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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2016	Oct. 28, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2016
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q3
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS CORP
Entity Central Index Key	0000092416
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,482,068

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Utility plant:
Gas plant	$ 6,118,900	$ 5,854,917
Less: accumulated depreciation	(2,158,099)	(2,084,007)
Acquisition adjustments, net	234	370
Construction work in progress	105,847	119,805
Net utility plant	4,066,882	3,891,085
Other property and investments	348,565	313,531
Current assets:
Cash and cash equivalents	85,154	35,997
Accounts receivable, net of allowances	286,696	314,512
Accrued utility revenue	33,000	74,700
Income taxes receivable, net	38,369	34,175
Deferred purchased gas costs	0	3,591
Prepaids and other current assets	101,081	95,199
Total current assets	544,300	558,174
Noncurrent assets:
Goodwill	142,439	126,145
Deferred income taxes	783	428
Deferred charges and other assets	429,827	469,322
Total noncurrent assets	573,049	595,895
Total assets	5,532,796	5,358,685
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 47,482,068 and 47,377,575 shares)	49,112	49,007
Additional paid-in capital	901,966	896,448
Accumulated other comprehensive income (loss), net	(46,300)	(50,268)
Retained earnings	720,401	699,221
Total Southwest Gas Corporation equity	1,625,179	1,594,408
Noncontrolling interest	(2,263)	(2,083)
Total equity	1,622,916	1,592,325
Redeemable noncontrolling interest	17,379	16,108
Long-term debt, less current maturities	1,592,946	1,551,204
Total capitalization	3,233,241	3,159,637
Current liabilities:
Current maturities of long-term debt	49,480	19,475
Short-term debt	0	18,000
Accounts payable	138,810	164,857
Customer deposits	72,039	72,631
Income taxes payable	219	940
Accrued general taxes	46,323	47,337
Accrued interest	20,152	16,173
Deferred purchased gas costs	123,399	45,601
Other current liabilities	162,622	150,031
Total current liabilities	613,044	535,045
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	814,978	769,445
Accumulated removal costs	305,000	303,000
Other deferred credits and other long-term liabilities	566,533	591,558
Total deferred income taxes and other credits	1,686,511	1,664,003
Total capitalization and liabilities	$ 5,532,796	$ 5,358,685

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	47,482,068	47,377,575
Common stock, outstanding	47,482,068	47,377,575

Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Operating revenues:
Gas operating revenues	$ 200,179	$ 219,420	$ 980,927	$ 1,059,178	$ 1,376,388	$ 1,457,266
Construction revenues	339,790	285,976	838,038	719,042	1,127,982	948,637
Total operating revenues	539,969	505,396	1,818,965	1,778,220	2,504,370	2,405,903
Operating expenses:
Net cost of gas sold	39,056	64,268	324,072	427,045	460,836	570,052
Operations and maintenance	102,438	100,102	301,979	294,956	400,222	385,271
Depreciation and amortization	69,845	66,476	217,764	200,898	286,977	266,811
Taxes other than income taxes	12,480	11,652	39,480	37,063	51,810	50,059
Construction expenses	300,611	246,755	757,919	647,512	1,009,188	850,591
Total operating expenses	524,430	489,253	1,641,214	1,607,474	2,209,033	2,122,784
Operating income	15,539	16,143	177,751	170,746	295,337	283,119
Other income and (expenses):
Net interest deductions	(18,158)	(18,401)	(54,100)	(54,095)	(71,884)	(73,919)
Other income (deductions)	2,565	(3,660)	6,756	(1,226)	10,861	966
Total other income and (expenses)	(15,593)	(22,061)	(47,344)	(55,321)	(61,023)	(72,953)
Income (loss) before income taxes	(54)	(5,918)	130,407	115,425	234,314	210,166
Income tax expense (benefit)	(2,961)	(1,708)	43,046	42,693	80,255	78,537
Net income (loss)	2,907	(4,210)	87,361	72,732	154,059	131,629
Net income attributable to noncontrolling interests	435	524	500	534	1,079	685
Net income (loss) attributable to Southwest Gas Corporation	$ 2,472	$ (4,734)	$ 86,861	$ 72,198	$ 152,980	$ 130,944
Basic earnings (loss) per share	$ 0.05	$ (0.10)	$ 1.83	$ 1.54	$ 3.22	$ 2.80
Diluted earnings (loss) per share	0.05	(0.10)	1.82	1.53	3.20	2.77
Dividends declared per share	$ 0.450	$ 0.405	$ 1.350	$ 1.215	$ 1.755	$ 1.580
Average number of common shares outstanding	47,481	47,102	47,464	46,863	47,442	46,777
Average shares outstanding (assuming dilution)	47,830	0	47,802	47,261	47,787	47,194

Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 2,907	$ (4,210)	$ 87,361	$ 72,732	$ 154,059	$ 131,629
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(18,922)	(107,661)
Amortization of prior service cost	207	207	621	621	828	676
Amortization of net actuarial loss	4,196	5,329	12,586	15,987	17,915	19,654
Prior service cost	0	0	0	0	0	(4,130)
Regulatory adjustment	(3,796)	(4,828)	(11,388)	(14,484)	(404)	82,137
Net defined benefit pension plans	607	708	1,819	2,124	(583)	(9,324)
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	519	1,556	1,556	2,073	2,074
Net forward-starting interest rate swaps	518	519	1,556	1,556	2,073	2,074
Foreign currency translation adjustments	(238)	(510)	614	(1,573)	233	(2,232)
Total other comprehensive income (loss), net of tax	887	717	3,989	2,107	1,723	(9,482)
Comprehensive income (loss)	3,794	(3,493)	91,350	74,839	155,782	122,147
Comprehensive income (loss) attributable to noncontrolling interests	427	504	521	479	1,089	608
Comprehensive income (loss) attributable to Southwest Gas Corporation	$ 3,367	$ (3,997)	$ 90,829	$ 74,360	$ 154,693	$ 121,539

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 87,361	$ 72,732	$ 154,059	$ 131,629
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	217,764	200,898	286,977	266,811
Deferred income taxes	43,702	5,961	86,526	32,580
Changes in current assets and liabilities:
Accounts receivable, net of allowances	28,531	6,570	(17,889)	(31,792)
Accrued utility revenue	41,700	41,700	(800)	(200)
Deferred purchased gas costs	81,389	131,495	79,460	122,257
Accounts payable	(24,942)	(38,878)	10,445	5,137
Accrued taxes	(7,055)	(4,427)	(11,033)	(1,131)
Other current assets and liabilities	9,903	8,333	19,870	(11,938)
Gains on sale	(4,117)	(3,019)	(4,200)	(3,529)
Changes in undistributed stock compensation	4,347	2,119	5,142	5,480
AFUDC	(1,893)	(2,011)	(2,890)	(2,435)
Changes in other assets and deferred charges	3,926	(14,423)	4,183	(18,735)
Changes in other liabilities and deferred credits	(4,813)	5,348	702	2,783
Net cash provided by operating activities	475,803	412,398	610,552	496,917
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(404,388)	(336,569)	(555,819)	(440,191)
Acquisition of businesses, net of cash acquired	(17,000)	(9,261)	(17,000)	(199,758)
Restricted cash	0	785	0	2,018
Changes in customer advances	5,445	14,300	9,445	21,539
Miscellaneous inflows	7,965	11,593	4,726	13,424
Miscellaneous outflows	0	0	0	(1,400)
Net cash used in investing activities	(407,978)	(319,152)	(558,648)	(604,368)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	530	35,419	507	35,405
Dividends paid	(61,950)	(55,060)	(81,138)	(72,040)
Centuri distribution to redeemable noncontrolling interest	(99)	0	(198)	0
Issuance of long-term debt, net	408,946	123,816	420,946	355,325
Retirement of long-term debt	(196,351)	(143,325)	(240,999)	(260,952)
Change in credit facility and commercial paper	(150,000)	(53,000)	(97,000)	47,000
Change in short-term debt	(18,000)	(5,000)	0	0
Principal payments on capital lease obligations	(1,125)	(1,096)	(1,449)	(1,530)
Other	(605)	(504)	(60)	(1,002)
Net cash provided by (used in) financing activities	(18,654)	(98,750)	609	102,206
Effects of currency translation on cash and cash equivalents	(14)	(1,103)	(318)	(961)
Change in cash and cash equivalents	49,157	(6,607)	52,195	(6,206)
Cash and cash equivalents at beginning of period	35,997	39,566	32,959	39,165
Cash and cash equivalents at end of period	85,154	32,959	85,154	32,959
Supplemental information:
Interest paid, net of amounts capitalized	47,134	45,312	68,445	68,169
Income taxes paid	$ 6,530	$ 39,856	$ 9,899	$ 49,280

Nature of Operations and Basis of Presentation

9 Months Ended

Sep. 30, 2016

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of Operations and Basis of Presentation

Note 1 – Nature of Operations and Basis of Presentation

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. Results for the natural gas operations segment are higher during winter periods due to the seasonality incorporated in its regulatory rate structures. Centuri Construction Group, Inc. (“Centuri” or the “construction services” segment), a 96.6% 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. (“NPL”), NPL Canada Ltd. (“NPL Canada”, formerly Link-Line Contractors Ltd.), W.S. Nicholls Construction, Inc. and related companies (“W.S. Nicholls”), and Brigadier Pipelines Inc. (“Brigadier”). In May 2016, Centuri completed the acquisition of two privately held, affiliated construction businesses by means of asset purchase: Enterprise Trenchless Technologies, Inc. and ETTI Holdings (collectively, “ETTI”). See Acquisition of Construction Services Businesses below for more information. Typically, Centuri revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.

Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with United States generally accepted accounting principles (“U.S. GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2015 Annual Report to Shareholders, which is incorporated by reference into the 2015 Form 10-K.

During the third quarter of 2016, Centuri evaluated the estimated useful lives of its depreciable assets, and in so doing determined that certain equipment lives should be extended. This change in estimate reduced current-quarter depreciation by approximately $2 million.

Prepaids and other current assets. Prepaids and other current assets includes gas pipe materials and operating supplies of $30 million at September 30, 2016 and $24 million at December 31, 2015 (carried at weighted average cost), and also includes natural gas stored underground and liquefied natural gas, in addition to prepaid assets.

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 September 30, 2016 and December 31, 2015 also include money market fund investments totaling approximately $68.3 million and $250,000, respectively, 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.

Significant non-cash investing and financing activities for the natural gas operations segment included the following: Upon contract expiration, customer advances of approximately $3.6 million and $2.8 million, during the first nine months of 2016 and 2015, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.

Goodwill. Goodwill is assessed as of October each year for impairment, or otherwise, if circumstances indicate impairment to the carrying value of goodwill may have occurred. No such circumstances indicating impairment were deemed to have occurred in the first nine months of 2016. The business acquisition of ETTI was deemed an asset purchase for tax purposes. As a result, approximately $11 million in goodwill associated with ETTI is expected to be tax deductible.


