Southwest Gas 10-Q 7 May 14 2014 Q1 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
	Mar. 31, 2014	Apr. 28, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	31-Mar-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q1	'
Trading Symbol	'SWX	'
Entity Registrant Name	'SOUTHWEST GAS CORP	'
Entity Central Index Key	'0000092416	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	46,503,111

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Utility plant:	'	'
Gas plant	$5,341,734	$5,252,469
Less: accumulated depreciation	-1,900,044	-1,868,504
Acquisition adjustments, net	685	730
Construction work in progress	70,285	101,413
Net utility plant	3,512,660	3,486,108
Other property and investments	262,187	260,871
Current assets:	'	'
Cash and cash equivalents	85,366	41,077
Accounts receivable, net of allowances	205,511	219,469
Accrued utility revenue	44,100	72,700
Income taxes receivable, net	'	3,790
Deferred income taxes	4,979	31,130
Deferred purchased gas costs	75,196	18,217
Prepaids and other current assets	84,661	108,289
Total current assets	499,813	494,672
Deferred charges and other assets	326,147	323,523
Total assets	4,600,807	4,565,174
Capitalization:	'	'
Common stock, $1 par (authorized- 60,000,000 shares; issued and outstanding- 46,495,191 and 46,356,125 shares)	48,125	47,986
Additional paid-in capital	843,463	840,521
Accumulated other comprehensive income (loss), net	-40,668	-41,698
Retained earnings	621,304	567,714
Total Southwest Gas Corporation equity	1,472,224	1,414,523
Noncontrolling interest	-2,214	-2,128
Total equity	1,470,010	1,412,395
Long-term debt, less current maturities	1,368,632	1,381,327
Total capitalization	2,838,642	2,793,722
Current liabilities:	'	'
Current maturities of long-term debt	11,158	11,105
Accounts payable	122,212	183,511
Customer deposits	73,669	73,367
Income taxes payable	14,588	'
Accrued general taxes	54,743	39,681
Accrued interest	23,563	17,920
Other current liabilities	117,141	108,580
Total current liabilities	417,074	434,164
Deferred income taxes and other credits:	'	'
Deferred income taxes and investment tax credits	662,583	674,411
Taxes payable	244	284
Accumulated removal costs	285,000	279,000
Other deferred credits	397,264	383,593
Total deferred income taxes and other credits	1,345,091	1,337,288
Total capitalization and liabilities	$4,600,807	$4,565,174

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]	'	'
Common stock, par	$1	$1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	46,495,191	46,356,125
Common stock, outstanding	46,495,191	46,356,125

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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Operating revenues:	'	'	'	'
Gas operating revenues	$486,493	$493,600	$1,293,047	$1,284,615
Construction revenues	121,903	119,905	652,626	599,023
Total operating revenues	608,396	613,505	1,945,673	1,883,638
Operating expenses:	'	'	'	'
Net cost of gas sold	191,377	200,608	426,770	437,463
Operations and maintenance	102,408	97,087	390,235	371,216
Depreciation and amortization	62,891	58,933	240,775	228,192
Taxes other than income taxes	11,456	11,795	45,212	42,792
Construction expenses	113,199	106,688	579,795	528,680
Total operating expenses	481,331	475,111	1,682,787	1,608,343
Operating income	127,065	138,394	262,886	275,295
Other income and (expenses):	'	'	'	'
Net interest deductions	-17,519	-15,878	-65,341	-66,739
Other income (deductions)	1,612	4,069	9,843	3,052
Total other income and (expenses)	-15,907	-11,809	-55,498	-63,687
Income before income taxes	111,158	126,585	207,388	211,608
Income tax expense	40,461	45,911	72,492	77,130
Net income	70,697	80,674	134,896	134,478
Net income (loss) attributable to noncontrolling interest	-86	-99	-434	-707
Net income attributable to Southwest Gas Corporation	$70,783	$80,773	$135,330	$135,185
Basic earnings per share	$1.52	$1.75	$2.92	$2.93
Diluted earnings per share	$1.51	$1.73	$2.89	$2.90
Dividends declared per share	$0.37	$0.33	$1.36	$1.22
Average number of common shares outstanding	46,440	46,251	46,364	46,160
Average shares outstanding (assuming dilution)	46,871	46,650	46,812	46,600

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]	'	'	'	'
Net income	$70,697	$80,674	$134,896	$134,478
Defined benefit pension plans:	'	'	'	'
Net actuarial gain (loss)	'	'	62,214	-46,409
Amortization of prior service cost	55	55	220	55
Amortization of transition obligation	'	'	'	404
Amortization of net actuarial loss	3,667	5,298	19,559	17,200
Prior service cost	'	'	'	-1,502
Regulatory adjustment	-3,210	-4,702	-75,159	25,442
Net defined benefit pension plans	512	651	6,834	-4,810
Forward-starting interest rate swaps:	'	'	'	'
Amounts reclassified into net income	518	518	2,074	2,074
Net forward-starting interest rate swaps	518	518	2,074	2,074
Total other comprehensive income (loss), net of tax	1,030	1,169	8,908	-2,736
Comprehensive income	71,727	81,843	143,804	131,742
Comprehensive income (loss) attributable to noncontrolling interest	-86	-99	-434	-707
Comprehensive income attributable to Southwest Gas Corporation	$71,813	$81,942	$144,238	$132,449

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
CASH FLOW FROM OPERATING ACTIVITIES:	'	'	'	'
Net income	$70,697	$80,674	$134,896	$134,478
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'	'
Depreciation and amortization	62,891	58,933	240,775	228,192
Deferred income taxes	13,692	40,241	42,090	66,983
Changes in current assets and liabilities:	'	'	'	'
Accounts receivable, net of allowances	13,958	-11,404	2,806	2,418
Accrued utility revenue	28,600	28,400	-500	100
Deferred purchased gas costs	-56,979	-51,444	-116,678	-34,233
Accounts payable	-66,641	-37,931	-1,042	-7,844
Accrued taxes	33,400	20,733	13,592	-1,042
Other current assets and liabilities	36,522	34,141	7,465	-21,220
Gains on sale	-2,429	-834	-5,707	-7,460
Changes in undistributed stock compensation	2,761	2,659	7,060	6,695
AFUDC and property-related changes	-498	-471	-2,301	-2,092
Changes in other assets and deferred charges	-11,134	-11,859	-20,994	-11,168
Changes in other liabilities and deferred credits	17,155	-18,237	53,141	-255
Net cash provided by operating activities	141,995	133,601	354,603	353,552
CASH FLOW FROM INVESTING ACTIVITIES:	'	'	'	'
Construction expenditures and property additions	-76,892	-53,237	-387,931	-363,984
Changes in customer advances	4,105	906	10,972	4,434
Miscellaneous inflows	3,593	1,600	10,458	13,299
Miscellaneous outflows	'	'	'	-2,004
Net cash used in investing activities	-69,194	-50,731	-366,501	-348,255
CASH FLOW FROM FINANCING ACTIVITIES:	'	'	'	'
Issuance of common stock, net	107	1,175	567	2,024
Dividends paid	-15,343	-13,658	-61,220	-54,483
Issuance of long-term debt, net	1,025	4,664	307,651	168,304
Retirement of long-term debt	-3,781	-69,500	-71,294	-419,121
Change in credit facility and commercial paper	-10,000	2,000	-113,000	113,000
Other	-520	'	1,479	'
Net cash provided by (used in) financing activities	-28,512	-75,319	64,183	-190,276
Change in cash and cash equivalents	44,289	7,551	52,285	-184,979
Cash and cash equivalents at beginning of period	41,077	25,530	33,081	218,060
Cash and cash equivalents at end of period	85,366	33,081	85,366	33,081
Supplemental information:	'	'	'	'
Interest paid, net of amounts capitalized	10,474	13,146	56,058	63,339
Income taxes paid	$238	$634	$6,454	$4,666

