Southwest Gas 10-Q 6 May 15 2015 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2015	Apr. 28, 2015
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS CORP
Entity Central Index Key	92416
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		46,842,753

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Utility plant:
Gas plant	$5,618,385	$5,556,599
Less: accumulated depreciation	-2,002,883	-1,973,098
Acquisition adjustments, net	505	550
Construction work in progress	81,001	74,332
Net utility plant	3,697,008	3,658,383
Other property and investments	319,689	326,743
Restricted cash	755	821
Current assets:
Cash and cash equivalents	38,013	39,566
Accounts receivable, net of allowances	314,682	281,824
Accrued utility revenue	44,400	73,900
Income taxes receivable, net	3,881	21,853
Deferred income taxes, net	8,429	2,109
Deferred purchased gas costs	28,523	87,556
Prepaids and other current assets	89,317	99,975
Total current assets	527,245	606,783
Noncurrent assets:
Goodwill	135,689	143,160
Deferred income taxes	1,418
Deferred charges and other assets	472,593	478,625
Total noncurrent assets	609,700	621,785
Total assets	5,154,397	5,214,515
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 46,840,289 and 46,523,184 shares)	48,470	48,153
Additional paid-in capital	862,955	851,381
Accumulated other comprehensive income (loss), net	-50,177	-50,175
Retained earnings	692,603	639,164
Total Southwest Gas Corporation equity	1,553,851	1,488,523
Noncontrolling interest	-2,116	-2,257
Total equity	1,551,735	1,486,266
Redeemable noncontrolling interest	19,096	20,042
Long-term debt, less current maturities	1,506,527	1,637,592
Total capitalization	3,077,358	3,143,900
Current liabilities:
Current maturities of long-term debt	18,266	19,192
Short-term debt		5,000
Accounts payable	119,425	167,988
Customer deposits	72,372	71,546
Income taxes payable	14,341
Accrued general taxes	61,819	44,339
Accrued interest	22,332	16,468
Deferred income taxes	16,299
Deferred purchased gas costs	390
Other current liabilities	152,338	145,584
Total current liabilities	477,582	470,117
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	721,924	723,688
Accumulated removal costs	309,000	304,000
Other deferred credits and other long-term liabilities	568,533	572,810
Total deferred income taxes and other credits	1,599,457	1,600,498
Total capitalization and liabilities	$5,154,397	$5,214,515

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 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	46,840,289	46,523,184
Common stock, outstanding	46,840,289	46,523,184

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (USD $)	3 Months Ended		12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Operating revenues:
Gas operating revenues	$553,115	$486,493	$1,448,709	$1,293,047
Construction revenues	181,105	121,903	798,822	652,626
Total operating revenues	734,220	608,396	2,247,531	1,945,673
Operating expenses:
Net cost of gas sold	253,762	191,377	567,741	426,770
Operations and maintenance	95,510	102,408	376,834	390,235
Depreciation and amortization	67,467	62,891	257,603	240,775
Taxes other than income taxes	12,997	11,456	48,793	45,212
Construction expenses	174,928	113,199	709,586	579,795
Total operating expenses	604,664	481,331	1,960,557	1,682,787
Operating income	129,556	127,065	286,974	262,886
Other income and (expenses):
Net interest deductions	-17,977	-17,519	-72,527	-65,341
Other income (deductions)	2,272	1,612	7,767	9,843
Total other income and (expenses)	-15,705	-15,907	-64,760	-55,498
Income before income taxes	113,851	111,158	222,214	207,388
Income tax expense	41,972	40,461	79,884	72,492
Net income	71,879	70,697	142,330	134,896
Net income (loss) attributable to noncontrolling interests	-104	-86	4	-434
Net income attributable to Southwest Gas Corporation	$71,983	$70,783	$142,326	$135,330
Basic earnings per share	$1.54	$1.52	$3.06	$2.92
Diluted earnings per share	$1.53	$1.51	$3.03	$2.89
Dividends declared per share	$0.41	$0.37	$1.50	$1.36
Average number of common shares outstanding	46,612	46,440	46,537	46,364
Average shares outstanding (assuming dilution)	47,036	46,871	46,986	46,812

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$71,879	$70,697	$142,330	$134,896
Defined benefit pension plans:
Net actuarial gain (loss)			-107,661	62,214
Amortization of prior service cost	206	55	371	220
Amortization of net actuarial loss	5,330	3,667	16,330	19,559
Prior service cost			-4,130
Regulatory adjustment	-4,828	-3,210	85,373	-75,159
Net defined benefit pension plans	708	512	-9,717	6,834
Forward-starting interest rate swaps:
Amounts reclassified into net income	519	518	2,074	2,074
Net forward-starting interest rate swaps	519	518	2,074	2,074
Foreign currency translation adjustments	-1,272		-1,931
Total other comprehensive income (loss), net of tax	-45	1,030	-9,574	8,908
Comprehensive income	71,834	71,727	132,756	143,804
Comprehensive income (loss) attributable to noncontrolling interests	-147	-86	-61	-434
Comprehensive income attributable to Southwest Gas Corporation	$71,981	$71,813	$132,817	$144,238

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$71,879	$70,697	$142,330	$134,896
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	67,467	62,891	257,603	240,775
Deferred income taxes	7,584	13,692	58,201	42,090
Changes in current assets and liabilities:
Accounts receivable, net of allowances	-31,850	13,958	-49,491	2,806
Accrued utility revenue	29,500	28,600	-300	-500
Deferred purchased gas costs	59,423	-56,979	47,063	-116,678
Accounts payable	-49,173	-66,641	-24,031	-1,042
Accrued taxes	49,782	33,400	2,809	13,592
Other current assets and liabilities	31,548	36,522	18,405	7,465
Gains on sale	-1,526	-2,429	-5,268	-5,707
Changes in undistributed stock compensation	462	2,761	5,674	7,060
AFUDC	-396	-498	-1,893	-2,301
Changes in other assets and deferred charges	-13,336	-11,134	-23,934	-20,994
Changes in other liabilities and deferred credits	876	17,155	-500	53,141
Net cash provided by operating activities	222,240	141,995	426,668	354,603
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	-90,391	-76,892	-410,397	-387,931
Acquisition of businesses, net of cash acquired	-9,261		-199,758
Restricted cash			1,233
Changes in customer advances	5,446	4,105	21,704	10,972
Miscellaneous inflows	2,835	3,593	10,853	10,458
Miscellaneous outflows			-1,400
Net cash used in investing activities	-91,371	-69,194	-577,765	-366,501
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	8,791	107	9,089	567
Dividends paid	-17,023	-15,343	-67,955	-61,220
Issuance of long-term debt, net	50,295	1,025	318,498	307,651
Retirement of long-term debt	-18,082	-3,781	-153,456	-71,294
Change in credit facility and commercial paper	-150,000	-10,000		-113,000
Change in short-term debt	-5,000
Principal payments on capital lease obligations	-386		-820
Other	-329	-520	-1,066	1,479
Net cash provided by (used in) financing activities	-131,734	-28,512	104,290	64,183
Effects of currency translation on cash and cash equivalents	-688		-546
Change in cash and cash equivalents	-1,553	44,289	-47,353	52,285
Cash and cash equivalents at beginning of period	39,566	41,077	85,366	33,081
Cash and cash equivalents at end of period	38,013	85,366	38,013	85,366
Supplemental information:
Interest paid, net of amounts capitalized	10,497	10,474	65,575	56,058
Income taxes paid	$2,101	$238	$26,110	$6,454