(In thousands of dollars)	Natural Gas Operations		Construction Services		Consolidated
December 31, 2015	$	10,095	$	116,050	$	126,145
Additional goodwill from ETTI acquisition		—		10,726		10,726
Foreign currency translation adjustment		—		5,568		5,568

September 30, 2016	$	10,095	$	132,344	$	142,439

Acquisition of Construction Services Businesses. In May 2016, Centuri completed the acquisition of ETTI, which is based in Lisbon Falls, Maine, and has a primary focus on underground utility installation using horizontal directional drilling technology. The acquisition of ETTI will provide complementary operational support to Brigadier and expand operations into Maine. Neither the acquisition itself nor the impacts to assets and operations are material to the construction services segment or the Company.

Assets acquired in the transaction were recorded at their acquisition date fair values. The final purchase accounting has not yet been completed. Further refinement could occur; however, no material changes are expected. The preliminary estimated fair values of assets acquired as of May 6, 2016, the acquisition date, are as follows (in millions of dollars):


	Acquisition Date
Property, plant and equipment	$	4.3
Intangible assets		2.9
Goodwill		10.7

Total assets acquired	$	17.9

The purchase price consisted of $17 million in cash on the acquisition date with the remaining amount being deferred over four years.

Intercompany Transactions. Centuri recognizes revenues generated from contracts with Southwest (see Note 3 - Segment Information below). Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars):


	September 30, 2016		December 31, 2015
Centuri accounts receivable for services provided to Southwest	$	12,402	$	10,006

The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.

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


	September 30, 2016			December 31, 2015
Centuri property and equipment	$	460,165		$	423,369
Centuri accumulated provision for depreciation and amortization		(228,230	)		(221,028	)
Net cash surrender value of COLI policies		104,940			99,276
Other property		11,690			11,914

Total	$	348,565		$	313,531

Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars):


	Three Months Ended						Nine Months Ended						Twelve Months Ended
	September 30						September 30						September 30
	2016			2015			2016			2015			2016			2015
Change in COLI policies	$	2,300		$	(3,900	)	$	5,400		$	(2,600	)	$	7,500		$	(200	)
Interest income		522			624			1,280			1,375			2,078			2,096
Equity AFUDC		611			878			1,893			2,011			2,890			2,435
Foreign transaction gain (loss)		(3	)		(302	)		(22	)		(874	)		28			(1,052	)
Miscellaneous income and (expense)		(865	)		(960	)		(1,795	)		(1,138	)		(1,635	)		(2,313	)

Total other income (deductions)	$	2,565		$	(3,660	)	$	6,756		$	(1,226	)	$	10,861		$	966

Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by 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 values of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences.

Income taxes. The Company claimed a federal income tax credit, which resulted in a recognized benefit of approximately $1.6 million during the third quarter of 2016.

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. In accordance with the update, an entity will be required to identify the contract with the 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. In July 2015, the FASB approved a one-year deferral of the effective date (annual periods beginning after December 15, 2017) and permitted entities to adopt one year earlier (i.e., the original effective date) if they choose. In March, April, and May of 2016, the FASB issued the updates “Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net)”, “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing”, and “Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients”. The amendments in the first two updates, respectively, provide guidance when another party, along with the entity, is involved in providing a good or service to a customer, and provide clarification with regard to identifying performance obligations and of the licensing implementation guidance in Topic 606. The third update includes narrow-scope improvements to the guidance on collectability, noncash consideration, and completed contracts at transition. In addition, a practical expedient is provided for contract modifications at transition and an accounting policy election related to the presentation of sales taxes and other similar taxes collected from customers. The combined amendments do not change the core principle of the guidance in Topic 606. The Company plans to adopt all of these updates at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2018. The Company is evaluating what impact these updates might have on its consolidated financial statements and disclosures.

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 assess 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 and changes thereto are not expected to have a material impact on the Company’s disclosures.

In January 2016, the FASB issued the update “Financial Instruments – Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities” in order to improve the recognition and measurement of financial instruments. The update makes targeted improvements to existing U.S. GAAP by: 1) requiring equity investments to be measured at fair value with changes in fair value recognized in net income; 2) requiring the use of the exit price notion when measuring the fair value of financial instruments for disclosure purposes; 3) requiring separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements; 4) eliminating the requirement to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet; and 5) requiring a reporting entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in instrument-specific credit risk when the organization has elected to measure the liability at fair value in accordance with the fair value option for financial instruments. The update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. All entities can early adopt the provision to record fair value changes for financial liabilities under the fair value option resulting from instrument-specific credit risk in other comprehensive income. The Company is evaluating what impact, if any, this update might have on its consolidated financial statements and disclosures.

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

•

A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and

•

A right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term.

Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Though companies have historically been required to make disclosures regarding leases and of contractual obligations, leases (with terms longer than a year) will no longer exist off-balance sheet. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing, and operating leases) must apply a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The modified retrospective approach would not require any transition accounting for leases that expired before the earliest comparative period presented. Lessees and lessors may not apply a full retrospective transition approach. Early application is permitted. The Company currently plans to adopt the update at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2019. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.

In March 2016, the FASB issued the update “Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting”. The amendments are intended to improve the accounting for employee share-based payments and affect all organizations that issue share-based payment awards to their employees. The update requires the recording of all of the tax effects related to share-based payments at settlement (or expiration) through the income statement. Currently, tax benefits in excess of compensation cost (“windfalls”) are recorded in equity, and tax deficiencies (“shortfalls”) are recorded in equity to the extent of previous windfalls, and then recorded in the income statement. While the simplification will reduce some of the administrative complexities by eliminating the need to track a “windfall pool,” it will increase the volatility of income tax expense. The update also allows entities to withhold shares for the employee tax burden up to the employees’ maximum individual tax rate in the relevant jurisdiction without resulting in a liability classification of the award (currently such withholding is limited to the employer’s minimum statutory withholding). The update clarifies that all cash payments made to taxing authorities

on the employees’ behalf for withheld shares should be presented as financing activities on the statement of cash flows. Also, the update requires all tax-related cash flows resulting from share-based payments be reported as operating activities on the statement of cash flows, a change from the current requirement to present windfall tax benefits as an inflow from financing activities and an outflow from operating activities. The update is effective for fiscal years beginning after December 15, 2016, including interim periods within those fiscal years. Early adoption is permitted. The Company issues share-based payment awards to its employees and is evaluating the impacts this update might have on its consolidated financial statements and disclosures.

In June 2016, the FASB issued the update “Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments”. The update amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. For assets held at amortized cost basis, the update eliminates the “probable” threshold for initial recognition of credit losses in current U.S. GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net amount expected to be collected. For available for sale debt securities, credit losses should be measured in a manner similar to current U.S. GAAP, however the update will require that credit losses be presented as an allowance rather than as a write-down. This update affects entities holding financial assets and net investment in leases that are not accounted for at fair value through net income. The update affects loans, debt securities, trade receivables, net investments in leases, off-balance sheet credit exposures, reinsurance receivables, and any other financial assets not excluded from the scope that have the contractual right to receive cash. The update is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. All entities may adopt the amendments in this update earlier as of fiscal years beginning after December 15, 2018, including interim periods within those fiscal years. The Company is evaluating what impact, if any, this update might have on its consolidated financial statements and disclosures.

In August 2016, the FASB issued the update “Classification of Certain Cash Receipts and Cash Payments”. This update addresses the following eight specific cash flow issues: debt prepayment or debt extinguishment costs; settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing; contingent consideration payments made after a business combination; proceeds from the settlement of insurance claims; proceeds from the settlement of corporate-owned life insurance (“COLI”) policies; distributions received from equity method investees; beneficial interests in securitization transactions; and separately identifiable cash flows, including identification of the predominant nature in cases where cash receipts and payments have aspects of more than one class of cash flows. The update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The Company is evaluating the impacts this update might have on its consolidated cash flow statements and disclosures.

In October 2016, the FASB issued the update “Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory.” This update eliminates the current U.S. GAAP exception for all intra-entity sales of assets other than inventory. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax effects of that transaction are eliminated in consolidation. Any deferred tax asset that arises in the buyer’s jurisdiction would also be recognized at the time of the transfer. The update is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted; however, the guidance can only be adopted in the first interim period of a fiscal year. The modified retrospective approach will be required for transition to the new guidance, with a cumulative-effect adjustment recorded in retained earnings as of the beginning of the period of adoption. The Company is evaluating the impacts this update might have on its consolidated financial statements.

In October 2016, the FASB issued the update “Consolidation (Topic 810): Interests Held through Related Parties That Are under Common Control.” The amendments affect reporting entities that are required to evaluate whether they should consolidate a variable interest entity in certain situations involving entities under common control. The update is effective for fiscal and interim periods beginning after December 15, 2016. Early adoption is permitted. The Company is evaluating what impacts, if any, this update might have on its consolidated financial statements.

Components of Net Periodic Benefit Cost

9 Months Ended

Sep. 30, 2016

Text Block [Abstract]

Components of Net Periodic Benefit Cost

Note 2 – Components of Net Periodic Benefit Cost

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.

During the third quarter of 2016, qualifying term-vested participants were offered a lump-sum present value payout of their pensions. The offer was primarily intended to reduce insurance and ongoing maintenance costs associated with qualifying participant balances. About one-half of the approximate 800 participants subject to the offer accepted the offer, resulting in an approximate $30 million payment from pension assets to be made in the fourth quarter of 2016. This lump sum payout will have no impact on net periodic benefit cost or pension funding requirements during 2016.

Net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets.


