Southwest Gas 10-Q 6 Aug 15 2015 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2015	Jul. 28, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q2
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		47,057,295

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Utility plant:
Gas plant	$ 5,681,417	$ 5,556,599
Less: accumulated depreciation	(2,029,322)	(1,973,098)
Acquisition adjustments, net	460	550
Construction work in progress	79,906	74,332
Net utility plant	3,732,461	3,658,383
Other property and investments	325,449	326,743
Restricted cash	0	821
Current assets:
Cash and cash equivalents	48,909	39,566
Accounts receivable, net of allowances	273,447	281,824
Accrued utility revenue	31,800	73,900
Income taxes receivable, net	13,997	21,853
Deferred income taxes, net	10,492	2,109
Deferred purchased gas costs	3,751	87,556
Prepaids and other current assets	98,445	99,975
Total current assets	480,841	606,783
Noncurrent assets:
Goodwill	138,257	143,160
Deferred income taxes	1,473	0
Deferred charges and other assets	468,381	478,625
Total noncurrent assets	608,111	621,785
Total assets	5,146,862	5,214,515
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 47,042,327 and 46,523,184 shares)	48,672	48,153
Additional paid-in capital	873,622	851,381
Accumulated other comprehensive income (loss), net	(48,750)	(50,175)
Retained earnings	678,288	639,164
Total Southwest Gas Corporation equity	1,551,832	1,488,523
Noncontrolling interest	(2,199)	(2,257)
Total equity	1,549,633	1,486,266
Redeemable noncontrolling interest	19,301	20,042
Long-term debt, less current maturities	1,521,683	1,637,592
Total capitalization	3,090,617	3,143,900
Current liabilities:
Current maturities of long-term debt	20,050	19,192
Short-term debt	0	5,000
Accounts payable	130,363	167,988
Customer deposits	72,744	71,546
Income taxes payable	4,271	0
Accrued general taxes	39,784	44,339
Accrued interest	16,159	16,468
Deferred income taxes	16,378	0
Deferred purchased gas costs	27,216	0
Other current liabilities	139,225	145,584
Total current liabilities	466,190	470,117
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	721,352	723,688
Accumulated removal costs	302,000	304,000
Other deferred credits and other long-term liabilities	566,703	572,810
Total deferred income taxes and other credits	1,590,055	1,600,498
Total capitalization and liabilities	$ 5,146,862	$ 5,214,515

Condensed Consolidated Balance3

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	47,042,327	46,523,184
Common stock, outstanding	47,042,327	46,523,184

Condensed Consolidated Statemen

Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating revenues:
Gas operating revenues	$ 286,643	$ 271,479	$ 839,758	$ 757,972	$ 1,463,873	$ 1,325,657
Construction revenues	251,961	181,674	433,066	303,577	869,109	661,595
Total operating revenues	538,604	453,153	1,272,824	1,061,549	2,332,982	1,987,252
Operating expenses:
Net cost of gas sold	109,015	97,985	362,777	289,362	578,771	455,367
Operations and maintenance	99,344	97,620	194,854	200,028	378,558	392,920
Depreciation and amortization	66,955	62,186	134,422	125,077	262,372	244,391
Taxes other than income taxes	12,414	10,965	25,411	22,421	50,242	45,104
Construction expenses	225,829	157,642	400,757	270,841	777,773	588,737
Total operating expenses	513,557	426,398	1,118,221	907,729	2,047,716	1,726,519
Operating income	25,047	26,755	154,603	153,820	285,266	260,733
Other income and (expenses):
Net interest deductions	(17,717)	(17,305)	(35,694)	(34,824)	(72,939)	(67,356)
Other income (deductions)	162	2,863	2,434	4,475	5,066	11,254
Total other income and (expenses)	(17,555)	(14,442)	(33,260)	(30,349)	(67,873)	(56,102)
Income before income taxes	7,492	12,313	121,343	123,471	217,393	204,631
Income tax expense	2,429	2,686	44,401	43,147	79,627	70,175
Net income	5,063	9,627	76,942	80,324	137,766	134,456
Net income (loss) attributable to noncontrolling interests	114	0	10	(86)	118	(393)
Net income attributable to Southwest Gas Corporation	$ 4,949	$ 9,627	$ 76,932	$ 80,410	$ 137,648	$ 134,849
Basic earnings per share	$ 0.11	$ 0.21	$ 1.65	$ 1.73	$ 2.95	$ 2.91
Diluted earnings per share	0.10	0.21	1.63	1.71	2.92	2.88
Dividends declared per share	$ 0.405	$ 0.365	$ 0.810	$ 0.730	$ 1.540	$ 1.390
Average number of common shares outstanding	46,869	46,502	46,741	46,471	46,628	46,407
Average shares outstanding (assuming dilution)	47,290	46,948	47,164	46,910	47,070	46,860

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 5,063	$ 9,627	$ 76,942	$ 80,324	$ 137,766	$ 134,456
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(107,661)	62,214
Amortization of prior service cost	208	55	414	110	524	221
Amortization of net actuarial loss	5,328	3,666	10,658	7,333	17,992	17,927
Prior service cost	0	0	0	0	(4,130)	0
Regulatory adjustment	(4,828)	(3,210)	(9,656)	(6,420)	83,755	(73,667)
Net defined benefit pension plans	708	511	1,416	1,023	(9,520)	6,695
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	519	1,037	1,037	2,073	2,074
Net forward-starting interest rate swaps	518	519	1,037	1,037	2,073	2,074
Foreign currency translation adjustments	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), net of tax	1,435	1,030	1,390	2,060	(9,169)	8,769
Comprehensive income	6,498	10,657	78,332	82,384	128,597	143,225
Comprehensive income (loss) attributable to noncontrolling interests	122	0	(25)	(86)	61	(393)
Comprehensive income attributable to Southwest Gas Corporation	$ 6,376	$ 10,657	$ 78,357	$ 82,470	$ 128,536	$ 143,618

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 76,942	$ 80,324	$ 137,766	$ 134,456
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	134,422	125,077	262,372	244,391
Deferred income taxes	3,896	24,493	43,712	47,351
Changes in current assets and liabilities:
Accounts receivable, net of allowances	6,018	35,997	(33,662)	(23,208)
Accrued utility revenue	42,100	41,100	(200)	(1,200)
Deferred purchased gas costs	111,021	(62,224)	103,906	(108,991)
Accounts payable	(38,471)	(91,674)	11,704	(10,082)
Accrued taxes	7,614	(4,801)	(1,158)	(1,424)
Other current assets and liabilities	5,163	46,238	(17,696)	27,234
Gains on sale	(2,563)	(4,137)	(4,597)	(5,964)
Changes in undistributed stock compensation	1,512	3,766	5,719	6,869
AFUDC	(1,133)	(1,024)	(2,104)	(2,296)
Changes in other assets and deferred charges	(15,239)	(15,150)	(21,821)	(20,476)
Changes in other liabilities and deferred credits	2,954	19,462	(729)	41,775
Net cash provided by operating activities	334,236	197,447	483,212	328,435
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(203,640)	(175,444)	(425,094)	(387,843)
Acquisition of businesses, net of cash acquired	(9,261)	0	(199,758)	0
Restricted cash	785	(16,649)	18,667	(16,649)
Changes in customer advances	9,689	8,947	21,105	13,593
Miscellaneous inflows	4,892	7,060	9,443	10,620
Miscellaneous outflows	0	0	(1,400)	0
Net cash used in investing activities	(197,535)	(176,086)	(577,037)	(380,279)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	20,713	237	20,881	515
Dividends paid	(36,001)	(32,316)	(69,960)	(62,904)
Issuance of long-term debt, net	93,165	17,719	344,674	287,336
Retirement of long-term debt	(78,409)	(10,716)	(206,848)	(46,460)
Change in credit facility and commercial paper	(120,000)	(10,000)	30,000	(119,000)
Change in short-term debt	(5,000)	0	0	0
Principal payments on capital lease obligations	(722)	0	(1,156)	0
Other	(534)	(472)	(1,319)	1,527
Net cash provided by (used in) financing activities	(126,788)	(35,548)	116,272	61,014
Effects of currency translation on cash and cash equivalents	(570)	0	(428)	0
Change in cash and cash equivalents	9,343	(14,187)	22,019	9,170
Cash and cash equivalents at beginning of period	39,566	41,077	26,890	17,720
Cash and cash equivalents at end of period	48,909	26,890	48,909	26,890
Supplemental information:
Interest paid, net of amounts capitalized	34,213	31,787	67,978	61,401
Income taxes paid	$ 28,479	$ 12,246	$ 40,480	$ 15,565

