Southwest Gas 10-Q 9 May 16 2016 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2016	Apr. 28, 2016
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q1
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS CORP
Entity Central Index Key	92,416
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,471,582

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Utility plant:
Gas plant	$ 5,961,648	$ 5,854,917
Less: accumulated depreciation	(2,110,238)	(2,084,007)
Acquisition adjustments, net	325	370
Construction work in progress	77,218	119,805
Net utility plant	3,928,953	3,891,085
Other property and investments	328,115	313,531
Current assets:
Cash and cash equivalents	60,809	35,997
Accounts receivable, net of allowances	271,314	314,512
Accrued utility revenue	45,100	74,700
Income taxes receivable, net	11,926	34,175
Deferred purchased gas costs		3,591
Prepaids and other current assets	81,079	95,199
Total current assets	470,228	558,174
Noncurrent assets:
Goodwill	132,774	126,145
Deferred income taxes	598	428
Deferred charges and other assets	455,340	469,322
Total noncurrent assets	588,712	595,895
Total assets	5,316,008	5,358,685
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 47,470,282 and 47,377,575 shares)	49,100	49,007
Additional paid-in capital	898,463	896,448
Accumulated other comprehensive income (loss), net	(48,386)	(50,268)
Retained earnings	753,105	699,221
Total Southwest Gas Corporation equity	1,652,282	1,594,408
Noncontrolling interest	(2,099)	(2,083)
Total equity	1,650,183	1,592,325
Redeemable noncontrolling interest	15,960	16,108
Long-term debt, less current maturities	1,388,968	1,551,204
Total capitalization	3,055,111	3,159,637
Current liabilities:
Current maturities of long-term debt	48,596	19,475
Short-term debt		18,000
Accounts payable	133,884	164,857
Customer deposits	72,903	72,631
Income taxes payable	8	940
Accrued general taxes	62,233	47,337
Accrued interest	21,814	16,173
Deferred purchased gas costs	100,987	45,601
Other current liabilities	137,642	150,031
Total current liabilities	578,067	535,045
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	789,961	769,445
Accumulated removal costs	304,000	303,000
Other deferred credits and other long-term liabilities	588,869	591,558
Total deferred income taxes and other credits	1,682,830	1,664,003
Total capitalization and liabilities	$ 5,316,008	$ 5,358,685

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 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	47,470,282	47,377,575
Common stock, outstanding	47,470,282	47,377,575

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Operating revenues:
Gas operating revenues	$ 525,100	$ 553,115	$ 1,426,624	$ 1,448,709
Construction revenues	206,148	181,105	1,034,029	798,822
Total operating revenues	731,248	734,220	2,460,653	2,247,531
Operating expenses:
Net cost of gas sold	213,600	253,762	523,647	567,741
Operations and maintenance	100,797	95,510	398,486	376,834
Depreciation and amortization	75,360	67,467	278,004	257,603
Taxes other than income taxes	14,013	12,997	50,409	48,793
Construction expenses	193,382	174,928	917,235	709,586
Total operating expenses	597,152	604,664	2,167,781	1,960,557
Operating income	134,096	129,556	292,872	286,974
Other income and (expenses):
Net interest deductions	(17,721)	(17,977)	(71,623)	(72,527)
Other income (deductions)	1,721	2,272	2,328	7,767
Total other income and (expenses)	(16,000)	(15,705)	(69,295)	(64,760)
Income before income taxes	118,096	113,851	223,577	222,214
Income tax expense	42,741	41,972	80,671	79,884
Net income	75,355	71,879	142,906	142,330
Net income (loss) attributable to noncontrolling interests	(91)	(104)	1,126	4
Net income attributable to Southwest Gas Corporation	$ 75,446	$ 71,983	$ 141,780	$ 142,326
Basic earnings per share	$ 1.59	$ 1.54	$ 3	$ 3.06
Diluted earnings per share	1.58	1.53	2.98	3.03
Dividends declared per share	$ 0.450	$ 0.405	$ 1.665	$ 1.500
Average number of common shares outstanding	47,437	46,612	47,196	46,537
Average shares outstanding (assuming dilution)	47,763	47,036	47,562	46,986

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 75,355	$ 71,879	$ 142,906	$ 142,330
Defined benefit pension plans:
Net actuarial gain (loss)			(18,922)	(107,661)
Amortization of prior service cost	207	206	829	371
Amortization of net actuarial loss	4,196	5,330	20,182	16,330
Prior service cost				(4,130)
Regulatory adjustment	(3,796)	(4,828)	(2,468)	85,373
Net defined benefit pension plans	607	708	(379)	(9,717)
Forward-starting interest rate swaps:
Amounts reclassified into net income	519	519	2,073	2,074
Net forward-starting interest rate swaps	519	519	2,073	2,074
Foreign currency translation adjustments	782	(1,272)	100	(1,931)
Total other comprehensive income (loss), net of tax	1,908	(45)	1,794	(9,574)
Comprehensive income	77,263	71,834	144,700	132,756
Comprehensive income (loss) attributable to noncontrolling interests	(65)	(147)	1,129	(61)
Comprehensive income attributable to Southwest Gas Corporation	$ 77,328	$ 71,981	$ 143,571	$ 132,817

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 75,355	$ 71,879	$ 142,906	$ 142,330
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	75,360	67,467	278,004	257,603
Deferred income taxes	18,931	7,584	60,132	58,201
Changes in current assets and liabilities:
Accounts receivable, net of allowances	45,525	(31,850)	37,525	(49,491)
Accrued utility revenue	29,600	29,500	(700)	(300)
Deferred purchased gas costs	58,977	59,423	129,120	47,063
Accounts payable	(35,126)	(49,173)	10,556	(24,031)
Accrued taxes	36,298	49,782	(21,889)	2,809
Other current assets and liabilities	6,776	31,548	(6,472)	18,405
Gains on sale	(1,333)	(1,526)	(2,909)	(5,268)
Changes in undistributed stock compensation	1,394	462	3,846	5,674
AFUDC	(532)	(396)	(3,144)	(1,893)
Changes in other assets and deferred charges	(291)	(13,336)	(1,121)	(23,934)
Changes in other liabilities and deferred credits	1,719	876	11,706	(500)
Net cash provided by operating activities	312,653	222,240	637,560	426,668
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(112,561)	(90,391)	(510,170)	(410,397)
Acquisition of businesses, net of cash acquired		(9,261)		(199,758)
Restricted cash			785	1,233
Changes in customer advances	3,661	5,446	16,515	21,704
Miscellaneous inflows	1,126	2,835	6,645	10,853
Miscellaneous outflows				(1,400)
Net cash used in investing activities	(107,774)	(91,371)	(486,225)	(577,765)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	401	8,791	27,006	9,089
Dividends paid	(19,220)	(17,023)	(76,445)	(67,955)
Centuri distribution to redeemable noncontrolling interest	(99)		(198)
Issuance of long-term debt, net	49,375	50,295	134,896	318,498
Retirement of long-term debt	(42,312)	(18,082)	(212,203)	(153,456)
Change in credit facility and commercial paper	(150,000)	(150,000)
Change in short-term debt	(18,000)	(5,000)
Principal payments on capital lease obligations	(313)	(386)	(1,347)	(820)
Other	142	(329)	512	(1,066)
Net cash provided by (used in) financing activities	(180,026)	(131,734)	(127,779)	104,290
Effects of currency translation on cash and cash equivalents	(41)	(688)	(760)	(546)
Change in cash and cash equivalents	24,812	(1,553)	22,796	(47,353)
Cash and cash equivalents at beginning of period	35,997	39,566	38,013	85,366
Cash and cash equivalents at end of period	60,809	38,013	60,809	38,013
Supplemental information:
Interest paid, net of amounts capitalized	10,593	10,497	66,719	65,575
Income taxes paid	$ 2,136	$ 2,101	$ 43,260	$ 26,110