Nature_of_Operations_and_Basis

Nature of Operations and Basis of Presentation	3 Months Ended
	Mar. 31, 2014
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 sales are seasonal, peaking during the winter months; therefore, results of operations for interim periods are not necessarily indicative of results for a full year. Natural gas purchases and the timing of related recoveries can materially impact liquidity. NPL Construction Co. (“NPL” 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. Typically, NPL 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 2013 Annual Report to Shareholders, which is incorporated by reference into the 2013 Form 10-K.
	Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $21 million at both March 31, 2014 and December 31, 2013.
	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. 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. Upon contract expiration, customer advances of approximately $1.6 million and $2.8 million, during the first quarters of 2014 and 2013, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.
	Intercompany Transactions. NPL 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, 2014			December 31, 2013
	Accounts receivable for NPL services	$	7,245		$	10,787
	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, 2014			December 31, 2013
	NPL property and equipment	$	325		$	320
	NPL accumulated provision for depreciation and amortization		(168	)		(163	)
	Net cash surrender value of COLI policies		94			93
	Other property		11			11
	Total	$	262		$	261
	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
		2014			2013			2014			2013
	Change in COLI policies	$	900		$	3,800		$	9,500		$	5,200
	Interest income		497			120			838			818
	Pipe replacement costs		—			(85	)		(47	)		(2,756	)
	Miscellaneous income and (expense)		215			234			(448	)		(210	)
	Total other income (deductions)	$	1,612		$	4,069		$	9,843		$	3,052
	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 value components of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work. The replacement program work subject to non-recoverability was substantially completed in 2012.
	Reclassifications. A reclassification was made to the prior year’s financial information between two categories on the Condensed Consolidated Statements of Comprehensive Income to present it on a basis comparable with the current year’s presentation, with no impact on comprehensive income overall.

Components_of_Net_Periodic_Ben

Components of Net Periodic Benefit Cost	3 Months Ended
	Mar. 31, 2014
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
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	5,340		$	5,764		$	22,632		$	21,004
	Interest cost		10,861			9,401			39,067			38,100
	Expected return on plan assets		(13,336	)		(12,460	)		(50,716	)		(46,795	)
	Amortization of net actuarial loss		5,718			8,066			29,913			25,978
	Net periodic benefit cost	$	8,583		$	10,771		$	40,896		$	38,287
		SERP
		Period Ended March 31,
		Three Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	73		$	94		$	352		$	299
	Interest cost		436			383			1,588			1,605
	Amortization of net actuarial loss		196			243			924			755
	Net periodic benefit cost	$	705		$	720		$	2,864		$	2,659
		PBOP
		Period Ended March 31,
		Three Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	276		$	305		$	1,191		$	1,037
	Interest cost		707			620			2,569			2,531
	Expected return on plan assets		(816	)		(706	)		(2,934	)		(2,509	)
	Amortization of prior service cost		89			89			355			89
	Amortization of transition obligation		—			—			—			650
	Amortization of net actuarial loss		—			236			709			1,009
	Net periodic benefit cost	$	256		$	544		$	1,890		$	2,807

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2014
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, 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
	Three months ended March 31, 2013
	Revenues from external customers	$	493,600	$	104,866		$	598,466
	Intersegment revenues		—		15,039			15,039
	Total	$	493,600	$	119,905		$	613,505
	Segment net income	$	79,292	$	1,481		$	80,773
		Natural Gas		Construction			Total
		Operations		Services
	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
	Twelve months ended March 31, 2013
	Revenues from external customers	$	1,284,615	$	516,655		$	1,801,270
	Intersegment revenues		—		82,368			82,368
	Total	$	1,284,615	$	599,023		$	1,883,638
	Segment net income	$	116,545	$	18,640		$	135,185

Derivatives_and_Fair_Value_Mea

Derivatives and Fair Value Measurements	3 Months Ended
	Mar. 31, 2014
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.
	As part of the June 2013 Nevada Annual Rate Adjustment and associated stipulation, the Company agreed to suspend further Swaps and fixed-price purchases pursuant to the Volatility Mitigation Program for its Nevada service territories. The decision will not impact previously executed purchase agreements. The Company, along with its regulators, will continue to evaluate this strategy in light of prevailing or anticipated changing market conditions.
	The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 35%, depending on the jurisdiction) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from April 2014 through March 2016. 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, 2014				December 31, 2013
	Contract notional amounts			7,664				13,571
	Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.
	The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three-and twelve-month periods ended March 31, 2014 and 2013 and their location in the Condensed Consolidated Statements of Income (thousands of dollars):
	Gains (losses) recognized in income for derivatives not designated as hedging instruments:
	(Thousands of dollars)
			Location of Gain or (Loss)		Three Months Ended							Twelve Months Ended
			Recognized in Income on Derivative		March 31							March 31
	Instrument				2014				2013			2014			2013
	Swaps		Net cost of gas sold		$	5,990			$	5,076		$	1,890		$	7,158
	Swaps		Net cost of gas sold			(5,990	)*			(5,076	)*		(1,890	)*		(7,158	)*
	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”) to hedge the risk of interest rate variability during the period leading up to the issuance of fixed-rate debt. At settlement of the first FSIRS in December 2010, Southwest paid an aggregate $11.7 million to the counterparties. The second FSIRS terminated in March 2012 at which time Southwest paid counterparties an aggregate $21.8 million. The 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, 2014		Balance Sheet Location		Asset				Liability			Net Total
	Instrument				Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	352			$	(16	)	$	336
	Swaps		Prepaids and other current assets			2,171				—			2,171
	Total				$	2,523			$	(16	)	$	2,507
	31-Dec-13		Balance Sheet Location		Asset				Liability			Net Total
	Instrument				Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	257			$	(77	)	$	180
	Swaps		Prepaids and other current assets			1,054				(253	)		801
	Swaps		Other current liabilities			126				(282	)		(156	)
	Swaps		Other deferred credits			7				(11	)		(4	)
	Total				$	1,444			$	(623	)	$	821
	The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). The Company has master netting arrangements with each counterparty that provide for the net settlement of all contracts through a single payment. As applicable, the Company has elected to reflect the net amounts in its balance sheets. The Company had no outstanding collateral associated with the Swaps during either period shown in the above table.
	Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income.
	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.
	(Thousands of dollars)		Three Months Ended				Twelve Months Ended
			March 31, 2014				March 31, 2014
	Paid to counterparties		$	15			$	2,294
	Received from counterparties		$	4,319			$	4,869
	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, 2014		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(336	)
	Swaps		Other current liabilities			(2,171	)
	31-Dec-13		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(180	)
	Swaps		Other current liabilities			(801	)
	Swaps		Prepaids and other current assets			156
	Swaps		Deferred charges and other assets			4
	Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at March 31, 2014 and December 31, 2013 using New York Mercantile Exchange (“NYMEX”) futures settlement prices for delivery of natural gas at Henry Hub adjusted by the price of NYMEX ClearPort basis Swaps, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs (inputs, other than quoted prices, for similar assets or liabilities) are observable in the marketplace throughout the full term of the Swaps, but have been credit-risk adjusted with no significant impact to the overall fair value measure.
	The following table sets forth by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the Company’s financial assets and liabilities recorded at fair value:
	Level 2 – Significant other observable inputs
	(Thousands of dollars)		March 31, 2014				December 31, 2013
	Assets at fair value:
	Prepaids and other current assets—Swaps		$	2,171			$	801
	Deferred charges and other assets—Swaps			336				180
	Liabilities at fair value:
	Other current liabilities—Swaps			—				(156	)
	Other deferred credits—Swaps			—				(4	)
	Net Assets (Liabilities)		$	2,507			$	821
	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.