Nature of Operations and Basis

Nature of Operations and Basis of Presentation	6 Months Ended
	Jun. 30, 2015
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. Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian construction services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The Company’s investment in Western is not significant in relation to its total assets included in the Condensed Consolidated Balance Sheets. 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 $2 million and $5.6 million, during the first six months of 2015 and 2014, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity. Goodwill. Note 8 – Acquisition of Construction Services Businesses (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2014 $ 10,095 $ 133,065 $ 143,160 Acquisition date adjustment — 1,380 1,380 Foreign currency translation adjustment — (6,283 ) (6,283 ) June 30, 2015 $ 10,095 $ 128,162 $ 138,257 Intercompany Transactions Note 3 – Segment Information June 30, 2015 December 31, 2014 Centuri accounts receivable for services provided to Southwest $ 10,531 $ 9,169 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. June 30, 2015 December 31, 2014 Centuri property and equipment $ 419 $ 405 Centuri accumulated provision for depreciation and amortization (206 ) (187 ) Net cash surrender value of COLI policies 101 99 Other property 11 10 Total $ 325 $ 327 Other Income (Deductions). Three Months Ended Six Months Ended Twelve Months Ended June 30 June 30 June 30 2015 2014 2015 2014 2015 2014 Change in COLI policies $ — $ 2,300 $ 1,300 $ 3,200 $ 3,400 $ 10,000 Interest income 161 612 751 1,109 2,244 1,306 Pipe replacement costs — — — — — (11 ) Foreign transaction gain (loss) (245 ) — (572 ) — (750 ) — Miscellaneous income and (expense) 246 (49 ) 955 166 172 (41 ) Total other income (deductions) $ 162 $ 2,863 $ 2,434 $ 4,475 $ 5,066 $ 11,254 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 April 2015, the FASB issued the update “Interest—Imputation of Interest (Subtopic 835-30) Simplifying the Presentation of Debt Issuance Costs.” To simplify presentation of debt issuance costs, the amendments in this update require that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs are not affected by the amendments in this update. Retrospective application of the update is required. The amendments in this update are effective for financial statements issued for fiscal years beginning after December 15, 2015, and interim periods within fiscal years beginning after December 15, 2016. Early adoption of the amendments in this update is permitted for financial statements that have not been previously issued. This update is not expected to have a material impact on the Company’s consolidated financial statements and disclosures.

Components of Net Periodic Bene

Components of Net Periodic Benefit Cost	6 Months Ended
	Jun. 30, 2015
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 June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 6,281 $ 5,340 $ 12,561 $ 10,680 $ 23,241 $ 22,208 Interest cost 11,057 10,860 22,115 21,721 43,834 40,525 Expected return on plan assets (14,452 ) (13,335 ) (28,904 ) (26,671 ) (55,575 ) (51,591 ) Amortization of net actuarial loss 8,186 5,718 16,371 11,436 27,808 27,566 Net periodic benefit cost $ 11,072 $ 8,583 $ 22,143 $ 17,166 $ 39,308 $ 38,708 SERP Period Ended June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 80 $ 73 $ 160 $ 146 $ 306 $ 332 Interest cost 424 436 847 872 1,720 1,640 Amortization of net actuarial loss 323 196 647 392 1,038 877 Net periodic benefit cost $ 827 $ 705 $ 1,654 $ 1,410 $ 3,064 $ 2,849 PBOP Period Ended June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 410 $ 275 $ 821 $ 551 $ 1,371 $ 1,161 Interest cost 750 708 1,499 1,415 2,913 2,656 Expected return on plan assets (866 ) (816 ) (1,732 ) (1,632 ) (3,364 ) (3,044 ) Amortization of prior service costs 334 88 667 177 845 355 Amortization of net actuarial loss 86 — 173 — 173 472 Net periodic benefit cost $ 714 $ 255 $ 1,428 $ 511 $ 1,938 $ 1,600

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2015
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 Operations Construction Services Total Three months ended June 30, 2015 Revenues from external customers $ 286,643 $ 229,112 $ 515,755 Intersegment revenues — 22,849 22,849 Total $ 286,643 $ 251,961 $ 538,604 Segment net income (loss) $ (657 ) $ 5,606 $ 4,949 Three months ended June 30, 2014 Revenues from external customers $ 271,479 $ 156,966 $ 428,445 Intersegment revenues — 24,708 24,708 Total $ 271,479 $ 181,674 $ 453,153 Segment net income $ 1,798 $ 7,829 $ 9,627 Natural Gas Operations Construction Services Total Six months ended June 30, 2015 Revenues from external customers $ 839,758 $ 390,201 $ 1,229,959 Intersegment revenues — 42,865 42,865 Total $ 839,758 $ 433,066 $ 1,272,824 Segment net income (loss) $ 78,264 $ (1,332 ) $ 76,932 Six months ended June 30, 2014 Revenues from external customers $ 757,972 $ 255,119 $ 1,013,091 Intersegment revenues — 48,458 48,458 Total $ 757,972 $ 303,577 $ 1,061,549 Segment net income $ 74,397 $ 6,013 $ 80,410 Natural Gas Operations Construction Services Total Twelve months ended June 30, 2015 Revenues from external customers $ 1,463,873 $ 782,514 $ 2,246,387 Intersegment revenues — 86,595 86,595 Total $ 1,463,873 $ 869,109 $ 2,332,982 Segment net income $ 120,739 $ 16,909 $ 137,648 Twelve months ended June 30, 2014 Revenues from external customers $ 1,325,657 $ 562,294 $ 1,887,951 Intersegment revenues — 99,301 99,301 Total $ 1,325,657 $ 661,595 $ 1,987,252 Segment net income $ 117,310 $ 17,539 $ 134,849

Derivatives and Fair Value Meas

Derivatives and Fair Value Measurements	6 Months Ended
	Jun. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [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 July 2015 through March 2017. Under such contracts, Southwest pays the counterparty at 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): June 30, 2015 December 31, 2014 Contract notional amounts 7,838 5,105 In late 2013, the Company suspended further Swaps and fixed-price purchases pursuant to the Volatility Mitigation Program (“VMP”) for its Nevada service territories. The decision did not impact previously executed purchase arrangements. Agreements, under the Nevada VMP, made prior to the suspension, terminated following the March 2015 delivery month. The Company, along with its regulators, will continue to evaluate this strategy in light of prevailing or anticipated changing market conditions. 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-, six-, and twelve-month periods ended June 30, 2015 and 2014 and their location in the Condensed Consolidated Statements of Income: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended Location of Gain or (Loss) June 30 June 30 June 30 Instrument Recognized in Income on Derivative 2015 2014 2015 2014 2015 2014 Swaps Net cost of gas sold $ 707 $ (83 ) $ (1,407 ) $ 5,907 $ (9,677 ) $ 9,476 Swaps Net cost of gas sold (707 )* 83 * 1,407 * (5,907 )* 9,677 * (9,476 )* Total $ — $ — $ — $ — $ — $ — * Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities. No gains (losses) were recognized in 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 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) and 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: June 30, 2015 Asset Liability Instrument Balance Sheet Location Derivatives Derivatives Net Total Swaps Other current liabilities $ 105 $ (2,098 ) $ (1,993 ) Swaps Other deferred credits 28 (208 ) (180 ) Total $ 133 $ (2,306 ) $ (2,173 ) December 31, 2014 Asset Liability Instrument Balance Sheet Location Derivatives Derivatives Net Total Swaps Other current liabilities $ — $ (5,062 ) $ (5,062 ) Swaps Other deferred credits — (363 ) (363 ) Total $ — $ (5,425 ) $ (5,425 ) The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). The Company has master netting arrangements with each counterparty that provide for the net settlement of all contracts through a single payment. As applicable, the Company has elected to reflect the net amounts in its balance sheets. The Company had no outstanding collateral associated with the Swaps during either period shown in the above table. Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income. The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps. Three Months Ended Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2015 June 30, 2015 June 30, 2015 Paid to counterparties $ 690 $ 4,660 $ 5,474 Received from counterparties $ — $ — $ 198 The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars). June 30, 2015 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 1,993 Swaps Deferred charges and other assets 180 December 31, 2014 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 5,062 Swaps Deferred charges and other assets 363 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) June 30, 2015 December 31, 2014 Liabilities at fair value: Other current liabilities—Swaps $ (1,993 ) $ (5,062 ) Other deferred credits—Swaps (180 ) (363 ) Net Assets (Liabilities) $ (2,173 ) $ (5,425 ) No financial assets or liabilities 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 2014. Refer to Note 9 – Pension and Other Post Retirement Benefits in the 2014 Annual Report to Shareholders on Form 10-K.

Common Stock

Common Stock	6 Months Ended
	Jun. 30, 2015
Equity [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 June 30, 2015, the Company sold, through the continuous equity offering program with BNY Mellon Capital Markets, LLC as agent, an aggregate of 180,247 shares of the Company’s common stock in the open market at a weighted average price of $53.90 per share, resulting in proceeds to the Company of $9,618,188, net of $97,154 in agent commissions. For the six months ended June 30, 2015, the Company sold an aggregate of 372,509 shares of common stock under this program resulting in proceeds to the Company of $20,557,144, net of $207,649 in agent commissions. As of June 30, 2015, the Company had up to $79,235,207 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 six months ended June 30, 2015, the Company issued approximately 145,000 shares of common stock through the Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan.