Nature_of_Operations_and_Basis

Nature of Operations and Basis of Presentation	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Nature of Operations and Basis of Presentation	Note 1 – Nature of Operations and Basis of Presentation
	Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. The timing and amount of rate relief can materially impact results of operations. Natural gas purchases and the timing of related recoveries can materially impact liquidity. Centuri Construction Group Inc. (“Centuri” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems, and industrial construction solutions. Centuri operations are generally conducted under the business names of NPL Construction Co. (“NPL”), Link-Line Contractors Ltd. (“Link-Line”), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Brigadier Pipelines Inc. (“Brigadier”). Typically, Centuri revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.
	Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2014 Annual Report to Shareholders, which is incorporated by reference into the 2014 Form 10-K.
	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 includes gas pipe inventory and operating supplies of $23 million at both March 31, 2015 and December 31, 2014.
	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. In general, cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. However, cash and cash equivalents at March 31, 2015 and December 31, 2014 also includes two money market fund investments totaling approximately $250,000 which fall within Level 2 (significant other observable inputs) of the fair value hierarchy, due to the asset valuation methods used by money market funds.
	Significant non-cash investing and financing activities for the natural gas operations segment included the following: Upon contract expiration, customer advances of approximately $700,000 and $1.6 million, during the first three months of 2015 and 2014, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity. In addition, approximately $2.3 million in proceeds from common stock sales during March 2015 associated with the Equity Shelf Program (see Note 5 – Common Stock) were received after March 31, 2015 and represent a non-cash financing activity at the reporting date.
	Goodwill. Goodwill is assessed for impairment annually, as required by U.S. GAAP, or otherwise, if circumstances indicate impairment to the carrying value of goodwill. No impairment was recorded in the first quarter of 2015. See Note 8 – Acquisition of Construction Services Businesses for more information on the acquisition date adjustment shown in the table below.
	(In thousands of dollars)	Natural Gas			Construction			Consolidated
		Operations			Services
	December 31, 2014	$	10,095		$	133,065		$	143,160
	Acquisition date adjustment		—			1,380			1,380
	Foreign currency translation adjustment		—			(8,851	)		(8,851	)
	March 31, 2015	$	10,095		$	125,594		$	135,689
	Intercompany Transactions. Centuri recognizes revenues generated from contracts with Southwest (see Note 3 – Segment Information below). Accounts receivable for these services are presented in the table below (thousands of dollars):
		March 31, 2015			December 31, 2014
	Accounts receivable for Centuri services	$	8,854		$	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 includes (millions of dollars):
		March 31, 2015			December 31, 2014
	Centuri property and equipment	$	402		$	405
	Centuri accumulated provision for depreciation and amortization		(195	)		(187	)
	Net cash surrender value of COLI policies		101			99
	Other property		12			10
	Total	$	320		$	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						Twelve Months Ended
		March 31						March 31
		2015			2014			2015			2014
	Change in COLI policies	$	1,300		$	900		$	5,700		$	9,500
	Interest income		590			497			2,695			838
	Pipe replacement costs		—			—			—			(47	)
	Foreign transaction gain (loss)		(327	)		—			(505	)		—
	Miscellaneous income and (expense)		709			215			(123	)		(448	)
	Total other income (deductions)	$	2,272		$	1,612		$	7,767		$	9,843
	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 May 2014, the Financial Accounting Standards Board (“FASB”) issued the update “Revenue from Contracts with Customers (Topic 606).” The update replaces much of the current guidance regarding revenue recognition including most industry-specific guidance. In accordance with the update, an entity will be required to identify the contract with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. In addition to the new revenue recognition requirements, entities will be required to disclose sufficient information to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities may choose between two retrospective transition methods when applying the update. The FASB voted on April 1, 2015 to propose a one-year deferral of the effective date (annual period ending after December 15, 2017), but to permit entities to adopt one year earlier if they choose (i.e., the original effective date). The FASB decided, based on its outreach to various stakeholders and the forthcoming exposure drafts, which amend the update, that a deferral is necessary to provide adequate time to effectively implement the update. If the proposed effective date deferral is approved, the Company plans to adopt the update at the required adoption date, which is expected to be for interim and annual reporting periods commencing January 1, 2018. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.
	In August 2014, the FASB issued the update “Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern,” which requires management to asses a company’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Under the update, disclosures are required when conditions give rise to substantial doubt about a company’s ability to continue as a going concern within one year from the financial statement issuance date. The update is effective for the annual period ending after December 15, 2016, and all annual and interim periods thereafter. This update 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. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.

Components_of_Net_Periodic_Ben

Components of Net Periodic Benefit Cost	3 Months Ended
	Mar. 31, 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 March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	6,280		$	5,340		$	22,300		$	22,632
	Interest cost		11,058			10,861			43,637			39,067
	Expected return on plan assets		(14,452	)		(13,336	)		(54,458	)		(50,716	)
	Amortization of net actuarial loss		8,185			5,718			25,340			29,913
	Net periodic benefit cost	$	11,071		$	8,583		$	36,819		$	40,896
		SERP
		Period Ended March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	80		$	73		$	299		$	352
	Interest cost		423			436			1,732			1,588
	Amortization of net actuarial loss		324			196			911			924
	Net periodic benefit cost	$	827		$	705		$	2,942		$	2,864
		PBOP
		Period Ended March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	411		$	276		$	1,236		$	1,191
	Interest cost		749			707			2,871			2,569
	Expected return on plan assets		(866	)		(816	)		(3,314	)		(2,934	)
	Amortization of prior service costs		333			89			599			355
	Amortization of net actuarial loss		87			—			87			709
	Net periodic benefit cost	$	714		$	256		$	1,479		$	1,890

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 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		Construction			Total
		Operations		Services
	Three months ended March 31, 2015
	Revenues from external customers	$	553,115	$	161,089		$	714,204
	Intersegment revenues		—		20,016			20,016
	Total	$	553,115	$	181,105		$	734,220
	Segment net income (loss)	$	78,921	$	(6,938	)	$	71,983
	Three months ended March 31, 2014
	Revenues from external customers	$	486,493	$	98,153		$	584,646
	Intersegment revenues		—		23,750			23,750
	Total	$	486,493	$	121,903		$	608,396
	Segment net income (loss)	$	72,599	$	(1,816	)	$	70,783
		Natural Gas		Construction			Total
		Operations		Services
	Twelve months ended March 31, 2015
	Revenues from external customers	$	1,448,709	$	710,368		$	2,159,077
	Intersegment revenues		—		88,454			88,454
	Total	$	1,448,709	$	798,822		$	2,247,531
	Segment net income	$	123,194	$	19,132		$	142,326
	Twelve months ended March 31, 2014
	Revenues from external customers	$	1,293,047	$	555,762		$	1,848,809
	Intersegment revenues		—		96,864			96,864
	Total	$	1,293,047	$	652,626		$	1,945,673
	Segment net income	$	117,476	$	17,854		$	135,330

Derivatives_and_Fair_Value_Mea

Derivatives and Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Fair Value Measurements	Note 4 – Derivatives and Fair Value Measurements
	Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts have no significant market value. The Swaps are recorded at fair value.
	The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 25% in the Arizona and California jurisdictions) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from April 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):
			March 31, 2015				December 31, 2014
	Contract notional amounts			5,954				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- and twelve-month periods ended March 31, 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							Twelve Months Ended
			Location of Gain or (Loss)		31-Mar							31-Mar
	Instrument		Recognized in Income on Derivative		2015				2014			2015			2014
	Swaps		Net cost of gas sold		$	(2,114	)		$	5,990		$	(10,467	)	$	1,890
	Swaps		Net cost of gas sold			2,114	*			(5,990	)*		10,467	*		(1,890	)*
	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:
	March 31, 2015				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives			Net Total
	Swaps		Other current liabilities		$	—			$	(3,346	)	$	(3,346	)
	Swaps		Other deferred credits			—				(223	)		(223	)
	Total				$	—			$	(3,569	)	$	(3,569	)
	31-Dec-14				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				Twelve Months Ended
	(Thousands of dollars)		March 31, 2015				March 31, 2015
	Paid to counterparties		$	3,970			$	4,784
	Received from counterparties		$	—			$	394
	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars).
	March 31, 2015
	Instrument		Balance Sheet Location		Net Total
	Swaps		Prepaids and other current assets		$	3,346
	Swaps		Deferred charges and other assets			223
	31-Dec-14
	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 estimated fair values of Southwest’s Swaps were determined at March 31, 2015 and December 31, 2014 using New York Mercantile Exchange (“NYMEX”) futures settlement prices for delivery of natural gas at Henry Hub adjusted by the price of NYMEX ClearPort basis Swaps, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs (inputs, other than quoted prices, for similar assets or liabilities) are observable in the marketplace throughout the full term of the Swaps, but have been credit-risk adjusted with no significant impact to the overall fair value measure.
	The following table sets forth by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the Company’s financial assets and liabilities that were accounted for at fair value:
	Level 2 - Significant other observable inputs
	(Thousands of dollars)		March 31, 2015				December 31, 2014
	Liabilities at fair value:
	Other current liabilities - Swaps		$	(3,346	)		$	(5,062	)
	Other deferred credits - Swaps			(223	)			(363	)
	Net Assets (Liabilities)		$	(3,569	)		$	(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.