LongTerm_Debt

Long-Term Debt	3 Months Ended
	Mar. 31, 2014
Debt Disclosure [Abstract]	'
Long-Term Debt	'
	Note 5 – Long-Term Debt
	Carrying amounts of the Company’s long-term debt and their related estimated fair values as of March 31, 2014 and December 31, 2013 are disclosed in the following table. The fair values of the revolving credit facility (including commercial paper), the NPL revolving credit facility, and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, and are categorized as Level 1 (quoted prices for identical financial instruments or liabilities that can be accessed at the measurement date) within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The market values of debentures (except the 6.1% Notes) and fixed-rate IDRBs are categorized as Level 2. The 6.1% Notes (private placement, not actively traded) and NPL other debt obligations (not actively traded) are categorized as Level 3 (based on significant unobservable inputs to their fair values). Fair values for the debentures, fixed-rate IDRBs, and NPL other debt obligations were determined through 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 and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.
		March 31, 2014					December 31, 2013
	(Thousands of dollars)	Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	134,433	$	125,000		$	130,953
	Notes, 6.1%, due 2041		125,000			148,894		125,000			141,873
	Notes, 3.875%, due 2022		250,000			255,923		250,000			252,485
	Notes, 4.875%, due 2043		250,000			268,873		250,000			257,280
	8% Series, due 2026		75,000			101,735		75,000			96,263
	Medium-term notes, 7.59% series, due 2017		25,000			28,691		25,000			28,741
	Medium-term notes, 7.78% series, due 2022		25,000			31,585		25,000			30,586
	Medium-term notes, 7.92% series, due 2027		25,000			33,402		25,000			31,497
	Medium-term notes, 6.76% series, due 2027		7,500			9,021		7,500			8,468
	Unamortized discount		(5,478	)				(5,560	)
			902,022					901,940
	Revolving credit facility and commercial paper		—			—		10,000			10,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,289		20,000			20,150
	5.25% 2004 Series A, due 2034		65,000			65,051		65,000			64,522
	5.00% 2004 Series B, due 2033		31,200			31,229		31,200			30,284
	4.85% 2005 Series A, due 2035		100,000			99,709		100,000			95,192
	4.75% 2006 Series A, due 2036		24,855			24,539		24,855			22,974
	Unamortized discount		(2,743	)				(2,776	)
			438,312					438,279
	NPL credit facility		—			—		—			—
	NPL other debt obligations		39,456			39,465		42,213			42,119
			1,379,790					1,392,432
	Less: current maturities		(11,158	)				(11,105	)
	Long-term debt, less current maturities	$	1,368,632				$	1,381,327
	In March 2014, the Company amended its $300 million credit facility. The facility was previously scheduled to expire in March 2017 and was extended to March 2019. The Company will continue to use $150 million of the facility as long-term debt and the remaining $150 million for working capital purposes. In addition to extending the credit facility, among other amendments, the applicable margins and unused commitment fees were reduced and the Pricing Level definitions were modified. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. At March 31, 2014, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate.
	Also in March 2014, the Company amended its 6.1% $125 million notes (“Notes”) note purchase agreement. The amendment modifies the Permitted Lien category permitting liens securing indebtedness not to exceed 10% of total capitalization as of the end of any quarter. This provision in the agreement prohibits liens on any property securing other indebtedness under bank facilities unless the Notes are secured in a similar manner. The provision was amended to clarify that it only applies to bank facilities of Southwest Gas Corporation.

Equity_Other_Comprehensive_Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	'
	Note 6 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income
	The table below provides details of activity in equity during the three months ended March 31, 2014.
			Southwest Gas Corporation Equity
			Common Stock						Additional			Accumulated			Retained			Non-			Total
									Paid-in			Other			Earnings			controlling
									Capital			Comprehensive						Interest
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-13			46,356		$	47,986		$	840,521		$	(41,698	)	$	567,714		$	(2,128	)	$	1,412,395
	Common stock issuances			139			139			2,942												3,081
	Net income (loss)															70,783			(86	)		70,697
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												512									512
	Amounts reclassified to net income, net of tax (FSIRS)												518									518
	Dividends declared
	Common: $0.365 per share															(17,193	)					(17,193	)
	31-Mar-14			46,495		$	48,125		$	843,463		$	(40,668	)	$	621,304		$	(2,214	)	$	1,470,010
	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
			March 31, 2014									March 31, 2013
			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 net actuarial (gain)/loss		$	5,914		$	(2,247	)	$	3,667		$	8,545		$	(3,247	)	$	5,298
	Amortization of prior service cost			89			(34	)		55			89			(34	)		55
	Regulatory adjustment			(5,178	)		1,968			(3,210	)		(7,584	)		2,882			(4,702	)
	Pension plans other comprehensive income (loss)			825			(313	)		512			1,050			(399	)		651
	FSIRS (designated hedging activities):
	Amounts reclassified into net income			836			(318	)		518			836			(318	)		518
	FSIRS other comprehensive income			836			(318	)		518			836			(318	)		518
	Total other comprehensive income (loss)		$	1,661		$	(631	)	$	1,030		$	1,886		$	(717	)	$	1,169
			Twelve Months Ended									Twelve Months Ended
			March 31, 2014									March 31, 2013
			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)		$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
	Amortization of prior service cost			355			(135	)		220			89			(34	)		55
	Amortization of transition obligation			—			—			—			650			(246	)		404
	Amortization of net actuarial (gain)/loss			31,546			(11,987	)		19,559			27,742			(10,542	)		17,200
	Prior service cost			—			—			—			(2,423	)		921			(1,502	)
	Regulatory adjustment			(121,224	)		46,065			(75,159	)		41,036			(15,594	)		25,442
	Pension plans other comprehensive income (loss)			11,022			(4,188	)		6,834			(7,759	)		2,949			(4,810	)
	FSIRS (designated hedging activities):
	Amounts reclassified 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
	Total other comprehensive income (loss)		$	14,367		$	(5,459	)	$	8,908		$	(4,414	)	$	1,678		$	(2,736	)
	Tax amounts are calculated using a 38% rate.
	Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (“AOCI”) at March 31, 2014, will be reclassified into interest expense within the next 12 months, as the related interest payments on long-term debt occur.
	The following represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets:
	AOCI—Rollforward
	(Thousands of dollars)
			Defined Benefit Plans									FSIRS
			Before-			Tax			After-			Before-			Tax			After-			AOCI
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
						Benefit									Benefit
	Beginning Balance AOCI December 31, 2013		$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	(41,698	)
	Other comprehensive income before reclassifications			—			—			—			—			—			—			—
	FSIRS amounts reclassified from AOCI (1)			—			—			—			836			(318	)		518			518
	Amortization of prior service cost (2)			89			(34	)		55			—			—			—			55
Amortization of net actuarial loss (2)			5,914			(2,247	)		3,667			—			—			—			3,667
Regulatory adjustment (3)			(5,178	)		1,968			(3,210	)		—			—			—			(3,210	)
Net current period other comprehensive income (loss)			825			(313	)		512			836			(318	)		518			1,030
Ending Balance AOCI March 31, 2014		$	(40,398	)	$	15,352		$	(25,046	)	$	(25,197	)	$	9,575		$	(15,622	)	$	(40,668	)
-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) 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, 2014			December 31, 2013
Net actuarial (loss) gain		$	(283,227	)	$	(289,141	)
Prior service cost			(1,978	)		(2,067	)
Less: amount recognized in regulatory assets			244,807			249,985
Recognized in AOCI		$	(40,398	)	$	(41,223	)