Nature of Operations and Basis

Nature of Operations and Basis of Presentation	3 Months Ended
	Mar. 31, 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. Basis of Presentation. Prepaids and other current assets. Cash and Cash Equivalents. Significant non-cash investing and financing activities for the natural gas operations segment included the following: Upon contract expiration, customer advances of approximately $900,000 and $700,000, during the first three months of 2016 and 2015, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity. In addition, approximately $2.3 million in proceeds from common stock sales during March 2015 associated with the Equity Shelf Program (see Note 5 – Common Stock) Goodwill. (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Foreign currency translation adjustment — 6,629 6,629 March 31, 2016 $ 10,095 $ 122,679 $ 132,774 Intercompany Transactions Note 3 - Segment Information March 31, 2016 December 31, 2015 Centuri accounts receivable for services provided to Southwest $ 11,089 $ 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. March 31, 2016 December 31, 2015 Centuri property and equipment $ 433 $ 423 Centuri accumulated provision for depreciation and amortization (216 ) (221 ) Net cash surrender value of COLI policies 100 99 Other property 11 13 Total $ 328 $ 314 Other Income (Deductions). Three Months Ended Twelve Months Ended March 31 March 31 2016 2015 2016 2015 Change in COLI policies $ 900 $ 1,300 $ (900 ) $ 5,700 Interest income 367 590 1,950 2,695 Equity AFUDC 532 396 3,144 1,894 Foreign transaction gain (loss) (10 ) (327 ) (507 ) (505 ) Miscellaneous income and (expense) (68 ) 313 (1,359 ) (2,017 ) Total other income (deductions) $ 1,721 $ 2,272 $ 2,328 $ 7,767 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. Recently Issued Accounting Standards Updates. 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 the 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 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 “Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net)”. The amendments relate to when another party, along with the entity, is involved in providing a good or service to a customer. Topic 606 Revenue from Contracts with Customers requires an entity to determine whether the nature of its promise is to provide that good or service to the customer (i.e., the entity is a principal) or to arrange for the good or service to be provided to the customer by the other party (i.e., the entity is an agent). The amendments are intended to improve the operability and understandability of the implementation guidance on principal versus agent considerations. The effective date and transition of these amendments is the same as the effective date and transition of ASU 2014-09, Revenue from Contracts with Customers (Topic 606). The Company plans to adopt the update at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2018. 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 April 2016, the FASB issued the update “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing”. The update clarifies the following aspects of Topic 606: (a) identifying performance obligations; and (b) the licensing implementation guidance. The amendments do not change the core principle of the guidance in Topic 606. Entities may choose between two retrospective transition methods when applying the update. The Company plans to adopt the update at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2018. The transition methods and the adoption date are the same as those disclosed for the May 2014 Topic 606 update noted above. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	3 Months Ended
	Mar. 31, 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. 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 March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 5,709 $ 6,280 $ 24,552 $ 22,300 Interest cost 11,506 11,058 44,677 43,637 Expected return on plan assets (14,140 ) (14,452 ) (57,496 ) (54,458 ) Amortization of net actuarial loss 6,317 8,185 30,875 25,340 Net periodic benefit cost $ 9,392 $ 11,071 $ 42,608 $ 36,819 SERP Period Ended March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 82 $ 80 $ 322 $ 299 Interest cost 465 423 1,737 1,732 Amortization of net actuarial loss 346 324 1,315 911 Net periodic benefit cost $ 893 $ 827 $ 3,374 $ 2,942 PBOP Period Ended March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 374 $ 411 $ 1,604 $ 1,236 Interest cost 796 749 3,046 2,871 Expected return on plan assets (788 ) (866 ) (3,386 ) (3,314 ) Amortization of prior service costs 334 333 1,336 599 Amortization of net actuarial loss 104 87 362 87 Net periodic benefit cost $ 820 $ 714 $ 2,962 $ 1,479

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 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 March 31, 2016 Revenues from external customers $ 525,100 $ 183,961 $ 709,061 Intersegment revenues — 22,187 22,187 Total $ 525,100 $ 206,148 $ 731,248 Segment net income (loss) $ 77,583 $ (2,137 ) $ 75,446 Three months ended March 31, 2015 Revenues from external customers $ 553,115 $ 161,089 $ 714,204 Intersegment revenues — 20,016 20,016 Total $ 553,115 $ 181,105 $ 734,220 Segment net income (loss) $ 78,921 $ (6,938 ) $ 71,983 Natural Gas Construction Operations Services Total Twelve months ended March 31, 2016 Revenues from external customers $ 1,426,624 $ 927,742 $ 2,354,366 Intersegment revenues — 106,287 106,287 Total $ 1,426,624 $ 1,034,029 $ 2,460,653 Segment net income $ 110,287 $ 31,493 $ 141,780 Twelve months ended March 31, 2015 Revenues from external customers $ 1,448,709 $ 710,368 $ 2,159,077 Intersegment revenues — 88,454 88,454 Total $ 1,448,709 $ 798,822 $ 2,247,531 Segment net income $ 123,194 $ 19,132 $ 142,326