Long-Term Debt

Long-Term Debt	6 Months Ended
	Jun. 30, 2015
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 June 30, 2015 and December 31, 2014 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 June 30, 2015 December 31, 2014 Carrying Market Carrying Market Amount Value Amount Value (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 133,183 $ 125,000 $ 133,403 Notes, 6.1%, due 2041 125,000 146,568 125,000 157,290 Notes, 3.875%, due 2022 250,000 259,723 250,000 262,030 Notes, 4.875%, due 2043 250,000 256,505 250,000 280,903 8% Series, due 2026 75,000 99,431 75,000 102,296 Medium-term notes, 7.59% series, due 2017 25,000 27,063 25,000 27,573 Medium-term notes, 7.78% series, due 2022 25,000 30,705 25,000 31,144 Medium-term notes, 7.92% series, due 2027 25,000 32,706 25,000 33,695 Medium-term notes, 6.76% series, due 2027 7,500 8,896 7,500 9,156 Unamortized discount (5,046 ) (5,223 ) 902,454 902,277 Revolving credit facility and commercial paper 30,000 30,000 150,000 150,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt Series A, due 2028 50,000 50,000 50,000 50,000 2003 Series A, due 2038 50,000 50,000 50,000 50,000 2008 Series A, due 2038 50,000 50,000 50,000 50,000 2009 Series A, due 2039 50,000 50,000 50,000 50,000 Fixed-rate bonds: 5.25% 2003 Series D, due 2038 20,000 20,103 20,000 20,277 5.00% 2004 Series B, due 2033 — — 31,200 31,223 4.85% 2005 Series A, due 2035 100,000 100,629 100,000 100,071 4.75% 2006 Series A, due 2036 24,855 25,005 24,855 25,399 Unamortized discount (1,645 ) (1,943 ) 343,210 374,112 Centuri secured revolving credit and term loan facility 234,598 235,542 199,267 200,341 Centuri other debt obligations 31,471 31,749 31,128 31,127 1,541,733 1,656,784 Less: current maturities (20,050 ) (19,192 ) Long-term debt, less current maturities $ 1,521,683 $ 1,637,592 In March 2015, the Company amended its $300 million credit facility. The facility was previously scheduled to expire in March 2019 and was extended to March 2020. 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 June 30, 2015, 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 June 30, 2015, $5 million was outstanding on the credit facility and $25 million was outstanding on the commercial paper program. In May 2015, the Company redeemed at par the $31.2 million 5.00% 2004 Series B IDRBs originally due in 2033. The Company facilitated the redemption primarily from cash on hand and borrowings under its $300 million credit facility. Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2019. At June 30, 2015, $235 million in borrowings were outstanding on the Centuri facility. Centuri assets securing the facility at June 30, 2015 totaled $492 million.

Equity, Other Comprehensive Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	6 Months Ended
	Jun. 30, 2015
Text Block [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 six months ended June 30, 2015. 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, 2014 46,523 $ 48,153 $ 851,381 $ (50,175 ) $ 639,164 $ (2,257 ) $ 1,486,266 $ 20,042 Common stock issuances 519 519 22,241 22,760 Net income (loss) 76,932 58 76,990 (48 ) Redemption value adjustments 658 658 (658 ) Foreign currency exchange translation adj. (1,028 ) (1,028 ) (35 ) Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,416 1,416 Amounts reclassified to net income,net of tax (FSIRS) 1,037 1,037 Dividends declared Common: $0.81 per share (38,466 ) (38,466 ) JUNE 30, 2015 47,042 $ 48,672 $ 873,622 $ (48,750 ) $ 678,288 $ (2,199 ) $ 1,549,633 $ 19,301 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. 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 Three Months Ended June 30, 2015 June 30, 2014 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 334 $ (126 ) $ 208 $ 88 $ (33 ) $ 55 Amortization of net actuarial (gain)/loss 8,595 (3,267 ) 5,328 5,914 (2,248 ) 3,666 Regulatory adjustment (7,787 ) 2,959 (4,828 ) (5,177 ) 1,967 (3,210 ) Pension plans other comprehensive income (loss) 1,142 (434 ) 708 825 (314 ) 511 FSIRS (designated hedging activities): Amounts reclassifed into net income 836 (318 ) 518 837 (318 ) 519 FSIRS other comprehensive income 836 (318 ) 518 837 (318 ) 519 Foreign currency translation adjustments: Translation adjustments 209 — 209 — — — Foreign currency other comprehensive income (loss) 209 — 209 — — — Total other comprehensive income (loss) $ 2,187 $ (752 ) $ 1,435 $ 1,662 $ (632 ) $ 1,030 Six Months Ended June 30, 2015 Six Months Ended June 30, 2014 Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 667 $ (253 ) $ 414 $ 177 $ (67 ) $ 110 Amortization of net actuarial (gain)/loss 17,191 (6,533 ) 10,658 11,828 (4,495 ) 7,333 Regulatory adjustment (15,574 ) 5,918 (9,656 ) (10,355 ) 3,935 (6,420 ) Pension plans other comprehensive income (loss) 2,284 (868 ) 1,416 1, 650 (627 ) 1,023 FSIRS (designated hedging activities): Amounts reclassifed into net income 1,672 (635 ) 1,037 1,673 (636 ) 1,037 FSIRS other comprehensive income 1,672 (635 ) 1,037 1,673 (636 ) 1,037 Foreign currency translation adjustments: Translation adjustments (1,063 ) — (1,063 ) — — — Foreign currency other comprehensive income (loss) (1,063 ) — (1,063 ) — — — Total other comprehensive income (loss) $ 2,893 $ (1,503 ) $ 1,390 $ 3,323 $ (1,263 ) $ 2,060 Twelve Months Ended June 30, 2015 Twelve Months Ended June 30, 2014 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) $ (173,646 ) $ 65,985 $ (107,661 ) $ 100,345 $ (38,131 ) $ 62,214 Amortization of prior service cost 845 (321 ) 524 355 (134 ) 221 Amortization of net actuarial (gain)/loss 29,019 (11,027 ) 17,992 28,915 (10,988 ) 17,927 Prior service cost (6,661 ) 2,531 (4,130 ) — — — Regulatory adjustment 135,089 (51,334 ) 83,755 (118,817 ) 45,150 (73,667 ) Pension plans other comprehensive income (loss) (15,354 ) 5,834 (9,520 ) 10,798 (4,103 ) 6,695 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 (1,722 ) — (1,722 ) — — — Foreign currency other comprehensive income (loss) (1,722 ) — (1,722 ) — — — Total other comprehensive income (loss) $ (13,732 ) $ 4,563 $ (9,169 ) $ 14,143 $ (5,374 ) $ 8,769 (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 June 30, 2015, 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, 2014 $ (57,211 ) $ 21,740 $ (35,471 ) $ (22,688 ) $ 8,621 $ (14,067 ) $ (637 ) $ — $ (637 ) $ (50,175 ) Translation adjustments — — — — — — (1,063 ) — (1,063 ) (1,063 ) Other comprehensive income before reclassifications — — — — — — (1,063 ) — (1,063 ) (1,063 ) FSIRS amounts reclassified from AOCI (1) — — — 1,672 (635 ) 1,037 — — — 1,037 Amortization of prior service cost (2) 667 (253 ) 414 — — — — — — 414 Amortization of net actuarial loss (2) 17,191 (6,533 ) 10,658 — — — — — — 10,658 Regulatory adjustment (3) (15,574 ) 5,918 (9,656 ) — — — — — — (9,656 ) Net current period other comprehensive income (loss) 2,284 (868 ) 1,416 1,672 (635 ) 1,037 (1,063 ) — (1,063 ) 1,390 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — (35 ) — (35 ) (35 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 2,284 (868 ) 1,416 1,672 (635 ) 1,037 (1,028 ) — (1,028 ) 1,425 Ending Balance AOCI June 30, 2015 $ (54,927 ) $ 20,872 $ (34,055 ) $ (21,016 ) $ 7,986 $ (13,030 ) $ (1,665 ) $ — $ (1,665 ) $ (48,750 ) (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). The following table represents amounts (before income tax impacts) associated with defined benefit plans and 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) June 30, 2015 December 31, 2014 Net actuarial (loss) gain $ (421,940 ) $ (439,131 ) Prior service cost (7,706 ) (8,373 ) Less: amount recognized in regulatory assets 374,719 390,293 Recognized in AOCI $ (54,927 ) $ (57,211 )

Acquisition of Construction Ser

Acquisition of Construction Services Businesses	6 Months Ended
	Jun. 30, 2015
Business Combinations [Abstract]
Acquisition of Construction Services Businesses	Note 8 – Acquisition of Construction Services Businesses In October 2014, the Company, through its subsidiaries, completed the acquisition of three privately held, affiliated construction businesses. See the Company’s 2014 Form 10-K for additional information about this acquisition. Assets acquired and liabilities assumed in the transaction were recorded, generally, at their acquisition date fair values. Transaction costs associated with the acquisition were expensed as incurred. The Company’s allocation of the purchase price was based on an evaluation of the appropriate fair values and represented management’s best estimate based on available data (including market data, data regarding customers of the acquired businesses, terms of acquisition-related agreements, analysis of historical and projected results, and other types of data). The analysis included the impacts of differences between Accounting Standards for Private Enterprises in Canada and U. S. GAAP applicable to public companies, as well as consideration of types of intangibles that were acquired, including non-competition agreements, customer relationships, trade names, and work backlog. While refinements were made to the estimated fair values of assets acquired and liabilities assumed during the first quarter of 2015, no adjustments were made to the acquisition-date values during the second quarter of 2015. The final purchase accounting has not yet been completed. Further refinement could occur; however, no material changes are expected. The revised preliminary estimated fair values of assets acquired and liabilities assumed as of October 1, 2014, are as follows (in millions of dollars): Revised Cash, cash equivalents, and restricted cash $ 3 Contracts receivable and other receivables 62 Property, plant and equipment 17 Other assets 15 Intangible assets 52 Goodwill 131 Total assets acquired 280 Current liabilities 40 Deferred income tax - long-term 17 Other long-term liabilities 4 Net assets acquired $ 219 The Company incurred and expensed acquisition costs of $5 million for the twelve months ended June 30, 2015. No acquisition costs were incurred during the six months ended June 30, 2015. The preliminary allocation of the purchase price of Link-Line, W.S. Nicholls, and Brigadier was accounted for in accordance with the applicable accounting guidance. Goodwill, which is generally not deductible for tax purposes, consists of the value associated with the assembled workforce and consolidation of operations. The business of Brigadier was acquired via asset purchase. Therefore, the $4.9 million of tax-basis goodwill assigned to Brigadier is expected to be deductible for tax purposes. All other goodwill associated with the acquisition is not deductible for tax purposes.