Common_Stock

Common Stock	3 Months Ended
	Mar. 31, 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 (“BNYMCM”). During the quarter ending March 31, 2015, the Company sold, through the continuous equity offering program with BNYMCM as agent, an aggregate of 192,262 shares of the Company’s common stock in the open market at a weighted average price of $57.47 per share, resulting in proceeds to the Company of $10,938,956, net of $110,495 in agent commissions. As of March 31, 2015, the Company had up to $88,950,549 of common stock available for sale under the program. Net proceeds from the sale of shares of common stock under the Equity Shelf Program will be used for general corporate purposes, including the acquisition of property for the construction, completion, extension or improvement of pipeline systems and facilities located in and around the communities Southwest serves.
	In addition, during the three months ended March 31, 2015, the Company issued approximately 125,000 shares of common stock through the Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan.

LongTerm_Debt

Long-Term Debt	3 Months Ended
	Mar. 31, 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 March 31, 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 fair value hierarchy that ranks the inputs used to measure fair value by their reliability, due to the Company’s ability to access similar debt arrangements at measurement dates with comparable terms, including variable rates. The fair values of debentures, senior notes, and fixed-rate IDRBs were determined utilizing a market-based valuation approach, where fair market values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves, credit ratings and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable. The market values of debentures and fixed-rate IDRBs are categorized as Level 2 (observable market inputs based on market prices of similar securities). The Centuri secured revolving credit and term loan facility and Centuri other debt obligations (not actively traded) are categorized as Level 3, based on significant unobservable inputs to their fair values. Since Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilizes current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s.
		March 31, 2015					December 31, 2014
		Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	(Thousands of dollars)
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	135,578	$	125,000		$	133,403
	Notes, 6.1%, due 2041		125,000			161,798		125,000			157,290
	Notes, 3.875%, due 2022		250,000			267,618		250,000			262,030
	Notes, 4.875%, due 2043		250,000			290,295		250,000			280,903
	8% Series, due 2026		75,000			103,938		75,000			102,296
	Medium-term notes, 7.59% series, due 2017		25,000			27,398		25,000			27,573
	Medium-term notes, 7.78% series, due 2022		25,000			31,575		25,000			31,144
	Medium-term notes, 7.92% series, due 2027		25,000			34,418		25,000			33,695
	Medium-term notes, 6.76% series, due 2027		7,500			9,375		7,500			9,156
	Unamortized discount		(5,135	)				(5,223	)
			902,365					902,277
	Revolving credit facility and commercial paper		—			—		150,000			150,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,216		20,000			20,277
	5.00% 2004 Series B, due 2033		31,200			31,329		31,200			31,223
	4.85% 2005 Series A, due 2035		100,000			100,651		100,000			100,071
	4.75% 2006 Series A, due 2036		24,855			25,406		24,855			25,399
	Unamortized discount		(1,920	)				(1,943	)
			374,135					374,112
	Centuri secured revolving credit and term loan facility		219,995			220,196		199,267			200,341
	Centuri other debt obligations		28,298			28,480		31,128			31,127
			1,524,793					1,656,784
	Less: current maturities		(18,266	)				(19,192	)
	Long-term debt, less current maturities	$	1,506,527				$	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 March 31, 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. No borrowings were outstanding on either the long-term or short-term portions of the credit facility at March 31, 2015.
	Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2020. At March 31, 2015, $220 million in borrowings were outstanding on the Centuri facility. Centuri assets securing the facility at March 31, 2015 totaled $447 million.

Equity_Other_Comprehensive_Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 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 three months ended March 31, 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)
	31-Dec-14			46,523		$	48,153		$	851,381		$	(50,175	)	$	639,164		$	(2,257	)	$	1,486,266		$	20,042
	Common stock issuances			317			317			11,574												11,891
	Net income (loss)															71,983			141			72,124			(245	)
	Redemption value adjustments															658						658			(658	)
	Foreign currency exchange translation adj.												(1,229	)								(1,229	)		(43	)
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												708									708
	Amounts reclassified to net income, net of tax (FSIRS)												519									519
	Dividends declared
	Common: $0.405 per share															(19,202	)					(19,202	)
	31-Mar-15			46,840		$	48,470		$	862,955		$	(50,177	)	$	692,603		$	(2,116	)	$	1,551,735		$	19,096
	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 for additional information on the FSIRS.
	Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)
	(Thousands of dollars)
			Three Months Ended									Three Months Ended
			31-Mar-15									31-Mar-14
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of prior service cost		$	333		$	(127	)	$	206		$	89		$	(34	)	$	55
	Amortization of net actuarial (gain)/loss			8,596			(3,266	)		5,330			5,914			(2,247	)		3,667
	Regulatory adjustment			(7,787	)		2,959			(4,828	)		(5,178	)		1,968			(3,210	)
	Pension plans other comprehensive income (loss)			1,142			(434	)		708			825			(313	)		512
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			836			(317	)		519			836			(318	)		518
	FSIRS other comprehensive income			836			(317	)		519			836			(318	)		518
	Foreign currency translation adjustments:
	Translation adjustments			(1,272	)		—			(1,272	)		—			—			—
	Foreign currency other comprehensive income (loss)			(1,272	)		—			(1,272	)		—			—			—
	Total other comprehensive income (loss)		$	706		$	(751	)	$	(45	)	$	1,661		$	(631	)	$	1,030
			Twelve Months Ended									Twelve Months Ended
			March 31, 2015									March 31, 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			599			(228	)		371			355			(135	)		220
	Amortization of net actuarial (gain)/loss			26,338			(10,008	)		16,330			31,546			(11,987	)		19,559
	Prior service cost			(6,661	)		2,531			(4,130	)		—			—			—
	Regulatory adjustment			137,699			(52,326	)		85,373			(121,224	)		46,065			(75,159	)
	Pension plans other comprehensive income (loss)			(15,671	)		5,954			(9,717	)		11,022			(4,188	)		6,834
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	Foreign currency translation adjustments:
	Translation adjustments			(1,931	)		—			(1,931	)		—			—			—
	Foreign currency other comprehensive income (loss)			(1,931	)		—			(1,931	)		—			—			—
	Total other comprehensive income (loss)		$	(14,257	)	$	4,683		$	(9,574	)	$	14,367		$	(5,459	)	$	8,908
	-1	Tax amounts are calculated using a 38% rate. The Company has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, the Company is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other Comprehensive Income, as repatriation of earnings is not anticipated.
	Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (“AOCI”) at March 31, 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			After-			Before-			Tax			After-			Before-			Tax			After-			AOCI
Tax			(Expense)			Tax			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Benefit									Benefit									Benefit
Beginning Balance AOCI December 31, 2014		$	(57,211	)	$	21,740		$	(35,471	)	$	(22,688	)	$	8,621		$	(14,067	)	$	(637	)	$	—		$	(637	)	$	(50,175	)
Translation adjustments			—			—			—			—			—			—			(1,272	)		—			(1,272	)		(1,272	)
Other comprehensive income before reclassifications			—			—			—			—			—			—			(1,272	)		—			(1,272	)		(1,272	)
FSIRS amounts reclassified from AOCI (1)			—			—			—			836			(317	)		519			—			—			—			519
Amortization of prior service cost (2)			333			(127	)		206			—			—			—			—			—			—			206
Amortization of net actuarial loss (2)			8,596			(3,266	)		5,330			—			—			—			—			—			—			5,330
Regulatory adjustment (3)			(7,787	)		2,959			(4,828	)		—			—			—			—			—			—			(4,828	)
Net current period other comprehensive income (loss)			1,142			(434	)		708			836			(317	)		519			(1,272	)		—			(1,272	)		(45	)
Less: Translation adjustment attributable to redeemable noncontrolling interest			—			—			—			—			—			—			(43	)		—			(43	)		(43	)
Net current period other comprehensive income (loss) attributable to
Southwest Gas Corporation			1,142			(434	)		708			836			(317	)		519			(1,229	)		—			(1,229	)		(2	)
Ending Balance AOCI March 31, 2015		$	(56,069	)	$	21,306		$	(34,763	)	$	(21,852	)	$	8,304		$	(13,548	)	$	(1,866	)	$	—		$	(1,866	)	$	(50,177	)
-1	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Condensed Consolidated Statements of Income.
-2	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details).
-3	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Condensed Consolidated Balance Sheets).
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)
		March 31, 2015			December 31, 2014
Net actuarial (loss) gain		$	(430,535	)	$	(439,131	)
Prior service cost			(8,040	)		(8,373	)
Less: amount recognized in regulatory assets			382,506			390,293
Recognized in AOCI		$	(56,069	)	$	(57,211	)

Acquisition_of_Construction_Se

Acquisition of Construction Services Businesses	3 Months Ended
	Mar. 31, 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. 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):
		Acquisition		Measurement			Revised
		Date		Period			Acquisition
				Adjustments			Date
	Cash, cash equivalents, and restricted cash	$	3	$	—		$	3
	Contracts receivable and other receivables		62		—			62
	Property, plant and equipment		17		—			17
	Other assets		17		(2	)		15
	Intangible assets		52		—			52
	Goodwill		130		1			131
	Total assets acquired		281		(1	)		280
	Current liabilities		39		1			40
	Deferred income tax - long-term		17		—			17
	Other long-term liabilities		4		—			4
	Net assets acquired	$	221	$	(2	)	$	219
	The Company incurred and expensed acquisition costs of $5 million for the twelve months ended March 31, 2015. No acquisition costs were incurred during the three months ended March 31, 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. In the first quarter of 2015, values at the acquisition date were adjusted as reflected in the table above, with no significant overall impact to the Company’s consolidated balance sheets.