Derivatives and Fair Value Meas

Derivatives and Fair Value Measurements	3 Months Ended
	Mar. 31, 2016
Fair Value Disclosures [Abstract]
Derivatives and Fair Value Measurements	Note 4 – Derivatives and Fair Value Measurements Derivatives. 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 April 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): March 31, 2016 December 31, 2015 Contract notional amounts 8,076 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- and twelve-month periods ended March 31, 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) Instrument Location of Gain or (Loss) Recognized in Income on Derivative Three Months Ended Twelve Months Ended March 31 March 31 2016 2015 2016 2015 Swaps Net cost of gas sold $ (1,212 ) $ (2,114 ) $ (6,696 ) $ (10,467 ) Swaps Net cost of gas sold 1,212 * 2,114 * 6,696 * 10,467 * 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. The second FSIRS terminated 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: March 31, 2016 Balance Sheet Location Asset Liability Net Total Swaps Prepaids and other current assets $ 116 $ — $ 116 Swaps Other current liabilities 120 (2,523 ) (2,403 ) Swaps Other deferred credits — (88 ) (88 ) Total $ 236 $ (2,611 ) $ (2,375 ) December 31, 2015 Balance Sheet Location Asset Liability 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 either period shown in the above table. Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income. The following table shows the amounts Southwest paid to counterparties for settlements of matured Swaps. (Thousands of dollars) Three Months Ended Twelve Months Ended Paid to counterparties $ 4,324 $ 7,891 No amounts were received from counterparties during either period indicated above. The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars). March 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other current liabilities $ (116 ) Swaps Prepaids and other current assets 2,403 Swaps Deferred charges and other assets 88 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 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) March 31, 2016 December 31, 2015 Assets at fair value: Prepaids and other current assets - Swaps $ 116 $ — Liabilities at fair value: Other current liabilities - Swaps (2,403 ) (4,267 ) Other deferred credits - Swaps (88 ) (1,219 ) Net Assets (Liabilities) $ (2,375 ) $ (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

Common Stock	3 Months Ended
	Mar. 31, 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 three months ending March 31, 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 March 31, 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 three months ended March 31, 2016, the Company issued approximately 93,000 shares of common stock through the Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan.

Long-Term Debt

Long-Term Debt	3 Months Ended
	Mar. 31, 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 March 31, 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 March 31, 2016 December 31, 2015 Carrying Market Carrying Market (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 132,708 $ 125,000 $ 130,273 Notes, 6.1%, due 2041 125,000 143,638 125,000 141,581 Notes, 3.875%, due 2022 250,000 265,240 250,000 253,600 Notes, 4.875%, due 2043 250,000 255,485 250,000 251,483 8% Series, due 2026 75,000 97,286 75,000 97,035 Medium-term notes, 7.59% series, due 2017 25,000 26,016 25,000 26,253 Medium-term notes, 7.78% series, due 2022 25,000 30,439 25,000 29,855 Medium-term notes, 7.92% series, due 2027 25,000 32,188 25,000 31,890 Medium-term notes, 6.76% series, due 2027 7,500 8,762 7,500 8,684 Unamortized discount and debt issuance costs (6,017 ) (6,137 ) 901,483 901,363 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 100,000 101,410 100,000 100,452 4.75% 2006 Series A, due 2036 24,855 25,084 24,855 25,130 Unamortized discount and debt issuance costs (3,765 ) (3,946 ) 321,090 320,909 Centuri term loan facility 117,556 117,658 112,571 112,665 Unamortized debt issuance costs (601 ) (692 ) 116,955 111,879 Centuri secured revolving credit facility 38,885 38,912 60,627 60,724 Centuri other debt obligations 59,151 58,963 25,901 26,059 1,437,564 1,570,679 Less: current maturities (48,596 ) (19,475 ) Long-term debt, less current maturities $ 1,388,968 $ 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 will continue to use $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 March 31, 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 March 31, 2016, no borrowings were outstanding on either the short-term or long-term portions of the credit facility. Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2019. At March 31, 2016, $156 million in borrowings were outstanding on the Centuri facility. Centuri assets securing the facility at March 31, 2016 totaled $450 million. In January 2016, Centuri entered into a $40 million equipment loan due in February 2021 under an existing master loan and security agreement.

Equity, Other Comprehensive Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 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 three months ended March 31, 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 93 93 2,015 2,108 Net income (loss) 75,446 (16 ) 75,430 (75 ) Foreign currency exchange translation adj. 756 756 26 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 607 607 Amounts reclassified to net income, net of tax (FSIRS) 519 519 Centuri distribution to redeemable noncontrolling interest (99 ) Dividends declared Common: $0.45 per share (21,562 ) (21,562 ) MARCH 31, 2016 47,470 $ 49,100 $ 898,463 $ (48,386 ) $ 753,105 $ (2,099 ) $ 1,650,183 $ 15,960 The following information provides insight into amounts impacting Other Comprehensive Income (Loss), both before and after-tax, 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 Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) Three Months Ended March 31, 2016 Three Months Ended March 31, 2015 Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 334 $ (127 ) $ 207 $ 333 $ (127 ) $ 206 Amortization of net actuarial (gain)/loss 6,767 (2,571 ) 4,196 8,596 (3,266 ) 5,330 Regulatory adjustment (6,123 ) 2,327 (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 836 (317 ) 519 836 (317 ) 519 FSIRS other comprehensive income 836 (317 ) 519 836 (317 ) 519 Foreign currency translation adjustments: Translation adjustments 782 — 782 (1,272 ) — (1,272 ) Foreign currency other comprehensive income (loss) 782 — 782 (1,272 ) — (1,272 ) Total other comprehensive income (loss) $ 2,596 $ (688 ) $ 1,908 $ 706 $ (751 ) $ (45 ) Twelve Months Ended March 31, 2016 Twelve Months Ended March 31, 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 1,336 (507 ) 829 599 (228 ) 371 Amortization of net actuarial (gain)/loss 32,552 (12,370 ) 20,182 26,338 (10,008 ) 16,330 Prior service cost — — — (6,661 ) 2,531 (4,130 ) Regulatory adjustment (3,982 ) 1,514 (2,468 ) 137,699 (52,326 ) 85,373 Pension plans other comprehensive income (loss) (613 ) 234 (379 ) (15,671 ) 5,954 (9,717 ) FSIRS (designated hedging activities): Amounts reclassifed into net income 3,344 (1,271 ) 2,073 3,345 (1,271 ) 2,074 FSIRS other comprehensive income (loss) 3,344 (1,271 ) 2,073 3,345 (1,271 ) 2,074 Foreign currency translation adjustments: Translation adjustments 100 — 100 (1,931 ) — (1,931 ) Foreign currency other comprehensive income (loss) 100 — 100 (1,931 ) — (1,931 ) Total other comprehensive income (loss) $ 2,831 $ (1,037 ) $ 1,794 $ (14,257 ) $ 4,683 $ (9,574 ) (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 March 31, 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 After- Tax Before- Tax Tax After-Tax Before- Tax Tax 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 — — — — — — 782 — 782 782 Other comprehensive income before reclassifications — — — — — — 782 — 782 782 FSIRS amounts reclassified from AOCI (1) — — — 836 (317 ) 519 — — — 519 Amortization of prior service cost (2) 334 (127 ) 207 — — — — — — 207 Amortization of net actuarial loss (2) 6,767 (2,571 ) 4,196 — — — — — — 4,196 Regulatory adjustment (3) (6,123 ) 2,327 (3,796 ) — — — — — — (3,796 ) Net current period other comprehensive income (loss) 978 (371 ) 607 836 (317 ) 519 782 — 782 1,908 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 26 — 26 26 Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 978 (371 ) 607 836 (317 ) 519 756 — 756 1,882 Ending Balance AOCI March 31, 2016 $ (56,682 ) $ 21,540 $ (35,142 ) $ (18,508 ) $ 7,033 $ (11,475 ) $ (1,769 ) $ — $ (1,769 ) $ (48,386 ) (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 (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) March 31, 2016 December 31, 2015 Net actuarial (loss) gain $ (428,502 ) $ (435,269 ) Prior service cost (6,704 ) (7,038 ) Less: amount recognized in regulatory assets 378,524 384,647 Recognized in AOCI $ (56,682 ) $ (57,660 )

