$ 976	$ (4,854)
Regulatory deferral accounting treatment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 2,363	$ (976)	$ 4,854

Derivatives and Fair Values Measurements - Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Amount of gain/(loss) realized/unrealized on FSIRS recognized in other comprehensive income on derivative			$ 2,959

Derivatives and Fair Value Measurements - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		$ 1,444
Liability derivatives not designated as hedging instruments	$ (5,425)	(623)
Net total not designated as hedging instruments	(5,425)	821
Swaps [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		126
Liability derivatives not designated as hedging instruments	(5,062)	(282)
Net total not designated as hedging instruments	(5,062)	(156)
Swaps [Member] \| Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		7
Liability derivatives not designated as hedging instruments	(363)	(11)
Net total not designated as hedging instruments	$ (363)	(4)
Swaps [Member] \| Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		257
Liability derivatives not designated as hedging instruments		(77)
Net total not designated as hedging instruments		180
Swaps [Member] \| Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		1,054
Liability derivatives not designated as hedging instruments		(253)
Net total not designated as hedging instruments		$ 801

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 829	$ 3,148	$ 14,843
Received from counterparties	$ 4,713	$ 915	$ 634

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 5,062	$ 156
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 363	4
Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Net Total		(180)
Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ (801)

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant other observable inputs [Member] - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (5,425)	$ 821
Swaps [Member] \| Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets		801
Swaps [Member] \| Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets		180
Swaps [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(5,062)	(156)
Swaps [Member] \| Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (363)	$ (4)

Segment Information - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Centuri Construction Group [Member]
Segment Reporting Information [Line Items]
Accounts receivable for Centuri services	$ 9,169	$ 10,787

Segment Information - Schedule of Revenues and Long-Lived Assets by Geographic Area (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total Revenues	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 538,357	$ 387,346	$ 411,574	$ 613,505	$ 488,566	$ 371,799	$ 409,768	$ 657,645	$ 2,121,707	$ 1,950,782	$ 1,927,778
Total Long-lived assets	6,035,951				5,674,449								6,035,951	5,674,449
United States [Member]
Segment Reporting Information [Line Items]
Total Revenues													2,069,513	1,950,782
Total Long-lived assets	6,021,476				$ 5,674,449								6,021,476	$ 5,674,449
Canada [Member]
Segment Reporting Information [Line Items]
Total Revenues													52,194
Total Long-lived assets	$ 14,475												$ 14,475

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total operating revenues	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 538,357	$ 387,346	$ 411,574	$ 613,505	$ 488,566	$ 371,799	$ 409,768	$ 657,645	$ 2,121,707	$ 1,950,782	$ 1,927,778
Interest revenue													2,602	461	924
Interest expense													72,069	63,700	68,020
Depreciation and amortization													253,027	236,817	223,422
Income tax expense													78,373	77,942	75,276
Segment net income	58,746	$ 1,970	$ 9,627	$ 70,783	57,303	$ (2,864)	$ 10,108	$ 80,773	62,393	$ (4,305)	$ (3,676)	$ 78,919	141,126	145,320	133,331
Segment assets	5,214,515				4,565,174				4,488,057				5,214,515	4,565,174	4,488,057
Capital expenditures													396,898	364,276	395,712
Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													2,029,519	1,862,629	1,844,404
Intersegment sales [Member]
Segment Reporting Information [Line Items]
Revenues													92,188	88,153	83,374
Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues													1,382,087	1,300,154	1,321,728
Interest revenue													2,596	456	915
Interest expense													68,299	62,555	66,957
Depreciation and amortization													204,144	193,848	186,035
Income tax expense													63,597	65,377	64,973
Segment net income													116,872	124,169	116,619
Segment assets	4,657,709				4,272,029				4,204,948				4,657,709	4,272,029	4,204,948
Capital expenditures													350,025	314,578	308,951
Gas Operations [Member] \| Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													1,382,087	1,300,154	1,321,728
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues													739,620	650,628	606,050
Interest revenue													6	5	9
Interest expense													3,770	1,145	1,063
Depreciation and amortization													48,883	42,969	37,387
Income tax expense													14,776	12,565	10,303
Segment net income													24,254	21,151	16,712
Segment assets	567,405				293,811				$ 283,109				567,405	293,811	283,109
Capital expenditures													46,873	49,698	86,761
Construction Services [Member] \| Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													647,432	562,475	522,676
Construction Services [Member] \| Intersegment sales [Member]
Segment Reporting Information [Line Items]
Revenues													92,188	88,153	$ 83,374
Adjustments [Member]
Segment Reporting Information [Line Items]
Segment assets	$ (10,599)				$ (666)								$ (10,599)	$ (666)