Construction_Services_Redeemab

Construction Services Redeemable Noncontrolling Interest	3 Months Ended
	Mar. 31, 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. Additionally, the previous owners may exit their investment retained by requiring Centuri to purchase a portion of their interest (in Lynxus Construction Group, Inc.) 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. 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 impacts retained earnings, but did not impact net income.
		Redeemable
		Noncontrolling
		Interest
	(Thousands of dollars):
	Balance, December 31, 2014	$	20,042
	Net income (loss) attributable to redeemable noncontrolling interest		(245	)
	Foreign currency exchange translation adjustment		(43	)
	Adjustment to redemption value		(658	)
	Balance, March 31, 2015	$	19,096
	The redemption value of the redeemable noncontrolling interest was determined using a Monte Carlo simulation method. First, a market approach was utilized to determine a construction services enterprise value as of the acquisition date. Potential guideline publicly-traded companies were identified by using a selection criteria, including actively traded equities, their financial solvency, and other factors. Once the guideline companies were determined, enterprise value was calculated using a weighted approach of projected earnings before interest expense and taxes (“EBIT”) and earnings before interest expense, taxes, and depreciation and amortization expense (“EBITDA”). After an estimated fair value was determined, a Monte Carlo simulation was used to assign a value to the noncontrolling interest of the sellers. Other assumptions used in this analysis included dividends, probability of events, and a discount due to lack of control (the sellers do not influence operations).

Nature_of_Operations_and_Basis1

Nature of Operations and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Nature of Operations	Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. The timing and amount of rate relief can materially impact results of operations. Natural gas purchases and the timing of related recoveries can materially impact liquidity. Centuri Construction Group Inc. (“Centuri” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems, and industrial construction solutions. Centuri operations are generally conducted under the business names of NPL Construction Co. (“NPL”), Link-Line Contractors Ltd. (“Link-Line”), W.S. Nicholls Construction, Inc. (“W.S. Nicholls”), and Brigadier Pipelines Inc. (“Brigadier”). Typically, Centuri revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.
Basis of Presentation	Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2014 Annual Report to Shareholders, which is incorporated by reference into the 2014 Form 10-K.
	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. Prepaids and other current assets includes gas pipe inventory and operating supplies of $23 million at both March 31, 2015 and December 31, 2014.
Cash and Cash Equivalents	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. In general, cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. However, cash and cash equivalents at March 31, 2015 and December 31, 2014 also includes two money market fund investments totaling approximately $250,000 which fall within Level 2 (significant other observable inputs) of the fair value hierarchy, due to the asset valuation methods used by money market funds.
	Significant non-cash investing and financing activities for the natural gas operations segment included the following: Upon contract expiration, customer advances of approximately $700,000 and $1.6 million, during the first three months of 2015 and 2014, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity. In addition, approximately $2.3 million in proceeds from common stock sales during March 2015 associated with the Equity Shelf Program (see Note 5 – Common Stock) were received after March 31, 2015 and represent a non-cash financing activity at the reporting date.
Goodwill	Goodwill. Goodwill is assessed for impairment annually, as required by U.S. GAAP, or otherwise, if circumstances indicate impairment to the carrying value of goodwill. No impairment was recorded in the first quarter of 2015. See Note 8 – Acquisition of Construction Services Businesses for more information on the acquisition date adjustment shown in the table below.
	(In thousands of dollars)	Natural Gas			Construction			Consolidated
		Operations			Services
	December 31, 2014	$	10,095		$	133,065		$	143,160
	Acquisition date adjustment		—			1,380			1,380
	Foreign currency translation adjustment		—			(8,851	)		(8,851	)
	March 31, 2015	$	10,095		$	125,594		$	135,689
Intercompany Transactions	Intercompany Transactions. Centuri recognizes revenues generated from contracts with Southwest (see Note 3 – Segment Information below). Accounts receivable for these services are presented in the table below (thousands of dollars):
		March 31, 2015			December 31, 2014
	Accounts receivable for Centuri services	$	8,854		$	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. Other property and investments includes (millions of dollars):
		March 31, 2015			December 31, 2014
	Centuri property and equipment	$	402		$	405
	Centuri accumulated provision for depreciation and amortization		(195	)		(187	)
	Net cash surrender value of COLI policies		101			99
	Other property		12			10
	Total	$	320		$	327
Other Income (Deductions)	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						Twelve Months Ended
		March 31						March 31
		2015			2014			2015			2014
	Change in COLI policies	$	1,300		$	900		$	5,700		$	9,500
	Interest income		590			497			2,695			838
	Pipe replacement costs		—			—			—			(47	)
	Foreign transaction gain (loss)		(327	)		—			(505	)		—
	Miscellaneous income and (expense)		709			215			(123	)		(448	)
	Total other income (deductions)	$	2,272		$	1,612		$	7,767		$	9,843
	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 May 2014, the Financial Accounting Standards Board (“FASB”) issued the update “Revenue from Contracts with Customers (Topic 606).” The update replaces much of the current guidance regarding revenue recognition including most industry-specific guidance. In accordance with the update, an entity will be required to identify the contract with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. In addition to the new revenue recognition requirements, entities will be required to disclose sufficient information to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities may choose between two retrospective transition methods when applying the update. The FASB voted on April 1, 2015 to propose a one-year deferral of the effective date (annual period ending after December 15, 2017), but to permit entities to adopt one year earlier if they choose (i.e., the original effective date). The FASB decided, based on its outreach to various stakeholders and the forthcoming exposure drafts, which amend the update, that a deferral is necessary to provide adequate time to effectively implement the update. If the proposed effective date deferral is approved, the Company plans to adopt the update at the required adoption date, which is expected to be for interim and annual reporting periods commencing January 1, 2018. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.
	In August 2014, the FASB issued the update “Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern,” which requires management to asses a company’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. Under the update, disclosures are required when conditions give rise to substantial doubt about a company’s ability to continue as a going concern within one year from the financial statement issuance date. The update is effective for the annual period ending after December 15, 2016, and all annual and interim periods thereafter. This update 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. The Company is evaluating what impact this update might have on its consolidated financial statements and disclosures.

Nature_of_Operations_and_Basis2

Nature of Operations and Basis of Presentation (Tables)	3 Months Ended
	Mar. 31, 2015
Accounting Policies [Abstract]
Schedule of Goodwill
	(In thousands of dollars)	Natural Gas			Construction			Consolidated
		Operations			Services
	December 31, 2014	$	10,095		$	133,065		$	143,160
	Acquisition date adjustment		—			1,380			1,380
	Foreign currency translation adjustment		—			(8,851	)		(8,851	)
	March 31, 2015	$	10,095		$	125,594		$	135,689
Intercompany Transactions	Accounts receivable for these services are presented in the table below (thousands of dollars):
		March 31, 2015			December 31, 2014
	Accounts receivable for Centuri services	$	8,854		$	9,169
Schedule of Other Property and Investments	Other Property and Investments. Other property and investments includes (millions of dollars):
		March 31, 2015			December 31, 2014
	Centuri property and equipment	$	402		$	405
	Centuri accumulated provision for depreciation and amortization		(195	)		(187	)
	Net cash surrender value of COLI policies		101			99
	Other property		12			10
	Total	$	320		$	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						Twelve Months Ended
		March 31						March 31
		2015			2014			2015			2014
	Change in COLI policies	$	1,300		$	900		$	5,700		$	9,500
	Interest income		590			497			2,695			838
	Pipe replacement costs		—			—			—			(47	)
	Foreign transaction gain (loss)		(327	)		—			(505	)		—
	Miscellaneous income and (expense)		709			215			(123	)		(448	)
	Total other income (deductions)	$	2,272		$	1,612		$	7,767		$	9,843