Construction Services Redeemabl

Construction Services Redeemable Noncontrolling Interest	3 Months Ended
	Mar. 31, 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 in Centuri. The previous owners 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 March 31, 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 (Thousands of dollars): Balance, December 31, 2015 $ 16,108 Net income (loss) attributable to redeemable noncontrolling interest (75 ) Foreign currency exchange translation adjustment 26 Centuri distribution to redeemable noncontrolling interest (99 ) Balance, March 31, 2016 $ 15,960

Nature of Operations and Basi15

Nature of Operations and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations.
Basis of Presentation	Basis of Presentation.
Prepaids and Other Current Assets	Prepaids and other current assets.
Cash and Cash Equivalents	Cash and Cash Equivalents. Significant non-cash investing and financing activities for the natural gas operations segment included the following: Upon contract expiration, customer advances of approximately $900,000 and $700,000, during the first three months of 2016 and 2015, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity. In addition, approximately $2.3 million in proceeds from common stock sales during March 2015 associated with the Equity Shelf Program (see Note 5 – Common Stock)
Goodwill	Goodwill. (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Foreign currency translation adjustment — 6,629 6,629 March 31, 2016 $ 10,095 $ 122,679 $ 132,774
Intercompany Transactions	Intercompany Transactions Note 3 - Segment Information March 31, 2016 December 31, 2015 Centuri accounts receivable for services provided to Southwest $ 11,089 $ 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. March 31, 2016 December 31, 2015 Centuri property and equipment $ 433 $ 423 Centuri accumulated provision for depreciation and amortization (216 ) (221 ) Net cash surrender value of COLI policies 100 99 Other property 11 13 Total $ 328 $ 314
Other Income (Deductions)	Other Income (Deductions). Three Months Ended Twelve Months Ended March 31 March 31 2016 2015 2016 2015 Change in COLI policies $ 900 $ 1,300 $ (900 ) $ 5,700 Interest income 367 590 1,950 2,695 Equity AFUDC 532 396 3,144 1,894 Foreign transaction gain (loss) (10 ) (327 ) (507 ) (505 ) Miscellaneous income and (expense) (68 ) 313 (1,359 ) (2,017 ) Total other income (deductions) $ 1,721 $ 2,272 $ 2,328 $ 7,767 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.
Recently Issued Accounting Standards Updates	Recently Issued Accounting Standards Updates. 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 the 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 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 “Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net)”. The amendments relate to when another party, along with the entity, is involved in providing a good or service to a customer. Topic 606 Revenue from Contracts with Customers requires an entity to determine whether the nature of its promise is to provide that good or service to the customer (i.e., the entity is a principal) or to arrange for the good or service to be provided to the customer by the other party (i.e., the entity is an agent). The amendments are intended to improve the operability and understandability of the implementation guidance on principal versus agent considerations. The effective date and transition of these amendments is the same as the effective date and transition of ASU 2014-09, Revenue from Contracts with Customers (Topic 606). The Company plans to adopt the update at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2018. 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 April 2016, the FASB issued the update “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing”. The update clarifies the following aspects of Topic 606: (a) identifying performance obligations; and (b) the licensing implementation guidance. The amendments do not change the core principle of the guidance in Topic 606. Entities may choose between two retrospective transition methods when applying the update. The Company plans to adopt the update at the required adoption date, which is for interim and annual reporting periods commencing January 1, 2018. The transition methods and the adoption date are the same as those disclosed for the May 2014 Topic 606 update noted above. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.

Nature of Operations and Basi16

Nature of Operations and Basis of Presentation (Tables)	3 Months Ended
	Mar. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Goodwill	(In thousands of dollars) Natural Gas Construction Consolidated December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Foreign currency translation adjustment — 6,629 6,629 March 31, 2016 $ 10,095 $ 122,679 $ 132,774
Intercompany Transactions	Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): March 31, 2016 December 31, 2015 Centuri accounts receivable for services provided to Southwest $ 11,089 $ 10,006
Schedule of Other Property and Investments	Other Property and Investments. March 31, 2016 December 31, 2015 Centuri property and equipment $ 433 $ 423 Centuri accumulated provision for depreciation and amortization (216 ) (221 ) Net cash surrender value of COLI policies 100 99 Other property 11 13 Total $ 328 $ 314
Other Income (Deductions)	Three Months Ended Twelve Months Ended March 31 March 31 2016 2015 2016 2015 Change in COLI policies $ 900 $ 1,300 $ (900 ) $ 5,700 Interest income 367 590 1,950 2,695 Equity AFUDC 532 396 3,144 1,894 Foreign transaction gain (loss) (10 ) (327 ) (507 ) (505 ) Miscellaneous income and (expense) (68 ) 313 (1,359 ) (2,017 ) Total other income (deductions) $ 1,721 $ 2,272 $ 2,328 $ 7,767