Construction Services Redeemabl

Construction Services Redeemable Noncontrolling Interest	6 Months Ended
	Jun. 30, 2015
Noncontrolling Interest [Abstract]
Construction Services Redeemable Noncontrolling Interest	Note 9 – Construction Services Redeemable Noncontrolling Interest At the close of the acquisition discussed above, previous owners of the acquired companies retained an approximate 10% equity interest in the Canadian businesses that were acquired. The agreement, associated with the approximate 10% equity interest of the sellers, provides special dividend rights which entitle the sellers, as holders, to dividends equal to 3.4% of dividends paid at the level of Centuri and subject to certain conditions, such interests may become exchangeable for a 3.4% equity interest in Centuri. In June 2015, the previous owners notified Centuri of their intent to exchange their full equity interest in the Canadian businesses for an equity interest in Centuri, in accordance with the agreement. The exchange is anticipated to be completed in the third quarter of 2015. Additionally, the previous owners may exit their investment retained by requiring the purchase of a portion of their interest commencing October 2016 and in incremental amounts each anniversary date thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election after September 2021. The Company has determined that this noncontrolling interest is a redeemable noncontrolling interest and, in accordance with SEC guidance, is classified as mezzanine equity (temporary equity) in the Consolidated Balance Sheets. Significant changes in the value of the redeemable noncontrolling interest are recognized as they occur, and the carrying value is adjusted as necessary at each reporting date. The adjustment to the redemption value in 2015 impacted retained earnings, but did not impact net income. Redeemable (Thousands of dollars): Balance, December 31, 2014 $ 20,042 Net income (loss) attributable to redeemable noncontrolling interest (48 ) Foreign currency exchange translation adjustment (35 ) Adjustment to redemption value (658 ) Balance, June 30, 2015 $ 19,301 The redemption value of the redeemable noncontrolling interest is based on a Monte Carlo simulation method. First, a market approach was utilized to determine a construction services enterprise value as of the acquisition date. Potential guideline publicly-traded companies were identified by using a selection criteria, including actively traded equities, their financial solvency, and other factors. Once the guideline companies were determined, enterprise value was calculated using a weighted approach of projected earnings before interest expense and taxes (“EBIT”) and earnings before interest expense, taxes, and depreciation and amortization expense (“EBITDA”). After an estimated fair value was determined, a Monte Carlo simulation was used to assign a value to the noncontrolling interest of the sellers. Other assumptions used in this analysis included dividends, probability of events, and a discount due to lack of control (the sellers do not influence operations). This method is employed no less frequently than annually. Each quarter, market changes in the guideline companies are considered and the weighted approach to projected EBIT and EBITDA, in relation to the guideline companies, is re-evaluated to determine if value changes are necessary at each quarterly reporting date.

Nature of Operations and Basi16

Nature of Operations and Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of Operations	Nature of Operations.
Basis of Presentation	Basis of Presentation. Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian construction services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The Company’s investment in Western is not significant in relation to its total assets included in the Condensed Consolidated Balance Sheets.
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 $2 million and $5.6 million, during the first six months of 2015 and 2014, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.
Goodwill	Goodwill. Note 8 – Acquisition of Construction Services Businesses (In thousands of dollars) Natural Gas Construction Consolidated December 31, 2014 $ 10,095 $ 133,065 $ 143,160 Acquisition date adjustment — 1,380 1,380 Foreign currency translation adjustment — (6,283 ) (6,283 ) June 30, 2015 $ 10,095 $ 128,162 $ 138,257
Intercompany Transactions	Intercompany Transactions Note 3 – Segment Information June 30, 2015 December 31, 2014 Centuri accounts receivable for services provided to Southwest $ 10,531 $ 9,169 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. June 30, 2015 December 31, 2014 Centuri property and equipment $ 419 $ 405 Centuri accumulated provision for depreciation and amortization (206 ) (187 ) Net cash surrender value of COLI policies 101 99 Other property 11 10 Total $ 325 $ 327
Other Income (Deductions)	Other Income (Deductions). Three Months Ended Six Months Ended Twelve Months Ended June 30 June 30 June 30 2015 2014 2015 2014 2015 2014 Change in COLI policies $ — $ 2,300 $ 1,300 $ 3,200 $ 3,400 $ 10,000 Interest income 161 612 751 1,109 2,244 1,306 Pipe replacement costs — — — — — (11 ) Foreign transaction gain (loss) (245 ) — (572 ) — (750 ) — Miscellaneous income and (expense) 246 (49 ) 955 166 172 (41 ) Total other income (deductions) $ 162 $ 2,863 $ 2,434 $ 4,475 $ 5,066 $ 11,254 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 April 2015, the FASB issued the update “Interest—Imputation of Interest (Subtopic 835-30) Simplifying the Presentation of Debt Issuance Costs.” To simplify presentation of debt issuance costs, the amendments in this update require that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The recognition and measurement guidance for debt issuance costs are not affected by the amendments in this update. Retrospective application of the update is required. The amendments in this update are effective for financial statements issued for fiscal years beginning after December 15, 2015, and interim periods within fiscal years beginning after December 15, 2016. Early adoption of the amendments in this update is permitted for financial statements that have not been previously issued. This update is not expected to have a material impact on the Company’s consolidated financial statements and disclosures.

Nature of Operations and Basi17

Nature of Operations and Basis of Presentation (Tables)	6 Months Ended
	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Goodwill	(In thousands of dollars) Natural Gas Construction Consolidated December 31, 2014 $ 10,095 $ 133,065 $ 143,160 Acquisition date adjustment — 1,380 1,380 Foreign currency translation adjustment — (6,283 ) (6,283 ) June 30, 2015 $ 10,095 $ 128,162 $ 138,257
Intercompany Transactions	Centuri’s accounts receivable for these services are presented in the table below (thousands of dollars): June 30, 2015 December 31, 2014 Centuri accounts receivable for services provided to Southwest $ 10,531 $ 9,169
Schedule of Other Property and Investments	Other Property and Investments. June 30, 2015 December 31, 2014 Centuri property and equipment $ 419 $ 405 Centuri accumulated provision for depreciation and amortization (206 ) (187 ) Net cash surrender value of COLI policies 101 99 Other property 11 10 Total $ 325 $ 327
Other Income (Deductions)	The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars): Three Months Ended Six Months Ended Twelve Months Ended June 30 June 30 June 30 2015 2014 2015 2014 2015 2014 Change in COLI policies $ — $ 2,300 $ 1,300 $ 3,200 $ 3,400 $ 10,000 Interest income 161 612 751 1,109 2,244 1,306 Pipe replacement costs — — — — — (11 ) Foreign transaction gain (loss) (245 ) — (572 ) — (750 ) — Miscellaneous income and (expense) 246 (49 ) 955 166 172 (41 ) Total other income (deductions) $ 162 $ 2,863 $ 2,434 $ 4,475 $ 5,066 $ 11,254

Components of Net Periodic Be18

Components of Net Periodic Benefit Cost (Tables)	6 Months Ended
	Jun. 30, 2015
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 June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 6,281 $ 5,340 $ 12,561 $ 10,680 $ 23,241 $ 22,208 Interest cost 11,057 10,860 22,115 21,721 43,834 40,525 Expected return on plan assets (14,452 ) (13,335 ) (28,904 ) (26,671 ) (55,575 ) (51,591 ) Amortization of net actuarial loss 8,186 5,718 16,371 11,436 27,808 27,566 Net periodic benefit cost $ 11,072 $ 8,583 $ 22,143 $ 17,166 $ 39,308 $ 38,708 SERP Period Ended June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 80 $ 73 $ 160 $ 146 $ 306 $ 332 Interest cost 424 436 847 872 1,720 1,640 Amortization of net actuarial loss 323 196 647 392 1,038 877 Net periodic benefit cost $ 827 $ 705 $ 1,654 $ 1,410 $ 3,064 $ 2,849 PBOP Period Ended June 30, Three Months Six Months Twelve Montls 2015 2014 2015 2014 2015 2014 (Thousands of dollars) Service cost $ 410 $ 275 $ 821 $ 551 $ 1,371 $ 1,161 Interest cost 750 708 1,499 1,415 2,913 2,656 Expected return on plan assets (866 ) (816 ) (1,732 ) (1,632 ) (3,364 ) (3,044 ) Amortization of prior service costs 334 88 667 177 845 355 Amortization of net actuarial loss 86 — 173 — 173 472 Net periodic benefit cost $ 714 $ 255 $ 1,428 $ 511 $ 1,938 $ 1,600