Components_of_Net_Periodic_Ben1

Components of Net Periodic Benefit Cost (Tables)	3 Months Ended
	Mar. 31, 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 March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	6,280		$	5,340		$	22,300		$	22,632
	Interest cost		11,058			10,861			43,637			39,067
	Expected return on plan assets		(14,452	)		(13,336	)		(54,458	)		(50,716	)
	Amortization of net actuarial loss		8,185			5,718			25,340			29,913
	Net periodic benefit cost	$	11,071		$	8,583		$	36,819		$	40,896
		SERP
		Period Ended March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	80		$	73		$	299		$	352
	Interest cost		423			436			1,732			1,588
	Amortization of net actuarial loss		324			196			911			924
	Net periodic benefit cost	$	827		$	705		$	2,942		$	2,864
		PBOP
		Period Ended March 31,
		Three Months						Twelve Months
		2015			2014			2015			2014
	(Thousands of dollars)
	Service cost	$	411		$	276		$	1,236		$	1,191
	Interest cost		749			707			2,871			2,569
	Expected return on plan assets		(866	)		(816	)		(3,314	)		(2,934	)
	Amortization of prior service costs		333			89			599			355
	Amortization of net actuarial loss		87			—			87			709
	Net periodic benefit cost	$	714		$	256		$	1,479		$	1,890

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 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		Construction			Total
		Operations		Services
	Three months ended March 31, 2015
	Revenues from external customers	$	553,115	$	161,089		$	714,204
	Intersegment revenues		—		20,016			20,016
	Total	$	553,115	$	181,105		$	734,220
	Segment net income (loss)	$	78,921	$	(6,938	)	$	71,983
	Three months ended March 31, 2014
	Revenues from external customers	$	486,493	$	98,153		$	584,646
	Intersegment revenues		—		23,750			23,750
	Total	$	486,493	$	121,903		$	608,396
	Segment net income (loss)	$	72,599	$	(1,816	)	$	70,783
		Natural Gas		Construction			Total
		Operations		Services
	Twelve months ended March 31, 2015
	Revenues from external customers	$	1,448,709	$	710,368		$	2,159,077
	Intersegment revenues		—		88,454			88,454
	Total	$	1,448,709	$	798,822		$	2,247,531
	Segment net income	$	123,194	$	19,132		$	142,326
	Twelve months ended March 31, 2014
	Revenues from external customers	$	1,293,047	$	555,762		$	1,848,809
	Intersegment revenues		—		96,864			96,864
	Total	$	1,293,047	$	652,626		$	1,945,673
	Segment net income	$	117,476	$	17,854		$	135,330

Derivatives_and_Fair_Value_Mea1

Derivatives and Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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):
			March 31, 2015				December 31, 2014
	Contract notional amounts			5,954				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- and twelve-month periods ended March 31, 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							Twelve Months Ended
			Location of Gain or (Loss)		31-Mar							31-Mar
	Instrument		Recognized in Income on Derivative		2015				2014			2015			2014
	Swaps		Net cost of gas sold		$	(2,114	)		$	5,990		$	(10,467	)	$	1,890
	Swaps		Net cost of gas sold			2,114	*			(5,990	)*		10,467	*		(1,890	)*
	Total				$	—			$	—		$	—		$	—
	*	Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
Fair Values of Swaps in Condensed Consolidated Balance Sheets	The following table sets forth the fair values of the Company’s Swaps and their location in the Condensed Consolidated Balance Sheets (thousands of dollars):
	Fair values of derivatives not designated as hedging instruments:
	March 31, 2015				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives			Net Total
	Swaps		Other current liabilities		$	—			$	(3,346	)	$	(3,346	)
	Swaps		Other deferred credits			—				(223	)		(223	)
	Total				$	—			$	(3,569	)	$	(3,569	)
	31-Dec-14				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				Twelve Months Ended
	(Thousands of dollars)		March 31, 2015				March 31, 2015
	Paid to counterparties		$	3,970			$	4,784
	Received from counterparties		$	—			$	394
Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars).
	March 31, 2015
	Instrument		Balance Sheet Location		Net Total
	Swaps		Prepaids and other current assets		$	3,346
	Swaps		Deferred charges and other assets			223
	31-Dec-14
	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)		March 31, 2015				December 31, 2014
	Liabilities at fair value:
	Other current liabilities - Swaps		$	(3,346	)		$	(5,062	)
	Other deferred credits - Swaps			(223	)			(363	)
	Net Assets (Liabilities)		$	(3,569	)		$	(5,425	)

LongTerm_Debt_Tables

Long-Term Debt (Tables)	3 Months Ended
	Mar. 31, 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.
		March 31, 2015					December 31, 2014
		Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	(Thousands of dollars)
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	135,578	$	125,000		$	133,403
	Notes, 6.1%, due 2041		125,000			161,798		125,000			157,290
	Notes, 3.875%, due 2022		250,000			267,618		250,000			262,030
	Notes, 4.875%, due 2043		250,000			290,295		250,000			280,903
	8% Series, due 2026		75,000			103,938		75,000			102,296
	Medium-term notes, 7.59% series, due 2017		25,000			27,398		25,000			27,573
	Medium-term notes, 7.78% series, due 2022		25,000			31,575		25,000			31,144
	Medium-term notes, 7.92% series, due 2027		25,000			34,418		25,000			33,695
	Medium-term notes, 6.76% series, due 2027		7,500			9,375		7,500			9,156
	Unamortized discount		(5,135	)				(5,223	)
			902,365					902,277
	Revolving credit facility and commercial paper		—			—		150,000			150,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,216		20,000			20,277
	5.00% 2004 Series B, due 2033		31,200			31,329		31,200			31,223
	4.85% 2005 Series A, due 2035		100,000			100,651		100,000			100,071
	4.75% 2006 Series A, due 2036		24,855			25,406		24,855			25,399
	Unamortized discount		(1,920	)				(1,943	)
			374,135					374,112
	Centuri secured revolving credit and term loan facility		219,995			220,196		199,267			200,341
	Centuri other debt obligations		28,298			28,480		31,128			31,127
			1,524,793					1,656,784
	Less: current maturities		(18,266	)				(19,192	)
	Long-term debt, less current maturities	$	1,506,527				$	1,637,592