	Qualified Retirement Plan
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	5,708		$	6,281		$	17,125		$	18,842		$	23,406		$	24,182
Interest cost		11,507			11,057			34,520			33,172			45,577			44,031
Expected return on plan assets		(14,140	)		(14,452	)		(42,419	)		(43,356	)		(56,871	)		(56,691	)
Amortization of net actuarial loss		6,317			8,186			18,950			24,557			27,136			30,276

Net periodic benefit cost	$	9,392		$	11,072		$	28,176		$	33,215		$	39,248		$	41,798


	SERP
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	83		$	80		$	248		$	240		$	328		$	313
Interest cost		464			424			1,394			1,271			1,818			1,708
Amortization of net actuarial loss		346			323			1,038			970			1,361			1,165

Net periodic benefit cost	$	893		$	827		$	2,680		$	2,481		$	3,507		$	3,186


	PBOP
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	375		$	410		$	1,124		$	1,231		$	1,534		$	1,506
Interest cost		795			750			2,386			2,249			3,136			2,956
Expected return on plan assets		(787	)		(866	)		(2,362	)		(2,598	)		(3,228	)		(3,414	)
Amortization of prior service costs		333			334			1,001			1,001			1,335			1,090
Amortization of net actuarial loss		104			86			312			259			398			259

Net periodic benefit cost	$	820		$	714		$	2,461		$	2,142		$	3,175		$	2,397

Segment Information

9 Months Ended

Sep. 30, 2016

Segment Reporting [Abstract]

Segment Information

Note 3 – Segment Information

The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):


	Natural Gas			Construction
	Operations			Services		Total
Three months ended September 30, 2016
Revenues from external customers	$	200,179		$	312,531	$	512,710
Intersegment revenues		—			27,259		27,259

Total	$	200,179		$	339,790	$	539,969

Segment net income (loss)	$	(12,405	)	$	14,877	$	2,472

Three months ended September 30, 2015
Revenues from external customers	$	219,420		$	255,088	$	474,508
Intersegment revenues		—			30,888		30,888

Total	$	219,420		$	285,976	$	505,396

Segment net income (loss)	$	(18,939	)	$	14,205	$	(4,734	)


	Natural Gas			Construction
	Operations			Services		Total
Nine months ended September 30, 2016
Revenues from external customers	$	980,927		$	762,835	$	1,743,762
Intersegment revenues		—			75,203		75,203

Total	$	980,927		$	838,038	$	1,818,965

Segment net income	$	67,536		$	19,325	$	86,861

Nine months ended September 30, 2015
Revenues from external customers	$	1,059,178		$	645,289	$	1,704,467
Intersegment revenues		—			73,753		73,753

Total	$	1,059,178		$	719,042	$	1,778,220

Segment net income	$	59,325		$	12,873	$	72,198


	Natural Gas			Construction
	Operations			Services		Total
Twelve months ended September 30, 2016
Revenues from external customers	$	1,376,388		$	1,022,416	$	2,398,804
Intersegment revenues		—			105,566		105,566

Total	$	1,376,388		$	1,127,982	$	2,504,370

Segment net income	$	119,836		$	33,144	$	152,980

Twelve months ended September 30, 2015
Revenues from external customers	$	1,457,266		$	854,312	$	2,311,578
Intersegment revenues		—			94,325		94,325

Total	$	1,457,266		$	948,637	$	2,405,903

Segment net income	$	113,252		$	17,692	$	130,944

- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8380-108599

Reference 6: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8933-108599

Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-SubTopic 10
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Reference 8: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8984-108599

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e9031-108599

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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Reference 12: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8864-108599

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8924-108599

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=68060357&loc=d3e8906-108599

+ Details

Derivatives and Fair Value Measurements

9 Months Ended

Sep. 30, 2016

Fair Value Disclosures [Abstract]

Derivatives and Fair Value Measurements

Note 4 – 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.

The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 25% in the Arizona and California jurisdictions) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from October 2016 through March 2018. 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):


	September 30, 2016		December 31, 2015
Contract notional amounts		11,985		7,407

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 three-, nine-, and twelve-month periods ended September 30, 2016 and 2015 and their location in the Condensed Consolidated Statements of Income:

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

(Thousands of dollars)


		Three Months Ended						Nine Months Ended						Twelve Months Ended
	Location of Gain or (Loss)	September 30						September 30						September 30
Instrument	Recognized in Income on Derivative	2016			2015			2016			2015			2016			2015
Swaps	Net cost of gas sold	$	(2,072	)	$	(3,282	)	$	2,253		$	(4,689	)	$	(656	)	$	(10,682	)
Swaps	Net cost of gas sold		2,072	*		3,282	*		(2,253	)*		4,689	*		656	*		10,682	*

Total		$	—		$	—		$	—		$	—		$	—		$	—

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

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

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

Fair values of derivatives not designated as hedging instruments:


September 30, 2016 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	226	$	(33	)	$	193
Swaps	Prepaids and other current assets		1,852		(182	)		1,670
Swaps	Other current liabilities		16		(982	)		(966	)

Total		$	2,094	$	(1,197	)	$	897


December 31, 2015 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Other current liabilities	$	—	$	(4,267	)	$	(4,267	)
Swaps	Other deferred credits		4		(1,223	)		(1,219	)

Total		$	4	$	(5,490	)	$	(5,486	)

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 (in the settlement month) 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 any period presented 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 counterparties for settlements of matured Swaps.


	Three Months Ended		Nine Months Ended		Twelve Months Ended
(Thousands of dollars)	September 30, 2016		September 30, 2016		September 30, 2016
Paid to counterparties	$	—	$	4,483	$	6,792

Received from counterparties	$	351	$	351	$	351

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


September 30, 2016 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(193	)
Swaps	Other current liabilities		(1,670	)
Swaps	Prepaids and other current assets		966

December 31, 2015 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	4,267
Swaps	Deferred charges and other assets		1,219

Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at September 30, 2016 and December 31, 2015 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 measurement.

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

Level 2 - Significant other observable inputs


(Thousands of dollars)	September 30, 2016			December 31, 2015
Assets at fair value:
Prepaids and other current assets - Swaps	$	1,670		$	—
Deferred charges and other assets - Swaps		193			—
Liabilities at fair value:
Other current liabilities - Swaps		(966	)		(4,267	)
Other deferred credits - Swaps		—			(1,219	)

Net Assets (Liabilities)	$	897		$	(5,486	)

No financial assets or liabilities associated with the Swaps, which were accounted for at fair value, fell within Level 1 (quoted prices in active markets for identical financial assets) or Level 3 (significant unobservable inputs) of the fair value hierarchy.

With regard to the fair values of assets associated with the Company’s pension and postretirement benefit plans, asset values were last updated as required as of December 2015. Refer to Note 10 – Pension and Other Post Retirement Benefits in the 2015 Annual Report to Shareholders on Form 10-K.

Common Stock	9 Months Ended
Common Stock	Sep. 30, 2016
Text Block [Abstract]
Common Stock	Note 5 – Common Stock On March 10, 2015, the Company filed with the Securities Exchange Commission (“SEC”) an automatic shelf registration statement on Form S-3 (File No. 333-202633), which became effective upon filing, for the offer and sale of up to $100,000,000 of the Company’s common stock from time to time in at-the-market offerings under the prospectus included therein and in accordance with the Sales Agency Agreement, dated March 10, 2015, between the Company and BNY Mellon Capital Markets, LLC (the “Equity Shelf Program”). During the twelve months ended September 30, 2016, the Company sold no shares through the continuous equity offering program. Since the start of the program in March 2015, the Company has sold an aggregate of 645,225 shares of common stock under this program resulting in proceeds to the Company of $35,167,584, net of $355,228 in agent commissions. As of September 30, 2016, the Company had up to $64,477,188 of common stock available for sale under the program. Net proceeds from the sale of shares of common stock under the Equity Shelf Program are intended for general corporate purposes, including the acquisition of property for the construction, completion, extension or improvement of pipeline systems and facilities located in and around the communities Southwest serves. In addition, during the nine months ended September 30, 2016, the Company issued approximately 105,000 shares of common stock through the Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan. No further issuances of the Company’s common stock will occur under the Stock Incentive Plan due to the expiration of the Stock Incentive Plan and all remaining options outstanding and granted thereunder.

Long-Term Debt

9 Months Ended

Sep. 30, 2016

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 September 30, 2016 and December 31, 2015 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 utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s.


	September 30, 2016					December 31, 2015
	Carrying			Market		Carrying			Market
	Amount			Value		Amount			Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	133,819	$	125,000		$	130,273
Notes, 6.1%, due 2041		125,000			162,340		125,000			141,581
Notes, 3.875%, due 2022		250,000			260,813		250,000			253,600
Notes, 4.875%, due 2043		250,000			295,000		250,000			251,483
Notes, 3.8%, due 2046		300,000			303,789		—			—
8% Series, due 2026		75,000			101,275		75,000			97,035
Medium-term notes, 7.59% series, due 2017		25,000			25,350		25,000			26,253
Medium-term notes, 7.78% series, due 2022		25,000			30,610		25,000			29,855
Medium-term notes, 7.92% series, due 2027		25,000			34,190		25,000			31,890
Medium-term notes, 6.76% series, due 2027		7,500			9,404		7,500			8,684
Unamortized discount and debt issuance costs		(9,901	)				(6,137	)

		1,197,599					901,363

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

Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
2003 Series A, due 2038		50,000			50,000		50,000			50,000
2008 Series A, due 2038		50,000			50,000		50,000			50,000
2009 Series A, due 2039		50,000			50,000		50,000			50,000
Fixed-rate bonds:
4.85% 2005 Series A, due 2035		—			—		100,000			100,452
4.75% 2006 Series A, due 2036		—			—		24,855			25,130
Unamortized discount and debt issuance costs		(2,591	)				(3,946	)

		197,409					320,909

Centuri term loan facility		111,612			111,767		112,571			112,665
Unamortized debt issuance costs		(560	)				(692	)

		111,052					111,879

Centuri secured revolving credit facility		79,932			80,479		60,627			60,724
Centuri other debt obligations		56,434			58,553		25,901			26,059

		1,642,426					1,570,679
Less: current maturities		(49,480	)				(19,475	)

Long-term debt, less current maturities	$	1,592,946				$	1,551,204

In March 2016, the Company amended its $300 million credit facility. The facility was previously scheduled to expire in March 2020 and was extended to March 2021. The Company uses $150 million of the facility as long-term debt and the remaining $150 million for working capital purposes. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. At September 30, 2016, 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 September 30, 2016, no borrowings were outstanding on the long-term or short-term portions of the credit facility.

The $100 million 2005 4.85% Series A fixed-rate IDRBs (originally due in 2035) were redeemed at par with accrued interest in July 2016. In September 2016, the $24.9 million 2006 Series A 4.75% fixed-rate IDRBs (originally due in 2036) were redeemed at par with accrued interest.

In September 2016, the Company issued $300 million in 3.8% Senior Notes at a discount of 0.302%. The notes will mature in September 2046. A portion of the net proceeds were used to temporarily pay down amounts outstanding under the credit facility. The remaining net proceeds are intended to be used for general corporate purposes.

Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2019. At September 30, 2016, $192 million in borrowings were outstanding on the Centuri facility. Centuri assets securing the facility at September 30, 2016 totaled $510 million.

In 2016, Centuri entered into equipment loans totaling $45 million due in 2021 under an existing master loan and security agreement.

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income

9 Months Ended

Sep. 30, 2016

Equity [Abstract]

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income

Note 7 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income

The table below provides details of activity in equity and the redeemable noncontrolling interest during the nine months ended September 30, 2016.