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2015
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 Operations Construction Services Total Three months ended June 30, 2015 Revenues from external customers $ 286,643 $ 229,112 $ 515,755 Intersegment revenues — 22,849 22,849 Total $ 286,643 $ 251,961 $ 538,604 Segment net income (loss) $ (657 ) $ 5,606 $ 4,949 Three months ended June 30, 2014 Revenues from external customers $ 271,479 $ 156,966 $ 428,445 Intersegment revenues — 24,708 24,708 Total $ 271,479 $ 181,674 $ 453,153 Segment net income $ 1,798 $ 7,829 $ 9,627 Natural Gas Operations Construction Services Total Six months ended June 30, 2015 Revenues from external customers $ 839,758 $ 390,201 $ 1,229,959 Intersegment revenues — 42,865 42,865 Total $ 839,758 $ 433,066 $ 1,272,824 Segment net income (loss) $ 78,264 $ (1,332 ) $ 76,932 Six months ended June 30, 2014 Revenues from external customers $ 757,972 $ 255,119 $ 1,013,091 Intersegment revenues — 48,458 48,458 Total $ 757,972 $ 303,577 $ 1,061,549 Segment net income $ 74,397 $ 6,013 $ 80,410 Natural Gas Operations Construction Services Total Twelve months ended June 30, 2015 Revenues from external customers $ 1,463,873 $ 782,514 $ 2,246,387 Intersegment revenues — 86,595 86,595 Total $ 1,463,873 $ 869,109 $ 2,332,982 Segment net income $ 120,739 $ 16,909 $ 137,648 Twelve months ended June 30, 2014 Revenues from external customers $ 1,325,657 $ 562,294 $ 1,887,951 Intersegment revenues — 99,301 99,301 Total $ 1,325,657 $ 661,595 $ 1,987,252 Segment net income $ 117,310 $ 17,539 $ 134,849

Derivatives and Fair Value Me20

Derivatives and Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Notional Amounts under Swaps Contracts	The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms): June 30, 2015 December 31, 2014 Contract notional amounts 7,838 5,105
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-, six-, and twelve-month periods ended June 30, 2015 and 2014 and their location in the Condensed Consolidated Statements of Income: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Three Months Ended Six Months Ended Twelve Months Ended Location of Gain or (Loss) June 30 June 30 June 30 Instrument Recognized in Income on Derivative 2015 2014 2015 2014 2015 2014 Swaps Net cost of gas sold $ 707 $ (83 ) $ (1,407 ) $ 5,907 $ (9,677 ) $ 9,476 Swaps Net cost of gas sold (707 )* 83 * 1,407 * (5,907 )* 9,677 * (9,476 )* 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: June 30, 2015 Asset Liability Instrument Balance Sheet Location Derivatives Derivatives Net Total Swaps Other current liabilities $ 105 $ (2,098 ) $ (1,993 ) Swaps Other deferred credits 28 (208 ) (180 ) Total $ 133 $ (2,306 ) $ (2,173 ) December 31, 2014 Asset Liability Instrument Balance Sheet Location Derivatives Derivatives Net Total Swaps Other current liabilities $ — $ (5,062 ) $ (5,062 ) Swaps Other deferred credits — (363 ) (363 ) Total $ — $ (5,425 ) $ (5,425 )
Paid to and Received from Counterparties for Settlements of Matured Swaps	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps. Three Months Ended Six Months Ended Twelve Months Ended (Thousands of dollars) June 30, 2015 June 30, 2015 June 30, 2015 Paid to counterparties $ 690 $ 4,660 $ 5,474 Received from counterparties $ — $ — $ 198
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). June 30, 2015 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 1,993 Swaps Deferred charges and other assets 180 December 31, 2014 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 5,062 Swaps Deferred charges and other assets 363
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) June 30, 2015 December 31, 2014 Liabilities at fair value: Other current liabilities—Swaps $ (1,993 ) $ (5,062 ) Other deferred credits—Swaps (180 ) (363 ) Net Assets (Liabilities) $ (2,173 ) $ (5,425 )

Long-Term Debt (Tables)

Long-Term Debt (Tables)	6 Months Ended
	Jun. 30, 2015
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 utilizes current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. June 30, 2015 December 31, 2014 Carrying Market Carrying Market Amount Value Amount Value (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 133,183 $ 125,000 $ 133,403 Notes, 6.1%, due 2041 125,000 146,568 125,000 157,290 Notes, 3.875%, due 2022 250,000 259,723 250,000 262,030 Notes, 4.875%, due 2043 250,000 256,505 250,000 280,903 8% Series, due 2026 75,000 99,431 75,000 102,296 Medium-term notes, 7.59% series, due 2017 25,000 27,063 25,000 27,573 Medium-term notes, 7.78% series, due 2022 25,000 30,705 25,000 31,144 Medium-term notes, 7.92% series, due 2027 25,000 32,706 25,000 33,695 Medium-term notes, 6.76% series, due 2027 7,500 8,896 7,500 9,156 Unamortized discount (5,046 ) (5,223 ) 902,454 902,277 Revolving credit facility and commercial paper 30,000 30,000 150,000 150,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt Series A, due 2028 50,000 50,000 50,000 50,000 2003 Series A, due 2038 50,000 50,000 50,000 50,000 2008 Series A, due 2038 50,000 50,000 50,000 50,000 2009 Series A, due 2039 50,000 50,000 50,000 50,000 Fixed-rate bonds: 5.25% 2003 Series D, due 2038 20,000 20,103 20,000 20,277 5.00% 2004 Series B, due 2033 — — 31,200 31,223 4.85% 2005 Series A, due 2035 100,000 100,629 100,000 100,071 4.75% 2006 Series A, due 2036 24,855 25,005 24,855 25,399 Unamortized discount (1,645 ) (1,943 ) 343,210 374,112 Centuri secured revolving credit and term loan facility 234,598 235,542 199,267 200,341 Centuri other debt obligations 31,471 31,749 31,128 31,127 1,541,733 1,656,784 Less: current maturities (20,050 ) (19,192 ) Long-term debt, less current maturities $ 1,521,683 $ 1,637,592