Nature_of_Operations_and_Basis1

Nature of Operations and Basis of Presentation (Policies)	3 Months Ended
	Mar. 31, 2014
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 sales are seasonal, peaking during the winter months; therefore, results of operations for interim periods are not necessarily indicative of results for a full year. Natural gas purchases and the timing of related recoveries can materially impact liquidity. NPL Construction Co. (“NPL” 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. Typically, NPL 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 2013 Annual Report to Shareholders, which is incorporated by reference into the 2013 Form 10-K.
Prepaids and other current assets	'
	Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $21 million at both March 31, 2014 and December 31, 2013.
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. 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. Upon contract expiration, customer advances of approximately $1.6 million and $2.8 million, during the first quarters of 2014 and 2013, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.
Intercompany Transactions	'
	Intercompany Transactions. NPL 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, 2014			December 31, 2013
	Accounts receivable for NPL services	$	7,245		$	10,787
	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, 2014			December 31, 2013
	NPL property and equipment	$	325		$	320
	NPL accumulated provision for depreciation and amortization		(168	)		(163	)
	Net cash surrender value of COLI policies		94			93
	Other property		11			11
	Total	$	262		$	261
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
		2014			2013			2014			2013
	Change in COLI policies	$	900		$	3,800		$	9,500		$	5,200
	Interest income		497			120			838			818
	Pipe replacement costs		—			(85	)		(47	)		(2,756	)
	Miscellaneous income and (expense)		215			234			(448	)		(210	)
	Total other income (deductions)	$	1,612		$	4,069		$	9,843		$	3,052
	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 value components of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work. The replacement program work subject to non-recoverability was substantially completed in 2012.
Reclassifications	'
	Reclassifications. A reclassification was made to the prior year’s financial information between two categories on the Condensed Consolidated Statements of Comprehensive Income to present it on a basis comparable with the current year’s presentation, with no impact on comprehensive income overall.

Nature_of_Operations_and_Basis2

Nature of Operations and Basis of Presentation (Tables)	3 Months Ended
	Mar. 31, 2014
Accounting Policies [Abstract]	'
Intercompany Transactions	'
	Accounts receivable for these services are presented in the table below (thousands of dollars):
		March 31, 2014			December 31, 2013
	Accounts receivable for NPL services	$	7,245		$	10,787
Schedule of Other Property and Investments	'
	Other property and investments includes (millions of dollars):
		March 31, 2014			December 31, 2013
	NPL property and equipment	$	325		$	320
	NPL accumulated provision for depreciation and amortization		(168	)		(163	)
	Net cash surrender value of COLI policies		94			93
	Other property		11			11
	Total	$	262		$	261
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
		2014			2013			2014			2013
	Change in COLI policies	$	900		$	3,800		$	9,500		$	5,200
	Interest income		497			120			838			818
	Pipe replacement costs		—			(85	)		(47	)		(2,756	)
	Miscellaneous income and (expense)		215			234			(448	)		(210	)
	Total other income (deductions)	$	1,612		$	4,069		$	9,843		$	3,052

Components_of_Net_Periodic_Ben1

Components of Net Periodic Benefit Cost (Tables)	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Schedule of Components of Net Periodic Benefit Costs	'
	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
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	5,340		$	5,764		$	22,632		$	21,004
	Interest cost		10,861			9,401			39,067			38,100
	Expected return on plan assets		(13,336	)		(12,460	)		(50,716	)		(46,795	)
	Amortization of net actuarial loss		5,718			8,066			29,913			25,978
	Net periodic benefit cost	$	8,583		$	10,771		$	40,896		$	38,287
		SERP
		Period Ended March 31,
		Three Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	73		$	94		$	352		$	299
	Interest cost		436			383			1,588			1,605
	Amortization of net actuarial loss		196			243			924			755
	Net periodic benefit cost	$	705		$	720		$	2,864		$	2,659
		PBOP
		Period Ended March 31,
		Three Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013
	Service cost	$	276		$	305		$	1,191		$	1,037
	Interest cost		707			620			2,569			2,531
	Expected return on plan assets		(816	)		(706	)		(2,934	)		(2,509	)
	Amortization of prior service cost		89			89			355			89
	Amortization of transition obligation		—			—			—			650
	Amortization of net actuarial loss		—			236			709			1,009
	Net periodic benefit cost	$	256		$	544		$	1,890		$	2,807

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2014
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, 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
	Three months ended March 31, 2013
	Revenues from external customers	$	493,600	$	104,866		$	598,466
	Intersegment revenues		—		15,039			15,039
	Total	$	493,600	$	119,905		$	613,505
	Segment net income	$	79,292	$	1,481		$	80,773
		Natural Gas		Construction			Total
		Operations		Services
	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
	Twelve months ended March 31, 2013
	Revenues from external customers	$	1,284,615	$	516,655		$	1,801,270
	Intersegment revenues		—		82,368			82,368
	Total	$	1,284,615	$	599,023		$	1,883,638
	Segment net income	$	116,545	$	18,640		$	135,185

Derivatives_and_Fair_Value_Mea1

Derivatives and Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Notional Amounts under Swaps Contracts	'
	The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):
			March 31, 2014				December 31, 2013
	Contract notional amounts			7,664				13,571
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, 2014 and 2013 and their location in the Condensed Consolidated Statements of Income (thousands of dollars):
	Gains (losses) recognized in income for derivatives not designated as hedging instruments:
	(Thousands of dollars)
			Location of Gain or (Loss)		Three Months Ended							Twelve Months Ended
			Recognized in Income on Derivative		March 31							March 31
	Instrument				2014				2013			2014			2013
	Swaps		Net cost of gas sold		$	5,990			$	5,076		$	1,890		$	7,158
	Swaps		Net cost of gas sold			(5,990	)*			(5,076	)*		(1,890	)*		(7,158	)*
	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, 2014		Balance Sheet Location		Asset				Liability			Net Total
	Instrument				Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	352			$	(16	)	$	336
	Swaps		Prepaids and other current assets			2,171				—			2,171
	Total				$	2,523			$	(16	)	$	2,507
	31-Dec-13		Balance Sheet Location		Asset				Liability			Net Total
	Instrument				Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	257			$	(77	)	$	180
	Swaps		Prepaids and other current assets			1,054				(253	)		801
	Swaps		Other current liabilities			126				(282	)		(156	)
	Swaps		Other deferred credits			7				(11	)		(4	)
	Total				$	1,444			$	(623	)	$	821
Paid to and Received from Counterparties for Settlements of Matured Swaps	'
	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.
	(Thousands of dollars)		Three Months Ended				Twelve Months Ended
			March 31, 2014				March 31, 2014
	Paid to counterparties		$	15			$	2,294
	Received from counterparties		$	4,319			$	4,869
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, 2014		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(336	)
	Swaps		Other current liabilities			(2,171	)
	31-Dec-13		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(180	)
	Swaps		Other current liabilities			(801	)
	Swaps		Prepaids and other current assets			156
	Swaps		Deferred charges and other assets			4
Significant Other Observable Inputs	'
	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 recorded at fair value:
	Level 2 – Significant other observable inputs
	(Thousands of dollars)		March 31, 2014				December 31, 2013
	Assets at fair value:
	Prepaids and other current assets—Swaps		$	2,171			$	801
	Deferred charges and other assets—Swaps			336				180
	Liabilities at fair value:
	Other current liabilities—Swaps			—				(156	)
	Other deferred credits—Swaps			—				(4	)
	Net Assets (Liabilities)		$	2,507			$	821