Equity_Other_Comprehensive_Inc1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 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 three months ended March 31, 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)
	31-Dec-14			46,523		$	48,153		$	851,381		$	(50,175	)	$	639,164		$	(2,257	)	$	1,486,266		$	20,042
	Common stock issuances			317			317			11,574												11,891
	Net income (loss)															71,983			141			72,124			(245	)
	Redemption value adjustments															658						658			(658	)
	Foreign currency exchange translation adj.												(1,229	)								(1,229	)		(43	)
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												708									708
	Amounts reclassified to net income, net of tax (FSIRS)												519									519
	Dividends declared
	Common: $0.405 per share															(19,202	)					(19,202	)
	31-Mar-15			46,840		$	48,470		$	862,955		$	(50,177	)	$	692,603		$	(2,116	)	$	1,551,735		$	19,096
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
			31-Mar-15									31-Mar-14
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of prior service cost		$	333		$	(127	)	$	206		$	89		$	(34	)	$	55
	Amortization of net actuarial (gain)/loss			8,596			(3,266	)		5,330			5,914			(2,247	)		3,667
	Regulatory adjustment			(7,787	)		2,959			(4,828	)		(5,178	)		1,968			(3,210	)
	Pension plans other comprehensive income (loss)			1,142			(434	)		708			825			(313	)		512
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			836			(317	)		519			836			(318	)		518
	FSIRS other comprehensive income			836			(317	)		519			836			(318	)		518
	Foreign currency translation adjustments:
	Translation adjustments			(1,272	)		—			(1,272	)		—			—			—
	Foreign currency other comprehensive income (loss)			(1,272	)		—			(1,272	)		—			—			—
	Total other comprehensive income (loss)		$	706		$	(751	)	$	(45	)	$	1,661		$	(631	)	$	1,030
			Twelve Months Ended									Twelve Months Ended
			March 31, 2015									March 31, 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			599			(228	)		371			355			(135	)		220
	Amortization of net actuarial (gain)/loss			26,338			(10,008	)		16,330			31,546			(11,987	)		19,559
	Prior service cost			(6,661	)		2,531			(4,130	)		—			—			—
	Regulatory adjustment			137,699			(52,326	)		85,373			(121,224	)		46,065			(75,159	)
	Pension plans other comprehensive income (loss)			(15,671	)		5,954			(9,717	)		11,022			(4,188	)		6,834
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	Foreign currency translation adjustments:
	Translation adjustments			(1,931	)		—			(1,931	)		—			—			—
	Foreign currency other comprehensive income (loss)			(1,931	)		—			(1,931	)		—			—			—
	Total other comprehensive income (loss)		$	(14,257	)	$	4,683		$	(9,574	)	$	14,367		$	(5,459	)	$	8,908
	-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			After-			Before-			Tax			After-			Before-			Tax			After-			AOCI
			Tax			(Expense)			Tax			Tax			(Expense)			Tax			Tax			(Expense)			Tax
						Benefit									Benefit									Benefit
	Beginning Balance AOCI December 31, 2014		$	(57,211	)	$	21,740		$	(35,471	)	$	(22,688	)	$	8,621		$	(14,067	)	$	(637	)	$	—		$	(637	)	$	(50,175	)
	Translation adjustments			—			—			—			—			—			—			(1,272	)		—			(1,272	)		(1,272	)
	Other comprehensive income before reclassifications			—			—			—			—			—			—			(1,272	)		—			(1,272	)		(1,272	)
	FSIRS amounts reclassified from AOCI (1)			—			—			—			836			(317	)		519			—			—			—			519
	Amortization of prior service cost (2)			333			(127	)		206			—			—			—			—			—			—			206
	Amortization of net actuarial loss (2)			8,596			(3,266	)		5,330			—			—			—			—			—			—			5,330
	Regulatory adjustment (3)			(7,787	)		2,959			(4,828	)		—			—			—			—			—			—			(4,828	)
	Net current period other comprehensive income (loss)			1,142			(434	)		708			836			(317	)		519			(1,272	)		—			(1,272	)		(45	)
	Less: Translation adjustment attributable to redeemable noncontrolling interest			—			—			—			—			—			—			(43	)		—			(43	)		(43	)
	Net current period other comprehensive income (loss) attributable to
	Southwest Gas Corporation			1,142			(434	)		708			836			(317	)		519			(1,229	)		—			(1,229	)		(2	)
	Ending Balance AOCI March 31, 2015		$	(56,069	)	$	21,306		$	(34,763	)	$	(21,852	)	$	8,304		$	(13,548	)	$	(1,866	)	$	—		$	(1,866	)	$	(50,177	)
	-1	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Condensed Consolidated Statements of Income.
	-2	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details).
	-3	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Condensed Consolidated Balance Sheets).
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)
			March 31, 2015			December 31, 2014
	Net actuarial (loss) gain		$	(430,535	)	$	(439,131	)
	Prior service cost			(8,040	)		(8,373	)
	Less: amount recognized in regulatory assets			382,506			390,293
	Recognized in AOCI		$	(56,069	)	$	(57,211	)

Acquisition_of_Construction_Se1

Acquisition of Construction Services Businesses (Tables)	3 Months Ended
	Mar. 31, 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):
		Acquisition		Measurement			Revised
		Date		Period			Acquisition
				Adjustments			Date
	Cash, cash equivalents, and restricted cash	$	3	$	—		$	3
	Contracts receivable and other receivables		62		—			62
	Property, plant and equipment		17		—			17
	Other assets		17		(2	)		15
	Intangible assets		52		—			52
	Goodwill		130		1			131
	Total assets acquired		281		(1	)		280
	Current liabilities		39		1			40
	Deferred income tax - long-term		17		—			17
	Other long-term liabilities		4		—			4
	Net assets acquired	$	221	$	(2	)	$	219

Construction_Services_Redeemab1

Construction Services Redeemable Noncontrolling Interest (Tables)	3 Months Ended
	Mar. 31, 2015
Noncontrolling Interest [Abstract]
Summary of Redeemable Noncontrolling Interest
		Redeemable
		Noncontrolling
		Interest
	(Thousands of dollars):
	Balance, December 31, 2014	$	20,042
	Net income (loss) attributable to redeemable noncontrolling interest		(245	)
	Foreign currency exchange translation adjustment		(43	)
	Adjustment to redemption value		(658	)
	Balance, March 31, 2015	$	19,096

Nature_of_Operations_and_Basis3

Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
			Segment
Significant Accounting Policies [Line Items]
Number of segments			2
Non-cash construction advances - non-cash investing item	$700,000	$1,600,000
Proceeds from common stock sales	2,300,000
Centuri Construction Group Inc [Member] | W.S. Nicholls Western Construction LTD. [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage	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
Gas Pipe Inventory and Operating Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe inventory and operating supplies	$23,000,000		23,000,000

Nature_of_Operations_and_Basis4

Nature of Operations and Basis of Presentation - Schedule of Goodwill (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill, Beginning balance	$143,160
Acquisition date adjustment	1,380
Foreign currency translation adjustment	-8,851
Goodwill, Ending balance	135,689
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance		10,095
Goodwill, Ending balance	10,095	10,095
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	133,065
Acquisition date adjustment	1,380
Foreign currency translation adjustment	-8,851
Goodwill, Ending balance	$125,594

Nature_of_Operations_and_Basis5

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) (Centuri Construction Group Inc [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Centuri Construction Group Inc [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable for Centuri services	$8,854	$9,169

Nature_of_Operations_and_Basis6

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Net cash surrender value of COLI policies	$101,000,000	$99,000,000
Other property	12,000,000	10,000,000
Total	319,689,000	326,743,000
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property and equipment	402,000,000	405,000,000
Centuri accumulated provision for depreciation and amortization	($195,000,000)	($187,000,000)

Nature_of_Operations_and_Basis7

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Other Income and Expenses [Abstract]
Change in COLI policies	$1,300	$900	$5,700	$9,500
Interest income	590	497	2,695	838
Pipe replacement costs				-47
Foreign transaction gain (loss)	-327		-505
Miscellaneous income and (expense)	709	215	-123	-448
Total other income (deductions)	$2,272	$1,612	$7,767	$9,843

Components_of_Net_Periodic_Ben2

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$6,280	$5,340	$22,300	$22,632
Interest cost	11,058	10,861	43,637	39,067
Expected return on plan assets	-14,452	-13,336	-54,458	-50,716
Amortization of net actuarial loss	8,185	5,718	25,340	29,913
Net periodic benefit cost	11,071	8,583	36,819	40,896
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	80	73	299	352
Interest cost	423	436	1,732	1,588
Amortization of net actuarial loss	324	196	911	924
Net periodic benefit cost	827	705	2,942	2,864
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	411	276	1,236	1,191
Interest cost	749	707	2,871	2,569
Expected return on plan assets	-866	-816	-3,314	-2,934
Amortization of prior service costs	333	89	599	355
Amortization of net actuarial loss	87		87	709
Net periodic benefit cost	$714	$256	$1,479	$1,890

Segment_Information_Schedule_o

Segment Information - Schedule of Segment Reporting Information (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$734,220	$608,396	$2,247,531	$1,945,673
Segment net income (loss)	71,983	70,783	142,326	135,330
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	714,204	584,646	2,159,077	1,848,809
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	20,016	23,750	88,454	96,864
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	553,115	486,493	1,448,709	1,293,047
Segment net income (loss)	78,921	72,599	123,194	117,476
Natural Gas Operations [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	553,115	486,493	1,448,709	1,293,047
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	181,105	121,903	798,822	652,626
Segment net income (loss)	-6,938	-1,816	19,132	17,854
Construction Services [Member] | Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	161,089	98,153	710,368	555,762
Construction Services [Member] | Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$20,016	$23,750	$88,454	$96,864

Derivatives_and_Fair_Value_Mea2

Derivatives and Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$519	$518	$2,074	$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	30-Apr-15
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	31-Mar-17
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_Mea3

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail)	Mar. 31, 2015	Dec. 31, 2014
	MMBTU	MMBTU
Offsetting [Abstract]
Contract notional amounts	5,954,000	5,105,000

Derivatives_and_Fair_Value_Mea4

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) (Cash flow hedging [Member], Net cost of gas sold [Member], USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	($2,114)	$5,990	($10,467)	$1,890
Regulatory deferral accounting treatment [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$2,114	($5,990)	$10,467	($1,890)