Components of Net Periodic Be17

Components of Net Periodic Benefit Cost (Tables)	3 Months Ended
	Mar. 31, 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 March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 5,709 $ 6,280 $ 24,552 $ 22,300 Interest cost 11,506 11,058 44,677 43,637 Expected return on plan assets (14,140 ) (14,452 ) (57,496 ) (54,458 ) Amortization of net actuarial loss 6,317 8,185 30,875 25,340 Net periodic benefit cost $ 9,392 $ 11,071 $ 42,608 $ 36,819 SERP Period Ended March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 82 $ 80 $ 322 $ 299 Interest cost 465 423 1,737 1,732 Amortization of net actuarial loss 346 324 1,315 911 Net periodic benefit cost $ 893 $ 827 $ 3,374 $ 2,942 PBOP Period Ended March 31, Three Months Twelve Months 2016 2015 2016 2015 (Thousands of dollars) Service cost $ 374 $ 411 $ 1,604 $ 1,236 Interest cost 796 749 3,046 2,871 Expected return on plan assets (788 ) (866 ) (3,386 ) (3,314 ) Amortization of prior service costs 334 333 1,336 599 Amortization of net actuarial loss 104 87 362 87 Net periodic benefit cost $ 820 $ 714 $ 2,962 $ 1,479

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2016 Revenues from external customers $ 525,100 $ 183,961 $ 709,061 Intersegment revenues — 22,187 22,187 Total $ 525,100 $ 206,148 $ 731,248 Segment net income (loss) $ 77,583 $ (2,137 ) $ 75,446 Three months ended March 31, 2015 Revenues from external customers $ 553,115 $ 161,089 $ 714,204 Intersegment revenues — 20,016 20,016 Total $ 553,115 $ 181,105 $ 734,220 Segment net income (loss) $ 78,921 $ (6,938 ) $ 71,983 Natural Gas Construction Operations Services Total Twelve months ended March 31, 2016 Revenues from external customers $ 1,426,624 $ 927,742 $ 2,354,366 Intersegment revenues — 106,287 106,287 Total $ 1,426,624 $ 1,034,029 $ 2,460,653 Segment net income $ 110,287 $ 31,493 $ 141,780 Twelve months ended March 31, 2015 Revenues from external customers $ 1,448,709 $ 710,368 $ 2,159,077 Intersegment revenues — 88,454 88,454 Total $ 1,448,709 $ 798,822 $ 2,247,531 Segment net income $ 123,194 $ 19,132 $ 142,326

Derivatives and Fair Value Me19

Derivatives and Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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): March 31, 2016 December 31, 2015 Contract notional amounts 8,076 7,407
Amount of Gain or Losses Recognized in Income on Derivatives	The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three- and twelve-month periods ended March 31, 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) Instrument Location of Gain or (Loss) Recognized in Income on Derivative Three Months Ended Twelve Months Ended March 31 March 31 2016 2015 2016 2015 Swaps Net cost of gas sold $ (1,212 ) $ (2,114 ) $ (6,696 ) $ (10,467 ) Swaps Net cost of gas sold 1,212 * 2,114 * 6,696 * 10,467 * 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: March 31, 2016 Balance Sheet Location Asset Liability Net Total Swaps Prepaids and other current assets $ 116 $ — $ 116 Swaps Other current liabilities 120 (2,523 ) (2,403 ) Swaps Other deferred credits — (88 ) (88 ) Total $ 236 $ (2,611 ) $ (2,375 ) December 31, 2015 Balance Sheet Location Asset Liability 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. (Thousands of dollars) Three Months Ended Twelve Months Ended Paid to counterparties $ 4,324 $ 7,891
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). March 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other current liabilities $ (116 ) Swaps Prepaids and other current assets 2,403 Swaps Deferred charges and other assets 88 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	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) March 31, 2016 December 31, 2015 Assets at fair value: Prepaids and other current assets - Swaps $ 116 $ — Liabilities at fair value: Other current liabilities - Swaps (2,403 ) (4,267 ) Other deferred credits - Swaps (88 ) (1,219 ) Net Assets (Liabilities) $ (2,375 ) $ (5,486 )

Long-Term Debt (Tables)

Long-Term Debt (Tables)	3 Months Ended
	Mar. 31, 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. March 31, 2016 December 31, 2015 Carrying Market Carrying Market (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 132,708 $ 125,000 $ 130,273 Notes, 6.1%, due 2041 125,000 143,638 125,000 141,581 Notes, 3.875%, due 2022 250,000 265,240 250,000 253,600 Notes, 4.875%, due 2043 250,000 255,485 250,000 251,483 8% Series, due 2026 75,000 97,286 75,000 97,035 Medium-term notes, 7.59% series, due 2017 25,000 26,016 25,000 26,253 Medium-term notes, 7.78% series, due 2022 25,000 30,439 25,000 29,855 Medium-term notes, 7.92% series, due 2027 25,000 32,188 25,000 31,890 Medium-term notes, 6.76% series, due 2027 7,500 8,762 7,500 8,684 Unamortized discount and debt issuance costs (6,017 ) (6,137 ) 901,483 901,363 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 100,000 101,410 100,000 100,452 4.75% 2006 Series A, due 2036 24,855 25,084 24,855 25,130 Unamortized discount and debt issuance costs (3,765 ) (3,946 ) 321,090 320,909 Centuri term loan facility 117,556 117,658 112,571 112,665 Unamortized debt issuance costs (601 ) (692 ) 116,955 111,879 Centuri secured revolving credit facility 38,885 38,912 60,627 60,724 Centuri other debt obligations 59,151 58,963 25,901 26,059 1,437,564 1,570,679 Less: current maturities (48,596 ) (19,475 ) Long-term debt, less current maturities $ 1,388,968 $ 1,551,204