	Southwest Gas Corporation Equity
							Accumulated												Redeemable
					Additional		Other						Non-						Noncontrolling
	Common Stock				Paid-in		Comprehensive			Retained			controlling						Interest
(In thousands, except per share amounts)	Shares		Amount		Capital		Income (Loss)			Earnings			Interest			Total			(Temporary Equity)
DECEMBER 31, 2015		47,377	$	49,007	$	896,448	$	(50,268	)	$	699,221		$	(2,083	)	$	1,592,325		$	16,108
Common stock issuances		105		105		5,518											5,623
Net income (loss)											86,861			(180	)		86,681			680
Redemption value adjustments											(1,009	)					(1,009	)		1,009
Foreign currency exchange translation adj.								593									593			21
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								1,819									1,819
Amounts reclassified to net income, net of tax (FSIRS)								1,556									1,556
Centuri dividend to redeemable noncontrolling interest																				(439	)
Dividends declared
Common: $1.35 per share											(64,672	)					(64,672	)

SEPTEMBER 30, 2016		47,482	$	49,112	$	901,966	$	(46,300	)	$	720,401		$	(2,263	)	$	1,622,916		$	17,379

The following information provides insight into amounts impacting Other Comprehensive Income (Loss), on both before- and after-tax bases, within the Condensed Consolidated Statements of Comprehensive Income, which also impact Accumulated Other Comprehensive Income in the Company’s Condensed Consolidated Balance Sheets and the associated column in the equity table above, as well as the Redeemable Noncontrolling Interest. See Note 4 – Derivatives and Fair Value Measurements for additional information on the FSIRS.

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

(Thousands of dollars)


	Three Months Ended									Three Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Amortization of prior service cost	$	333		$	(126	)	$	207		$	334		$	(127	)	$	207
Amortization of net actuarial (gain)/loss		6,767			(2,571	)		4,196			8,595			(3,266	)		5,329
Regulatory adjustment		(6,122	)		2,326			(3,796	)		(7,787	)		2,959			(4,828	)

Pension plans other comprehensive income (loss)		978			(371	)		607			1,142			(434	)		708
FSIRS (designated hedging activities):
Amounts reclassifed into net income		835			(317	)		518			836			(317	)		519

FSIRS other comprehensive income		835			(317	)		518			836			(317	)		519
Foreign currency translation adjustments:
Translation adjustments		(238	)		—			(238	)		(510	)		—			(510	)

Foreign currency other comprehensive income (loss)		(238	)		—			(238	)		(510	)		—			(510	)

Total other comprehensive income (loss)	$	1,575		$	(688	)	$	887		$	1,468		$	(751	)	$	717


	Nine Months Ended									Nine Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Amortization of prior service cost	$	1,001		$	(380	)	$	621		$	1,001		$	(380	)	$	621
Amortization of net actuarial (gain)/loss		20,300			(7,714	)		12,586			25,786			(9,799	)		15,987
Regulatory adjustment		(18,368	)		6,980			(11,388	)		(23,361	)		8,877			(14,484	)

Pension plans other comprehensive income (loss)		2,933			(1,114	)		1,819			3,426			(1,302	)		2,124
FSIRS (designated hedging activities):
Amounts reclassifed into net income		2,508			(952	)		1,556			2,508			(952	)		1,556

FSIRS other comprehensive income		2,508			(952	)		1,556			2,508			(952	)		1,556
Foreign currency translation adjustments:
Translation adjustments		614			—			614			(1,573	)		—			(1,573	)

Foreign currency other comprehensive income (loss)		614			—			614			(1,573	)		—			(1,573	)

Total other comprehensive income (loss)	$	6,055		$	(2,066	)	$	3,989		$	4,361		$	(2,254	)	$	2,107


	Twelve Months Ended									Twelve Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(30,519	)	$	11,597		$	(18,922	)	$	(173,646	)	$	65,985		$	(107,661	)
Amortization of prior service cost		1,335			(507	)		828			1,090			(414	)		676
Amortization of net actuarial (gain)/loss		28,895			(10,980	)		17,915			31,700			(12,046	)		19,654
Prior service cost		—			—			—			(6,661	)		2,531			(4,130	)
Regulatory adjustment		(653	)		249			(404	)		132,480			(50,343	)		82,137

Pension plans other comprehensive income (loss)		(942	)		359			(583	)		(15,037	)		5,713			(9,324	)
FSIRS (designated hedging activities):
Amounts reclassifed into net income		3,344			(1,271	)		2,073			3,344			(1,270	)		2,074

FSIRS other comprehensive income (loss)		3,344			(1,271	)		2,073			3,344			(1,270	)		2,074
Foreign currency translation adjustments:
Translation adjustments		233			—			233			(2,232	)		—			(2,232	)

Foreign currency other comprehensive income (loss)		233			—			233			(2,232	)		—			(2,232	)

Total other comprehensive income (loss)	$	2,635		$	(912	)	$	1,723		$	(13,925	)	$	4,443		$	(9,482	)

(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.

Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (“AOCI”) at September 30, 2016, will be reclassified into interest expense within the next 12 months as the related interest payments on long-term debt occur.

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

AOCI - Rollforward

(Thousands of dollars)


	Defined Benefit Plans									FSIRS									Foreign Currency Items
	Before- Tax			Tax (Expense) Benefit (4)			After- Tax			Before- Tax			Tax (Expense) Benefit (4)			After- Tax			Before- Tax			Tax (Expense) Benefit		After- Tax			AOCI
Beginning Balance AOCI December 31, 2015	$	(57,660	)	$	21,911		$	(35,749	)	$	(19,344	)	$	7,350		$	(11,994	)	$	(2,525	)	$	—	$	(2,525	)	$	(50,268	)

Translation adjustments		—			—			—			—			—			—			614			—		614			614

Other comprehensive income before reclassifications		—			—			—			—			—			—			614			—		614			614
FSIRS amounts reclassified from AOCI (1)		—			—			—			2,508			(952	)		1,556			—			—		—			1,556
Amortization of prior service cost (2)		1,001			(380	)		621			—			—			—			—			—		—			621
Amortization of net actuarial loss (2)		20,300			(7,714	)		12,586			—			—			—			—			—		—			12,586
Regulatory adjustment (3)		(18,368	)		6,980			(11,388	)		—			—			—			—			—		—			(11,388	)

Net current period other comprehensive income (loss)		2,933			(1,114	)		1,819			2,508			(952	)		1,556			614			—		614			3,989
Less: Translation adjustment attributable to redeemable noncontrolling interest		—			—			—			—			—			—			21			—		21			21

Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation		2,933			(1,114	)		1,819			2,508			(952	)		1,556			593			—		593			3,968

Ending Balance AOCI September 30, 2016	$	(54,727	)	$	20,797		$	(33,930	)	$	(16,836	)	$	6,398		$	(10,438	)	$	(1,932	)	$	—	$	(1,932	)	$	(46,300	)

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

(2)	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost 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 Condensed Consolidated Balance Sheets).

(4)	Tax amounts are calculated using a 38% rate.

The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)


	September 30, 2016			December 31, 2015
Net actuarial (loss) gain	$	(414,969	)	$	(435,269	)
Prior service cost		(6,037	)		(7,038	)
Less: amount recognized in regulatory assets		366,279			384,647

Recognized in AOCI	$	(54,727	)	$	(57,660	)

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 235
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-Publisher FASB
-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 4.C)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187143-122770

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Paragraph 1
-Subparagraph (SX 210.5-02(30))
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Reference 9: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 10: 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(d))
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

Construction Services Redeemable Noncontrolling Interest

9 Months Ended

Sep. 30, 2016

Noncontrolling Interest [Abstract]

Construction Services Redeemable Noncontrolling Interest

Note 8 – Construction Services Redeemable Noncontrolling Interest

In conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies currently hold a 3.4% equity interest (a noncontrolling interest) in Centuri. The noncontrolling parties are able to exit their investment retained by requiring the purchase of a portion of their interest commencing July 2017 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 beginning in July 2022. Due to the ability of the noncontrolling parties to redeem their interest for cash, their interest is presented on the Company’s Condensed Consolidated Balance Sheet at September 30, 2016 as a Redeemable noncontrolling interest, a category of mezzanine equity (temporary equity). The following depicts changes to the balance of the redeemable noncontrolling interest between the indicated periods.


	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2015	$	16,108
Net income (loss) attributable to redeemable noncontrolling interest		680
Foreign currency exchange translation adjustment		21
Centuri dividend to redeemable noncontrolling interest		(439	)
Adjustment to redemption value		1,009

Balance, September 30, 2016	$	17,379

Nature of Operations and Basis of Presentation (Policies)

9 Months Ended

Sep. 30, 2016

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of Operations

Basis of Presentation

Prepaids and Other Current Assets

Cash and Cash Equivalents

Goodwill


(In thousands of dollars)	Natural Gas Operations		Construction Services		Consolidated
December 31, 2015	$	10,095	$	116,050	$	126,145
Additional goodwill from ETTI acquisition		—		10,726		10,726
Foreign currency translation adjustment		—		5,568		5,568

September 30, 2016	$	10,095	$	132,344	$	142,439


	Acquisition Date
Property, plant and equipment	$	4.3
Intangible assets		2.9
Goodwill		10.7

Total assets acquired	$	17.9

The purchase price consisted of $17 million in cash on the acquisition date with the remaining amount being deferred over four years.

Intercompany Transactions


	September 30, 2016		December 31, 2015
Centuri accounts receivable for services provided to Southwest	$	12,402	$	10,006

Other Property and Investments

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


	September 30, 2016			December 31, 2015
Centuri property and equipment	$	460,165		$	423,369
Centuri accumulated provision for depreciation and amortization		(228,230	)		(221,028	)
Net cash surrender value of COLI policies		104,940			99,276
Other property		11,690			11,914

Total	$	348,565		$	313,531

Other Income (Deductions)


	Three Months Ended						Nine Months Ended						Twelve Months Ended
	September 30						September 30						September 30
	2016			2015			2016			2015			2016			2015
Change in COLI policies	$	2,300		$	(3,900	)	$	5,400		$	(2,600	)	$	7,500		$	(200	)
Interest income		522			624			1,280			1,375			2,078			2,096
Equity AFUDC		611			878			1,893			2,011			2,890			2,435
Foreign transaction gain (loss)		(3	)		(302	)		(22	)		(874	)		28			(1,052	)
Miscellaneous income and (expense)		(865	)		(960	)		(1,795	)		(1,138	)		(1,635	)		(2,313	)

Total other income (deductions)	$	2,565		$	(3,660	)	$	6,756		$	(1,226	)	$	10,861		$	966

Income Taxes

Income taxes. The Company claimed a federal income tax credit, which resulted in a recognized benefit of approximately $1.6 million during the third quarter of 2016.

Recently Issued Accounting Standards Updates

•

A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and

•

A right-of-use asset, which is an asset that represents the lessee’s right to use, or control the use of, a specified asset for the lease term.

