Southwest Gas 10-Q 7 Nov 13 2013 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2013	Oct. 28, 2013
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Sep-13	'
Document Fiscal Year Focus	'2013	'
Document Fiscal Period Focus	'Q3	'
Trading Symbol	'SWX	'
Entity Registrant Name	'SOUTHWEST GAS CORP	'
Entity Central Index Key	'0000092416	'
Current Fiscal Year End Date	'--12-31	'
Entity Filer Category	'Large Accelerated Filer	'
Entity Common Stock, Shares Outstanding	'	46,350,863

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Utility plant:	'	'
Gas plant	$5,158,793	$5,019,500
Less: accumulated depreciation	-1,840,092	-1,750,795
Acquisition adjustments, net	776	911
Construction work in progress	108,913	74,178
Net utility plant	3,428,390	3,343,794
Other property and investments	257,338	242,096
Current assets:	'	'
Cash and cash equivalents	13,853	25,530
Accounts receivable, net of allowances	155,603	196,913
Accrued utility revenue	31,300	72,000
Income taxes receivable, net	4,700	2,945
Deferred income taxes	48,365	47,088
Deferred purchased gas costs	2,698	6,031
Prepaids and other current assets	91,676	107,910
Total current assets	348,195	458,417
Deferred charges and other assets	431,132	443,750
Total assets	4,465,055	4,488,057
Capitalization:	'	'
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 46,340,002 and 46,147,788 shares)	47,970	47,778
Additional paid-in capital	835,975	828,777
Accumulated other comprehensive income (loss), net	-47,238	-50,745
Retained earnings	525,971	484,369
Total Southwest Gas Corporation equity	1,362,678	1,310,179
Noncontrolling interest	-2,014	-1,681
Total equity	1,360,664	1,308,498
Long-term debt, less current maturities	1,280,392	1,268,373
Total capitalization	2,641,056	2,576,871
Current liabilities:	'	'
Current maturities of long-term debt	11,052	50,137
Short-term debt	33,000	0
Accounts payable	101,135	155,667
Customer deposits	73,729	77,858
Accrued general taxes	38,359	37,644
Accrued interest	17,067	16,080
Deferred purchased gas costs	16,915	98,957
Other current liabilities	115,136	98,786
Total current liabilities	406,393	535,129
Deferred income taxes and other credits:	'	'
Deferred income taxes and investment tax credits	663,315	616,184
Taxes payable	351	551
Accumulated removal costs	272,000	256,000
Other deferred credits	481,940	503,322
Total deferred income taxes and other credits	1,417,606	1,376,057
Total capitalization and liabilities	$4,465,055	$4,488,057

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]	'	'
Common stock, par	$1	$1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	46,340,002	46,147,788
Common stock, outstanding	46,340,002	46,147,788

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Operating revenues:	'	'	'	'	'	'
Gas operating revenues	$195,031	$195,573	$927,500	$982,203	$1,267,025	$1,362,655
Construction revenues	192,315	176,226	484,925	457,009	633,966	594,208
Total operating revenues	387,346	371,799	1,412,425	1,439,212	1,900,991	1,956,863
Operating expenses:	'	'	'	'	'	'
Net cost of gas sold	47,746	53,277	317,742	387,983	409,361	533,446
Operations and maintenance	95,981	90,627	288,003	278,361	379,621	368,114
Depreciation and amortization	58,744	56,718	176,247	166,418	233,251	218,269
Taxes other than income taxes	11,153	10,600	34,021	31,065	44,684	41,264
Construction expenses	167,581	154,267	422,969	419,072	545,420	537,533
Total operating expenses	381,205	365,489	1,238,982	1,282,899	1,612,337	1,698,626
Operating income	6,141	6,310	173,443	156,313	288,654	258,237
Other income and (expenses):	'	'	'	'	'	'
Net interest deductions	-15,097	-16,410	-46,265	-51,821	-62,464	-68,802
Other income (deductions)	2,668	1,636	8,189	4,574	8,026	5,976
Total other income and (expenses)	-12,429	-14,774	-38,076	-47,247	-54,438	-62,826
Income (loss) before income taxes	-6,288	-8,464	135,367	109,066	234,216	195,411
Income tax expense (benefit)	-3,231	-4,050	47,683	38,533	84,426	69,735
Net income (loss)	-3,057	-4,414	87,684	70,533	149,790	125,676
Net income (loss) attributable to noncontrolling interest	-193	-109	-333	-405	-620	-586
Net income (loss) attributable to Southwest Gas Corporation	($2,864)	($4,305)	$88,017	$70,938	$150,410	$126,262
Basic earnings (loss) per share	($0.06)	($0.09)	$1.90	$1.54	$3.25	$2.74
Diluted earnings (loss) per share	($0.06)	($0.09)	$1.88	$1.52	$3.22	$2.72
Dividends declared per share	$0.33	$0.30	$0.99	$0.89	$1.28	$1.15
Average number of common shares outstanding	46,337	46,134	46,306	46,106	46,265	46,059
Average shares outstanding (assuming dilution)	'	'	46,732	46,534	46,704	46,493

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]	'	'	'	'	'	'
Net income (loss)	($3,057)	($4,414)	$87,684	$70,533	$149,790	$125,676
Defined benefit pension plans:	'	'	'	'	'	'
Net actuarial gain (loss)	'	'	'	'	-46,409	-84,005
Amortization of transition obligation	'	135	'	403	135	537
Amortization of net actuarial loss	5,297	3,966	15,893	11,902	19,861	14,316
Prior service cost	55	'	164	'	-1,338	'
Regulatory adjustment	-4,701	-3,625	-14,105	-10,877	23,290	61,482
Net defined benefit pension plans	651	476	1,952	1,428	-4,461	-7,670
Forward-starting interest rate swaps:	'	'	'	'	'	'
Unrealized/realized gain (loss)	'	'	'	1,834	'	857
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
Net forward-starting interest rate swaps	518	518	1,555	3,052	2,074	2,256
Total other comprehensive income (loss), net of tax	1,169	994	3,507	4,480	-2,387	-5,414
Comprehensive income (loss)	-1,888	-3,420	91,191	75,013	147,403	120,262
Comprehensive income (loss) attributable to noncontrolling interest	-193	-109	-333	-405	-620	-586
Comprehensive income (loss) attributable to Southwest Gas Corporation	($1,695)	($3,311)	$91,524	$75,418	$148,023	$120,848

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (USD $)	9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
CASH FLOW FROM OPERATING ACTIVITIES:	'	'	'	'
Net income	$87,684	$70,533	$149,790	$125,676
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'	'
Depreciation and amortization	176,247	166,418	233,251	218,269
Deferred income taxes	43,704	33,343	76,641	62,320
Changes in current assets and liabilities:	'	'	'	'
Accounts receivable, net of allowances	41,310	57,498	-3,855	-32,172
Accrued utility revenue	40,700	40,100	-1,100	1,100
Deferred purchased gas costs	-78,709	52,850	-108,736	29,301
Accounts payable	-54,817	-93,608	12,793	-11,845
Accrued taxes	-1,240	-938	-189	645
Other current assets and liabilities	27,793	20,785	-11,940	2,609
Gains on sale	-2,990	-4,506	-6,524	-5,853
Changes in undistributed stock compensation	5,444	4,215	6,366	6,062
AFUDC and property-related changes	-1,658	-1,289	-2,312	-1,824
Changes in other assets and deferred charges	-22,454	-12,733	-25,088	-9,242
Changes in other liabilities and deferred credits	7,524	-2,352	5,449	-47,021
Net cash provided by operating activities	268,538	330,316	324,546	338,025
CASH FLOW FROM INVESTING ACTIVITIES:	'	'	'	'
Construction expenditures and property additions	-254,437	-275,970	-374,179	-393,335
Restricted cash	'	12,785	'	37,783
Changes in customer advances	5,203	-21,210	23,388	-24,217
Miscellaneous inflows	6,407	8,767	11,603	12,650
Miscellaneous outflows	'	-2,004	'	-2,004
Net cash used in investing activities	-242,827	-277,632	-339,188	-369,123
CASH FLOW FROM FINANCING ACTIVITIES:	'	'	'	'
Issuance of common stock, net	1,446	1,459	1,568	4,133
Dividends paid	-44,238	-39,428	-57,850	-51,597
Interest rate swap settlement	'	-21,754	'	-21,754
Issuance of long-term debt, net	63,673	453,638	99,553	515,424
Retirement of long-term debt	-130,269	-377,457	-179,855	-432,865
Change in credit facility and commercial paper	39,000	-69,000	110,000	20,000
Change in short-term debt	33,000	'	33,000	'
Net cash provided by (used in) financing activities	-37,388	-52,542	6,416	33,341
Change in cash and cash equivalents	-11,677	142	-8,226	2,243
Cash and cash equivalents at beginning of period	25,530	21,937	22,079	19,836
Cash and cash equivalents at end of period	13,853	22,079	13,853	22,079
Supplemental information:	'	'	'	'
Interest paid, net of amounts capitalized	41,161	70,522	58,078	89,555
Income taxes paid	$4,929	$1,971	$5,801	$2,050