Equity, Other Comprehensive I21

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 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 three months ended March 31, 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 93 93 2,015 2,108 Net income (loss) 75,446 (16 ) 75,430 (75 ) Foreign currency exchange translation adj. 756 756 26 Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 607 607 Amounts reclassified to net income, net of tax (FSIRS) 519 519 Centuri distribution to redeemable noncontrolling interest (99 ) Dividends declared Common: $0.45 per share (21,562 ) (21,562 ) MARCH 31, 2016 47,470 $ 49,100 $ 898,463 $ (48,386 ) $ 753,105 $ (2,099 ) $ 1,650,183 $ 15,960
Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)	Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) Three Months Ended March 31, 2016 Three Months Ended March 31, 2015 Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 334 $ (127 ) $ 207 $ 333 $ (127 ) $ 206 Amortization of net actuarial (gain)/loss 6,767 (2,571 ) 4,196 8,596 (3,266 ) 5,330 Regulatory adjustment (6,123 ) 2,327 (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 836 (317 ) 519 836 (317 ) 519 FSIRS other comprehensive income 836 (317 ) 519 836 (317 ) 519 Foreign currency translation adjustments: Translation adjustments 782 — 782 (1,272 ) — (1,272 ) Foreign currency other comprehensive income (loss) 782 — 782 (1,272 ) — (1,272 ) Total other comprehensive income (loss) $ 2,596 $ (688 ) $ 1,908 $ 706 $ (751 ) $ (45 ) Twelve Months Ended March 31, 2016 Twelve Months Ended March 31, 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 1,336 (507 ) 829 599 (228 ) 371 Amortization of net actuarial (gain)/loss 32,552 (12,370 ) 20,182 26,338 (10,008 ) 16,330 Prior service cost — — — (6,661 ) 2,531 (4,130 ) Regulatory adjustment (3,982 ) 1,514 (2,468 ) 137,699 (52,326 ) 85,373 Pension plans other comprehensive income (loss) (613 ) 234 (379 ) (15,671 ) 5,954 (9,717 ) FSIRS (designated hedging activities): Amounts reclassifed into net income 3,344 (1,271 ) 2,073 3,345 (1,271 ) 2,074 FSIRS other comprehensive income (loss) 3,344 (1,271 ) 2,073 3,345 (1,271 ) 2,074 Foreign currency translation adjustments: Translation adjustments 100 — 100 (1,931 ) — (1,931 ) Foreign currency other comprehensive income (loss) 100 — 100 (1,931 ) — (1,931 ) Total other comprehensive income (loss) $ 2,831 $ (1,037 ) $ 1,794 $ (14,257 ) $ 4,683 $ (9,574 ) (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.
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 After- Tax Before- Tax Tax After-Tax Before- Tax Tax 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 — — — — — — 782 — 782 782 Other comprehensive income before reclassifications — — — — — — 782 — 782 782 FSIRS amounts reclassified from AOCI (1) — — — 836 (317 ) 519 — — — 519 Amortization of prior service cost (2) 334 (127 ) 207 — — — — — — 207 Amortization of net actuarial loss (2) 6,767 (2,571 ) 4,196 — — — — — — 4,196 Regulatory adjustment (3) (6,123 ) 2,327 (3,796 ) — — — — — — (3,796 ) Net current period other comprehensive income (loss) 978 (371 ) 607 836 (317 ) 519 782 — 782 1,908 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 26 — 26 26 Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 978 (371 ) 607 836 (317 ) 519 756 — 756 1,882 Ending Balance AOCI March 31, 2016 $ (56,682 ) $ 21,540 $ (35,142 ) $ (18,508 ) $ 7,033 $ (11,475 ) $ (1,769 ) $ — $ (1,769 ) $ (48,386 ) (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 (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) March 31, 2016 December 31, 2015 Net actuarial (loss) gain $ (428,502 ) $ (435,269 ) Prior service cost (6,704 ) (7,038 ) Less: amount recognized in regulatory assets 378,524 384,647 Recognized in AOCI $ (56,682 ) $ (57,660 )

Construction Services Redeema22

Construction Services Redeemable Noncontrolling Interest (Tables)	3 Months Ended
	Mar. 31, 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 (Thousands of dollars): Balance, December 31, 2015 $ 16,108 Net income (loss) attributable to redeemable noncontrolling interest (75 ) Foreign currency exchange translation adjustment 26 Centuri distribution to redeemable noncontrolling interest (99 ) Balance, March 31, 2016 $ 15,960

Nature of Operations and Basi23

Nature of Operations and Basis of Presentation - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016USD ($)Segment	Mar. 31, 2015USD ($)	Dec. 31, 2015USD ($)
Significant Accounting Policies [Line Items]
Number of segments | Segment	2
Non-cash construction advances - non-cash investing item	$ 900,000	$ 700,000
Proceeds from common stock sales		$ 2,300,000
Goodwill impairment charges	$ 0
Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary	96.60%
Pooled Funds and Mutual Funds [Member] | Level 2 - Significant Other Observable Inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments	$ 11,300,000		$ 250,000
Gas Pipe Materials and Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies	$ 27,000,000		$ 24,000,000

Nature of Operations and Basi24

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016USD ($)
Goodwill [Line Items]
Goodwill, Beginning balance	$ 126,145
Foreign currency translation adjustment	6,629
Goodwill, Ending balance	132,774
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	10,095
Goodwill, Ending balance	10,095
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	116,050
Foreign currency translation adjustment	6,629
Goodwill, Ending balance	$ 122,679

Nature of Operations and Basi25

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) - USD ($) $ in Thousands	Mar. 31, 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	$ 11,089	$ 10,006

Nature of Operations and Basi26

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Net cash surrender value of COLI policies	$ 100,000	$ 99,000
Other property	11,000	13,000
Total	328,115	313,531
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property and equipment	433,000	423,000
Centuri accumulated provision for depreciation and amortization	$ (216,000)	$ (221,000)

Nature of Operations and Basi27

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Other Income and Expenses [Abstract]
Change in COLI policies	$ 900	$ 1,300	$ (900)	$ 5,700
Interest income	367	590	1,950	2,695
Equity AFUDC	532	396	3,144	1,894
Foreign transaction gain (loss)	(10)	(327)	(507)	(505)
Miscellaneous income and (expense)	(68)	313	(1,359)	(2,017)
Total other income (deductions)	$ 1,721	$ 2,272	$ 2,328	$ 7,767

Components of Net Periodic Be28

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,709	$ 6,280	$ 24,552	$ 22,300
Interest cost	11,506	11,058	44,677	43,637
Expected return on plan assets	(14,140)	(14,452)	(57,496)	(54,458)
Amortization of net actuarial loss	6,317	8,185	30,875	25,340
Net periodic benefit cost	9,392	11,071	42,608	36,819
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	82	80	322	299
Interest cost	465	423	1,737	1,732
Amortization of net actuarial loss	346	324	1,315	911
Net periodic benefit cost	893	827	3,374	2,942
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	374	411	1,604	1,236
Interest cost	796	749	3,046	2,871
Expected return on plan assets	(788)	(866)	(3,386)	(3,314)
Amortization of prior service costs	334	333	1,336	599
Amortization of net actuarial loss	104	87	362	87
Net periodic benefit cost	$ 820	$ 714	$ 2,962	$ 1,479

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Total operating revenues	$ 731,248	$ 734,220	$ 2,460,653	$ 2,247,531
Segment net income (loss)	75,446	71,983	141,780	142,326
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	709,061	714,204	2,354,366	2,159,077
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	22,187	20,016	106,287	88,454
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	525,100	553,115	1,426,624	1,448,709
Segment net income (loss)	77,583	78,921	110,287	123,194
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	525,100	553,115	1,426,624	1,448,709
Natural Gas Operations [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	206,148	181,105	1,034,029	798,822
Segment net income (loss)	(2,137)	(6,938)	31,493	19,132
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	183,961	161,089	927,742	710,368
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 22,187	$ 20,016	$ 106,287	$ 88,454