LongTerm_Debt_Tables

Long-Term Debt (Tables)	3 Months Ended
	Mar. 31, 2014
Debt Disclosure [Abstract]	'
Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt	'
	The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.
		March 31, 2014					December 31, 2013
	(Thousands of dollars)	Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	134,433	$	125,000		$	130,953
	Notes, 6.1%, due 2041		125,000			148,894		125,000			141,873
	Notes, 3.875%, due 2022		250,000			255,923		250,000			252,485
	Notes, 4.875%, due 2043		250,000			268,873		250,000			257,280
	8% Series, due 2026		75,000			101,735		75,000			96,263
	Medium-term notes, 7.59% series, due 2017		25,000			28,691		25,000			28,741
	Medium-term notes, 7.78% series, due 2022		25,000			31,585		25,000			30,586
	Medium-term notes, 7.92% series, due 2027		25,000			33,402		25,000			31,497
	Medium-term notes, 6.76% series, due 2027		7,500			9,021		7,500			8,468
	Unamortized discount		(5,478	)				(5,560	)
			902,022					901,940
	Revolving credit facility and commercial paper		—			—		10,000			10,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,289		20,000			20,150
	5.25% 2004 Series A, due 2034		65,000			65,051		65,000			64,522
	5.00% 2004 Series B, due 2033		31,200			31,229		31,200			30,284
	4.85% 2005 Series A, due 2035		100,000			99,709		100,000			95,192
	4.75% 2006 Series A, due 2036		24,855			24,539		24,855			22,974
	Unamortized discount		(2,743	)				(2,776	)
			438,312					438,279
	NPL credit facility		—			—		—			—
	NPL other debt obligations		39,456			39,465		42,213			42,119
			1,379,790					1,392,432
	Less: current maturities		(11,158	)				(11,105	)
	Long-term debt, less current maturities	$	1,368,632				$	1,381,327

Equity_Other_Comprehensive_Inc1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	3 Months Ended
	Mar. 31, 2014
Text Block [Abstract]	'
Detailed Activities of Equity	'
	The table below provides details of activity in equity during the three months ended March 31, 2014.
			Southwest Gas Corporation Equity
			Common Stock						Additional			Accumulated			Retained			Non-			Total
									Paid-in			Other			Earnings			controlling
									Capital			Comprehensive						Interest
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-13			46,356		$	47,986		$	840,521		$	(41,698	)	$	567,714		$	(2,128	)	$	1,412,395
	Common stock issuances			139			139			2,942												3,081
	Net income (loss)															70,783			(86	)		70,697
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												512									512
	Amounts reclassified to net income, net of tax (FSIRS)												518									518
	Dividends declared
	Common: $0.365 per share															(17,193	)					(17,193	)
	31-Mar-14			46,495		$	48,125		$	843,463		$	(40,668	)	$	621,304		$	(2,214	)	$	1,470,010
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
			March 31, 2014									March 31, 2013
			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 net actuarial (gain)/loss		$	5,914		$	(2,247	)	$	3,667		$	8,545		$	(3,247	)	$	5,298
	Amortization of prior service cost			89			(34	)		55			89			(34	)		55
	Regulatory adjustment			(5,178	)		1,968			(3,210	)		(7,584	)		2,882			(4,702	)
	Pension plans other comprehensive income (loss)			825			(313	)		512			1,050			(399	)		651
	FSIRS (designated hedging activities):
	Amounts reclassified into net income			836			(318	)		518			836			(318	)		518
	FSIRS other comprehensive income			836			(318	)		518			836			(318	)		518
	Total other comprehensive income (loss)		$	1,661		$	(631	)	$	1,030		$	1,886		$	(717	)	$	1,169
			Twelve Months Ended									Twelve Months Ended
			March 31, 2014									March 31, 2013
			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)		$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
	Amortization of prior service cost			355			(135	)		220			89			(34	)		55
	Amortization of transition obligation			—			—			—			650			(246	)		404
	Amortization of net actuarial (gain)/loss			31,546			(11,987	)		19,559			27,742			(10,542	)		17,200
	Prior service cost			—			—			—			(2,423	)		921			(1,502	)
	Regulatory adjustment			(121,224	)		46,065			(75,159	)		41,036			(15,594	)		25,442
	Pension plans other comprehensive income (loss)			11,022			(4,188	)		6,834			(7,759	)		2,949			(4,810	)
	FSIRS (designated hedging activities):
	Amounts reclassified 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
	Total other comprehensive income (loss)		$	14,367		$	(5,459	)	$	8,908		$	(4,414	)	$	1,678		$	(2,736	)
	Tax amounts are calculated using a 38% rate.
Rollforward of Accumulated Other Comprehensive Income	'
	The following represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets:
	AOCI—Rollforward
	(Thousands of dollars)
			Defined Benefit Plans									FSIRS
			Before-			Tax			After-			Before-			Tax			After-			AOCI
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
						Benefit									Benefit
	Beginning Balance AOCI December 31, 2013		$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	(41,698	)
	Other comprehensive income before reclassifications			—			—			—			—			—			—			—
	FSIRS amounts reclassified from AOCI (1)			—			—			—			836			(318	)		518			518
	Amortization of prior service cost (2)			89			(34	)		55			—			—			—			55
	Amortization of net actuarial loss (2)			5,914			(2,247	)		3,667			—			—			—			3,667
	Regulatory adjustment (3)			(5,178	)		1,968			(3,210	)		—			—			—			(3,210	)
	Net current period other comprehensive income (loss)			825			(313	)		512			836			(318	)		518			1,030
	Ending Balance AOCI March 31, 2014		$	(40,398	)	$	15,352		$	(25,046	)	$	(25,197	)	$	9,575		$	(15,622	)	$	(40,668	)
	-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 in Accumulated Other Comprehensive Income	'
	The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:
	Amounts Recognized in AOCI (Before Tax)
	(Thousands of dollars)
			March 31, 2014			December 31, 2013
	Net actuarial (loss) gain		$	(283,227	)	$	(289,141	)
	Prior service cost			(1,978	)		(2,067	)
	Less: amount recognized in regulatory assets			244,807			249,985
	Recognized in AOCI		$	(40,398	)	$	(41,223	)