Nature_of_Operations_and_Basis

Nature of Operations and Basis of Presentation	9 Months Ended
	Sep. 30, 2013
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 2012 Annual Report to Shareholders, which is incorporated by reference into the 2012 Form 10-K, and the first and second quarter 2013 reports on Form 10-Q.
	Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $24 million at September 30, 2013 and $25 million at December 31, 2012.
	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. During the nine months ended September 30, 2013, approximately $7.9 million of customer advances, upon contract expiration, were applied as contributions toward utility construction activity and represent a 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):
		September 30, 2013			December 31, 2012
	Accounts receivable for NPL services	$	11,517		$	8,179
	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):
		September 30, 2013			December 31, 2012
	NPL property and equipment	$	315		$	287
	NPL accumulated provision for depreciation and amortization		(157	)		(136	)
	Net cash surrender value of COLI policies		88			80
	Other property		11			11
	Total	$	257		$	242
	Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):
		Three Months Ended						Nine Months Ended						Twelve Months Ended
		September 30						September 30						September 30
		2013			2012			2013			2012			2013			2012
	Change in COLI policies	$	2,500		$	2,200		$	8,100		$	5,500		$	9,200		$	8,100
	Interest income		158			280			422			785			561			962
	Pipe replacement costs		(42	)		(973	)		(163	)		(2,015	)		(828	)		(3,663	)
	Miscellaneous income and (expense)		52			129			(170	)		304			(907	)		577
	Total other income (deductions)	$	2,668		$	1,636		$	8,189		$	4,574		$	8,026		$	5,976
	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.
	Final and Proposed Income Tax Regulations. In September 2013, the United States Department of the Treasury and the Internal Revenue Service (“IRS”) issued final and proposed regulations for the tax treatment of tangible property. The final regulations include standards for determining whether and when a taxpayer must capitalize costs incurred in acquiring, maintaining, or improving tangible property. The final regulations are generally effective for tax years beginning on or after January 1, 2014, and may be adopted in earlier years under certain circumstances. Proposed regulations were also released that revise the rules for dispositions of tangible property and general asset accounts. The proposed regulations addressing dispositions and general asset accounts are also expected (when finalized) to be effective for tax years starting on or after January 1, 2014, and may be adopted in earlier years under certain circumstances. The Company expects the IRS to issue natural gas industry guidance which will facilitate its analysis regarding the regulations’ impact on natural gas distribution networks. Based upon preliminary analysis of the final and proposed regulations, and in anticipation of specific guidance for the natural gas industry, the Company expects the regulations could result in a modest acceleration of tax deductibility, and the deferral of tax payments.

Components_of_Net_Periodic_Ben

Components of Net Periodic Benefit Cost	9 Months Ended
	Sep. 30, 2013
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 September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	5,764		$	5,080		$	17,292		$	15,239		$	22,372		$	19,671
	Interest cost		9,402			9,567			28,205			28,700			37,771			38,019
	Expected return on plan assets		(12,460	)		(11,445	)		(37,380	)		(34,335	)		(48,825	)		(44,364	)
	Amortization of net actuarial loss		8,065			5,970			24,196			17,912			30,167			21,499
	Net periodic benefit cost	$	10,771		$	9,172		$	32,313		$	27,516		$	41,485		$	34,825
		SERP
		Period Ended September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	93		$	69		$	280		$	206		$	348		$	260
	Interest cost		384			407			1,151			1,221			1,559			1,663
	Amortization of net actuarial loss		243			171			729			513			899			671
	Net periodic benefit cost	$	720		$	647		$	2,160		$	1,940		$	2,806		$	2,594
		PBOP
		Period Ended September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	305		$	244		$	915		$	733		$	1,159		$	947
	Interest cost		620			637			1,861			1,910			2,498			2,567
	Expected return on plan assets		(706	)		(601	)		(2,118	)		(1,803	)		(2,719	)		(2,397	)
	Amortization of prior service cost		89			—			266			—			266			—
	Amortization of transition obligation		—			217			—			650			217			867
	Amortization of net actuarial loss		236			257			709			773			967			921
	Net periodic benefit cost	$	544		$	754		$	1,633		$	2,263		$	2,388		$	2,905

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013
	Revenues from external customers	$	195,031		$	168,200	$	363,231
	Intersegment revenues		—			24,115		24,115
	Total	$	195,031		$	192,315	$	387,346
	Segment net income (loss)	$	(11,939	)	$	9,075	$	(2,864	)
	Three months ended September 30, 2012
	Revenues from external customers	$	195,573		$	148,855	$	344,428
	Intersegment revenues		—			27,371		27,371
	Total	$	195,573		$	176,226	$	371,799
	Segment net income (loss)	$	(11,389	)	$	7,084	$	(4,305	)
		Natural Gas			Construction		Total
		Operations			Services
	Nine months ended September 30, 2013
	Revenues from external customers	$	927,500		$	423,500	$	1,351,000
	Intersegment revenues		—			61,425		61,425
	Total	$	927,500		$	484,925	$	1,412,425
	Segment net income	$	69,317		$	18,700	$	88,017
	Nine months ended September 30, 2012
	Revenues from external customers	$	982,203		$	394,144	$	1,376,347
	Intersegment revenues		—			62,865		62,865
	Total	$	982,203		$	457,009	$	1,439,212
	Segment net income	$	64,609		$	6,329	$	70,938
		Natural Gas			Construction		Total
		Operations			Services
	Twelve months ended September 30, 2013
	Revenues from external customers	$	1,267,025		$	552,032	$	1,819,057
	Intersegment revenues		—			81,934		81,934
	Total	$	1,267,025		$	633,966	$	1,900,991
	Segment net income	$	121,327		$	29,083	$	150,410
	Twelve months ended September 30, 2012
	Revenues from external customers	$	1,362,655		$	505,210	$	1,867,865
	Intersegment revenues		—			88,998		88,998
	Total	$	1,362,655		$	594,208	$	1,956,863
	Segment net income	$	113,381		$	12,881	$	126,262

Derivatives_and_Fair_Value_Mea

Derivatives and Fair Value Measurements	9 Months Ended
	Sep. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Derivatives and Fair Value Measurements	'
	Note 4 – Derivatives and Fair Value Measurements
	Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts have no significant market value. The Swaps are recorded at fair value.
	The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (currently ranging from 25% 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 October 2013 through March 2015. 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):
			September 30, 2013				December 31, 2012
	Contract notional amounts			15,662				14,579
	Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.
	The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three-, nine-, and twelve-month periods ended September 30, 2013 and 2012 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)
					Three Months Ended								Nine Months Ended								Twelve Months Ended
			Location of Gain or (Loss)		September 30								September 30								September 30
	Instrument		Recognized in Income on Derivative		2013				2012				2013				2012				2013				2012
	Swaps		Net cost of gas sold		$	(353	)		$	3,617			$	(2,946	)		$	(945	)		$	(6,855	)		$	(9,607	)
	Swaps		Net cost of gas sold			353	*			(3,617	)*			2,946	*			945	*			6,855	*			9,607	*
	Total				$	—			$	—			$	—			$	—			$	—			$	—
	*	Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
	In January 2010, 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 planned issuance of fixed-rate debt to replace $200 million of debt that matured in February 2011 and $200 million that matured in May 2012. Both were designated cash flow hedges. The first FSIRS terminated in December 2010 concurrent with the related issuance of $125 million 4.45% 10-year Senior Notes. The second FSIRS had a notional amount of $100 million, and terminated in March 2012 concurrent with the related issuance of $250 million 3.875% 10-year Senior Notes. No gain or loss was recognized in income (ineffective portion) for either FSIRS during any period, including the periods presented in the following table. See Note 7 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income for additional information on both FSIRS contracts.
	Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:
	(Thousands of dollars)
			Three Months Ended								Nine Months Ended								Twelve Months Ended
			September 30								September 30								September 30
			2013				2012				2013				2012				2013				2012
	Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative		$	—			$	—			$	—			$	2,959			$	—			$	1,383
	The following table sets forth the fair values of the Company’s Swaps and their location in the balance sheets (thousands of dollars):
	Fair values of derivatives not designated as hedging instruments:
	September 30, 2013				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives				Net Total
	Swaps		Other current liabilities		$	25			$	(4,282	)		$	(4,257	)
	Swaps		Other deferred credits			8				(787	)			(779	)
	Total				$	33			$	(5,069	)		$	(5,036	)
	31-Dec-12				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives				Net Total
	Swaps		Deferred charges and other assets		$	132			$	(126	)		$	6
	Swaps		Other current liabilities			391				(2,467	)			(2,076	)
	Swaps		Other deferred credits			233				(552	)			(319	)
	Total				$	756			$	(3,145	)		$	(2,389	)
	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				Nine Months Ended				Twelve Months Ended
			30-Sep-13				30-Sep-13				30-Sep-13
	Paid to counterparties		$	342			$	1,214			$	2,260
	Received from counterparties		$	7			$	915			$	1,549
	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the balance sheets (thousands of dollars).
	September 30, 2013
	Instrument		Balance Sheet Location		Net Total
	Swaps		Prepaids and other current assets		$	4,257
	Swaps		Deferred charges and other assets			779
	31-Dec-12
	Instrument		Balance Sheet Location		Net Total
	Swaps		Other deferred credits		$	(6	)
	Swaps		Prepaids and other current assets			2,076
	Swaps		Deferred charges and other assets			319
	Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at September 30, 2013 and December 31, 2012 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 the Company’s Level 2 financial assets and liabilities recorded at fair value:
	Level 2 – Significant other observable inputs
	(Thousands of dollars)		September 30, 2013				December 31, 2012
	Assets at fair value:
	Deferred charges and other assets - Swaps		$	—			$	6
	Liabilities at fair value:
	Other current liabilities - Swaps			(4,257	)			(2,076	)
	Other deferred credits - Swaps			(779	)			(319	)
	Net Assets (Liabilities)		$	(5,036	)		$	(2,389	)
No financial assets or liabilities accounted for at fair value fell within Level 1 or Level 3 of the fair value hierarchy.