Nature of Operations and Basis of Presentation (Tables)

9 Months Ended

Sep. 30, 2016

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Goodwill


(In thousands of dollars)	Natural Gas Operations		Construction Services		Consolidated
December 31, 2015	$	10,095	$	116,050	$	126,145
Additional goodwill from ETTI acquisition		—		10,726		10,726
Foreign currency translation adjustment		—		5,568		5,568

September 30, 2016	$	10,095	$	132,344	$	142,439

Estimated Fair Values of Assets Acquired as of Acquisition Date

The preliminary estimated fair values of assets acquired as of May 6, 2016, the acquisition date, are as follows (in millions of dollars):


	Acquisition Date
Property, plant and equipment	$	4.3
Intangible assets		2.9
Goodwill		10.7

Total assets acquired	$	17.9

Intercompany Transactions

Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars):


	September 30, 2016		December 31, 2015
Centuri accounts receivable for services provided to Southwest	$	12,402	$	10,006

Schedule of Other Property and Investments

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


	September 30, 2016			December 31, 2015
Centuri property and equipment	$	460,165		$	423,369
Centuri accumulated provision for depreciation and amortization		(228,230	)		(221,028	)
Net cash surrender value of COLI policies		104,940			99,276
Other property		11,690			11,914

Total	$	348,565		$	313,531

Other Income (Deductions)

The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars):


	Three Months Ended						Nine Months Ended						Twelve Months Ended
	September 30						September 30						September 30
	2016			2015			2016			2015			2016			2015
Change in COLI policies	$	2,300		$	(3,900	)	$	5,400		$	(2,600	)	$	7,500		$	(200	)
Interest income		522			624			1,280			1,375			2,078			2,096
Equity AFUDC		611			878			1,893			2,011			2,890			2,435
Foreign transaction gain (loss)		(3	)		(302	)		(22	)		(874	)		28			(1,052	)
Miscellaneous income and (expense)		(865	)		(960	)		(1,795	)		(1,138	)		(1,635	)		(2,313	)

Total other income (deductions)	$	2,565		$	(3,660	)	$	6,756		$	(1,226	)	$	10,861		$	966

Components of Net Periodic Benefit Cost (Tables)

9 Months Ended

Sep. 30, 2016

Text Block [Abstract]

Schedule of Components of Net Periodic Benefit Costs

As a result, net periodic benefit costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets.


	Qualified Retirement Plan
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	5,708		$	6,281		$	17,125		$	18,842		$	23,406		$	24,182
Interest cost		11,507			11,057			34,520			33,172			45,577			44,031
Expected return on plan assets		(14,140	)		(14,452	)		(42,419	)		(43,356	)		(56,871	)		(56,691	)
Amortization of net actuarial loss		6,317			8,186			18,950			24,557			27,136			30,276

Net periodic benefit cost	$	9,392		$	11,072		$	28,176		$	33,215		$	39,248		$	41,798


	SERP
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	83		$	80		$	248		$	240		$	328		$	313
Interest cost		464			424			1,394			1,271			1,818			1,708
Amortization of net actuarial loss		346			323			1,038			970			1,361			1,165

Net periodic benefit cost	$	893		$	827		$	2,680		$	2,481		$	3,507		$	3,186


	PBOP
	Period Ended September 30,
	Three Months						Nine Months						Twelve Months
	2016			2015			2016			2015			2016			2015
(Thousands of dollars)
Service cost	$	375		$	410		$	1,124		$	1,231		$	1,534		$	1,506
Interest cost		795			750			2,386			2,249			3,136			2,956
Expected return on plan assets		(787	)		(866	)		(2,362	)		(2,598	)		(3,228	)		(3,414	)
Amortization of prior service costs		333			334			1,001			1,001			1,335			1,090
Amortization of net actuarial loss		104			86			312			259			398			259

Net periodic benefit cost	$	820		$	714		$	2,461		$	2,142		$	3,175		$	2,397

Segment Information (Tables)

9 Months Ended

Sep. 30, 2016

Segment Reporting [Abstract]

Schedule of Segment Reporting Information

The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):


	Natural Gas			Construction
	Operations			Services		Total
Three months ended September 30, 2016
Revenues from external customers	$	200,179		$	312,531	$	512,710
Intersegment revenues		—			27,259		27,259

Total	$	200,179		$	339,790	$	539,969

Segment net income (loss)	$	(12,405	)	$	14,877	$	2,472

Three months ended September 30, 2015
Revenues from external customers	$	219,420		$	255,088	$	474,508
Intersegment revenues		—			30,888		30,888

Total	$	219,420		$	285,976	$	505,396

Segment net income (loss)	$	(18,939	)	$	14,205	$	(4,734	)


	Natural Gas			Construction
	Operations			Services		Total
Nine months ended September 30, 2016
Revenues from external customers	$	980,927		$	762,835	$	1,743,762
Intersegment revenues		—			75,203		75,203

Total	$	980,927		$	838,038	$	1,818,965

Segment net income	$	67,536		$	19,325	$	86,861

Nine months ended September 30, 2015
Revenues from external customers	$	1,059,178		$	645,289	$	1,704,467
Intersegment revenues		—			73,753		73,753

Total	$	1,059,178		$	719,042	$	1,778,220

Segment net income	$	59,325		$	12,873	$	72,198


	Natural Gas			Construction
	Operations			Services		Total
Twelve months ended September 30, 2016
Revenues from external customers	$	1,376,388		$	1,022,416	$	2,398,804
Intersegment revenues		—			105,566		105,566

Total	$	1,376,388		$	1,127,982	$	2,504,370

Segment net income	$	119,836		$	33,144	$	152,980

Twelve months ended September 30, 2015
Revenues from external customers	$	1,457,266		$	854,312	$	2,311,578
Intersegment revenues		—			94,325		94,325

Total	$	1,457,266		$	948,637	$	2,405,903

Segment net income	$	113,252		$	17,692	$	130,944

Derivatives and Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2016

Fair Value Disclosures [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):


	September 30, 2016		December 31, 2015
Contract notional amounts		11,985		7,407

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)


		Three Months Ended						Nine Months Ended						Twelve Months Ended
	Location of Gain or (Loss)	September 30						September 30						September 30
Instrument	Recognized in Income on Derivative	2016			2015			2016			2015			2016			2015
Swaps	Net cost of gas sold	$	(2,072	)	$	(3,282	)	$	2,253		$	(4,689	)	$	(656	)	$	(10,682	)
Swaps	Net cost of gas sold		2,072	*		3,282	*		(2,253	)*		4,689	*		656	*		10,682	*

Total		$	—		$	—		$	—		$	—		$	—		$	—

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

Fair Values of Swaps in Condensed Consolidated Balance Sheets

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

Fair values of derivatives not designated as hedging instruments:


September 30, 2016 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Deferred charges and other assets	$	226	$	(33	)	$	193
Swaps	Prepaids and other current assets		1,852		(182	)		1,670
Swaps	Other current liabilities		16		(982	)		(966	)

Total		$	2,094	$	(1,197	)	$	897


December 31, 2015 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives			Net Total
Swaps	Other current liabilities	$	—	$	(4,267	)	$	(4,267	)
Swaps	Other deferred credits		4		(1,223	)		(1,219	)

Total		$	4	$	(5,490	)	$	(5,486	)

Paid to and Received from Counterparties for Settlements of Matured Swaps

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


	Three Months Ended		Nine Months Ended		Twelve Months Ended
(Thousands of dollars)	September 30, 2016		September 30, 2016		September 30, 2016
Paid to counterparties	$	—	$	4,483	$	6,792

Received from counterparties	$	351	$	351	$	351

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

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


September 30, 2016 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(193	)
Swaps	Other current liabilities		(1,670	)
Swaps	Prepaids and other current assets		966

December 31, 2015 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	4,267
Swaps	Deferred charges and other assets		1,219

Significant Other Observable Inputs

Level 2 - Significant other observable inputs


(Thousands of dollars)	September 30, 2016			December 31, 2015
Assets at fair value:
Prepaids and other current assets - Swaps	$	1,670		$	—
Deferred charges and other assets - Swaps		193			—
Liabilities at fair value:
Other current liabilities - Swaps		(966	)		(4,267	)
Other deferred credits - Swaps		—			(1,219	)

Net Assets (Liabilities)	$	897		$	(5,486	)

Long-Term Debt (Tables)

9 Months Ended

Sep. 30, 2016

Debt Disclosure [Abstract]

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

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 utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s.


	September 30, 2016					December 31, 2015
	Carrying			Market		Carrying			Market
	Amount			Value		Amount			Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000		$	133,819	$	125,000		$	130,273
Notes, 6.1%, due 2041		125,000			162,340		125,000			141,581
Notes, 3.875%, due 2022		250,000			260,813		250,000			253,600
Notes, 4.875%, due 2043		250,000			295,000		250,000			251,483
Notes, 3.8%, due 2046		300,000			303,789		—			—
8% Series, due 2026		75,000			101,275		75,000			97,035
Medium-term notes, 7.59% series, due 2017		25,000			25,350		25,000			26,253
Medium-term notes, 7.78% series, due 2022		25,000			30,610		25,000			29,855
Medium-term notes, 7.92% series, due 2027		25,000			34,190		25,000			31,890
Medium-term notes, 6.76% series, due 2027		7,500			9,404		7,500			8,684
Unamortized discount and debt issuance costs		(9,901	)				(6,137	)

		1,197,599					901,363

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

Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
2003 Series A, due 2038		50,000			50,000		50,000			50,000
2008 Series A, due 2038		50,000			50,000		50,000			50,000
2009 Series A, due 2039		50,000			50,000		50,000			50,000
Fixed-rate bonds:
4.85% 2005 Series A, due 2035		—			—		100,000			100,452
4.75% 2006 Series A, due 2036		—			—		24,855			25,130
Unamortized discount and debt issuance costs		(2,591	)				(3,946	)

		197,409					320,909

Centuri term loan facility		111,612			111,767		112,571			112,665
Unamortized debt issuance costs		(560	)				(692	)

		111,052					111,879

Centuri secured revolving credit facility		79,932			80,479		60,627			60,724
Centuri other debt obligations		56,434			58,553		25,901			26,059

		1,642,426					1,570,679
Less: current maturities		(49,480	)				(19,475	)

Long-term debt, less current maturities	$	1,592,946				$	1,551,204

- Definition

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=64930739&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=64930785&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=64930785&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

+ Details

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)

9 Months Ended

Sep. 30, 2016

Equity [Abstract]

Detailed Activities of Equity and Redeemable Noncontrolling Interest

The table below provides details of activity in equity and the redeemable noncontrolling interest during the nine months ended September 30, 2016.