Derivatives and Fair Value Me30

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 519,000	$ 519,000	$ 2,073,000	$ 2,074,000
Derivative instrument loss at settlement amortization period	10 years
Received from counterparties	$ 0		$ 0
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	Apr. 30, 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 Me31

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Mar. 31, 2016	Dec. 31, 2015
Offsetting [Abstract]
Contract notional amounts	8,076,000	7,407,000

Derivatives and Fair Value Me32

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 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	(1,212)	(2,114)	(6,696)	(10,467)
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	$ 1,212	$ 2,114	$ 6,696	$ 10,467

Derivatives and Fair Value Me33

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 236	$ 4
Liability derivatives not designated as hedging instruments	(2,611)	(5,490)
Net total not designated as hedging instruments	(2,375)	(5,486)
Swaps [Member] | Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	116
Liability derivatives not designated as hedging instruments	0
Net total not designated as hedging instruments	116
Swaps [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	120	0
Liability derivatives not designated as hedging instruments	(2,523)	(4,267)
Net total not designated as hedging instruments	(2,403)	(4,267)
Swaps [Member] | Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	0	4
Liability derivatives not designated as hedging instruments	(88)	(1,223)
Net total not designated as hedging instruments	$ (88)	$ (1,219)

Derivatives and Fair Value Me34

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Mar. 31, 2016
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 4,324	$ 7,891

Derivatives and Fair Value Me35

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ (116)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	2,403	$ 4,267
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 88	$ 1,219

Derivatives and Fair Value Me36

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant Other Observable Inputs [Member] - Swaps [Member] - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (2,375)	$ (5,486)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	116	0
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(2,403)	(4,267)
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (88)	$ (1,219)

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($)	Mar. 10, 2015	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Value of common stock	$ 100,000,000
Net proceeds to Company		$ 401,000	$ 8,791,000	$ 27,006,000	$ 9,089,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan		93,000
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

Long-Term Debt - Schedule of Ca

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	$ (48,596)	$ (19,475)
Long-term debt, less current maturities	1,388,968	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	(601)	(692)
Total long-term debt	1,437,564	1,570,679
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(48,596)	(19,475)
Long-term debt, less current maturities	1,388,968	1,551,204
Total long-term debt	1,437,564	1,570,679
Carrying Amount [Member] | Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	38,885	60,627
Carrying Amount [Member] | IDRBs [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	116,955	111,879
Unsecured debt	321,090	320,909
Carrying Amount [Member] | Centuri Term Loan Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	117,556	112,571
Carrying Amount [Member] | Centuri Other Debt Obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	59,151	25,901
Carrying Amount [Member] | Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(6,017)	(6,137)
Notes payable	901,483	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] | 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	(3,765)	(3,946)
Carrying Amount [Member] | Fixed-Rate Bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000	100,000
Carrying Amount [Member] | Fixed-Rate Bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855	24,855
Market Value [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	117,658	112,665
Centuri other debt obligations	58,963	26,059
Market Value [Member] | Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	38,912	60,724
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	132,708	130,273
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	143,638	141,581
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	265,240	253,600
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	255,485	251,483
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	97,286	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	26,016	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	30,439	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	32,188	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	8,762	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	101,410	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	$ 25,084	$ 25,130

Long-Term Debt - Schedule of 39

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 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. 1, 2020	Dec. 1, 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. 1, 2022	Apr. 1, 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. 1, 2043	Oct. 1, 2043
Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 1, 2026	Aug. 1, 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. 3, 2022	Feb. 3, 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. 1, 2028	Dec. 1, 2028
Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 1, 2038	Mar. 1, 2038
Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 1, 2038	Mar. 1, 2038
Variable-rate bonds [Member] | Market Value [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 1, 2039	Dec. 1, 2039
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. 1, 2035	Oct. 1, 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. 1, 2036	Sep. 1, 2036

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	Jan. 31, 2016	Mar. 31, 2016
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
Credit facility expiration date		Oct. 31, 2019
Borrowings outstanding under facility		$ 156,000,000
Debt secured by assets		450,000,000
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. 1, 2021
Borrowings outstanding under facility		$ 0
Southwest Gas Credit Facility [Member] | Before Amendment [Member]
Debt Instrument [Line Items]
Previous credit facility expiration date		Mar. 1, 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 loan	$ 40,000,000
Debt instrument due date	Feb. 15, 2021

Equity, Other Comprehensive I41

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Schedule Of Stockholders Equity [Line Items]
Balance	$ 1,592,325
Balance	$ 16,108
Balance, shares	47,377,575
Common stock issuances	$ 2,108
Net income (loss)	75,430
Net income (loss)	75,355	$ 71,879	$ 142,906	$ 142,330
Foreign currency exchange translation adj.	756
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	607	708	(379)	(9,717)
Amounts reclassified into net income	519	$ 519	2,073	$ 2,074
Dividends declared Common	(21,562)
Balance	15,960		15,960
Balance	$ 1,650,183		$ 1,650,183
Balance, shares	47,470,282		47,470,282
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 16,108
Net income (loss) attributable to temporary equity	(75)
Foreign currency exchange translation adj attributable to temporary equity	26
Other comprehensive income (loss):
Centuri distribution to redeemable noncontrolling interest	(99)
Centuri distribution to redeemable noncontrolling interest	(99)
Balance	15,960		$ 15,960
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(2,083)
Net income (loss)	(16)
Other comprehensive income (loss):
Balance	(2,099)		(2,099)
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 49,007
Balance, shares	47,377,000
Common stock issuances	$ 93
Common stock issuances, Shares	93,000
Other comprehensive income (loss):
Balance	$ 49,100		$ 49,100
Balance, shares	47,470,000		47,470,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 896,448
Common stock issuances	2,015
Other comprehensive income (loss):
Balance	898,463		$ 898,463
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(50,268)
Foreign currency exchange translation adj.	756
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	607
Amounts reclassified into net income	519
Balance	(48,386)		(48,386)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	699,221
Net income (loss)	75,446
Other comprehensive income (loss):
Dividends declared Common	(21,562)
Balance	$ 753,105		$ 753,105

Equity, Other Comprehensive I42

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Parenthetical) (Detail) - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 0.450	$ 0.405	$ 1.665	$ 1.500