LongTerm_Debt

Long-Term Debt	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013 and December 31, 2012 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) 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 and NPL other debt obligations 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.
		September 30, 2013					December 31, 2012
		Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	(Thousands of dollars)
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	133,784	$	125,000		$	141,771
	Notes, 6.1%, due 2041		125,000			142,243		125,000			165,779
	Notes, 3.875%, due 2022		250,000			256,543		250,000			277,950
	8% Series, due 2026		75,000			98,735		75,000			111,501
	Medium-term notes, 7.59% series, due 2017		25,000			29,106		25,000			30,710
	Medium-term notes, 7.78% series, due 2022		25,000			31,356		25,000			34,637
	Medium-term notes, 7.92% series, due 2027		25,000			32,302		25,000			36,953
	Medium-term notes, 6.76% series, due 2027		7,500			8,680		7,500			10,058
	Unamortized discount		(3,259	)				(3,403	)
			654,241					654,097
	Revolving credit facility and commercial paper		150,000			150,000		111,000			111,000
	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:
	5.55% 1999 Series D, due 2038		—			—		8,270			8,375
	5.45% 2003 Series C, due 2038		—			—		30,000			30,152
	5.25% 2003 Series D, due 2038		20,000			20,095		20,000			20,571
	5.80% 2003 Series E, due 2038		—			—		15,000			15,102
	5.25% 2004 Series A, due 2034		65,000			61,914		65,000			66,955
	5.00% 2004 Series B, due 2033		31,200			29,886		31,200			31,655
	4.85% 2005 Series A, due 2035		100,000			94,475		100,000			101,184
	4.75% 2006 Series A, due 2036		24,855			23,280		24,855			25,189
	Unamortized discount		(2,808	)				(3,195	)
			438,247					491,130
	NPL credit facility		4,000			4,000		41,562			41,562
	NPL other debt obligations		44,956			44,977		20,721			20,991
			1,291,444					1,318,510
	Less: current maturities		(11,052	)				(50,137	)
	Long-term debt, less current maturities	$	1,280,392				$	1,268,373
	In February 2013, a notice of mandatory tender was sent to holders of the Clark County, Nevada 5.45% Series 2003C and 5.80% Series 2003E IDRBs. These IDRBs (totaling $45 million) were subject to mandatory tender on March 1, 2013 at a price of 100% plus accrued interest, and the Company tendered these IDRBs to the trustee for cancellation immediately following the mandatory tender, thereby extinguishing this debt. The Company facilitated the redemption primarily from borrowings under its $300 million credit facility.
	In September 2013, the Company redeemed at par the $8.27 million 5.55% 1999 Series D IDRBs originally due in 2038. The Company facilitated the redemption primarily from borrowings under its $300 million credit facility.
	In October 2013, the Company issued $250 million in 4.875% Senior Notes at a 0.078% discount. The notes will mature in October 2043. A portion of the net proceeds were used to temporarily pay down amounts outstanding under the credit facility. The remaining net proceeds are intended to be used for general corporate purposes.

ShortTerm_Debt

Short-Term Debt	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Short-Term Debt	'
	Note 6 – Short-Term Debt
	In March 2012, Southwest replaced the existing $300 million credit facility that was to expire in May 2012 with a $300 million facility that is scheduled to expire in March 2017. Of the $300 million available under the facility, $150 million was designated by management for working capital purposes. Southwest had $33 million in short-term borrowings outstanding on the credit facility at September 30, 2013 and no outstanding short-term borrowings at December 31, 2012. The weighted-average interest rate on these borrowings was 1.62% at September 30, 2013.

Equity_Other_Comprehensive_Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	'
	Note 7 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income
	The table below provides details of activity in equity during the nine months ended September 30, 2013.
			Southwest Gas Corporation Equity
									Additional			Accumulated			Retained			Non-			Total
									Paid-in			Other			Earnings			controlling
			Common Stock						Capital			Comprehensive						Interest
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-12			46,148		$	47,778		$	828,777		$	(50,745	)	$	484,369		$	(1,681	)	$	1,308,498
	Common stock issuances			192			192			7,198												7,390
	Net income (loss)															88,017			(333	)		87,684
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												1,952									1,952
	Amounts reclassified to net income, net of tax (FSIRS)												1,555									1,555
	Dividends declared
	Common: $0.99 per share															(46,415	)					(46,415	)
	30-Sep-13			46,340		$	47,970		$	835,975		$	(47,238	)	$	525,971		$	(2,014	)	$	1,360,664
	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
			September 30, 2013									September 30, 2012
			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 transition obligation		$	—		$	—		$	—		$	217		$	(82	)	$	135
	Amortization of net actuarial (gain)/loss			8,544			(3,247	)		5,297			6,398			(2,432	)		3,966
	Prior service cost			89			(34	)		55			—			—			—
	Regulatory adjustment			(7,583	)		2,882			(4,701	)		(5,847	)		2,222			(3,625	)
	Pension plans other comprehensive income (loss)			1,050			(399	)		651			768			(292	)		476
	FSIRS (designated hedging activities):
	Unrealized/realized gain			—			—			—			—			—			—
	Amounts reclassifed into net income			836			(318	)		518			836			(318	)		518
	FSIRS other comprehensive income			836			(318	)		518			836			(318	)		518
	Total other comprehensive income (loss)		$	1,886		$	(717	)	$	1,169		$	1,604		$	(610	)	$	994
			Nine Months Ended									Nine Months Ended
			September 30, 2013									September 30, 2012
			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 transition obligation		$	—		$	—		$	—		$	650		$	(247	)	$	403
	Amortization of net actuarial (gain)/loss			25,634			(9,741	)		15,893			19,198			(7,296	)		11,902
	Prior service cost			266			(102	)		164			—			—			—
	Regulatory adjustment			(22,751	)		8,646			(14,105	)		(17,544	)		6,667			(10,877	)
	Pension plans other comprehensive income (loss)			3,149			(1,197	)		1,952			2,304			(876	)		1,428
	FSIRS (designated hedging activities):
	Unrealized/realized gain			—			—			—			2,959			(1,125	)		1,834
	Amounts reclassifed into net income			2,509			(954	)		1,555			1,965			(747	)		1,218
	FSIRS other comprehensive income			2,509			(954	)		1,555			4,924			(1,872	)		3,052
	Total other comprehensive income (loss)		$	5,658		$	(2,151	)	$	3,507		$	7,228		$	(2,748	)	$	4,480
			Twelve Months Ended									Twelve Months Ended
			September 30, 2013									September 30, 2012
			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)		$	(74,853	)	$	28,444		$	(46,409	)	$	(135,492	)	$	51,487		$	(84,005	)
	Amortization of transition obligation			217			(82	)		135			867			(330	)		537
	Amortization of net actuarial (gain)/loss			32,033			(12,172	)		19,861			23,091			(8,775	)		14,316
	Prior service cost			(2,157	)		819			(1,338	)		—			—			—
	Regulatory adjustment			37,564			(14,274	)		23,290			99,164			(37,682	)		61,482
	Pension plans other comprehensive income (loss)			(7,196	)		2,735			(4,461	)		(12,370	)		4,700			(7,670	)
FSIRS (designated hedging activities):
Unrealized/realized gain			—			—			—			1,383			(526	)		857
Amounts reclassifed into net income			3,345			(1,271	)		2,074			2,257			(858	)		1,399
FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,640			(1,384	)		2,256
Total other comprehensive income (loss)		$	(3,851	)	$	1,464		$	(2,387	)	$	(8,730	)	$	3,316		$	(5,414	)
-1	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 September 30, 2013, 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			Tax			After-Tax			Before-Tax			Tax			After-Tax			AOCI
					(Expense)									(Expense)
					Benefit									Benefit
Beginning Balance AOCI December 31, 2012		$	(52,470	)	$	19,939		$	(32,531	)	$	(29,378	)	$	11,164		$	(18,214	)	$	(50,745	)
Other comprehensive income before reclassifications			—			—			—			—			—			—			—
FSIRS amounts reclassified from AOCI (1)			—			—			—			2,509			(954	)		1,555			1,555
Amortization of prior service costs (2)			266			(102	)		164			—			—			—			164
Amortization of net actuarial loss (2)			25,634			(9,741	)		15,893			—			—			—			15,893
Regulatory adjustment (3)			(22,751	)		8,646			(14,105	)		—			—			—			(14,105	)
Net current period other comprehensive income (loss)			3,149			(1,197	)		1,952			2,509			(954	)		1,555			3,507
Ending Balance AOCI September 30, 2013		$	(49,321	)	$	18,742		$	(30,579	)	$	(26,869	)	$	10,210		$	(16,659	)	$	(47,238	)
-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 defined benefit plan reclassifications above that relates to the regulatory asset 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)
		September 30, 2013			December 31, 2012
Net actuarial (loss) gain		$	(398,028	)	$	(423,662	)
Prior service cost			(2,157	)		(2,423	)
Less: amount recognized in regulatory assets			350,864			373,615
Recognized in AOCI		$	(49,321	)	$	(52,470	)