	Southwest Gas Corporation Equity
							Accumulated												Redeemable
					Additional		Other						Non-						Noncontrolling
	Common Stock				Paid-in		Comprehensive			Retained			controlling						Interest
(In thousands, except per share amounts)	Shares		Amount		Capital		Income (Loss)			Earnings			Interest			Total			(Temporary Equity)
DECEMBER 31, 2015		47,377	$	49,007	$	896,448	$	(50,268	)	$	699,221		$	(2,083	)	$	1,592,325		$	16,108
Common stock issuances		105		105		5,518											5,623
Net income (loss)											86,861			(180	)		86,681			680
Redemption value adjustments											(1,009	)					(1,009	)		1,009
Foreign currency exchange translation adj.								593									593			21
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								1,819									1,819
Amounts reclassified to net income, net of tax (FSIRS)								1,556									1,556
Centuri dividend to redeemable noncontrolling interest																				(439	)
Dividends declared
Common: $1.35 per share											(64,672	)					(64,672	)

SEPTEMBER 30, 2016		47,482	$	49,112	$	901,966	$	(46,300	)	$	720,401		$	(2,263	)	$	1,622,916		$	17,379

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

(Thousands of dollars)


	Three Months Ended									Three Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Amortization of prior service cost	$	333		$	(126	)	$	207		$	334		$	(127	)	$	207
Amortization of net actuarial (gain)/loss		6,767			(2,571	)		4,196			8,595			(3,266	)		5,329
Regulatory adjustment		(6,122	)		2,326			(3,796	)		(7,787	)		2,959			(4,828	)

Pension plans other comprehensive income (loss)		978			(371	)		607			1,142			(434	)		708
FSIRS (designated hedging activities):
Amounts reclassifed into net income		835			(317	)		518			836			(317	)		519

FSIRS other comprehensive income		835			(317	)		518			836			(317	)		519
Foreign currency translation adjustments:
Translation adjustments		(238	)		—			(238	)		(510	)		—			(510	)

Foreign currency other comprehensive income (loss)		(238	)		—			(238	)		(510	)		—			(510	)

Total other comprehensive income (loss)	$	1,575		$	(688	)	$	887		$	1,468		$	(751	)	$	717


	Nine Months Ended									Nine Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Amortization of prior service cost	$	1,001		$	(380	)	$	621		$	1,001		$	(380	)	$	621
Amortization of net actuarial (gain)/loss		20,300			(7,714	)		12,586			25,786			(9,799	)		15,987
Regulatory adjustment		(18,368	)		6,980			(11,388	)		(23,361	)		8,877			(14,484	)

Pension plans other comprehensive income (loss)		2,933			(1,114	)		1,819			3,426			(1,302	)		2,124
FSIRS (designated hedging activities):
Amounts reclassifed into net income		2,508			(952	)		1,556			2,508			(952	)		1,556

FSIRS other comprehensive income		2,508			(952	)		1,556			2,508			(952	)		1,556
Foreign currency translation adjustments:
Translation adjustments		614			—			614			(1,573	)		—			(1,573	)

Foreign currency other comprehensive income (loss)		614			—			614			(1,573	)		—			(1,573	)

Total other comprehensive income (loss)	$	6,055		$	(2,066	)	$	3,989		$	4,361		$	(2,254	)	$	2,107


	Twelve Months Ended									Twelve Months Ended
	September 30, 2016									September 30, 2015
	Before-			Tax			Net-of-			Before-			Tax			Net-of-
	Tax			(Expense)			Tax			Tax			(Expense)			Tax
	Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(30,519	)	$	11,597		$	(18,922	)	$	(173,646	)	$	65,985		$	(107,661	)
Amortization of prior service cost		1,335			(507	)		828			1,090			(414	)		676
Amortization of net actuarial (gain)/loss		28,895			(10,980	)		17,915			31,700			(12,046	)		19,654
Prior service cost		—			—			—			(6,661	)		2,531			(4,130	)
Regulatory adjustment		(653	)		249			(404	)		132,480			(50,343	)		82,137

Pension plans other comprehensive income (loss)		(942	)		359			(583	)		(15,037	)		5,713			(9,324	)
FSIRS (designated hedging activities):
Amounts reclassifed into net income		3,344			(1,271	)		2,073			3,344			(1,270	)		2,074

FSIRS other comprehensive income (loss)		3,344			(1,271	)		2,073			3,344			(1,270	)		2,074
Foreign currency translation adjustments:
Translation adjustments		233			—			233			(2,232	)		—			(2,232	)

Foreign currency other comprehensive income (loss)		233			—			233			(2,232	)		—			(2,232	)

Total other comprehensive income (loss)	$	2,635		$	(912	)	$	1,723		$	(13,925	)	$	4,443		$	(9,482	)

(1)

Rollforward of Accumulated Other Comprehensive Income

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

AOCI - Rollforward

(Thousands of dollars)


	Defined Benefit Plans									FSIRS									Foreign Currency Items
	Before- Tax			Tax (Expense) Benefit (4)			After- Tax			Before- Tax			Tax (Expense) Benefit (4)			After- Tax			Before- Tax			Tax (Expense) Benefit		After- Tax			AOCI
Beginning Balance AOCI December 31, 2015	$	(57,660	)	$	21,911		$	(35,749	)	$	(19,344	)	$	7,350		$	(11,994	)	$	(2,525	)	$	—	$	(2,525	)	$	(50,268	)

Translation adjustments		—			—			—			—			—			—			614			—		614			614

Other comprehensive income before reclassifications		—			—			—			—			—			—			614			—		614			614
FSIRS amounts reclassified from AOCI (1)		—			—			—			2,508			(952	)		1,556			—			—		—			1,556
Amortization of prior service cost (2)		1,001			(380	)		621			—			—			—			—			—		—			621
Amortization of net actuarial loss (2)		20,300			(7,714	)		12,586			—			—			—			—			—		—			12,586
Regulatory adjustment (3)		(18,368	)		6,980			(11,388	)		—			—			—			—			—		—			(11,388	)

Net current period other comprehensive income (loss)		2,933			(1,114	)		1,819			2,508			(952	)		1,556			614			—		614			3,989
Less: Translation adjustment attributable to redeemable noncontrolling interest		—			—			—			—			—			—			21			—		21			21

Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation		2,933			(1,114	)		1,819			2,508			(952	)		1,556			593			—		593			3,968

Ending Balance AOCI September 30, 2016	$	(54,727	)	$	20,797		$	(33,930	)	$	(16,836	)	$	6,398		$	(10,438	)	$	(1,932	)	$	—	$	(1,932	)	$	(46,300	)

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

(2)	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost 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 Condensed Consolidated Balance Sheets).

(4)	Tax amounts are calculated using a 38% rate.

Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income

The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:

Amounts Recognized in AOCI (Before Tax)

(Thousands of dollars)


	September 30, 2016			December 31, 2015
Net actuarial (loss) gain	$	(414,969	)	$	(435,269	)
Prior service cost		(6,037	)		(7,038	)
Less: amount recognized in regulatory assets		366,279			384,647

Recognized in AOCI	$	(54,727	)	$	(57,660	)

Construction Services Redeemable Noncontrolling Interest (Tables)

9 Months Ended

Sep. 30, 2016

Noncontrolling Interest [Abstract]

Summary of Redeemable Noncontrolling Interest

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


	Redeemable Noncontrolling Interest
(Thousands of dollars):
Balance, December 31, 2015	$	16,108
Net income (loss) attributable to redeemable noncontrolling interest		680
Foreign currency exchange translation adjustment		21
Centuri dividend to redeemable noncontrolling interest		(439	)
Adjustment to redemption value		1,009

Balance, September 30, 2016	$	17,379

Nature of Operations and Basis of Presentation - Additional Information (Detail)		3 Months Ended	9 Months Ended
	May 06, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($) Segment	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Significant Accounting Policies [Line Items]
Number of segments \| Segment			2
Non-cash construction advances - non-cash investing item			$ 3,600,000	$ 2,800,000
Goodwill impairment charges			$ 0
Purchase price paid in cash	$ 17,000,000
Business acquisition deferred payment period	4 years
Federal income tax credit, benefit recognized		$ (1,600,000)
Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary			96.60%
ETTI [Member]
Significant Accounting Policies [Line Items]
ETTI tax deductible goodwill		11,000,000	$ 11,000,000
Service Life [Member]
Significant Accounting Policies [Line Items]
Reduction in depreciation due to change in estimate		(2,000,000)
Pooled Funds and Mutual Funds [Member] \| Level 2 - Significant Other Observable Inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments		68,300,000	68,300,000		$ 250,000
Gas Pipe Materials and Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies		$ 30,000,000	$ 30,000,000		$ 24,000,000

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($)
Goodwill [Line Items]
Goodwill, Beginning balance	$ 126,145
Foreign currency translation adjustment	5,568
Goodwill, Ending balance	142,439
ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	10,726
Goodwill, Ending balance	10,700
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	10,095
Foreign currency translation adjustment	0
Goodwill, Ending balance	10,095
Natural Gas Operations [Member] \| ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	0
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	116,050
Foreign currency translation adjustment	5,568
Goodwill, Ending balance	132,344
Construction Services [Member] \| ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	$ 10,726

Nature of Operations and Basis of Presentation - Estimated Fair Values of Assets Acquired as of Acquisition Date (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 142,439	$ 126,145
ETTI [Member]
Business Acquisition [Line Items]
Property, plant and equipment	4,300
Intangible assets	2,900
Goodwill	10,700
Total assets acquired	$ 17,900

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Centuri Construction Group Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 12,402	$ 10,006

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Net cash surrender value of COLI policies	$ 104,940	$ 99,276
Other property	11,690	11,914
Total	348,565	313,531
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property and equipment	460,165	423,369
Centuri accumulated provision for depreciation and amortization	$ (228,230)	$ (221,028)

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Other Income and Expenses [Abstract]
Change in COLI policies	$ 2,300	$ (3,900)	$ 5,400	$ (2,600)	$ 7,500	$ (200)
Interest income	522	624	1,280	1,375	2,078	2,096
Equity AFUDC	611	878	1,893	2,011	2,890	2,435
Foreign transaction gain (loss)	(3)	(302)	(22)	(874)	28	(1,052)
Miscellaneous income and (expense)	(865)	(960)	(1,795)	(1,138)	(1,635)	(2,313)
Total other income (deductions)	$ 2,565	$ (3,660)	$ 6,756	$ (1,226)	$ 10,861	$ 966

Components of Net Periodic Benefit Cost - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 Participants	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Description of specific offer to select employees			About one-half of the approximate 800 participants subject to the offer accepted the offer, resulting in an approximate $30 million payment from pension assets to be made in the fourth quarter of 2016. This lump sum payout will have no impact on net periodic benefit cost or pension funding requirements during 2016.
Number of employees offered a lump-sum payout \| Participants		800
Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Lump sum pension payment to be paid \| $	$ 30