Segment Information - Schedule of Segment Reporting Information (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Income tax payable netted		$ 666,000
Gas Operations [Member]
Segment Reporting Information [Line Items]
Deferred income tax liability	$ 4,500,000
Construction Services [Member]
Segment Reporting Information [Line Items]
Income tax payable netted	3,300,000
Deferred income tax liability	1,400,000
Long-term deferred tax benefit	$ 1,400,000

Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 538,357	$ 387,346	$ 411,574	$ 613,505	$ 488,566	$ 371,799	$ 409,768	$ 657,645	$ 2,121,707	$ 1,950,782	$ 1,927,778
Operating income	112,373	18,290	26,755	127,065	100,772	6,141	28,908	138,394	115,211	6,310	15,380	134,623	284,483	274,215	271,524
Net income (loss)	58,897	1,927	9,627	70,697	57,189	(3,057)	10,067	80,674	62,106	(4,414)	(3,888)	78,835	141,148	144,873	132,639
Net income (loss) attributable to Southwest Gas Corporation	$ 58,746	$ 1,970	$ 9,627	$ 70,783	$ 57,303	$ (2,864)	$ 10,108	$ 80,773	$ 62,393	$ (4,305)	$ (3,676)	$ 78,919	$ 141,126	$ 145,320	$ 133,331
Basic earnings (loss) per common share	$ 1.26	$ 0.04	$ 0.21	$ 1.52	$ 1.24	$ (0.06)	$ 0.22	$ 1.75	$ 1.35	$ (0.09)	$ (0.08)	$ 1.71	$ 3.04	$ 3.14	$ 2.89
Diluted earnings (loss) per common share	$ 1.25	$ 0.04	$ 0.21	$ 1.51	$ 1.22	$ (0.06)	$ 0.22	$ 1.73	$ 1.34	$ (0.09)	$ (0.08)	$ 1.70	$ 3.01	$ 3.11	$ 2.86

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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