Nature_of_Operations_and_Basis1

Nature of Operations and Basis of Presentation (Policies)	9 Months Ended
	Sep. 30, 2013
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 2012 Annual Report to Shareholders, which is incorporated by reference into the 2012 Form 10-K, and the first and second quarter 2013 reports on Form 10-Q.
Prepaids and Other Current Assets	'
	Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $24 million at September 30, 2013 and $25 million at December 31, 2012.
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. During the nine months ended September 30, 2013, approximately $7.9 million of customer advances, upon contract expiration, were applied as contributions toward utility construction activity and represent a 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):
		September 30, 2013			December 31, 2012
	Accounts receivable for NPL services	$	11,517		$	8,179
	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):
		September 30, 2013			December 31, 2012
	NPL property and equipment	$	315		$	287
	NPL accumulated provision for depreciation and amortization		(157	)		(136	)
	Net cash surrender value of COLI policies		88			80
	Other property		11			11
	Total	$	257		$	242
Other Income (Deductions)	'
	Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):
		Three Months Ended						Nine Months Ended						Twelve Months Ended
		September 30						September 30						September 30
		2013			2012			2013			2012			2013			2012
	Change in COLI policies	$	2,500		$	2,200		$	8,100		$	5,500		$	9,200		$	8,100
	Interest income		158			280			422			785			561			962
	Pipe replacement costs		(42	)		(973	)		(163	)		(2,015	)		(828	)		(3,663	)
	Miscellaneous income and (expense)		52			129			(170	)		304			(907	)		577
	Total other income (deductions)	$	2,668		$	1,636		$	8,189		$	4,574		$	8,026		$	5,976
	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.
Final and Proposed Income Tax Regulations	'
	Final and Proposed Income Tax Regulations. In September 2013, the United States Department of the Treasury and the Internal Revenue Service (“IRS”) issued final and proposed regulations for the tax treatment of tangible property. The final regulations include standards for determining whether and when a taxpayer must capitalize costs incurred in acquiring, maintaining, or improving tangible property. The final regulations are generally effective for tax years beginning on or after January 1, 2014, and may be adopted in earlier years under certain circumstances. Proposed regulations were also released that revise the rules for dispositions of tangible property and general asset accounts. The proposed regulations addressing dispositions and general asset accounts are also expected (when finalized) to be effective for tax years starting on or after January 1, 2014, and may be adopted in earlier years under certain circumstances. The Company expects the IRS to issue natural gas industry guidance which will facilitate its analysis regarding the regulations’ impact on natural gas distribution networks. Based upon preliminary analysis of the final and proposed regulations, and in anticipation of specific guidance for the natural gas industry, the Company expects the regulations could result in a modest acceleration of tax deductibility, and the deferral of tax payments.

Nature_of_Operations_and_Basis2

Nature of Operations and Basis of Presentation (Tables)	9 Months Ended
	Sep. 30, 2013
Accounting Policies [Abstract]	'
Intercompany Transactions	'
	Accounts receivable for these services are presented in the table below (thousands of dollars):
		September 30, 2013			December 31, 2012
	Accounts receivable for NPL services	$	11,517		$	8,179
Schedule of Other Property and Investments	'
	Other property and investments includes (millions of dollars):
		September 30, 2013			December 31, 2012
	NPL property and equipment	$	315		$	287
	NPL accumulated provision for depreciation and amortization		(157	)		(136	)
	Net cash surrender value of COLI policies		88			80
	Other property		11			11
	Total	$	257		$	242
Other Income (Deductions)	'
	The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):
		Three Months Ended						Nine Months Ended						Twelve Months Ended
		September 30						September 30						September 30
		2013			2012			2013			2012			2013			2012
	Change in COLI policies	$	2,500		$	2,200		$	8,100		$	5,500		$	9,200		$	8,100
	Interest income		158			280			422			785			561			962
	Pipe replacement costs		(42	)		(973	)		(163	)		(2,015	)		(828	)		(3,663	)
	Miscellaneous income and (expense)		52			129			(170	)		304			(907	)		577
	Total other income (deductions)	$	2,668		$	1,636		$	8,189		$	4,574		$	8,026		$	5,976

Components_of_Net_Periodic_Ben1

Components of Net Periodic Benefit Cost (Tables)	9 Months Ended
	Sep. 30, 2013
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 September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	5,764		$	5,080		$	17,292		$	15,239		$	22,372		$	19,671
	Interest cost		9,402			9,567			28,205			28,700			37,771			38,019
	Expected return on plan assets		(12,460	)		(11,445	)		(37,380	)		(34,335	)		(48,825	)		(44,364	)
	Amortization of net actuarial loss		8,065			5,970			24,196			17,912			30,167			21,499
	Net periodic benefit cost	$	10,771		$	9,172		$	32,313		$	27,516		$	41,485		$	34,825
		SERP
		Period Ended September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	93		$	69		$	280		$	206		$	348		$	260
	Interest cost		384			407			1,151			1,221			1,559			1,663
	Amortization of net actuarial loss		243			171			729			513			899			671
	Net periodic benefit cost	$	720		$	647		$	2,160		$	1,940		$	2,806		$	2,594
		PBOP
		Period Ended September 30,
		Three Months						Nine Months						Twelve Months
		2013			2012			2013			2012			2013			2012
	(Thousands of dollars)
	Service cost	$	305		$	244		$	915		$	733		$	1,159		$	947
	Interest cost		620			637			1,861			1,910			2,498			2,567
	Expected return on plan assets		(706	)		(601	)		(2,118	)		(1,803	)		(2,719	)		(2,397	)
	Amortization of prior service cost		89			—			266			—			266			—
	Amortization of transition obligation		—			217			—			650			217			867
	Amortization of net actuarial loss		236			257			709			773			967			921
	Net periodic benefit cost	$	544		$	754		$	1,633		$	2,263		$	2,388		$	2,905