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,708	$ 6,281	$ 17,125	$ 18,842	$ 23,406	$ 24,182
Interest cost	11,507	11,057	34,520	33,172	45,577	44,031
Expected return on plan assets	(14,140)	(14,452)	(42,419)	(43,356)	(56,871)	(56,691)
Amortization of net actuarial loss	6,317	8,186	18,950	24,557	27,136	30,276
Net periodic benefit cost	9,392	11,072	28,176	33,215	39,248	41,798
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	83	80	248	240	328	313
Interest cost	464	424	1,394	1,271	1,818	1,708
Amortization of net actuarial loss	346	323	1,038	970	1,361	1,165
Net periodic benefit cost	893	827	2,680	2,481	3,507	3,186
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	375	410	1,124	1,231	1,534	1,506
Interest cost	795	750	2,386	2,249	3,136	2,956
Expected return on plan assets	(787)	(866)	(2,362)	(2,598)	(3,228)	(3,414)
Amortization of prior service costs	333	334	1,001	1,001	1,335	1,090
Amortization of net actuarial loss	104	86	312	259	398	259
Net periodic benefit cost	$ 820	$ 714	$ 2,461	$ 2,142	$ 3,175	$ 2,397

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Total operating revenues	$ 539,969	$ 505,396	$ 1,818,965	$ 1,778,220	$ 2,504,370	$ 2,405,903
Segment net income (loss)	2,472	(4,734)	86,861	72,198	152,980	130,944
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	512,710	474,508	1,743,762	1,704,467	2,398,804	2,311,578
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	27,259	30,888	75,203	73,753	105,566	94,325
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	200,179	219,420	980,927	1,059,178	1,376,388	1,457,266
Segment net income (loss)	(12,405)	(18,939)	67,536	59,325	119,836	113,252
Natural Gas Operations [Member] \| Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	200,179	219,420	980,927	1,059,178	1,376,388	1,457,266
Natural Gas Operations [Member] \| Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	339,790	285,976	838,038	719,042	1,127,982	948,637
Segment net income (loss)	14,877	14,205	19,325	12,873	33,144	17,692
Construction Services [Member] \| Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	312,531	255,088	762,835	645,289	1,022,416	854,312
Construction Services [Member] \| Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 27,259	$ 30,888	$ 75,203	$ 73,753	$ 105,566	$ 94,325

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 518,000	$ 519,000	$ 1,556,000	$ 1,556,000	$ 2,073,000	$ 2,074,000
Derivative instrument loss at settlement amortization period			10 years
Cash Flow Hedging [Member]
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges			$ 0
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Oct. 31, 2016
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Mar. 31, 2018
Arizona [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate			25.00%
California [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate			25.00%

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Sep. 30, 2016	Dec. 31, 2015
Offsetting [Abstract]
Contract notional amounts	11,985,000	7,407,000

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Cash Flow Hedging [Member] \| Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	(2,072)	(3,282)	2,253	(4,689)	(656)	(10,682)
Regulatory Deferral Accounting Treatment [Member] \| Cash Flow Hedging [Member] \| Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 2,072	$ 3,282	$ (2,253)	$ 4,689	$ 656	$ 10,682

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 2,094	$ 4
Liability derivatives not designated as hedging instruments	(1,197)	(5,490)
Net total not designated as hedging instruments	897	(5,486)
Swaps [Member] \| Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	226
Liability derivatives not designated as hedging instruments	(33)
Net total not designated as hedging instruments	193
Swaps [Member] \| Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	1,852
Liability derivatives not designated as hedging instruments	(182)
Net total not designated as hedging instruments	1,670
Swaps [Member] \| Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	16	0
Liability derivatives not designated as hedging instruments	(982)	(4,267)
Net total not designated as hedging instruments	$ (966)	(4,267)
Swaps [Member] \| Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		4
Liability derivatives not designated as hedging instruments		(1,223)
Net total not designated as hedging instruments		$ (1,219)

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 0	$ 4,483	$ 6,792
Received from counterparties	$ 351	$ 351	$ 351

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ (193)
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Net Total	(1,670)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 966	$ 4,267
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ 1,219

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant Other Observable Inputs [Member] - Swaps [Member] - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ 897	$ (5,486)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	1,670	0
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	193	0
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(966)	(4,267)
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ 0	$ (1,219)

Common Stock - Additional Information (Detail) - USD ($)		9 Months Ended		12 Months Ended		19 Months Ended
	Mar. 10, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016
Value of common stock	$ 100,000,000
Net proceeds to Company		$ 530,000	$ 35,419,000	$ 507,000	$ 35,405,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan		105,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent				0		645,225
Net proceeds to Company						$ 35,167,584
Agent commissions						355,228
Common stock available for sale under the program		$ 64,477,188		$ 64,477,188		$ 64,477,188

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	$ (49,480)	$ (19,475)
Long-term debt, less current maturities	1,592,946	1,551,204
Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs	(560)	(692)
Total long-term debt	1,642,426	1,570,679
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(49,480)	(19,475)
Long-term debt, less current maturities	1,592,946	1,551,204
Total long-term debt	1,642,426	1,570,679
Carrying Amount [Member] \| Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	79,932	60,627
Carrying Amount [Member] \| IDRBs [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	111,052	111,879
Unsecured debt	197,409	320,909
Carrying Amount [Member] \| Centuri Term Loan Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	111,612	112,571
Carrying Amount [Member] \| Centuri Other Debt Obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	56,434	25,901
Carrying Amount [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(9,901)	(6,137)
Notes payable	1,197,599	901,363
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] \| Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Notes payable	300,000	0
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	0	150,000
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 and debt issuance costs	(2,591)	(3,946)
Carrying Amount [Member] \| Fixed-Rate Bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	0	100,000
Carrying Amount [Member] \| Fixed-Rate Bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	0	24,855
Market Value [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	111,767	112,665
Centuri other debt obligations	58,553	26,059
Market Value [Member] \| Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	80,479	60,724
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	133,819	130,273
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	162,340	141,581
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	260,813	253,600
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	295,000	251,483
Market Value [Member] \| Debentures [Member] \| Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	303,789	0
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	101,275	97,035
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	25,350	26,253
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	30,610	29,855
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	34,190	31,890
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,404	8,684
Market Value [Member] \| Debentures [Member] \| Revolving Credit Facility and Commercial Paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	0	150,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000	50,000
Market Value [Member] \| Fixed-Rate Bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	0	100,452
Market Value [Member] \| Fixed-Rate Bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 0	$ 25,130

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	Dec. 01, 2020	Dec. 01, 2020
Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	Feb. 15, 2041	Feb. 15, 2041
Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%	3.875%
Debt instrument due date	Apr. 01, 2022	Apr. 01, 2022
Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.875%	4.875%
Debt instrument due date	Oct. 01, 2043	Oct. 01, 2043
Debentures [Member] \| Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.80%	3.80%
Debt instrument due date	Sep. 30, 2046	Sep. 30, 2046
Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 01, 2026	Aug. 01, 2026
Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	Jan. 17, 2017	Jan. 17, 2017
Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	Feb. 03, 2022	Feb. 03, 2022
Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	Jun. 24, 2027	Jun. 24, 2027
Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	Sep. 24, 2027	Sep. 24, 2027
Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2028	Dec. 01, 2028
Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-Rate Bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	Oct. 01, 2035	Oct. 01, 2035
Fixed-Rate Bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	Sep. 01, 2036	Sep. 01, 2036
Market Value [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2039	Dec. 01, 2039

Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Jul. 31, 2016	Mar. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Debentures [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable	$ 1,197,599,000			$ 1,197,599,000	$ 901,363,000
Centuri Construction Group Inc [Member] \| Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	300,000,000			$ 300,000,000
Credit facility expiration date				Oct. 31, 2019
Borrowings outstanding under facility	192,000,000			$ 192,000,000
Assets securing debt	510,000,000			510,000,000
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity			$ 300,000,000
Line of credit designated as long term debt			150,000,000
Line of credit designated for working capital purposes			$ 150,000,000
Credit facility expiration date			Mar. 01, 2021
Southwest Gas Credit Facility [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility	0			0
Southwest Gas Credit Facility [Member] \| Short-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility	0			$ 0
Southwest Gas Credit Facility [Member] \| Before Amendment [Member]
Debt Instrument [Line Items]
Previous credit facility expiration date			Mar. 01, 2020
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%
Loans Payable [Member] \| Centuri Construction Group Inc [Member] \| Equipment Loan [Member]
Debt Instrument [Line Items]
Equipment loans	$ 45,000,000			$ 45,000,000
Debt instrument maturity year				2021
4.85% 2005 Series A, due 2035 [Member] \| Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%			4.85%	4.85%
Debt instrument due date				Oct. 01, 2035	Oct. 01, 2035
4.85% 2005 Series A, due 2035 [Member] \| Fixed-Rate Bonds [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt	$ 0			$ 0	$ 100,000,000
4.85% 2005 Series A, due 2035 [Member] \| Early Redemption of Debt [Member] \| Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt		$ 100,000,000
Debt instrument, original debt due date of debt year		2035
Debt instrument interest rate		4.85%
4.75% 2006 Series A, due 2036 [Member] \| Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%			4.75%	4.75%
Debt instrument due date				Sep. 01, 2036	Sep. 01, 2036
4.75% 2006 Series A, due 2036 [Member] \| Fixed-Rate Bonds [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt	$ 0			$ 0	$ 24,855,000
4.75% 2006 Series A, due 2036 [Member] \| Early Redemption of Debt [Member] \| Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	$ 24,900,000			$ 24,900,000
Debt instrument, original debt due date of debt year	2036
Debt instrument interest rate	4.75%			4.75%
Notes, 3.8%, due 2046 [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.80%			3.80%	3.80%
Debt instrument due date				Sep. 30, 2046	Sep. 30, 2046
Debt instrument discount rate	0.302%			0.302%
Notes, 3.8%, due 2046 [Member] \| Debentures [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable	$ 300,000,000			$ 300,000,000	$ 0

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,592,325
Balance			$ 16,108
Balance, shares			47,377,575
Common stock issuances			$ 5,623
Net income (loss)			86,681
Net income (loss)	$ 2,907	$ (4,210)	87,361	$ 72,732	$ 154,059	$ 131,629
Redemption value adjustments			(1,009)
Foreign currency exchange translation adj.			593
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	607	708	1,819	2,124	(583)	(9,324)
Amounts reclassified into net income	518	$ 519	1,556	$ 1,556	2,073	$ 2,074
Dividends declared Common			(64,672)
Balance	1,622,916		1,622,916		1,622,916
Balance	$ 17,379		$ 17,379		$ 17,379
Balance, shares	47,482,068		47,482,068		47,482,068
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 16,108
Net income (loss) attributable to temporary equity			680
Redemption value adjustments			1,009
Foreign currency exchange translation adj attributable to temporary equity			21
Other comprehensive income (loss):
Centuri dividend to redeemable noncontrolling interest			(439)
Centuri dividend to redeemable noncontrolling interest			(439)
Balance	$ 17,379		17,379		$ 17,379
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,083)
Net income (loss)			(180)
Other comprehensive income (loss):
Balance	(2,263)		(2,263)		(2,263)
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 49,007
Balance, shares			47,377,000
Common stock issuances			$ 105
Common stock issuances, Shares			105,000
Other comprehensive income (loss):
Balance	$ 49,112		$ 49,112		$ 49,112
Balance, shares	47,482,000		47,482,000		47,482,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 896,448
Common stock issuances			5,518
Other comprehensive income (loss):
Balance	$ 901,966		901,966		$ 901,966
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,268)
Foreign currency exchange translation adj.			593
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,819
Amounts reclassified into net income			1,556
Balance	(46,300)		(46,300)		(46,300)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			699,221
Net income (loss)			86,861
Redemption value adjustments			(1,009)
Other comprehensive income (loss):
Dividends declared Common			(64,672)
Balance	$ 720,401		$ 720,401		$ 720,401