Derivatives_and_Fair_Value_Mea5

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$0	$0
Liability derivatives not designated as hedging instruments	-3,569	-5,425
Net total not designated as hedging instruments	-3,569	-5,425
Swaps [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	0	0
Liability derivatives not designated as hedging instruments	-3,346	-5,062
Net total not designated as hedging instruments	-3,346	-5,062
Swaps [Member] | Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	0	0
Liability derivatives not designated as hedging instruments	-223	-363
Net total not designated as hedging instruments	($223)	($363)

Derivatives_and_Fair_Value_Mea6

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) (USD $)	3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$3,970	$4,784
Received from counterparties		$394

Derivatives_and_Fair_Value_Mea7

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) (Swaps [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$3,346	$5,062
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$223	$363

Derivatives_and_Fair_Value_Mea8

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) (Level 2 - Significant other observable inputs [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	($3,569)	($5,425)
Swaps [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	-3,346	-5,062
Swaps [Member] | Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	($223)	($363)

Common_Stock_Additional_Inform

Common Stock - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Mar. 10, 2015
Value of common shares planning to sell	$11,891,000
Net proceeds to Company	8,791,000	107,000	9,089,000	567,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan	125,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent	192,262
Common stock weighted average price	$57.47
Net proceeds to Company	10,938,956
Agent commissions	110,495
Common stock available for sale under the program	88,950,549		88,950,549
Maximum [Member]
Value of common shares planning to sell					$100,000,000

LongTerm_Debt_Schedule_of_Carr

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Long-term debt
Less: current maturities	($18,266,000)	($19,192,000)
Long-term debt, less current maturities	1,506,527,000	1,637,592,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Total long-term debt	1,524,793,000	1,656,784,000
Long-term debt
Less: current maturities	-18,266,000	-19,192,000
Long-term debt, less current maturities	1,506,527,000	1,637,592,000
Total long-term debt	1,524,793,000	1,656,784,000
Carrying Amount [Member] | Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	374,135,000	374,112,000
Carrying Amount [Member] | Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	219,995,000	199,267,000
Carrying Amount [Member] | Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	28,298,000	31,128,000
Carrying Amount [Member] | Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount	-5,135,000	-5,223,000
Notes payable	902,365,000	902,277,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable	75,000,000	75,000,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	7,500,000	7,500,000
Carrying Amount [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility		150,000,000
Carrying Amount [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] | Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Unamortized discount	-1,920,000	-1,943,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	20,000,000	20,000,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured debt	31,200,000	31,200,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000,000	100,000,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855,000	24,855,000
Market Value [Member] | Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	220,196,000	200,341,000
Market Value [Member] | Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	28,480,000	31,127,000
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	135,578,000	133,403,000
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	161,798,000	157,290,000
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	267,618,000	262,030,000
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	290,295,000	280,903,000
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	103,938,000	102,296,000
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,398,000	27,573,000
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,575,000	31,144,000
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	34,418,000	33,695,000
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,375,000	9,156,000
Market Value [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value		150,000,000
Market Value [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
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	20,216,000	20,277,000
Market Value [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	31,329,000	31,223,000
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	100,651,000	100,071,000
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	$25,406,000	$25,399,000

LongTerm_Debt_Schedule_of_Carr1

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Notes, 4.45%, due 2020 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	1-Dec-20	1-Dec-20
Notes, 6.1%, due 2041 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	15-Feb-41	15-Feb-41
Notes, 3.875%, due 2022 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.88%	3.88%
Debt instrument due date	1-Apr-22	1-Apr-22
Notes, 4.875%, due 2043 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.88%	4.88%
Debt instrument due date	1-Oct-43	1-Oct-43
8% Series, due 2026 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	1-Aug-26	1-Aug-26
Medium-term notes, 7.59% series, due 2017 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	17-Jan-17	17-Jan-17
Medium-term notes, 7.78% series, due 2022 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	3-Feb-22	3-Feb-22
Medium-term notes, 7.92% series, due 2027 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	4-Jun-27	4-Jun-27
Medium-term notes, 6.76% series, due 2027 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	24-Sep-27	24-Sep-27
Tax-exempt Series A, due 2028 [Member] | Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	1-Dec-28	1-Dec-28
2003 Series A, due 2038 [Member] | Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	1-Mar-38	1-Mar-38
2008 Series A, due 2038 [Member] | Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	1-Mar-38	1-Mar-38
2009 Series A, due 2039 [Member] | Variable-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument due date	1-Dec-39	1-Dec-39
5.25% 2003 Series D, due 2038 [Member] | Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Mar-38	1-Mar-38
5.00% 2004 Series B, due 2033 [Member] | Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	1-Dec-33	1-Dec-33
4.85% 2005 Series A, due 2035 [Member] | Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	1-Oct-35	1-Oct-35
4.75% 2006 Series A, due 2036 [Member] | Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	1-Sep-36	1-Sep-36

LongTerm_Debt_Additional_Infor

Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$300,000,000
Line of credit designated as long term debt	150,000,000
Line of credit designated for working capital purposes	150,000,000
Credit facility expiration date	1-Mar-20
Borrowings outstanding under facility	0
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	1-Mar-19
Centuri Construction Group Inc [Member] | Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	300,000,000
Credit facility expiration date	31-Oct-20
Borrowings outstanding under facility	220,000,000
Debt secured by assets	$447,000,000

Equity_Other_Comprehensive_Inc2

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Schedule Of Stockholders Equity [Line Items]
Balance	$1,486,266,000
Balance, shares	46,523,184
Common stock issuances	11,891,000
Net income (loss)	71,879,000	70,697,000	142,330,000	134,896,000
Redemption value adjustments	658,000
Foreign currency exchange translation adj.	-1,229,000
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708,000	512,000	-9,717,000	6,834,000
Amounts reclassified into net income	519,000	518,000	2,074,000	2,074,000
Dividends declared Common $0.405 per share	-19,202,000
Balance	1,551,735,000		1,551,735,000
Balance, shares	46,840,289		46,840,289
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	48,153,000
Balance, shares	46,523,000
Common stock issuances	317,000
Common stock issuance, shares	317,000
Other comprehensive income (loss):
Balance	48,470,000		48,470,000
Balance, shares	46,840,000		46,840,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	851,381,000
Common stock issuances	11,574,000
Other comprehensive income (loss):
Balance	862,955,000		862,955,000
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	-50,175,000
Foreign currency exchange translation adj.	-1,229,000
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	708,000
Amounts reclassified into net income	519,000
Balance	-50,177,000		-50,177,000
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	639,164,000
Net income (loss)	71,983,000
Redemption value adjustments	658,000
Other comprehensive income (loss):
Dividends declared Common $0.405 per share	-19,202,000
Balance	692,603,000		692,603,000
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	-2,257,000
Net income (loss)	141,000
Other comprehensive income (loss):
Balance	-2,116,000		-2,116,000
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	20,042,000
Net income (loss)	-245,000
Redemption value adjustments	-658,000
Foreign currency exchange translation adj.	-43,000
Other comprehensive income (loss):
Balance	$19,096,000		$19,096,000

Equity_Other_Comprehensive_Inc3

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity and Redeemable Noncontrolling Interest (Parenthetical) (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$0.41	$0.37	$1.50	$1.36

Equity_Other_Comprehensive_Inc4

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount			($173,646)	$100,345
Amortization of prior service cost, Before-Tax Amount	333	89	599	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,596	5,914	26,338	31,546
Prior service cost, Before-Tax Amount			-6,661
Regulatory adjustment, Before-Tax Amount	-7,787	-5,178	137,699	-121,224
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	-15,671	11,022
Amounts reclassified into net income, Before-Tax Amount	836	836	3,345	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	3,345	3,345
Translation adjustments, Before-Tax Amount	-1,272		-1,931
Foreign currency other comprehensive income (loss), Before-Tax Amount	-1,272		-1,931
Total other comprehensive income (loss), Before-Tax Amount	706	1,661	-14,257	14,367
Net actuarial gain/(loss), Tax (Expense) or Benefit			65,985	-38,131
Amortization of prior service cost, Tax (Expense) or Benefit	-127	-34	-228	-135
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	-3,266	-2,247	-10,008	-11,987
Prior service cost, Tax (Expense) or Benefit			2,531
Regulatory adjustment, Tax (Expense) or Benefit	2,959	1,968	-52,326	46,065
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	-434	-313	5,954	-4,188
Amounts reclassified into net income, Tax (Expense) or Benefit	-317	-318	-1,271	-1,271
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	-317	-318	-1,271	-1,271
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	-751	-631	4,683	-5,459
Net actuarial gain/(loss), Net-of-Tax Amount			-107,661	62,214
Amortization of prior service cost, Net-of-Tax Amount	206	55	371	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,330	3,667	16,330	19,559
Prior service cost, Net-of-Tax Amount			-4,130
Regulatory adjustment, Net-of-Tax Amount	-4,828	-3,210	85,373	-75,159
Pension plans other comprehensive income (loss), Net-of-Tax Amount	708	512	-9,717	6,834
Amounts reclassified into net income	519	518	2,074	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	518	2,074	2,074
Translation adjustments, Net-of-Tax Amount	-1,272		-1,931
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	-1,272		-1,931
Total other comprehensive income (loss), net of tax	($45)	$1,030	($9,574)	$8,908