Nature_of_Operations_and_Basis3

Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013
			Gas pipe inventory and operating supplies [Member]	Gas pipe inventory and operating supplies [Member]
Significant Accounting Policies [Line Items]	'	'	'	'
Gas pipe inventory and operating supplies	'	'	$21	$21
Non-cash construction advances - non-cash investing item	$1.60	$2.80	'	'

Nature_of_Operations_and_Basis4

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) (NPL Construction Co. [Member], USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
NPL Construction Co. [Member]	'	'
Accounts receivable for NPL services	$7,245	$10,787

Nature_of_Operations_and_Basis5

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Investment [Line Items]	'	'
Net cash surrender value of COLI policies	$94,000,000	$93,000,000
Other property	11,000,000	11,000,000
Total	262,187,000	260,871,000
NPL Construction Co. [Member]	'	'
Investment [Line Items]	'	'
NPL property and equipment	325,000,000	320,000,000
NPL accumulated provision for depreciation and amortization	($168,000,000)	($163,000,000)

Nature_of_Operations_and_Basis6

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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Other Income And Expenses [Abstract]	'	'	'	'
Change in COLI policies	$900	$3,800	$9,500	$5,200
Interest income	497	120	838	818
Pipe replacement costs	'	-85	-47	-2,756
Miscellaneous income and (expense)	215	234	-448	-210
Total other income (deductions)	$1,612	$4,069	$9,843	$3,052

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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Qualified Retirement Plan [Member]	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'
Service cost	$5,340	$5,764	$22,632	$21,004
Interest cost	10,861	9,401	39,067	38,100
Expected return on plan assets	-13,336	-12,460	-50,716	-46,795
Amortization of net actuarial loss	5,718	8,066	29,913	25,978
Net periodic benefit cost	8,583	10,771	40,896	38,287
SERP [Member]	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'
Service cost	73	94	352	299
Interest cost	436	383	1,588	1,605
Amortization of net actuarial loss	196	243	924	755
Net periodic benefit cost	705	720	2,864	2,659
PBOP [Member]	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'
Service cost	276	305	1,191	1,037
Interest cost	707	620	2,569	2,531
Expected return on plan assets	-816	-706	-2,934	-2,509
Amortization of prior service cost	89	89	355	89
Amortization of transition obligation	'	'	'	650
Amortization of net actuarial loss	'	236	709	1,009
Net periodic benefit cost	$256	$544	$1,890	$2,807

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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]	'	'	'	'
Total operating revenues	$608,396	$613,505	$1,945,673	$1,883,638
Segment net income (loss)	70,783	80,773	135,330	135,185
Revenues from external customers [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	584,646	598,466	1,848,809	1,801,270
Intersegment revenues [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	23,750	15,039	96,864	82,368
Natural Gas Operations [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total operating revenues	486,493	493,600	1,293,047	1,284,615
Segment net income (loss)	72,599	79,292	117,476	116,545
Natural Gas Operations [Member] | Revenues from external customers [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	486,493	493,600	1,293,047	1,284,615
Natural Gas Operations [Member] | Intersegment revenues [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	'	'	'	'
Construction Services [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Total operating revenues	121,903	119,905	652,626	599,023
Segment net income (loss)	-1,816	1,481	17,854	18,640
Construction Services [Member] | Revenues from external customers [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	98,153	104,866	555,762	516,655
Construction Services [Member] | Intersegment revenues [Member]	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'
Revenues	$23,750	$15,039	$96,864	$82,368

Derivatives_and_Fair_Value_Mea2

Derivatives and Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Derivative [Line Items]	'	'	'	'
Natural gas portfolios, maximum % rate	35.00%	'	'	'
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$518,000	$518,000	$2,074,000	$2,074,000
Derivative instrument loss at settlement amortization period	'10 years	'	'	'
Cash flow hedging [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	0	'	'	'
Settlement at maturity of the first FSIRS [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Agreement settlement paid	11,700,000	'	'	'
Settlement at maturity of the second FSIRS [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Agreement settlement paid	$21,800,000	'	'	'
Minimum [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Maturities of natural gas swaps	1-Apr-14	'	'	'
Maximum [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Maturities of natural gas swaps	1-Mar-16	'	'	'

Derivatives_and_Fair_Value_Mea3

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail)	Mar. 31, 2014	Dec. 31, 2013
	MMBTU	MMBTU
Offsetting [Abstract]	'	'
Contract notional amounts	7,664,000	13,571,000

Derivatives_and_Fair_Value_Mea4

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	'	'	'	'
Cash flow hedging [Member] | Net cost of gas sold [Member]	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	5,990	5,076	1,890	7,158
Regulatory deferral accounting treatment [Member] | Cash flow hedging [Member] | Net cost of gas sold [Member]	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	($5,990)	($5,076)	($1,890)	($7,158)

Derivatives_and_Fair_Value_Mea5

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	$2,523	$1,444
Liability derivatives not designated as hedging instruments	-16	-623
Net total not designated as hedging instruments	2,507	821
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	352	257
Liability derivatives not designated as hedging instruments	-16	-77
Net total not designated as hedging instruments	336	180
Swaps [Member] | Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	2,171	1,054
Liability derivatives not designated as hedging instruments	'	-253
Net total not designated as hedging instruments	2,171	801
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	'	126
Liability derivatives not designated as hedging instruments	'	-282
Net total not designated as hedging instruments	'	-156
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	'	7
Liability derivatives not designated as hedging instruments	'	-11
Net total not designated as hedging instruments	'	($4)

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, 2014	Mar. 31, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'	'
Paid to counterparties	$15	$2,294
Received from counterparties	$4,319	$4,869

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, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	($336)	($180)
Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	-2,171	-801
Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	'	156
Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	'	$4

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, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Net Assets (Liabilities)	$2,507	$821
Swaps [Member] | Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Assets	2,171	801
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Assets	336	180
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Liabilities	'	-156
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Liabilities	'	($4)

LongTerm_Debt_Schedule_of_Carr

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Debt Instrument [Line Items]	'	'
Less: current maturities	($11,158)	($11,105)
Long-term debt, less current maturities	1,368,632	1,381,327
Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	125,000	'
Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL credit facility	'	'
Unamortized discount	-2,743	-2,776
Long-term debt, gross	1,379,790	1,392,432
Less: current maturities	-11,158	-11,105
Long-term debt, less current maturities	1,368,632	1,381,327
Carrying Amount [Member] | Bonds [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	438,312	438,279
Carrying Amount [Member] | NPL other debt obligations [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL other debt obligations	39,456	42,213
Carrying Amount [Member] | Debentures [Member]	'	'
Debt Instrument [Line Items]	'	'
Unamortized discount	-5,478	-5,560
Carrying Amount [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	125,000	125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	125,000	125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	250,000	250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	250,000	250,000
Carrying Amount [Member] | Debentures [Member] | 8% Series, due 2026 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	75,000	75,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	25,000	25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	25,000	25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	25,000	25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	7,500	7,500
Carrying Amount [Member] | Debentures [Member] | Notes Payable [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable	902,022	901,940
Carrying Amount [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]	'	'
Debt Instrument [Line Items]	'	'
Line of credit facility	'	10,000
Carrying Amount [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	50,000	50,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	20,000	20,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.25% 2004 Series A, due 2034 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	65,000	65,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	31,200	31,200
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	100,000	100,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	24,855	24,855
Market Value [Member] | NPL other debt obligations [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL other debt obligations	39,465	42,119
Market Value [Member] | NPL credit facility [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL credit facility	'	'
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	134,433	130,953
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	148,894	141,873
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	255,923	252,485
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	268,873	257,280
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	101,735	96,263
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	28,691	28,741
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	31,585	30,586
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	33,402	31,497
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	9,021	8,468
Market Value [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]	'	'
Debt Instrument [Line Items]	'	'
Line of credit facility, fair value	'	10,000
Market Value [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	20,289	20,150
Market Value [Member] | Fixed-rate bonds [Member] | 5.25% 2004 Series A, due 2034 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	65,051	64,522
Market Value [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	31,229	30,284
Market Value [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	99,709	95,192
Market Value [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	$24,539	$22,974