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Parenthetical) (Detail) - $ / shares	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 0.450	$ 0.405	$ 1.350	$ 1.215	$ 1.755	$ 1.580

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Amortization of prior service cost, Before-Tax Amount	$ 333	$ 334	$ 1,001	$ 1,001	$ 1,335	$ 1,090
Amortization of net actuarial (gain)/loss, Before-Tax Amount	6,767	8,595	20,300	25,786	28,895	31,700
Regulatory adjustment, Before-Tax Amount	(6,122)	(7,787)	(18,368)	(23,361)	(653)	132,480
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	978	1,142	2,933	3,426	(942)	(15,037)
Amounts reclassified into net income, Before-Tax Amount	835	836	2,508	2,508	3,344	3,344
FSIRS other comprehensive income (loss), Before-Tax Amount	835	836	2,508	2,508	3,344	3,344
Translation adjustments, Before-Tax Amount	(238)	(510)	614	(1,573)	233	(2,232)
Foreign currency other comprehensive income (loss), Before-Tax Amount	(238)	(510)	614	(1,573)	233	(2,232)
Total other comprehensive income (loss), Before-Tax Amount	1,575	1,468	6,055	4,361	2,635	(13,925)
Amortization of prior service cost, Tax (Expense) or Benefit	(126)	(127)	(380)	(380)	(507)	(414)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,571)	(3,266)	(7,714)	(9,799)	(10,980)	(12,046)
Regulatory adjustment, Tax (Expense) or Benefit	2,326	2,959	6,980	8,877	249	(50,343)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(371)	(434)	(1,114)	(1,302)	359	5,713
Amounts reclassified into net income, Tax (Expense) or Benefit	(317)	(317)	(952)	(952)	(1,271)	(1,270)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(317)	(317)	(952)	(952)	(1,271)	(1,270)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(688)	(751)	(2,066)	(2,254)	(912)	4,443
Amortization of prior service cost, Net-of-Tax Amount	207	207	621	621	828	676
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	4,196	5,329	12,586	15,987	17,915	19,654
Regulatory adjustment, Net-of-Tax Amount	(3,796)	(4,828)	(11,388)	(14,484)	(404)	82,137
Pension plans other comprehensive income (loss), Net-of-Tax Amount	607	708	1,819	2,124	(583)	(9,324)
Amounts reclassified into net income	518	519	1,556	1,556	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	519	1,556	1,556	2,073	2,074
Translation adjustments, Net-of-Tax Amount	(238)	(510)	614	(1,573)	233	(2,232)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(238)	(510)	614	(1,573)	233	(2,232)
Total other comprehensive income (loss), net of tax	887	717	3,989	2,107	1,723	(9,482)
Net actuarial gain/(loss), Before-Tax Amount					(30,519)	(173,646)
Prior service cost, Before-Tax Amount					0	(6,661)
Net actuarial gain/(loss), Tax (Expense) or Benefit					11,597	65,985
Prior service cost, Tax (Expense) or Benefit					0	2,531
Net actuarial gain/(loss), Net-of-Tax Amount	0	0	0	0	(18,922)	(107,661)
Prior service cost, Net-of-Tax Amount	$ 0	$ 0	$ 0	$ 0	$ 0	$ (4,130)

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2016 USD ($)
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Sep. 30, 2016 USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Equity [Abstract]
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans Before-Tax			$ (57,660)
Translation adjustments, Defined Benefit Plans Before-Tax			0
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax			0
Amortization of prior service cost, Defined Benefit Plans Before-Tax	$ 333	$ 334	1,001	$ 1,001	$ 1,335	$ 1,090
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	6,767	8,595	20,300	25,786	28,895	31,700
Regulatory adjustment, Defined Benefit Plans Before-Tax	(6,122)	(7,787)	(18,368)	(23,361)	(653)	132,480
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	978	1,142	2,933	3,426	(942)	(15,037)
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			2,933
Ending Balance AOCI September 30, 2016, Defined Benefit Plans Before-Tax	(54,727)		(54,727)		(54,727)
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans Tax (Expense) Benefit			21,911
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit			0
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit	(126)	(127)	(380)	(380)	(507)	(414)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(2,571)	(3,266)	(7,714)	(9,799)	(10,980)	(12,046)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,326	2,959	6,980	8,877	249	(50,343)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(371)	(434)	(1,114)	(1,302)	359	5,713
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			(1,114)
Ending Balance AOCI September 30, 2016, Defined Benefit Plans Tax (Expense) Benefit	20,797		20,797		20,797
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans After-Tax			(35,749)
Translation adjustments, Defined Benefit Plans After-Tax			0
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax			0
Amortization of prior service cost, Defined Benefit Plans After-Tax	207	207	621	621	828	676
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	4,196	5,329	12,586	15,987	17,915	19,654
Regulatory adjustment, Defined Benefit Plans After-Tax			(11,388)
Net defined benefit pension plans	607	708	1,819	2,124	(583)	(9,324)
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			1,819
Ending Balance AOCI September 30, 2016, Defined Benefit Plans After-Tax	(33,930)		(33,930)		(33,930)
Beginning Balance AOCI December 31, 2015, FSIRS - Before-Tax			(19,344)
Translation adjustments, FSIRS - Before-Tax			0
Other comprehensive income before reclassifications, FSIRS - Before-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	835	836	2,508	2,508	3,344	3,344
Net current period other comprehensive income (loss), FSIRS - Before-Tax	835	836	2,508	2,508	3,344	3,344
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			2,508
Ending Balance AOCI September 30, 2016, FSIRS - Before-Tax	(16,836)		(16,836)		(16,836)
Beginning Balance AOCI December 31, 2015, FSIRS - Tax (Expense) Benefit			7,350
Translation adjustments, FSIRS - Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(317)	(317)	(952)	(952)	(1,271)	(1,270)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(317)	(317)	(952)	(952)	(1,271)	(1,270)
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			(952)
Ending Balance AOCI September 30, 2016, FSIRS - Tax (Expense) Benefit	6,398		6,398		6,398
Beginning Balance AOCI December 31, 2015, FSIRS - After-Tax			(11,994)
Translation adjustments, FSIRS - After-Tax			0
Other comprehensive income before reclassifications			0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	519	1,556	1,556	2,073	2,074
Net current period other comprehensive income (loss)	518	519	1,556	1,556	2,073	2,074
Less: Translation adjustment attributable to redeemable noncontrolling interest			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax			1,556
Ending Balance AOCI September 30, 2016, FSIRS - After-Tax	(10,438)		(10,438)		(10,438)
Beginning Balance AOCI December 31, 2015, Foreign Currency Items Before-Tax			(2,525)
Translation adjustments, Foreign Currency Items Before-Tax	(238)	(510)	614	(1,573)	233	(2,232)
Other comprehensive income before reclassifications	(238)	(510)	614	(1,573)	233	(2,232)
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
Regulatory adjustment, Foreign Currency Items Before-Tax			0
Net current period other comprehensive income (loss)			614
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax			21
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax			593
Ending Balance AOCI September 30, 2016	(1,932)		(1,932)		(1,932)
Beginning Balance AOCI December 31, 2015, Foreign Currency items - Tax (Expense) Benefit			0
Translation adjustments, Foreign Currency items - Tax (Expense) Benefit	0	0	0	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
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	0	0	0
Ending Balance AOCI September 30, 2016, Foreign Currency items - Tax (Expense) Benefit	0		0		0
Beginning Balance AOCI December 31, 2015, Foreign Currency items - After-Tax			(2,525)
Translation adjustments, Foreign Currency items - After-Tax	(238)	(510)	614	(1,573)	233	(2,232)
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	(238)	(510)	614	(1,573)	233	(2,232)
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
Regulatory adjustment, Foreign Currency items - After-Tax			0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax			614
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax			21
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax			593
Ending Balance AOCI September 30, 2016, Foreign Currency items - After-Tax	(1,932)		(1,932)		(1,932)
Beginning Balance AOCI December 31, 2015, AOCI			(50,268)
Translation adjustments, AOCI	614		614		614
Other comprehensive income before reclassifications, AOCI			614
Amounts reclassified into net income	518	519	1,556	1,556	2,073	2,074
Amortization of prior service cost, AOCI	207	207	621	621	828	676
Amortization of net actuarial loss	4,196	5,329	12,586	15,987	17,915	19,654
Regulatory adjustment, AOCI			(11,388)
Total other comprehensive income (loss), net of tax	887	$ 717	3,989	$ 2,107	1,723	$ (9,482)
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	21		21		21
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	3,968		3,968		3,968
Ending Balance AOCI September 30, 2016, AOCI	$ (46,300)		$ (46,300)		$ (46,300)

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail)	9 Months Ended
	Sep. 30, 2016
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (54,727)	$ (57,660)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(414,969)	(435,269)
Prior service cost	(6,037)	(7,038)
Less: amount recognized in regulatory assets	366,279	384,647
Recognized in AOCI	$ (54,727)	$ (57,660)

Construction Services Noncontrolling Interests - Additional Information (Detail) - Principal Previous Owners [Member] - Link-Line and WS Nicholls [Member]	1 Months Ended	9 Months Ended
	Oct. 31, 2014	Sep. 30, 2016
Noncontrolling Interest [Line Items]
First redemption date to exchange holdings for cash		2017-07
Description of equity interest		In conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies currently hold a 3.4% equity interest (a noncontrolling interest) in Centuri. The noncontrolling parties are able to exit their investment retained by requiring the purchase of a portion of their interest commencing July 2017 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 beginning in July 2022.
Construction Services [Member]
Noncontrolling Interest [Line Items]
Percentage of earnings attribution by Centuri to previous owners	3.40%
Construction Services [Member] \| Maximum [Member]
Noncontrolling Interest [Line Items]
Percentage of retained interest may elect to sell to Centuri		100.00%

Construction Services Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2016 USD ($)
Redeemable Noncontrolling Interest [Line Items]
Balance	$ 16,108
Adjustment to redemption value	(1,009)
Balance	17,379
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Redeemable Noncontrolling Interest [Line Items]
Balance	16,108
Net income (loss) attributable to redeemable noncontrolling interest	680
Foreign currency exchange translation adjustment	21
Centuri dividend to redeemable noncontrolling interest	(439)
Adjustment to redemption value	1,009
Balance	$ 17,379