Equity_Other_Comprehensive_Inc5

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$0

Equity_Other_Comprehensive_Inc6

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$2.10

Equity_Other_Comprehensive_Inc7

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 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	333	89	599	355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,596	5,914	26,338	31,546
Regulatory adjustment, Defined Benefit Plans Before-Tax	-7,787	-5,178	137,699	-121,224
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	-15,671	11,022
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	1,142
Ending Balance AOCI March 31, 2015, Defined Benefit Plans Before-Tax	-56,069		-56,069
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	-127	-34	-228	-135
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	-3,266	-2,247	-10,008	-11,987
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,959	1,968	-52,326	46,065
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	-434	-313	5,954	-4,188
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	-434
Ending Balance AOCI March 31, 2015 Defined Benefit Plans Tax (Expense) Benefit	21,306		21,306
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	206	55	371	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,330	3,667	16,330	19,559
Regulatory adjustment, Defined Benefit Plans After-Tax	-4,828
Net defined benefit pension plans	708	512	-9,717	6,834
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	708
Ending Balance AOCI March 31, 2015, Defined Benefit Plans After-Tax	-34,763		-34,763
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	836	3,345	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	836	836	3,345	3,345
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Before-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax	836
Ending Balance AOCI March 31, 2015, FSIRS - Before-Tax	-21,852		-21,852
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	-317	-318	-1,271	-1,271
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	-317	-318	-1,271	-1,271
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit	-317
Ending Balance AOCI March 31, 2015, FSIRS - Tax (Expense) Benefit	8,304		8,304
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	519	518	2,074	2,074
Net forward-starting interest rate swaps	519	518	2,074	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	519
Ending Balance AOCI March 31, 2015, FSIRS - After-Tax	-13,548		-13,548
Beginning Balance AOCI December 31, 2014,Foreign Currency Items Before-Tax	-637
Translation adjustments, Foreign Currency Items Before-Tax	-1,272		-1,931
Other comprehensive income before reclassifications, Foreign Currency Items Before-Tax	-1,272		-1,931
FSIRS amounts reclassified from AOCI, Foreign Currency Items Before-Tax	0
Amortization of prior service cost, Foreign Currency Items Before-Tax	0
Amortization of net actuarial loss, Foreign Currency Items Before-Tax	0
Regulatory adjustment, Foreign Currency Items Before-Tax	0
Net current period other comprehensive income (loss), Foreign Currency Items Before-Tax	-1,272
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax	-43
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax	-1,229
Ending Balance AOCI March 31, 2015, Foreign Currency Items Before-Tax	-1,866		-1,866
Beginning Balance AOCI December 31, 2013,Foreign Currency items - Tax (Expense) Benefit	0
Translation adjustments,Foreign Currency items - Tax (Expense) Benefit	0	0	0	0
Other comprehensive income before reclassifications,Foreign Currency items - Tax (Expense) Benefit	0
FSIRS amounts reclassified from AOCI ,Foreign Currency items - Tax (Expense) Benefit	0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit	0
Amortization of net actuarial loss,Foreign Currency items - Tax (Expense) Benefit	0
Regulatory adjustment,Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss),Foreign Currency items - Tax (Expense) Benefit	0
Less: Translation adjustment attributable to redeemable noncontrolling interest ,Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation , Foreign Currency items - Tax (Expense) Benefit	0	0	0	0
Ending Balance AOCI December 31, 2014,Foreign Currency items - Tax (Expense) Benefit	0		0
Beginning Balance AOCI December 31, 2013, Foreign Currency items - After-Tax	-637
Translation adjustments, Foreign Currency items - After-Tax	-1,272		-1,931
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	-1,272		-1,931
FSIRS amounts reclassified from AOCI, Foreign Currency items - After-Tax	0
Amortization of prior service cost, Foreign Currency items - After-Tax	0
Amortization of net actuarial loss, Foreign Currency items - After-Tax	0
Regulatory adjustment, Foreign Currency items - After-Tax	0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax	-1,272
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax	-43
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax	-1,229
Ending Balance AOCI December 31, 2014, Foreign Currency items - After-Tax	-1,866		-1,866
Beginning Balance AOCI December 31, 2014, AOCI	-50,175
Translation adjustments, AOCI	-1,272		-1,272
Other comprehensive income before reclassifications, AOCI	-1,272
Amounts reclassified into net income	519	518	2,074	2,074
Amortization of prior service cost, AOCI	206	55	371	220
Amortization of net actuarial loss	5,330	3,667	16,330	19,559
Regulatory adjustment, AOCI	-4,828
Total other comprehensive income (loss), net of tax	-45	1,030	-9,574	8,908
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	-43		-43
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	-2		-2
Ending Balance AOCI March 31, 2015, AOCI	($50,177)		($50,177)

Equity_Other_Comprehensive_Inc8

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 $)	Mar. 31, 2015	Dec. 31, 2014
In Thousands, unless otherwise specified
Accumulated Other Comprehensive Income (Loss) [Line Items]
Recognized in AOCI	($56,069)	($57,211)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	-430,535	-439,131
Prior service cost	-8,040	-8,373
Less: amount recognized in regulatory assets	382,506	390,293
Recognized in AOCI	($56,069)	($57,211)

Acquisition_of_Construction_Se2

Acquisition of Construction Services Businesses - Revised Preliminary Estimated Fair Values of Assets Acquired and Liabilities Assumed (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014	Oct. 01, 2014
Business Acquisition [Line Items]
Goodwill	$135,689,000	$143,160,000
Link-Line, W.S. Nicholls, and Brigadier [Member]
Business Acquisition [Line Items]
Cash, cash equivalents, and restricted cash			3,000,000
Contracts receivable and other receivables			62,000,000
Property, plant and equipment			17,000,000
Other assets			15,000,000
Intangible assets			52,000,000
Goodwill			131,000,000
Total assets acquired			280,000,000
Current liabilities			40,000,000
Deferred income tax - long-term			17,000,000
Other long-term liabilities			4,000,000
Net assets acquired			219,000,000
Link-Line, W.S. Nicholls, and Brigadier [Member] | Acquisition Date [Member]
Business Acquisition [Line Items]
Cash, cash equivalents, and restricted cash			3,000,000
Contracts receivable and other receivables			62,000,000
Property, plant and equipment			17,000,000
Other assets			17,000,000
Intangible assets			52,000,000
Goodwill			130,000,000
Total assets acquired			281,000,000
Current liabilities			39,000,000
Deferred income tax - long-term			17,000,000
Other long-term liabilities			4,000,000
Net assets acquired			221,000,000
Link-Line, W.S. Nicholls, and Brigadier [Member] | Measurement Period Adjustments [Member]
Business Acquisition [Line Items]
Other assets			-2,000,000
Goodwill			1,000,000
Total assets acquired			-1,000,000
Current liabilities			1,000,000
Net assets acquired			($2,000,000)

Acquisition_of_Construction_Se3

Acquisition of Construction Services Businesses - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2015
Business Acquisition [Line Items]
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

Construction_Services_Noncontr

Construction Services Noncontrolling Interests - Additional Information (Detail) (Principal Previous Owners [Member], Link-Line and WS Nicholls [Member])	1 Months Ended	3 Months Ended
	Oct. 31, 2014	Mar. 31, 2015
Noncontrolling Interest [Line Items]
Equity interest percentage	10.00%
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. Additionally, the previous owners may exit their investment retained by requiring Centuri to purchase a portion of their interest (in Lynxus Construction Group, Inc.) 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_Noncontr1

Construction Services Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) (USD $)	3 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2015
Redeemable Noncontrolling Interest [Line Items]
Balance, December 31, 2014	$20,042
Foreign currency exchange translation adjustment	-1,229
Adjustment to redemption value	658
Balance, March 31, 2015	19,096
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	-245
Foreign currency exchange translation adjustment	-43
Adjustment to redemption value	-658
Balance, March 31, 2015	$19,096