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2013
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 September 30, 2013
	Revenues from external customers	$	195,031		$	168,200	$	363,231
	Intersegment revenues		—			24,115		24,115
	Total	$	195,031		$	192,315	$	387,346
	Segment net income (loss)	$	(11,939	)	$	9,075	$	(2,864	)
	Three months ended September 30, 2012
	Revenues from external customers	$	195,573		$	148,855	$	344,428
	Intersegment revenues		—			27,371		27,371
	Total	$	195,573		$	176,226	$	371,799
	Segment net income (loss)	$	(11,389	)	$	7,084	$	(4,305	)
		Natural Gas			Construction		Total
		Operations			Services
	Nine months ended September 30, 2013
	Revenues from external customers	$	927,500		$	423,500	$	1,351,000
	Intersegment revenues		—			61,425		61,425
	Total	$	927,500		$	484,925	$	1,412,425
	Segment net income	$	69,317		$	18,700	$	88,017
	Nine months ended September 30, 2012
	Revenues from external customers	$	982,203		$	394,144	$	1,376,347
	Intersegment revenues		—			62,865		62,865
	Total	$	982,203		$	457,009	$	1,439,212
	Segment net income	$	64,609		$	6,329	$	70,938
		Natural Gas			Construction		Total
		Operations			Services
	Twelve months ended September 30, 2013
	Revenues from external customers	$	1,267,025		$	552,032	$	1,819,057
	Intersegment revenues		—			81,934		81,934
	Total	$	1,267,025		$	633,966	$	1,900,991
	Segment net income	$	121,327		$	29,083	$	150,410
	Twelve months ended September 30, 2012
	Revenues from external customers	$	1,362,655		$	505,210	$	1,867,865
	Intersegment revenues		—			88,998		88,998
	Total	$	1,362,655		$	594,208	$	1,956,863
	Segment net income	$	113,381		$	12,881	$	126,262

Derivatives_and_Fair_Value_Mea1

Derivatives and Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2013
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):
			September 30, 2013				December 31, 2012
	Contract notional amounts			15,662				14,579
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-, nine-, and twelve-month periods ended September 30, 2013 and 2012 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)
					Three Months Ended								Nine Months Ended								Twelve Months Ended
			Location of Gain or (Loss)		September 30								September 30								September 30
	Instrument		Recognized in Income on Derivative		2013				2012				2013				2012				2013				2012
	Swaps		Net cost of gas sold		$	(353	)		$	3,617			$	(2,946	)		$	(945	)		$	(6,855	)		$	(9,607	)
	Swaps		Net cost of gas sold			353	*			(3,617	)*			2,946	*			945	*			6,855	*			9,607	*
	Total				$	—			$	—			$	—			$	—			$	—			$	—
	*	Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments	'
	Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:
	(Thousands of dollars)
			Three Months Ended								Nine Months Ended								Twelve Months Ended
			September 30								September 30								September 30
			2013				2012				2013				2012				2013				2012
	Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative		$	—			$	—			$	—			$	2,959			$	—			$	1,383
Fair Values of Swaps in Balance Sheet Location	'
	The following table sets forth the fair values of the Company’s Swaps and their location in the balance sheets (thousands of dollars):
	Fair values of derivatives not designated as hedging instruments:
	September 30, 2013				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives				Net Total
	Swaps		Other current liabilities		$	25			$	(4,282	)		$	(4,257	)
	Swaps		Other deferred credits			8				(787	)			(779	)
	Total				$	33			$	(5,069	)		$	(5,036	)
	31-Dec-12				Asset				Liability
	Instrument		Balance Sheet Location		Derivatives				Derivatives				Net Total
	Swaps		Deferred charges and other assets		$	132			$	(126	)		$	6
	Swaps		Other current liabilities			391				(2,467	)			(2,076	)
	Swaps		Other deferred credits			233				(552	)			(319	)
	Total				$	756			$	(3,145	)		$	(2,389	)
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				Nine Months Ended				Twelve Months Ended
			September 30, 2013				September 30, 2013				September 30, 2013
	Paid to counterparties		$	342			$	1,214			$	2,260
	Received from counterparties		$	7			$	915			$	1,549
Regulatory Assets/Liabilities Offsetting the Derivatives at Fair Value in the Balance Sheet	'
	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the balance sheets (thousands of dollars).
	September 30, 2013
	Instrument		Balance Sheet Location		Net Total
	Swaps		Prepaids and other current assets		$	4,257
	Swaps		Deferred charges and other assets			779
	31-Dec-12
	Instrument		Balance Sheet Location		Net Total
	Swaps		Other deferred credits		$	(6	)
	Swaps		Prepaids and other current assets			2,076
	Swaps		Deferred charges and other assets			319
Significant Other Observable Inputs	'
	The following table sets forth the Company’s Level 2 financial assets and liabilities recorded at fair value:
	Level 2 - Significant other observable inputs
	(Thousands of dollars)		September 30, 2013				December 31, 2012
	Assets at fair value:
	Deferred charges and other assets - Swaps		$	—			$	6
	Liabilities at fair value:
	Other current liabilities - Swaps			(4,257	)			(2,076	)
	Other deferred credits - Swaps			(779	)			(319	)
	Net Assets (Liabilities)		$	(5,036	)		$	(2,389	)

LongTerm_Debt_Tables

Long-Term Debt (Tables)	9 Months Ended
	Sep. 30, 2013
Debt Disclosure [Abstract]	'
Schedule of 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.
		September 30, 2013					December 31, 2012
		Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	(Thousands of dollars)
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	133,784	$	125,000		$	141,771
	Notes, 6.1%, due 2041		125,000			142,243		125,000			165,779
	Notes, 3.875%, due 2022		250,000			256,543		250,000			277,950
	8% Series, due 2026		75,000			98,735		75,000			111,501
	Medium-term notes, 7.59% series, due 2017		25,000			29,106		25,000			30,710
	Medium-term notes, 7.78% series, due 2022		25,000			31,356		25,000			34,637
	Medium-term notes, 7.92% series, due 2027		25,000			32,302		25,000			36,953
	Medium-term notes, 6.76% series, due 2027		7,500			8,680		7,500			10,058
	Unamortized discount		(3,259	)				(3,403	)
			654,241					654,097
	Revolving credit facility and commercial paper		150,000			150,000		111,000			111,000
	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:
	5.55% 1999 Series D, due 2038		—			—		8,270			8,375
	5.45% 2003 Series C, due 2038		—			—		30,000			30,152
	5.25% 2003 Series D, due 2038		20,000			20,095		20,000			20,571
	5.80% 2003 Series E, due 2038		—			—		15,000			15,102
	5.25% 2004 Series A, due 2034		65,000			61,914		65,000			66,955
	5.00% 2004 Series B, due 2033		31,200			29,886		31,200			31,655
	4.85% 2005 Series A, due 2035		100,000			94,475		100,000			101,184
	4.75% 2006 Series A, due 2036		24,855			23,280		24,855			25,189
	Unamortized discount		(2,808	)				(3,195	)
			438,247					491,130
	NPL credit facility		4,000			4,000		41,562			41,562
	NPL other debt obligations		44,956			44,977		20,721			20,991
			1,291,444					1,318,510
	Less: current maturities		(11,052	)				(50,137	)
	Long-term debt, less current maturities	$	1,280,392				$	1,268,373