Acquisition of Construction Services Businesses - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Oct. 31, 2014 Business	Dec. 31, 2014 USD ($) Business	Oct. 01, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Business acquisition, consideration		$ 221,000,000
Number of business acquisitions \| Business	3	3
Goodwill		$ 143,160,000		$ 17,810,000
Construction Services [Member]
Business Acquisition [Line Items]
Goodwill		133,000,000
Brigadier [Member]
Business Acquisition [Line Items]
Goodwill assigned for deductible tax purpose		4,900,000
Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Acquisition [Line Items]
Goodwill		125,000,000	$ 130,000,000
Intangible assets		48,200,000
Foreign currency adjustments		1,900,000
Accumulated amortization		1,500,000
Incurred acquisition costs		$ 5,000,000
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Indirect equity interest percentage		100.00%
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Non-Competition Agreements [Member]
Business Acquisition [Line Items]
Intangible assets		$ 500,000
Accumulated amortization		$ 40,000
Intangible assets useful life		5 years
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Customer Relationships [Member]
Business Acquisition [Line Items]
Intangible assets		$ 36,500,000
Accumulated amortization		$ 550,000
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Customer Relationships [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Intangible assets useful life		12 years
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Customer Relationships [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Intangible assets useful life		21 years
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Trade Names [Member]
Business Acquisition [Line Items]
Intangible assets		$ 10,000,000
Accumulated amortization		$ 250,000
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Trade Names [Member] \| Minimum [Member]
Business Acquisition [Line Items]
Intangible assets useful life		4 years
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Trade Names [Member] \| Maximum [Member]
Business Acquisition [Line Items]
Intangible assets useful life		20 years
Link-Line, W.S. Nicholls, and Brigadier [Member] \| Work Backlog [Member]
Business Acquisition [Line Items]
Intangible assets		$ 1,200,000
Accumulated amortization		$ 700,000
Intangible assets useful life		8 months
Centuri Construction Group [Member]
Business Acquisition [Line Items]
Indirect equity interest convertible for percentage of holding company equity interest		96.60%
Principal Previous Owners [Member] \| Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Acquisition [Line Items]
Indirect equity interest percentage		10.00%
Indirect equity interest dividend rights, percentage of cash dividend paid by Centuri to the Company		3.40%
Description of indirect equity interest		However, while the actual ownership interest was approximately 10% of Lynxus (Canadian operations), the terms of the underlying equity agreements include dividend participation rights equal to 3.4% of dividends declared at the level of Centuri. Additionally, these same agreements include, among other terms, the ability of the prior owners to exit their investment retained by requiring Centuri to purchase a portion of their interest (in Lynxus) commencing October 2016 and in incremental amounts each anniversary date thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election after September 2021. Furthermore, the equity agreements include an exchange feature such that the noncontrolling ownership interest retained by the parties, in the Canadian subsidiaries, may be convertible into shares equivalent to a 3.4% interest in Centuri (redeemable noncontrolling interest). In consideration of these circumstances and the underlying agreements, it was deemed appropriate to allocate earnings to the redeemable noncontrolling interest at an amount equivalent to approximately 3.4% of total Centuri earnings (rather than based on the actual current proportional ownership). Through this earnings allocation process, approximately 96.6% of Centuri earnings are attributable to the Company.
Principal Previous Owners [Member] \| Link-Line, W.S. Nicholls, and Brigadier [Member] \| Construction Services [Member]
Business Acquisition [Line Items]
Indirect equity interest convertible for percentage of holding company equity interest		3.40%
Principal Previous Owners [Member] \| Centuri Construction Group [Member]
Business Acquisition [Line Items]
Indirect equity interest percentage		10.00%

Acquisition of Construction Services Businesses - Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Dec. 31, 2014	Oct. 01, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 143,160		$ 17,810
Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Acquisition [Line Items]
Cash, cash equivalents, and restricted cash		$ 3,000
Contracts receivable and other receivables		62,000
Property, plant and equipment		17,000
Other assets		17,000
Intangible assets		52,000
Goodwill	$ 125,000	130,000
Total assets acquired		281,000
Current liabilities		39,000
Deferred income tax-long-term		17,000
Other long-term liabilities		4,000
Net assets acquired		$ 221,000

Acquisition of Construction Services Businesses - Schedule of Estimated Future Amortization of Intangible Assets Acquired (Detail) - Link-Line, W.S. Nicholls, and Brigadier [Member] $ in Thousands	Dec. 31, 2014 USD ($)
Finite-Lived Intangible Assets [Line Items]
2015	$ 4,615
2016	3,371
2017	3,371
2018	3,123
2019	$ 2,370

Acquisition of Construction Services Businesses - Schedule of Unaudited Pro Forma Consolidated Financial Information (Detail) - Link-Line, W.S. Nicholls, and Brigadier [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Total operating revenues	$ 2,295,318	$ 2,203,272
Net income attributable to Southwest Gas Corporation	$ 149,588	$ 143,424
Basic earnings per share	$ 3.22	$ 3.10
Diluted earnings per share	$ 3.19	$ 3.07

Acquisition of Construction Services Businesses - Schedule of Actual Results Included In Consolidated Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Business Combination, Separately Recognized Transactions [Line Items]
Construction revenues													$ 739,620	$ 650,628	$ 606,050
Net income attributable to Link-Line, W.S. Nicholls, and Brigadier	$ 58,746	$ 1,970	$ 9,627	$ 70,783	$ 57,303	$ (2,864)	$ 10,108	$ 80,773	$ 62,393	$ (4,305)	$ (3,676)	$ 78,919	141,126	$ 145,320	$ 133,331
Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Construction revenues													54,264
Net income attributable to Link-Line, W.S. Nicholls, and Brigadier													$ 1,859

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