Equity, Other Comprehensive I22

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2015
Text Block [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 six months ended June 30, 2015. 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, 2014 46,523 $ 48,153 $ 851,381 $ (50,175 ) $ 639,164 $ (2,257 ) $ 1,486,266 $ 20,042 Common stock issuances 519 519 22,241 22,760 Net income (loss) 76,932 58 76,990 (48 ) Redemption value adjustments 658 658 (658 ) Foreign currency exchange translation adj. (1,028 ) (1,028 ) (35 ) Other comprehensive income (loss): Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax 1,416 1,416 Amounts reclassified to net income,net of tax (FSIRS) 1,037 1,037 Dividends declared Common: $0.81 per share (38,466 ) (38,466 ) JUNE 30, 2015 47,042 $ 48,672 $ 873,622 $ (48,750 ) $ 678,288 $ (2,199 ) $ 1,549,633 $ 19,301
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 Three Months Ended June 30, 2015 June 30, 2014 Before- Tax Net-of- Before- Tax Net-of- Tax (Expense) Tax Tax (Expense) Tax Amount or Benefit (1) Amount Amount or Benefit (1) Amount Defined benefit pension plans: Amortization of prior service cost $ 334 $ (126 ) $ 208 $ 88 $ (33 ) $ 55 Amortization of net actuarial (gain)/loss 8,595 (3,267 ) 5,328 5,914 (2,248 ) 3,666 Regulatory adjustment (7,787 ) 2,959 (4,828 ) (5,177 ) 1,967 (3,210 ) Pension plans other comprehensive income (loss) 1,142 (434 ) 708 825 (314 ) 511 FSIRS (designated hedging activities): Amounts reclassifed into net income 836 (318 ) 518 837 (318 ) 519 FSIRS other comprehensive income 836 (318 ) 518 837 (318 ) 519 Foreign currency translation adjustments: Translation adjustments 209 — 209 — — — Foreign currency other comprehensive income (loss) 209 — 209 — — — Total other comprehensive income (loss) $ 2,187 $ (752 ) $ 1,435 $ 1,662 $ (632 ) $ 1,030 Six Months Ended June 30, 2015 Six Months Ended June 30, 2014 Before- Tax Net-of- Before- Tax Net-of- Defined benefit pension plans: Amortization of prior service cost $ 667 $ (253 ) $ 414 $ 177 $ (67 ) $ 110 Amortization of net actuarial (gain)/loss 17,191 (6,533 ) 10,658 11,828 (4,495 ) 7,333 Regulatory adjustment (15,574 ) 5,918 (9,656 ) (10,355 ) 3,935 (6,420 ) Pension plans other comprehensive income (loss) 2,284 (868 ) 1,416 1, 650 (627 ) 1,023 FSIRS (designated hedging activities): Amounts reclassifed into net income 1,672 (635 ) 1,037 1,673 (636 ) 1,037 FSIRS other comprehensive income 1,672 (635 ) 1,037 1,673 (636 ) 1,037 Foreign currency translation adjustments: Translation adjustments (1,063 ) — (1,063 ) — — — Foreign currency other comprehensive income (loss) (1,063 ) — (1,063 ) — — — Total other comprehensive income (loss) $ 2,893 $ (1,503 ) $ 1,390 $ 3,323 $ (1,263 ) $ 2,060 Twelve Months Ended June 30, 2015 Twelve Months Ended June 30, 2014 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) $ (173,646 ) $ 65,985 $ (107,661 ) $ 100,345 $ (38,131 ) $ 62,214 Amortization of prior service cost 845 (321 ) 524 355 (134 ) 221 Amortization of net actuarial (gain)/loss 29,019 (11,027 ) 17,992 28,915 (10,988 ) 17,927 Prior service cost (6,661 ) 2,531 (4,130 ) — — — Regulatory adjustment 135,089 (51,334 ) 83,755 (118,817 ) 45,150 (73,667 ) Pension plans other comprehensive income (loss) (15,354 ) 5,834 (9,520 ) 10,798 (4,103 ) 6,695 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 (1,722 ) — (1,722 ) — — — Foreign currency other comprehensive income (loss) (1,722 ) — (1,722 ) — — — Total other comprehensive income (loss) $ (13,732 ) $ 4,563 $ (9,169 ) $ 14,143 $ (5,374 ) $ 8,769 (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, 2014 $ (57,211 ) $ 21,740 $ (35,471 ) $ (22,688 ) $ 8,621 $ (14,067 ) $ (637 ) $ — $ (637 ) $ (50,175 ) Translation adjustments — — — — — — (1,063 ) — (1,063 ) (1,063 ) Other comprehensive income before reclassifications — — — — — — (1,063 ) — (1,063 ) (1,063 ) FSIRS amounts reclassified from AOCI (1) — — — 1,672 (635 ) 1,037 — — — 1,037 Amortization of prior service cost (2) 667 (253 ) 414 — — — — — — 414 Amortization of net actuarial loss (2) 17,191 (6,533 ) 10,658 — — — — — — 10,658 Regulatory adjustment (3) (15,574 ) 5,918 (9,656 ) — — — — — — (9,656 ) Net current period other comprehensive income (loss) 2,284 (868 ) 1,416 1,672 (635 ) 1,037 (1,063 ) — (1,063 ) 1,390 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — (35 ) — (35 ) (35 ) Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 2,284 (868 ) 1,416 1,672 (635 ) 1,037 (1,028 ) — (1,028 ) 1,425 Ending Balance AOCI June 30, 2015 $ (54,927 ) $ 20,872 $ (34,055 ) $ (21,016 ) $ 7,986 $ (13,030 ) $ (1,665 ) $ — $ (1,665 ) $ (48,750 ) (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).
Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income	The following table represents amounts (before income tax impacts) associated with defined benefit plans and 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) June 30, 2015 December 31, 2014 Net actuarial (loss) gain $ (421,940 ) $ (439,131 ) Prior service cost (7,706 ) (8,373 ) Less: amount recognized in regulatory assets 374,719 390,293 Recognized in AOCI $ (54,927 ) $ (57,211 )

Acquisition of Construction S23

Acquisition of Construction Services Businesses (Tables)	6 Months Ended
	Jun. 30, 2015
Business Combinations [Abstract]
Revised Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed	The revised preliminary estimated fair values of assets acquired and liabilities assumed as of October 1, 2014, are as follows (in millions of dollars): Revised Cash, cash equivalents, and restricted cash $ 3 Contracts receivable and other receivables 62 Property, plant and equipment 17 Other assets 15 Intangible assets 52 Goodwill 131 Total assets acquired 280 Current liabilities 40 Deferred income tax - long-term 17 Other long-term liabilities 4 Net assets acquired $ 219

Construction Services Redeema24

Construction Services Redeemable Noncontrolling Interest (Tables)	6 Months Ended
	Jun. 30, 2015
Noncontrolling Interest [Abstract]
Summary of Redeemable Noncontrolling Interest	Redeemable (Thousands of dollars): Balance, December 31, 2014 $ 20,042 Net income (loss) attributable to redeemable noncontrolling interest (48 ) Foreign currency exchange translation adjustment (35 ) Adjustment to redemption value (658 ) Balance, June 30, 2015 $ 19,301

Nature of Operations and Basi25

Nature of Operations and Basis of Presentation - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2015USD ($)	Jun. 30, 2015USD ($)Segment	Jun. 30, 2014USD ($)	Dec. 31, 2014USD ($)
Significant Accounting Policies [Line Items]
Number of segments | Segment		2
Non-cash construction advances - non-cash investing item		$ 2,000,000	$ 5,600,000
Goodwill impairment charges		0
Acquisition date adjustments to goodwill	$ 0	$ 1,380,000
Centuri Construction Group Inc [Member] | W.S. Nicholls Western Construction LTD. [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage	50.00%	50.00%
Pooled funds and mutual funds [Member] | Level 2 - Significant other observable inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments	$ 250,000	$ 250,000		$ 250,000
Gas Pipe Inventory and Operating Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe inventory and operating supplies	$ 27,000,000	$ 27,000,000		$ 23,000,000

Nature of Operations and Basi26

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) - Jun. 30, 2015 - USD ($)	Total	Total
Goodwill [Line Items]
Goodwill, Beginning balance		$ 143,160,000
Acquisition date adjustment	$ 0	1,380,000
Foreign currency translation adjustment		(6,283,000)
Goodwill, Ending balance	138,257,000	138,257,000
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance		10,095,000
Acquisition date adjustment		0
Foreign currency translation adjustment		0
Goodwill, Ending balance	10,095,000	10,095,000
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance		133,065,000
Acquisition date adjustment		1,380,000
Foreign currency translation adjustment		(6,283,000)
Goodwill, Ending balance	$ 128,162,000	$ 128,162,000

Nature of Operations and Basi27

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Centuri Construction Group Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Centuri accounts receivable for services provided to Southwest	$ 10,531	$ 9,169

Nature of Operations and Basi28

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investment [Line Items]
Net cash surrender value of COLI policies	$ 101,000	$ 99,000
Other property	11,000	10,000
Total	325,449	326,743
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property and equipment	419,000	405,000
Centuri accumulated provision for depreciation and amortization	$ (206,000)	$ (187,000)

Nature of Operations and Basi29

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Income and Expenses [Abstract]
Change in COLI policies	$ 0	$ 2,300	$ 1,300	$ 3,200	$ 3,400	$ 10,000
Interest income	161	612	751	1,109	2,244	1,306
Pipe replacement costs	0	0	0	0	0	(11)
Foreign transaction gain (loss)	(245)	0	(572)	0	(750)	0
Miscellaneous income and (expense)	246	(49)	955	166	172	(41)
Total other income (deductions)	$ 162	$ 2,863	$ 2,434	$ 4,475	$ 5,066	$ 11,254

Components of Net Periodic Be30

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,281	$ 5,340	$ 12,561	$ 10,680	$ 23,241	$ 22,208
Interest cost	11,057	10,860	22,115	21,721	43,834	40,525
Expected return on plan assets	(14,452)	(13,335)	(28,904)	(26,671)	(55,575)	(51,591)
Amortization of net actuarial loss	8,186	5,718	16,371	11,436	27,808	27,566
Net periodic benefit cost	11,072	8,583	22,143	17,166	39,308	38,708
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	73	160	146	306	332
Interest cost	424	436	847	872	1,720	1,640
Amortization of net actuarial loss	323	196	647	392	1,038	877
Net periodic benefit cost	827	705	1,654	1,410	3,064	2,849
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	410	275	821	551	1,371	1,161
Interest cost	750	708	1,499	1,415	2,913	2,656
Expected return on plan assets	(866)	(816)	(1,732)	(1,632)	(3,364)	(3,044)
Amortization of prior service costs	334	88	667	177	845	355
Amortization of net actuarial loss	86	0	173	0	173	472
Net periodic benefit cost	$ 714	$ 255	$ 1,428	$ 511	$ 1,938	$ 1,600

Segment Information - Schedule

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$ 538,604	$ 453,153	$ 1,272,824	$ 1,061,549	$ 2,332,982	$ 1,987,252
Segment net income (loss)	4,949	9,627	76,932	80,410	137,648	134,849
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	515,755	428,445	1,229,959	1,013,091	2,246,387	1,887,951
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	22,849	24,708	42,865	48,458	86,595	99,301
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	286,643	271,479	839,758	757,972	1,463,873	1,325,657
Segment net income (loss)	(657)	1,798	78,264	74,397	120,739	117,310
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	286,643	271,479	839,758	757,972	1,463,873	1,325,657
Natural Gas Operations [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	0	0	0	0	0	0
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	251,961	181,674	433,066	303,577	869,109	661,595
Segment net income (loss)	5,606	7,829	(1,332)	6,013	16,909	17,539
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	229,112	156,966	390,201	255,119	782,514	562,294
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 22,849	$ 24,708	$ 42,865	$ 48,458	$ 86,595	$ 99,301

Derivatives and Fair Value Me32

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 518	$ 519	$ 1,037	$ 1,037	$ 2,073	$ 2,074
Derivative instrument loss at settlement amortization period			10 years
Cash flow hedging [Member]
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges			$ 0
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Jul. 31, 2015
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps			Mar. 31, 2017
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 Me33