Equity_Other_Comprehensive_Inc1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	9 Months Ended
	Sep. 30, 2013
Text Block [Abstract]	'
Details of Activities in Equity	'
	The table below provides details of activity in equity during the nine months ended September 30, 2013.
			Southwest Gas Corporation Equity
									Additional			Accumulated			Retained			Non-			Total
									Paid-in			Other			Earnings			controlling
			Common Stock						Capital			Comprehensive						Interest
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-12			46,148		$	47,778		$	828,777		$	(50,745	)	$	484,369		$	(1,681	)	$	1,308,498
	Common stock issuances			192			192			7,198												7,390
	Net income (loss)															88,017			(333	)		87,684
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												1,952									1,952
	Amounts reclassified to net income, net of tax (FSIRS)												1,555									1,555
	Dividends declared
	Common: $0.99 per share															(46,415	)					(46,415	)
	30-Sep-13			46,340		$	47,970		$	835,975		$	(47,238	)	$	525,971		$	(2,014	)	$	1,360,664
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
			September 30, 2013									September 30, 2012
			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 transition obligation		$	—		$	—		$	—		$	217		$	(82	)	$	135
	Amortization of net actuarial (gain)/loss			8,544			(3,247	)		5,297			6,398			(2,432	)		3,966
	Prior service cost			89			(34	)		55			—			—			—
	Regulatory adjustment			(7,583	)		2,882			(4,701	)		(5,847	)		2,222			(3,625	)
	Pension plans other comprehensive income (loss)			1,050			(399	)		651			768			(292	)		476
	FSIRS (designated hedging activities):
	Unrealized/realized gain			—			—			—			—			—			—
	Amounts reclassifed into net income			836			(318	)		518			836			(318	)		518
	FSIRS other comprehensive income			836			(318	)		518			836			(318	)		518
	Total other comprehensive income (loss)		$	1,886		$	(717	)	$	1,169		$	1,604		$	(610	)	$	994
			Nine Months Ended									Nine Months Ended
			30-Sep-13									30-Sep-12
			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 transition obligation		$	—		$	—		$	—		$	650		$	(247	)	$	403
	Amortization of net actuarial (gain)/loss			25,634			(9,741	)		15,893			19,198			(7,296	)		11,902
	Prior service cost			266			(102	)		164			—			—			—
	Regulatory adjustment			(22,751	)		8,646			(14,105	)		(17,544	)		6,667			(10,877	)
	Pension plans other comprehensive income (loss)			3,149			(1,197	)		1,952			2,304			(876	)		1,428
	FSIRS (designated hedging activities):
	Unrealized/realized gain			—			—			—			2,959			(1,125	)		1,834
	Amounts reclassifed into net income			2,509			(954	)		1,555			1,965			(747	)		1,218
	FSIRS other comprehensive income			2,509			(954	)		1,555			4,924			(1,872	)		3,052
	Total other comprehensive income (loss)		$	5,658		$	(2,151	)	$	3,507		$	7,228		$	(2,748	)	$	4,480
			Twelve Months Ended									Twelve Months Ended
			30-Sep-13									30-Sep-12
			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)		$	(74,853	)	$	28,444		$	(46,409	)	$	(135,492	)	$	51,487		$	(84,005	)
	Amortization of transition obligation			217			(82	)		135			867			(330	)		537
	Amortization of net actuarial (gain)/loss			32,033			(12,172	)		19,861			23,091			(8,775	)		14,316
	Prior service cost			(2,157	)		819			(1,338	)		—			—			—
	Regulatory adjustment			37,564			(14,274	)		23,290			99,164			(37,682	)		61,482
	Pension plans other comprehensive income (loss)			(7,196	)		2,735			(4,461	)		(12,370	)		4,700			(7,670	)
	FSIRS (designated hedging activities):
	Unrealized/realized gain			—			—			—			1,383			(526	)		857
	Amounts reclassifed into net income			3,345			(1,271	)		2,074			2,257			(858	)		1,399
	FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,640			(1,384	)		2,256
	Total other comprehensive income (loss)		$	(3,851	)	$	1,464		$	(2,387	)	$	(8,730	)	$	3,316		$	(5,414	)
	-1	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, 2012		$	(52,470	)	$	19,939		$	(32,531	)	$	(29,378	)	$	11,164		$	(18,214	)	$	(50,745	)
	Other comprehensive income before reclassifications			—			—			—			—			—			—			—
	FSIRS amounts reclassified from AOCI (1)			—			—			—			2,509			(954	)		1,555			1,555
	Amortization of prior service costs (2)			266			(102	)		164			—			—			—			164
	Amortization of net actuarial loss (2)			25,634			(9,741	)		15,893			—			—			—			15,893
	Regulatory adjustment (3)			(22,751	)		8,646			(14,105	)		—			—			—			(14,105	)
	Net current period other comprehensive income (loss)			3,149			(1,197	)		1,952			2,509			(954	)		1,555			3,507
	Ending Balance AOCI September 30, 2013		$	(49,321	)	$	18,742		$	(30,579	)	$	(26,869	)	$	10,210		$	(16,659	)	$	(47,238	)
	-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 defined benefit plan reclassifications above that relates to the regulatory asset 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)
			September 30, 2013			December 31, 2012
	Net actuarial (loss) gain		$	(398,028	)	$	(423,662	)
	Prior service cost			(2,157	)		(2,423	)
	Less: amount recognized in regulatory assets			350,864			373,615
	Recognized in AOCI		$	(49,321	)	$	(52,470	)

Nature_of_Operations_and_Basis3

Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
		Gas pipe inventory and operating supplies [Member]	Gas pipe inventory and operating supplies [Member]
Gas pipe inventory and operating supplies	'	$24	$25
Non-cash construction advances - non-cash investing item	$7.90	'	'

Nature_of_Operations_and_Basis4

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) (NPL Construction Co. [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
NPL Construction Co. [Member]	'	'
Accounts receivable for NPL services	$11,517	$8,179

Nature_of_Operations_and_Basis5

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Net cash surrender value of COLI policies	$88,000,000	$80,000,000
Other property	11,000,000	11,000,000
Total	257,338,000	242,096,000
NPL Construction Co. [Member]	'	'
NPL property and equipment	315,000,000	287,000,000
NPL accumulated provision for depreciation and amortization	($157,000,000)	($136,000,000)

Nature_of_Operations_and_Basis6

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Income And Expenses [Abstract]	'	'	'	'	'	'
Change in COLI policies	$2,500	$2,200	$8,100	$5,500	$9,200	$8,100
Interest income	158	280	422	785	561	962
Pipe replacement costs	-42	-973	-163	-2,015	-828	-3,663
Miscellaneous income and (expense)	52	129	-170	304	-907	577
Total other income (deductions)	$2,668	$1,636	$8,189	$4,574	$8,026	$5,976

Components_of_Net_Periodic_Ben2

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Amortization of prior service cost	'	'	($266)	'	'	'
Qualified Retirement Plan [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	5,764	5,080	17,292	15,239	22,372	19,671
Interest cost	9,402	9,567	28,205	28,700	37,771	38,019
Expected return on plan assets	-12,460	-11,445	-37,380	-34,335	-48,825	-44,364
Amortization of net actuarial loss	8,065	5,970	24,196	17,912	30,167	21,499
Net periodic benefit cost	10,771	9,172	32,313	27,516	41,485	34,825
SERP [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	93	69	280	206	348	260
Interest cost	384	407	1,151	1,221	1,559	1,663
Amortization of net actuarial loss	243	171	729	513	899	671
Net periodic benefit cost	720	647	2,160	1,940	2,806	2,594
PBOP [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	305	244	915	733	1,159	947
Interest cost	620	637	1,861	1,910	2,498	2,567
Expected return on plan assets	-706	-601	-2,118	-1,803	-2,719	-2,397
Amortization of prior service cost	89	'	266	'	266	'
Amortization of transition obligation	'	217	'	650	217	867
Amortization of net actuarial loss	236	257	709	773	967	921
Net periodic benefit cost	$544	$754	$1,633	$2,263	$2,388	$2,905

Segment_Information_Schedule_o

Segment Information - Schedule of Segment Reporting Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	$387,346	$371,799	$1,412,425	$1,439,212	$1,900,991	$1,956,863
Segment net income (loss)	-2,864	-4,305	88,017	70,938	150,410	126,262
Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	363,231	344,428	1,351,000	1,376,347	1,819,057	1,867,865
Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	24,115	27,371	61,425	62,865	81,934	88,998
Natural Gas Operations [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	195,031	195,573	927,500	982,203	1,267,025	1,362,655
Segment net income (loss)	-11,939	-11,389	69,317	64,609	121,327	113,381
Natural Gas Operations [Member] | Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	195,031	195,573	927,500	982,203	1,267,025	1,362,655
Natural Gas Operations [Member] | Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	'	'	'	'	'	'
Construction Services [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	192,315	176,226	484,925	457,009	633,966	594,208
Segment net income (loss)	9,075	7,084	18,700	6,329	29,083	12,881
Construction Services [Member] | Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	168,200	148,855	423,500	394,144	552,032	505,210
Construction Services [Member] | Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	$24,115	$27,371	$61,425	$62,865	$81,934	$88,998

Derivatives_and_Fair_Value_Mea2

Derivatives and Fair Value Measurements - Additional Information (Detail) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative [Line Items]	'	'	'	'
Natural gas portfolios, minimum % rate	25.00%	'	'	'
Natural gas portfolios, maximum % rate	35.00%	'	'	'
Retirement of long-term debt	$130,269,000	$377,457,000	$179,855,000	$432,865,000
February 2011 [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Debt maturing date	'February 2011	'	'February 2011	'
Retirement of long-term debt	200,000,000	'	'	'
May 2012 [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Debt maturing date	'May 2012	'	'May 2012	'
Retirement of long-term debt	200,000,000	'	'	'
Senior Notes, 4.45%, Due 2020 [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Notes payable	125,000,000	'	125,000,000	'
Debt instrument interest rate	4.45%	'	4.45%	'
Issuance of fixed rate debt (in years)	'10 years	'	'	'
Notional amount	100,000,000	'	100,000,000	'
Senior Notes, 3.875%, Due 2022 [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Notes payable	$250,000,000	'	$250,000,000	'
Debt instrument interest rate	3.88%	'	3.88%	'
Issuance of fixed rate debt (in years)	'10 years	'	'	'
Minimum [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Maturities of natural gas swaps	1-Oct-13	'	'	'
Maximum [Member]	'	'	'	'
Derivative [Line Items]	'	'	'	'
Maturities of natural gas swaps	1-Mar-15	'	'	'