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, 2014	Dec. 31, 2013
Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	6.10%	'
Debentures [Member] | Carrying Amount [Member] | Notes, 4.45%, due 2020 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	1-Dec-20	1-Dec-20
Debentures [Member] | Carrying Amount [Member] | Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	15-Feb-41	15-Feb-41
Debentures [Member] | Carrying Amount [Member] | Notes, 3.875%, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	3.88%	3.88%
Debt instrument due date	1-Apr-22	1-Apr-22
Debentures [Member] | Carrying Amount [Member] | Notes, 4.875%, due 2043 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.88%	4.88%
Debt instrument due date	1-Oct-43	1-Oct-43
Debentures [Member] | Carrying Amount [Member] | 8% Series, due 2026 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	1-Aug-26	1-Aug-26
Debentures [Member] | Carrying Amount [Member] | Medium-term notes, 7.59% series, due 2017 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	17-Jan-17	17-Jan-17
Debentures [Member] | Carrying Amount [Member] | Medium-term notes, 7.78% series, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	3-Feb-22	3-Feb-22
Debentures [Member] | Carrying Amount [Member] | Medium-term notes, 7.92% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	4-Jun-27	4-Jun-27
Debentures [Member] | Carrying Amount [Member] | Medium-term notes, 6.76% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	24-Sep-27	24-Sep-27
Debentures [Member] | Market Value [Member] | Notes, 4.45%, due 2020 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	1-Dec-20	1-Dec-20
Debentures [Member] | Market Value [Member] | Notes, 6.1%, due 2041 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	15-Feb-41	15-Feb-41
Debentures [Member] | Market Value [Member] | Notes, 3.875%, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	3.88%	3.88%
Debt instrument due date	1-Apr-22	1-Apr-22
Debentures [Member] | Market Value [Member] | Notes, 4.875%, due 2043 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.88%	4.88%
Debt instrument due date	1-Oct-43	1-Oct-43
Debentures [Member] | Market Value [Member] | 8% Series, due 2026 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	1-Aug-26	1-Aug-26
Debentures [Member] | Market Value [Member] | Medium-term notes, 7.59% series, due 2017 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	17-Jan-17	17-Jan-17
Debentures [Member] | Market Value [Member] | Medium-term notes, 7.78% series, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	3-Feb-22	3-Feb-22
Debentures [Member] | Market Value [Member] | Medium-term notes, 7.92% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	4-Jun-27	4-Jun-27
Debentures [Member] | Market Value [Member] | Medium-term notes, 6.76% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	24-Sep-27	24-Sep-27
Variable-rate bonds [Member] | Carrying Amount [Member] | Tax-exempt Series A, due 2028 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-28	1-Dec-28
Variable-rate bonds [Member] | Carrying Amount [Member] | 2003 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | Carrying Amount [Member] | 2008 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | Carrying Amount [Member] | 2009 Series A, due 2039 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-39	1-Dec-39
Variable-rate bonds [Member] | Market Value [Member] | Tax-exempt Series A, due 2028 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-28	1-Dec-28
Variable-rate bonds [Member] | Market Value [Member] | 2003 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | Market Value [Member] | 2008 Series A, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | Market Value [Member] | 2009 Series A, due 2039 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-39	1-Dec-39
Fixed-rate bonds [Member] | Carrying Amount [Member] | 5.25% 2003 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Dec-38	1-Dec-38
Fixed-rate bonds [Member] | Carrying Amount [Member] | 5.25% 2004 Series A, due 2034 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Jul-34	1-Jul-34
Fixed-rate bonds [Member] | Carrying Amount [Member] | 5.00% 2004 Series B, due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	1-Dec-33	1-Dec-33
Fixed-rate bonds [Member] | Carrying Amount [Member] | 4.85% 2005 Series A, due 2035 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	1-Oct-35	1-Oct-35
Fixed-rate bonds [Member] | Carrying Amount [Member] | 4.75% 2006 Series A, due 2036 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	1-Sep-36	1-Sep-36
Fixed-rate bonds [Member] | Market Value [Member] | 5.25% 2003 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Dec-38	1-Dec-38
Fixed-rate bonds [Member] | Market Value [Member] | 5.25% 2004 Series A, due 2034 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Jul-34	1-Jul-34
Fixed-rate bonds [Member] | Market Value [Member] | 5.00% 2004 Series B, due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	1-Dec-33	1-Dec-33
Fixed-rate bonds [Member] | Market Value [Member] | 4.85% 2005 Series A, due 2035 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	1-Oct-35	1-Oct-35
Fixed-rate bonds [Member] | Market Value [Member] | 4.75% 2006 Series A, due 2036 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	1-Sep-36	1-Sep-36

LongTerm_Debt_Additional_Infor

Long-Term Debt - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014
Debt Instrument [Line Items]	'
Line of credit designated as long term debt	$150,000,000
Line of credit designated for working capital purposes	150,000,000
Two Thousand And Nineteen [Member]	'
Debt Instrument [Line Items]	'
Credit facility maximum borrowing capacity	300,000,000
Credit facility expiration date	1-Mar-19
LIBOR [Member] | Southwest Gas Credit Facility [Member]	'
Debt Instrument [Line Items]	'
Applicable margin	1.00%
Alternative base rate [Member] | Southwest Gas Credit Facility [Member]	'
Debt Instrument [Line Items]	'
Applicable margin	0.00%
Notes, 6.1%, due 2041 [Member]	'
Debt Instrument [Line Items]	'
Debt instrument interest rate	6.10%
Notes payable	$125,000,000
Notes, 6.1%, due 2041 [Member] | Maximum [Member]	'
Debt Instrument [Line Items]	'
Percentage limit on liens securing indebtedness	10.00%

Equity_Other_Comprehensive_Inc2

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) (USD $)	3 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	$1,412,395	'	'	'
Balance, shares	46,356,125	'	'	'
Common stock issuances	3,081	'	'	'
Net income (loss)	70,697	80,674	134,896	134,478
Other comprehensive income (loss):	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	512	651	6,834	-4,810
Amounts reclassified into net income	518	518	2,074	2,074
Dividends declared Common: $0.365 per share	-17,193	'	'	'
Balance	1,470,010	'	1,470,010	'
Balance, shares	46,495,191	'	46,495,191	'
Common Stock [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	47,986	'	'	'
Balance, shares	46,356,000	'	'	'
Common stock issuances	139	'	'	'
Common stock issuances, shares	139	'	'	'
Other comprehensive income (loss):	'	'	'	'
Balance	48,125	'	48,125	'
Balance, shares	46,495,000	'	46,495,000	'
Additional Paid-in Capital [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	840,521	'	'	'
Common stock issuances	2,942	'	'	'
Other comprehensive income (loss):	'	'	'	'
Balance	843,463	'	843,463	'
Accumulated Other Comprehensive Income (Loss) [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	-41,698	'	'	'
Other comprehensive income (loss):	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	512	'	'	'
Amounts reclassified into net income	518	'	'	'
Balance	-40,668	'	-40,668	'
Retained Earnings [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	567,714	'	'	'
Net income (loss)	70,783	'	'	'
Other comprehensive income (loss):	'	'	'	'
Dividends declared Common: $0.365 per share	-17,193	'	'	'
Balance	621,304	'	621,304	'
Non-controlling Interest [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Balance	-2,128	'	'	'
Net income (loss)	-86	'	'	'
Other comprehensive income (loss):	'	'	'	'
Balance	($2,214)	'	($2,214)	'