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Jun. 30, 2015	Dec. 31, 2014
Offsetting [Abstract]
Contract notional amounts	7,838,000	5,105,000

Derivatives and Fair Value Me34

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Cash flow hedging [Member] | Net cost of gas sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	707	(83)	(1,407)	5,907	(9,677)	9,476
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	$ (707)	$ 83	$ 1,407	$ (5,907)	$ 9,677	$ (9,476)

Derivatives and Fair Value Me35

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 133	$ 0
Liability derivatives not designated as hedging instruments	(2,306)	(5,425)
Net total not designated as hedging instruments	(2,173)	(5,425)
Swaps [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	105	0
Liability derivatives not designated as hedging instruments	(2,098)	(5,062)
Net total not designated as hedging instruments	(1,993)	(5,062)
Swaps [Member] | Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	28	0
Liability derivatives not designated as hedging instruments	(208)	(363)
Net total not designated as hedging instruments	$ (180)	$ (363)

Derivatives and Fair Value Me36

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - Jun. 30, 2015 - USD ($) $ in Thousands	Total	Total	Total
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 690	$ 4,660	$ 5,474
Received from counterparties	$ 0	$ 0	$ 198

Derivatives and Fair Value Me37

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 1,993	$ 5,062
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 180	$ 363

Derivatives and Fair Value Me38

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant other observable inputs [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (2,173)	$ (5,425)
Swaps [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(1,993)	(5,062)
Swaps [Member] | Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (180)	$ (363)

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($)	Mar. 10, 2015	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Value of common shares planning to sell			$ 22,760,000
Net proceeds to Company			$ 20,713,000	$ 237,000	$ 20,881,000	$ 515,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan			145,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent		180,247	372,509
Common stock weighted average price		$ 53.90
Net proceeds to Company		$ 9,618,188	$ 20,557,144
Agent commissions		97,154	207,649
Common stock available for sale under the program		$ 79,235,207	$ 79,235,207		$ 79,235,207
Maximum [Member]
Value of common shares planning to sell	$ 100,000,000

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	Jun. 30, 2015	Dec. 31, 2014
Long-term debt
Less: current maturities	$ (20,050)	$ (19,192)
Long-term debt, less current maturities	1,521,683	1,637,592
Carrying Amount [Member]
Debt Instrument [Line Items]
Total long-term debt	1,541,733	1,656,784
Long-term debt
Less: current maturities	(20,050)	(19,192)
Long-term debt, less current maturities	1,521,683	1,637,592
Total long-term debt	1,541,733	1,656,784
Carrying Amount [Member] | Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	343,210	374,112
Carrying Amount [Member] | Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	234,598	199,267
Carrying Amount [Member] | Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	31,471	31,128
Carrying Amount [Member] | Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount	(5,046)	(5,223)
Notes payable	902,454	902,277
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	30,000	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	(1,645)	(1,943)
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	20,000	20,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured debt	0	31,200
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000	100,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855	24,855
Market Value [Member] | Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	235,542	200,341
Market Value [Member] | Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	31,749	31,127
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	133,183	133,403
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	146,568	157,290
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	259,723	262,030
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	256,505	280,903
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	99,431	102,296
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,063	27,573
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	30,705	31,144
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	32,706	33,695
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,896	9,156
Market Value [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	30,000	150,000
Market Value [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000	50,000
Market Value [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	20,103	20,277
Market Value [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	0	31,223
Market Value [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	100,629	100,071
Market Value [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	$ 25,005	$ 25,399

Long-Term Debt - Schedule of 41

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	May. 31, 2015
5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			5.00%
Debt instrument due date	Dec. 1, 2033
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. 4, 2027	Jun. 4, 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] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 1, 2039	Dec. 1, 2039
Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Mar. 1, 2038	Mar. 1, 2038
Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	Dec. 1, 2033	Dec. 1, 2033
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 ($)	1 Months Ended	6 Months Ended		12 Months Ended
	May. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Debt Instrument [Line Items]
Redemption of long term debt		$ 78,409,000	$ 10,716,000	$ 206,848,000	$ 46,460,000
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		300,000,000		300,000,000
Line of credit designated as long term debt		150,000,000		150,000,000
Line of credit designated for working capital purposes		$ 150,000,000		150,000,000
Credit facility expiration date		Mar. 1, 2020
Borrowings outstanding under facility		$ 5,000,000		5,000,000
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%
Southwest Gas Credit Facility [Member] | Before Amendment [Member]
Debt Instrument [Line Items]
Credit facility expiration date		Mar. 1, 2019
Centuri Construction Group Inc [Member] | Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity		$ 300,000,000		300,000,000
Credit facility expiration date		Oct. 31, 2019
Borrowings outstanding under facility		$ 235,000,000		235,000,000
Debt secured by assets		$ 492,000,000		492,000,000
5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Redemption of long term debt	$ 31,200,000
Debt instrument interest rate	5.00%
Debt instrument original due date		Dec. 1, 2033
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility		$ 25,000,000		$ 25,000,000

Equity, Other Comprehensive I43

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,486,266
Balance, shares			46,523,184
Common stock issuances			$ 22,760
Net income (loss)	$ 4,949	$ 9,627	76,932	$ 80,410	$ 137,648	$ 134,849
Redemption value adjustments			658
Foreign currency exchange translation adj.			(1,028)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708	511	1,416	1,023	(9,520)	6,695
Amounts reclassified into net income	518	$ 519	1,037	$ 1,037	2,073	$ 2,074
Dividends declared Common $0.81 per share			(38,466)
Balance	$ 1,549,633		$ 1,549,633		$ 1,549,633
Balance, shares	47,042,327		47,042,327		47,042,327
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 48,153
Balance, shares			46,523,000
Common stock issuances			$ 519
Common stock issuance, shares			519,000
Other comprehensive income (loss):
Balance	$ 48,672		$ 48,672		$ 48,672
Balance, shares	47,042,000		47,042,000		47,042,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 851,381
Common stock issuances			22,241
Other comprehensive income (loss):
Balance	$ 873,622		873,622		$ 873,622
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,175)
Foreign currency exchange translation adj.			(1,028)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,416
Amounts reclassified into net income			1,037
Balance	(48,750)		(48,750)		(48,750)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			639,164
Net income (loss)			76,932
Redemption value adjustments			658
Other comprehensive income (loss):
Dividends declared Common $0.81 per share			(38,466)
Balance	678,288		678,288		678,288
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(2,257)
Net income (loss)			58
Other comprehensive income (loss):
Balance	(2,199)		(2,199)		(2,199)
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			20,042
Net income (loss)			(48)
Redemption value adjustments			(658)
Foreign currency exchange translation adj.			(35)
Other comprehensive income (loss):
Balance	$ 19,301		$ 19,301		$ 19,301

Equity, Other Comprehensive I44

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Parenthetical) (Detail) - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 0.405	$ 0.365	$ 0.810	$ 0.730	$ 1.540	$ 1.390

Equity, Other Comprehensive I45

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		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount					$ (173,646)	$ 100,345
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 88	$ 667	$ 177	845	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,595	5,914	17,191	11,828	29,019	28,915
Prior service cost, Before-Tax Amount					(6,661)	0
Regulatory adjustment, Before-Tax Amount	(7,787)	(5,177)	(15,574)	(10,355)	135,089	(118,817)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	2,284	1,650	(15,354)	10,798
Amounts reclassified into net income, Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
Translation adjustments, Before-Tax Amount	209	0	(1,063)	0	(1,722)	0
Foreign currency other comprehensive income (loss), Before-Tax Amount	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), Before-Tax Amount	2,187	1,662	2,893	3,323	(13,732)	14,143
Net actuarial gain/(loss), Tax (Expense) or Benefit					65,985	(38,131)
Amortization of prior service cost, Tax (Expense) or Benefit	(126)	(33)	(253)	(67)	(321)	(134)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(3,267)	(2,248)	(6,533)	(4,495)	(11,027)	(10,988)
Prior service cost, Tax (Expense) or Benefit					2,531	0
Regulatory adjustment, Tax (Expense) or Benefit	2,959	1,967	5,918	3,935	(51,334)	45,150
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(314)	(868)	(627)	5,834	(4,103)
Amounts reclassified into net income, Tax (Expense) or Benefit	(318)	(318)	(635)	(636)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(318)	(318)	(635)	(636)	(1,271)	(1,271)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(752)	(632)	(1,503)	(1,263)	4,563	(5,374)
Net actuarial gain/(loss), Net-of-Tax Amount	0	0	0	0	(107,661)	62,214
Amortization of prior service cost, Net-of-Tax Amount	208	55	414	110	524	221
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,328	3,666	10,658	7,333	17,992	17,927
Prior service cost, Net-of-Tax Amount	0	0	0	0	(4,130)	0
Regulatory adjustment, Net-of-Tax Amount	(4,828)	(3,210)	(9,656)	(6,420)	83,755	(73,667)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	708	511	1,416	1,023	(9,520)	6,695
Amounts reclassified into net income	518	519	1,037	1,037	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	519	1,037	1,037	2,073	2,074
Translation adjustments, Net-of-Tax Amount	209	0	(1,063)	0	(1,722)	0
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), net of tax	$ 1,435	$ 1,030	$ 1,390	$ 2,060	$ (9,169)	$ 8,769