Equity, Other Comprehensive I43

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		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Equity [Abstract]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 333	$ 1,336	$ 599
Amortization of net actuarial (gain)/loss, Before-Tax Amount	6,767	8,596	32,552	26,338
Regulatory adjustment, Before-Tax Amount	(6,123)	(7,787)	(3,982)	137,699
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	978	1,142	(613)	(15,671)
Amounts reclassified into net income, Before-Tax Amount	836	836	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	3,344	3,345
Translation adjustments, Before-Tax Amount	782	(1,272)	100	(1,931)
Foreign currency other comprehensive income (loss), Before-Tax Amount	782	(1,272)	100	(1,931)
Total other comprehensive income (loss), Before-Tax Amount	2,596	706	2,831	(14,257)
Amortization of prior service cost, Tax (Expense) or Benefit	(127)	(127)	(507)	(228)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,571)	(3,266)	(12,370)	(10,008)
Regulatory adjustment, Tax (Expense) or Benefit	2,327	2,959	1,514	(52,326)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(371)	(434)	234	5,954
Amounts reclassified into net income, Tax (Expense) or Benefit	(317)	(317)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(317)	(317)	(1,271)	(1,271)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(688)	(751)	(1,037)	4,683
Amortization of prior service cost, Net-of-Tax Amount	207	206	829	371
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	4,196	5,330	20,182	16,330
Regulatory adjustment, Net-of-Tax Amount	(3,796)	(4,828)	(2,468)	85,373
Pension plans other comprehensive income (loss), Net-of-Tax Amount	607	708	(379)	(9,717)
Amounts reclassified into net income	519	519	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	519	2,073	2,074
Translation adjustments, Net-of-Tax Amount	782	(1,272)	100	(1,931)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	782	(1,272)	100	(1,931)
Total other comprehensive income (loss), net of tax	$ 1,908	$ (45)	1,794	(9,574)
Net actuarial gain/(loss), Before-Tax Amount			(30,519)	(173,646)
Prior service cost, Before-Tax Amount				(6,661)
Net actuarial gain/(loss), Tax (Expense) or Benefit			11,597	65,985
Prior service cost, Tax (Expense) or Benefit				2,531
Net actuarial gain/(loss), Net-of-Tax Amount			$ (18,922)	(107,661)
Prior service cost, Net-of-Tax Amount				$ (4,130)

Equity, Other Comprehensive I44

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2016USD ($)
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Equity, Other Comprehensive I45

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Mar. 31, 2016USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

Equity, Other Comprehensive I46

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 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	334	$ 333	$ 1,336	$ 599
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	6,767	8,596	32,552	26,338
Regulatory adjustment, Defined Benefit Plans Before-Tax	(6,123)	(7,787)	(3,982)	137,699
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	978	1,142	(613)	(15,671)
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	978
Ending Balance AOCI March 31, 2016, Defined Benefit Plans Before-Tax	(56,682)		(56,682)
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	(127)	(127)	(507)	(228)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(2,571)	(3,266)	(12,370)	(10,008)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,327	2,959	1,514	(52,326)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(371)	(434)	234	5,954
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	(371)
Ending Balance AOCI March 31, 2016, Defined Benefit Plans Tax (Expense) Benefit	21,540		21,540
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	206	829	371
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	4,196	5,330	20,182	16,330
Regulatory adjustment, Defined Benefit Plans After-Tax	(3,796)
Net defined benefit pension plans	607	708	(379)	(9,717)
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	607
Ending Balance AOCI March 31, 2016, Defined Benefit Plans After-Tax	(35,142)		(35,142)
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	836	836	3,344	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	836	836	3,344	3,345
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	836
Ending Balance AOCI March 31, 2016, FSIRS - Before-Tax	(18,508)		(18,508)
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)	(1,271)	(1,271)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(317)	(317)	(1,271)	(1,271)
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	(317)
Ending Balance AOCI March 31, 2016, FSIRS - Tax (Expense) Benefit	7,033		7,033
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	519	519	2,073	2,074
Net current period other comprehensive income (loss)	519	519	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	519
Ending Balance AOCI March 31, 2016, FSIRS - After-Tax	(11,475)		(11,475)
Beginning Balance AOCI December 31, 2015, Foreign Currency Items Before-Tax	(2,525)
Translation adjustments, Foreign Currency Items Before-Tax	782	(1,272)	100	(1,931)
Other comprehensive income before reclassifications	782	(1,272)	100	(1,931)
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)	782
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax	26
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax	756
Ending Balance AOCI March 31, 2016	(1,769)		(1,769)
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
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
Ending Balance AOCI March 31, 2016, Foreign Currency items - Tax (Expense) Benefit	0		0
Beginning Balance AOCI December 31, 2015, Foreign Currency items - After-Tax	(2,525)
Translation adjustments, Foreign Currency items - After-Tax	782	(1,272)	100	(1,931)
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	782	(1,272)	100	(1,931)
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	782
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax	26
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax	756
Ending Balance AOCI March 31, 2016, Foreign Currency items - After-Tax	(1,769)		(1,769)
Beginning Balance AOCI December 31, 2015, AOCI	(50,268)
Translation adjustments, AOCI	782		782
Other comprehensive income before reclassifications, AOCI	782
Amounts reclassified into net income	519	519	2,073	2,074
Amortization of prior service cost, AOCI	207	206	829	371
Amortization of net actuarial loss	4,196	5,330	20,182	16,330
Regulatory adjustment, AOCI	(3,796)
Total other comprehensive income (loss), net of tax	1,908	$ (45)	1,794	$ (9,574)
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	26		26
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	1,882		1,882
Ending Balance AOCI March 31, 2016, AOCI	$ (48,386)		$ (48,386)

Equity, Other Comprehensive I47

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2016
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%

Equity, Other Comprehensive I48

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	Mar. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ 56,682	$ 57,660
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(428,502)	(435,269)
Prior service cost	(6,704)	(7,038)
Less: amount recognized in regulatory assets	378,524	384,647
Recognized in AOCI	$ (56,682)	$ (57,660)

Construction Services Noncontro

Construction Services Noncontrolling Interests - Additional Information (Detail) - Principal Previous Owners [Member] - Link-Line and WS Nicholls [Member]	1 Months Ended	3 Months Ended
	Oct. 31, 2014	Mar. 31, 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 in Centuri. The previous owners 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 Noncont50

Construction Services Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2016USD ($)
Redeemable Noncontrolling Interest [Line Items]
Balance	$ 16,108
Balance	15,960
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Redeemable Noncontrolling Interest [Line Items]
Balance	16,108
Net income (loss) attributable to redeemable noncontrolling interest	(75)
Foreign currency exchange translation adjustment	26
Centuri distribution to redeemable noncontrolling interest	(99)
Balance	$ 15,960