Derivatives_and_Fair_Value_Mea3

Derivatives and Fair Value Measurements - Notional Amounts Under Swaps Contracts (Detail)	Sep. 30, 2013	Dec. 31, 2012
	MMBTU	MMBTU
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'	'
Contract notional amounts	15,662,000	14,579,000

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		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	'	'	'	'	'	'
Swaps Contracts [Member] | Net cost of gas sold [Member]	'	'	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	-353	3,617	-2,946	-945	-6,855	-9,607
Regulatory Deferral Accounting Treatment [Member] | Swaps Contracts [Member] | Net cost of gas sold [Member]	'	'	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	$353	($3,617)	$2,946	$945	$6,855	$9,607

Derivatives_and_Fair_Value_Mea5

Derivatives and Fair Value Measurements - Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Offsetting [Abstract]	'	'	'	'	'	'
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	'	'	'	$2,959	'	$1,383

Derivatives_and_Fair_Value_Mea6

Derivatives and Fair Value Measurements - Fair Values of Swaps in Balance Sheet Location (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	$33	$756
Liability derivatives not designated as hedging instruments	-5,069	-3,145
Net total not designated as hedging instruments	-5,036	-2,389
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	25	391
Liability derivatives not designated as hedging instruments	-4,282	-2,467
Net total not designated as hedging instruments	-4,257	-2,076
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	8	233
Liability derivatives not designated as hedging instruments	-787	-552
Net total not designated as hedging instruments	-779	-319
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	'	132
Liability derivatives not designated as hedging instruments	'	-126
Net total not designated as hedging instruments	'	$6

Derivatives_and_Fair_Value_Mea7

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'	'	'
Paid to counterparties	$342	$1,214	$2,260
Received from counterparties	$7	$915	$1,549

Derivatives_and_Fair_Value_Mea8

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in the Balance Sheet (Detail) (Swaps [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	$4,257	$2,076
Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	779	319
Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	'	($6)

Derivatives_and_Fair_Value_Mea9

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) (Level 2 - Significant other observable inputs [Member], USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Net Assets (Liabilities)	($5,036)	($2,389)
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative assets	'	6
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative liabilities	-4,257	-2,076
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative liabilities	($779)	($319)

LongTerm_Debt_Schedule_of_Esti

Long-Term Debt - Schedule of Estimated Fair Values of Long-Term Debt (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Debt Instrument [Line Items]	'	'
Less: current maturities	($11,052)	($50,137)
Long-term debt, less current maturities	1,280,392	1,268,373
Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL credit facility	4,000	41,562
Unamortized discount	-2,808	-3,195
Long-term debt, gross	1,291,444	1,318,510
Less: current maturities	-11,052	-50,137
Long-term debt, less current maturities	1,280,392	1,268,373
Carrying Amount [Member] | Bonds [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	438,247	491,130
Carrying Amount [Member] | NPL other debt obligations [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL other debt obligations	44,956	20,721
Carrying Amount [Member] | Debentures [Member]	'	'
Debt Instrument [Line Items]	'	'
Unamortized discount	-3,259	-3,403
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] | 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	654,241	654,097
Carrying Amount [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]	'	'
Debt Instrument [Line Items]	'	'
Line of credit facility, fair value	150,000	111,000
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] | 5.55% 1999 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	'	8,270
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.45% 2003 Series C, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	'	30,000
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] | 5.80% 2003 Series E, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured debt	'	15,000
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	44,977	20,991
Market Value [Member] | NPL credit facility [Member]	'	'
Debt Instrument [Line Items]	'	'
NPL credit facility	4,000	41,562
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	133,784	141,771
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041[Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	142,243	165,779
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	256,543	277,950
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	98,735	111,501
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	29,106	30,710
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	31,356	34,637
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	32,302	36,953
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]	'	'
Debt Instrument [Line Items]	'	'
Notes payable, fair value disclosure	8,680	10,058
Market Value [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]	'	'
Debt Instrument [Line Items]	'	'
Line of credit facility, fair value	150,000	111,000
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] | 5.55% 1999 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	'	8,375
Market Value [Member] | Fixed-rate bonds [Member] | 5.45% 2003 Series C, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	'	30,152
Market Value [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	20,095	20,571
Market Value [Member] | Fixed-rate bonds [Member] | 5.80% 2003 Series E, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	'	15,102
Market Value [Member] | Fixed-rate bonds [Member] | 5.25% 2004 Series A, due 2034 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	61,914	66,955
Market Value [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	29,886	31,655
Market Value [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	94,475	101,184
Market Value [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]	'	'
Debt Instrument [Line Items]	'	'
Unsecured Debt Fair Value	$23,280	$25,189

LongTerm_Debt_Schedule_of_Esti1

Long-Term Debt - Schedule of Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
5.55% 1999 Series D, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.55%	'
5.45% 2003 Series C, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.45%	'
5.80% 2003 Series E, due 2038 [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.80%	'
Debentures [Member] | Notes, 4.45%, due 2020 [Member] | Carrying Amount [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] | Notes, 4.45%, due 2020 [Member] | Market Value [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] | Notes, 6.1%, due 2041[Member] | Carrying Amount [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] | Notes, 6.1%, due 2041[Member] | Market Value [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] | Notes, 3.875%, due 2022 [Member] | Carrying Amount [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] | Notes, 3.875%, due 2022 [Member] | Market Value [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] | 8% Series, due 2026 [Member] | Carrying Amount [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] | 8% Series, due 2026 [Member] | Market Value [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] | Medium-term notes, 7.59% series, due 2017 [Member] | Carrying Amount [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] | Medium-term notes, 7.59% series, due 2017 [Member] | Market Value [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] | Medium-term notes, 7.78% series, due 2022 [Member] | Carrying Amount [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] | Medium-term notes, 7.78% series, due 2022 [Member] | Market Value [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] | Medium-term notes, 7.92% series, due 2027 [Member] | Carrying Amount [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] | Medium-term notes, 7.92% series, due 2027 [Member] | Market Value [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] | Medium-term notes, 6.76% series, due 2027 [Member] | Carrying Amount [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] | Medium-term notes, 6.76% series, due 2027 [Member] | Market Value [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] | Tax-exempt Series A, due 2028 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-28	1-Dec-28
Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-28	1-Dec-28
Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Mar-38	1-Mar-38
Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-39	1-Dec-39
Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument due date	1-Dec-39	1-Dec-39
Fixed-rate bonds [Member] | 5.55% 1999 Series D, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.55%	5.55%
Debt instrument due date	1-Dec-38	1-Dec-38
Fixed-rate bonds [Member] | 5.55% 1999 Series D, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.55%	5.55%
Debt instrument due date	1-Dec-38	1-Dec-38
Fixed-rate bonds [Member] | 5.45% 2003 Series C, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.45%	5.45%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.45% 2003 Series C, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.45%	5.45%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.80% 2003 Series E, due 2038 [Member] | Carrying Amount [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.80%	5.80%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.80% 2003 Series E, due 2038 [Member] | Market Value [Member]	'	'
Debt Instrument [Line Items]	'	'
Debt instrument interest rate	5.80%	5.80%
Debt instrument due date	1-Mar-38	1-Mar-38
Fixed-rate bonds [Member] | 5.25% 2004 Series A, due 2034 [Member] | Carrying Amount [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] | 5.25% 2004 Series A, due 2034 [Member] | Market Value [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] | 5.00% 2004 Series B, due 2033 [Member] | Carrying Amount [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] | 5.00% 2004 Series B, due 2033 [Member] | Market Value [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] | 4.85% 2005 Series A, due 2035 [Member] | Carrying Amount [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] | 4.85% 2005 Series A, due 2035 [Member] | Market Value [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] | 4.75% 2006 Series A, due 2036 [Member] | Carrying Amount [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] | 4.75% 2006 Series A, due 2036 [Member] | Market Value [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 $)	9 Months Ended
	Sep. 30, 2013
Debt Instrument [Line Items]	'
Credit facility maximum borrowing capacity	$300,000,000
IDRBs [Member]	'
Debt Instrument [Line Items]	'
Debt redeemed	45,000,000
5.45% 2003 Series C, due 2038 [Member]	'
Debt Instrument [Line Items]	'
Debt instrument interest rate	5.45%
5.80% 2003 Series E, due 2038 [Member]	'
Debt Instrument [Line Items]	'
Debt instrument interest rate	5.80%
5.55% 1999 Series D, due 2038 [Member]	'
Debt Instrument [Line Items]	'
Debt redeemed	8,270,000
Debt instrument interest rate	5.55%
4.875% Senior Notes, Due 2043 [Member]	'
Debt Instrument [Line Items]	'
Debt instrument interest rate	4.88%
Notes payable	$250,000,000
Senior Notes issued maturity date	'October 2043
Discount on issuance of debt	0.08%