Equity, Other Comprehensive I46

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail) - 6 months ended Jun. 30, 2015 - USD ($)	Total
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Equity, Other Comprehensive I47

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Jun. 30, 2015USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

Equity, Other Comprehensive I48

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Equity [Abstract]
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans Before-Tax			$ (57,211)
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	$ 88	667	$ 177	$ 845	$ 355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,595	5,914	17,191	11,828	29,019	28,915
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,787)	(5,177)	(15,574)	(10,355)	135,089	(118,817)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	2,284	1,650	(15,354)	10,798
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Before-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax			2,284
Ending Balance AOCI June 30, 2015, Defined Benefit Plans Before-Tax	(54,927)		(54,927)		(54,927)
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans Tax (Expense) Benefit			21,740
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit			0
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit			0
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit	(126)	(33)	(253)	(67)	(321)	(134)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(3,267)	(2,248)	(6,533)	(4,495)	(11,027)	(10,988)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,959	1,967	5,918	3,935	(51,334)	45,150
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(314)	(868)	(627)	5,834	(4,103)
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			(868)
Ending Balance AOCI June 30, 2015, Defined Benefit Plans Tax (Expense) Benefit	20,872		20,872		20,872
Beginning Balance AOCI December 31, 2014, Defined Benefit Plans After-Tax			(35,471)
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	208	55	414	110	524	221
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,328	3,666	10,658	7,333	17,992	17,927
Regulatory adjustment, Defined Benefit Plans After-Tax			(9,656)
Net defined benefit pension plans	708	511	1,416	1,023	(9,520)	6,695
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans After-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax			1,416
Ending Balance AOCI June 30, 2015, Defined Benefit Plans After-Tax	(34,055)		(34,055)		(34,055)
Beginning Balance AOCI December 31, 2014, FSIRS - Before-Tax			(22,688)
Translation adjustments, FSIRS - Before-Tax			0
Other comprehensive income before reclassifications, FSIRS - Before-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	837	1,672	1,673	3,344	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	836	837	1,672	1,673	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			1,672
Ending Balance AOCI June 30, 2015, FSIRS - Before-Tax	(21,016)		(21,016)		(21,016)
Beginning Balance AOCI December 31, 2014, FSIRS - Tax (Expense) Benefit			8,621
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	(318)	(318)	(635)	(636)	(1,271)	(1,271)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(318)	(318)	(635)	(636)	(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			(635)
Ending Balance AOCI June 30, 2015, FSIRS - Tax (Expense) Benefit	7,986		7,986		7,986
Beginning Balance AOCI December 31, 2014, FSIRS - After-Tax			(14,067)
Translation adjustments, FSIRS - After-Tax			0
Other comprehensive income before reclassifications, FSIRS - After-Tax			0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	519	1,037	1,037	2,073	2,074
Net forward-starting interest rate swaps	518	519	1,037	1,037	2,073	2,074
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - After-Tax			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax			1,037
Ending Balance AOCI June 30, 2015, FSIRS - After-Tax	(13,030)		(13,030)		(13,030)
Beginning Balance AOCI December 31, 2014, Foreign Currency Items Before-Tax			(637)
Translation adjustments, Foreign Currency Items Before-Tax	209	0	(1,063)	0	(1,722)	0
Other comprehensive income before reclassifications, Foreign Currency Items Before-Tax	209	0	(1,063)	0	(1,722)	0
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), Foreign Currency Items Before-Tax			(1,063)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax			(35)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax			(1,028)
Ending Balance AOCI June 30, 2015, Foreign Currency Items Before-Tax	(1,665)		(1,665)		(1,665)
Beginning Balance AOCI December 31, 2014, Foreign Currency items - Tax (Expense) Benefit			0
Translation adjustments, Foreign Currency items - Tax (Expense) Benefit	0	0	0	0	0	0
Other comprehensive income before reclassifications, Foreign Currency items - Tax (Expense) Benefit			0
FSIRS amounts reclassified from AOCI, Foreign Currency items - Tax (Expense) Benefit			0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit			0
Amortization of net actuarial loss, Foreign Currency items - Tax (Expense) Benefit			0
Regulatory adjustment, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss), Foreign Currency items - Tax (Expense) Benefit			0
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - Tax (Expense) Benefit			0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency items - Tax (Expense) Benefit	0	0	0	0	0	0
Ending Balance AOCI June 30, 2015, Foreign Currency items - Tax (Expense) Benefit	0		0		0
Beginning Balance AOCI December 31, 2014, Foreign Currency items - After-Tax			(637)
Translation adjustments, Foreign Currency items - After-Tax	209	0	(1,063)	0	(1,722)	0
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	209	0	(1,063)	0	(1,722)	0
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			(1,063)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax			(35)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax			(1,028)
Ending Balance AOCI June 30, 2015, Foreign Currency items - After-Tax	(1,665)		(1,665)		(1,665)
Beginning Balance AOCI December 31, 2014, AOCI			(50,175)
Translation adjustments, AOCI	(1,063)		(1,063)		(1,063)
Other comprehensive income before reclassifications, AOCI			(1,063)
Amounts reclassified into net income	518	519	1,037	1,037	2,073	2,074
Amortization of prior service cost, AOCI	208	55	414	110	524	221
Amortization of net actuarial loss	5,328	3,666	10,658	7,333	17,992	17,927
Regulatory adjustment, AOCI			(9,656)
Total other comprehensive income (loss), net of tax	1,435	$ 1,030	1,390	$ 2,060	(9,169)	$ 8,769
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	(35)		(35)		(35)
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	1,425		1,425		1,425
Ending Balance AOCI June 30, 2015, AOCI	$ (48,750)		$ (48,750)		$ (48,750)

Equity, Other Comprehensive I49

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	Jun. 30, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	$ (54,927)	$ (57,211)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(421,940)	(439,131)
Prior service cost	(7,706)	(8,373)
Less: amount recognized in regulatory assets	374,719	390,293
Recognized in AOCI	$ (54,927)	$ (57,211)

Acquisition of Construction S50

Acquisition of Construction Services Businesses - Additional Information (Detail) - Jun. 30, 2015 - USD ($)	Total	Total	Total
Business Acquisition [Line Items]
Acquisition date adjustment	$ 0	$ 1,380,000
Business acquisition cost		0	$ 5,000,000
Brigadier [Member]
Business Acquisition [Line Items]
Goodwill assigned for deductible tax purpose	$ 4,900,000	$ 4,900,000	$ 4,900,000

Acquisition of Construction S51

Acquisition of Construction Services Businesses - Revised Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Oct. 01, 2014
Business Acquisition [Line Items]
Goodwill	$ 138,257	$ 143,160
Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Acquisition [Line Items]
Cash, cash equivalents, and restricted cash			$ 3,000
Contracts receivable and other receivables			62,000
Property, plant and equipment			17,000
Other assets			15,000
Intangible assets			52,000
Goodwill			131,000
Total assets acquired			280,000
Current liabilities			40,000
Deferred income tax - long-term			17,000
Other long-term liabilities			4,000
Net assets acquired			$ 219,000

Construction Services Redeema52

Construction Services Redeemable Noncontrolling Interests - Additional Information (Detail) - Principal Previous Owners [Member] - Link-Line and WS Nicholls [Member]	1 Months Ended	6 Months Ended
	Oct. 31, 2014	Jun. 30, 2015
Noncontrolling Interest [Line Items]
Equity interest percentage	10.00%
Equity interest exchange transaction completion period		Third quarter of 2015
Description of equity interest		The agreement, associated with the approximate 10% equity interest of the sellers, provides special dividend rights which entitle the sellers, as holders, to dividends equal to 3.4% of dividends paid at the level of Centuri and subject to certain conditions, such interests may become exchangeable for a 3.4% equity interest in Centuri. In June 2015, the previous owners notified Centuri of their intent to exchange their full equity interest in the Canadian businesses for an equity interest in Centuri, in accordance with the agreement. The exchange is anticipated to be completed in the third quarter of 2015. Additionally, the previous owners may exit their investment retained by requiring the purchase of a portion of their interest commencing October 2016 and in incremental amounts each anniversary date thereafter. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election after September 2021.
Equity interest dividend rights, percentage of dividends paid by Centuri	3.40%
Noncontrolling interest purchase date		2016-10
Maximum [Member]
Noncontrolling Interest [Line Items]
Equity interest percentage		100.00%
Construction Services [Member]
Noncontrolling Interest [Line Items]
Equity interest exchangeable percentage in Centuri	3.40%

Construction Services Redeema53

Construction Services Redeemable Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2015USD ($)
Redeemable Noncontrolling Interest [Line Items]
Balance, December 31, 2014	$ 20,042
Foreign currency exchange translation adjustment	(1,028)
Adjustment to redemption value	658
Balance, June 30, 2015	19,301
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Redeemable Noncontrolling Interest [Line Items]
Balance, December 31, 2014	20,042
Net Income (Loss) attributable to redeemable noncontrolling interest	(48)
Foreign currency exchange translation adjustment	(35)
Adjustment to redemption value	(658)
Balance, June 30, 2015	$ 19,301