Equity_Other_Comprehensive_Inc3

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Parenthetical) (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Statement Of Stockholders Equity [Abstract]	'	'	'	'
Dividends declared per share	$0.37	$0.33	$1.36	$1.22

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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Equity [Abstract]	'	'	'	'
Amortization of net actuarial (gain)/loss, Before-Tax Amount	$5,914	$8,545	$31,546	$27,742
Amortization of prior service cost, Before-Tax Amount	89	89	355	89
Regulatory adjustment, Before-Tax Amount	-5,178	-7,584	-121,224	41,036
Pension plans other comprehensive income (loss), Before-Tax Amount	825	1,050	11,022	-7,759
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
Total other comprehensive income (loss), Before-Tax Amount	1,661	1,886	14,367	-4,414
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	-2,247	-3,247	-11,987	-10,542
Amortization of prior service cost, Tax (Expense) or Benefit	-34	-34	-135	-34
Regulatory adjustment, Tax (Expense) or Benefit	1,968	2,882	46,065	-15,594
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	-313	-399	-4,188	2,949
Amounts reclassified into net income, Tax (Expense) or Benefit	-318	-318	-1,271	-1,271
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	-318	-318	-1,271	-1,271
Total other comprehensive income (loss), Tax (Expense) or Benefit	-631	-717	-5,459	1,678
Amortization of net actuarial (gain)/loss), Net-of-Tax Amount	3,667	5,298	19,559	17,200
Amortization of prior service cost, Net-of-Tax Amount	55	55	220	55
Regulatory adjustment, Net-of-Tax Amount	-3,210	-4,702	-75,159	25,442
Net defined benefit pension plans	512	651	6,834	-4,810
Amounts reclassified into net income	518	518	2,074	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	518	2,074	2,074
Total other comprehensive income (loss), Net-of-Tax Amount	1,030	1,169	8,908	-2,736
Net actuarial gain/(loss), Before-Tax Amount	'	'	100,345	-74,853
Amortization of transition obligation, Before-Tax Amount	'	'	'	650
Prior service cost, Before-Tax Amount	'	'	'	-2,423
Net actuarial gain/(loss), Tax (Expense) or Benefit	'	'	-38,131	28,444
Amortization of transition obligation, Tax (Expense) or Benefit	'	'	'	-246
Prior service cost, Tax (Expense) or Benefit	'	'	'	921
Net actuarial gain/(loss), Net-of-Tax Amount	'	'	62,214	-46,409
Amortization of transition obligation, Net-of-Tax Amount	'	'	'	404
Prior service cost, Net-of-Tax Amount	'	'	'	($1,502)

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)	3 Months Ended
	Mar. 31, 2014
Equity [Abstract]	'
Other comprehensive income loss, effective income tax rate	38.00%

Equity_Other_Comprehensive_Inc6

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) (USD $)	Mar. 31, 2014
In Millions, unless otherwise specified
Equity [Abstract]	'
Amount of FSIRS existing AOCI losses expected to reclassified income in next 12 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, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Before-Tax	($41,223)	'	'	'
Amortization of prior service cost, Defined Benefit Plans Before-Tax	89	89	355	89
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	5,914	8,545	31,546	27,742
Regulatory adjustment, Defined Benefit Plans Before-Tax	-5,178	-7,584	-121,224	41,036
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	825	1,050	11,022	-7,759
Ending Balance AOCI March 31, 2014, Defined Benefit Plans Before-Tax	-40,398	'	-40,398	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Tax (Expense) Benefit	15,665	'	'	'
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit	-34	-34	-135	-34
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	-2,247	-3,247	-11,987	-10,542
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	1,968	2,882	46,065	-15,594
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	-313	-399	-4,188	2,949
Ending Balance AOCI March 31, 2013 Defined Benefit Plans Tax (Expense) Benefit	15,352	'	15,352	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans After-Tax	-25,558	'	'	'
Amortization of prior service costs, Defined Benefit Plans After-Tax	55	55	220	55
Amortization of net actuarial (gain)/loss), Net-of-Tax Amount	3,667	5,298	19,559	17,200
Regulatory adjustment, AOCI	-3,210	-4,702	-75,159	25,442
Net current period other comprehensive income (loss), Defined Benefit Plans After-Tax	512	651	6,834	-4,810
Ending Balance AOCI March 31, 2014, Defined Benefit Plans After-Tax	-25,046	'	-25,046	'
Beginning Balance AOCI December 31, 2013, FSIRS - Before-Tax	-26,033	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	836	3,345	3,345
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	-318	-318	-1,271	-1,271
Ending Balance AOCI March 31, 2014, FSIRS - Before-Tax	-25,197	'	-25,197	'
Beginning Balance AOCI December 31, 2013, FSIRS - Tax (Expense) Benefit	9,893	'	'	'
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	-318	-318	-1,271	-1,271
Ending Balance AOCI March 31, 2014, FSIRS - Tax (Expense) Benefit	9,575	'	9,575	'
Beginning Balance AOCI December 31, 2013, FSIRS - After-Tax	-16,140	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	518	2,074	2,074
Net forward-starting interest rate swaps	518	518	2,074	2,074
Ending Balance AOCI March 31, 2014, FSIRS - After-Tax	-15,622	'	-15,622	'
Beginning Balance AOCI December 31, 2013, AOCI	-41,698	'	'	'
Amounts reclassified into net income	518	518	2,074	2,074
Amortization of prior service costs, AOCI	55	55	220	55
Amortization of net actuarial loss	3,667	5,298	19,559	17,200
Regulatory adjustment, Net-of-Tax Amount	-3,210	-4,702	-75,159	25,442
Net current period other comprehensive income (loss), AOCI	1,030	'	'	'
Ending Balance AOCI March 31, 2014, AOCI	-40,668	'	-40,668	'
Reclassification out of Accumulated Other Comprehensive Income [Member]	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax	'	'	'	'
Other comprehensive income before reclassifications, FSIRS - Before-Tax	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	318	'	'	'
Other comprehensive income before reclassifications, FSIRS - After-Tax	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	'	'	'
Other comprehensive income before reclassifications, AOCI	'	'	'	'
Amounts reclassified into net income	$518	'	'	'

Equity_Other_Comprehensive_Inc8

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income (Detail) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Mar. 31, 2013	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014	Dec. 31, 2013
				Defined Benefit Plans [Member]	Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'	'
Net actuarial (loss) gain	'	'	'	($283,227)	($289,141)
Prior service cost	2,423	'	'	-1,978	-2,067
Less: amount recognized in regulatory assets	'	'	'	244,807	249,985
Recognized in AOCI	'	($40,398)	($41,223)	($40,398)	($41,223)