ShortTerm_Debt_Additional_Info

Short-Term Debt - Additional Information (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013	Sep. 30, 2013
			May 2012 [Member]	March 2017 [Member]
Credit Facilities [Line Items]	'	'	'	'
Short-term debt expiration date	'	'	1-May-12	1-Mar-17
Working capital purposes	$150,000,000	'	'	'
Short-term borrowings outstanding on the credit facility	33,000,000	0	'	'
Maximum borrowing capacity	300,000,000	'	300,000,000	'
Current borrowing capacity	'	'	'	$300,000,000
Weighted-average interest rate	1.62%	'	'	'

Equity_Other_Comprehensive_Inc2

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Details of Activities in Equity (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	$1,308,498	'	'	'
Balance, shares	'	'	46,147,788	'	'	'
Common stock issuances	'	'	7,390	'	'	'
Net income (loss)	-3,057	-4,414	87,684	70,533	149,790	125,676
Other comprehensive income (loss):	'	'	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	651	476	1,952	1,428	-4,461	-7,670
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
Dividends declared Common: $0.99 per share	'	'	-46,415	'	'	'
Balance	1,360,664	'	1,360,664	'	1,360,664	'
Balance, shares	46,340,002	'	46,340,002	'	46,340,002	'
Common Stock [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	47,778	'	'	'
Balance, shares	'	'	46,148	'	'	'
Common stock issuances	'	'	192	'	'	'
Common stock issuances, shares	'	'	192	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	47,970	'	47,970	'	47,970	'
Balance, shares	46,340	'	46,340	'	46,340	'
Additional Paid-in Capital [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	828,777	'	'	'
Common stock issuances	'	'	7,198	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	835,975	'	835,975	'	835,975	'
Accumulated Other Comprehensive Income (Loss) [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	-50,745	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	'	'	1,952	'	'	'
Amounts reclassified into net income	'	'	1,555	'	'	'
Balance	-47,238	'	-47,238	'	-47,238	'
Retained Earnings [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	484,369	'	'	'
Net income (loss)	'	'	88,017	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Dividends declared Common: $0.99 per share	'	'	-46,415	'	'	'
Balance	525,971	'	525,971	'	525,971	'
Non-controlling Interest [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	-1,681	'	'	'
Net income (loss)	'	'	-333	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	($2,014)	'	($2,014)	'	($2,014)	'

Equity_Other_Comprehensive_Inc3

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Details of Activities in Equity (Parenthetical) (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Statement Of Stockholders Equity [Abstract]	'	'	'	'	'	'
Dividends declared per share	$0.33	$0.30	$0.99	$0.89	$1.28	$1.15

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		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Defined benefit pension plans:	'	'	'	'	'	'
Amortization of transition obligation, Before-Tax Amount	'	$217	'	$650	$217	$867
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,544	6,398	25,634	19,198	32,033	23,091
Prior service cost, Before-Tax Amount	89	'	266	'	-2,157	'
Regulatory adjustment, Before-Tax Amount	-7,583	-5,847	-22,751	-17,544	37,564	99,164
Pension plans other comprehensive income (loss), Before-Tax Amount	1,050	768	3,149	2,304	-7,196	-12,370
Net actuarial gain/(loss), Before-Tax Amount	'	'	'	'	-74,853	-135,492
FSIRS (designated hedging activities):	'	'	'	'	'	'
Unrealized/realized gain, Before-Tax Amount	'	'	'	2,959	'	1,383
Amounts reclassified into net income, Before-Tax Amount	836	836	2,509	1,965	3,345	2,257
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	2,509	4,924	3,345	3,640
Total other comprehensive income (loss), Before-Tax Amount	1,886	1,604	5,658	7,228	-3,851	-8,730
Amortization of transition obligation, Tax (Expense) or Benefit	'	-82	'	-247	-82	-330
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	-3,247	-2,432	-9,741	-7,296	-12,172	-8,775
Prior service cost, Tax (Expense) or Benefit	-34	'	-102	'	819	'
Regulatory adjustment, Tax (Expense) or Benefit	2,882	2,222	8,646	6,667	-14,274	-37,682
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	-399	-292	-1,197	-876	2,735	4,700
Unrealized/realized gain, Tax (Expense) or Benefit	'	'	'	-1,125	'	-526
Amounts reclassified into net income, Tax (Expense) or Benefit	-318	-318	-954	-747	-1,271	-858
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	-318	-318	-954	-1,872	-1,271	-1,384
Total other comprehensive income (loss), Tax (Expense) or Benefit	-717	-610	-2,151	-2,748	1,464	3,316
Amortization of transition obligation, Net-of-Tax Amount	'	135	'	403	135	537
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	5,297	3,966	15,893	11,902	19,861	14,316
Prior service cost, Net-of-Tax Amount	55	'	164	'	-1,338	'
Regulatory adjustment, Net-of-Tax Amount	-4,701	-3,625	-14,105	-10,877	23,290	61,482
Net defined benefit pension plans	651	476	1,952	1,428	-4,461	-7,670
Unrealized/realized gain, Net-of-Tax Amount	'	'	'	1,834	'	857
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	518	1,555	3,052	2,074	2,256
Total other comprehensive income (loss), Net-of-Tax Amount	1,169	994	3,507	4,480	-2,387	-5,414
Net actuarial gain/(loss), Tax (Expense) or Benefit	'	'	'	'	28,444	51,487
Net actuarial gain/(loss), Net-of-Tax Amount	'	'	'	'	($46,409)	($84,005)

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)	9 Months Ended
	Sep. 30, 2013
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 $)	Sep. 30, 2013
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		9 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Before-Tax	'	'	($52,470)	'	'	'
Amortization of prior service costs, Defined Benefit Plans Before-Tax	'	'	266	'	'	'
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,544	6,398	25,634	19,198	32,033	23,091
Regulatory adjustment, Defined Benefit Plans Before-Tax	-7,583	-5,847	-22,751	-17,544	37,564	99,164
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	'	'	3,149	'	'	'
Ending Balance AOCI September 30, 2013 Defined Benefit Plans Before-Tax	-49,321	'	-49,321	'	-49,321	'
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Tax (Expense) Benefit	'	'	19,939	'	'	'
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit	'	'	-102	'	'	'
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	'	'	-9,741	'	'	'
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,882	2,222	8,646	6,667	-14,274	-37,682
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	-399	-292	-1,197	-876	2,735	4,700
Ending Balance AOCI September 30, 2013 Defined Benefit Plans Tax (Expense) Benefit	18,742	'	18,742	'	18,742	'
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans After-Tax	'	'	-32,531	'	'	'
Amortization of prior service costs, Defined Benefit Plans After-Tax	'	'	164	'	'	'
Amortization of net actuarial loss, Defined Benefit Plans After-Tax	'	'	15,893	'	'	'
Regulatory adjustment, AOCI	-4,701	-3,625	-14,105	-10,877	23,290	61,482
Net current period other comprehensive income (loss), Defined Benefit Plans After-Tax	'	'	1,952	'	'	'
Ending Balance AOCI September 30, 2013 Defined Benefit Plans After-Tax	-30,579	'	-30,579	'	-30,579	'
Beginning Balance AOCI December 31, 2012, FSIRS - Before-Tax	'	'	-29,378	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	836	2,509	1,965	3,345	2,257
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	2,509	4,924	3,345	3,640
Ending Balance AOCI September 30, 2013, FSIRS - Before-Tax	-26,869	'	-26,869	'	-26,869	'
Beginning Balance AOCI December 31, 2012, FSIRS - Tax (Expense) Benefit	'	'	11,164	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	-318	-318	-954	-747	-1,271	-858
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	-318	-318	-954	-1,872	-1,271	-1,384
Ending Balance AOCI September 30, 2013, FSIRS - Tax (Expense) Benefit	10,210	'	10,210	'	10,210	'
Beginning Balance AOCI December 31, 2012, FSIRS - After-Tax	'	'	-18,214	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	518	1,555	1,218	2,074	1,399
Net forward-starting interest rate swaps	518	518	1,555	3,052	2,074	2,256
Ending Balance AOCI September 30, 2013, FSIRS - After-Tax	-16,659	'	-16,659	'	-16,659	'
Beginning Balance AOCI December 31, 2012, AOCI	'	'	-50,745	'	'	'
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
Amortization of prior service costs, AOCI	'	'	164	'	'	'
Amortization of net actuarial loss, AOCI	'	'	15,893	'	'	'
Regulatory adjustment, Net-of-Tax Amount	-4,701	-3,625	-14,105	-10,877	23,290	61,482
Net current period other comprehensive income (loss), AOCI	'	'	3,507	'	'	'
Ending Balance AOCI September 30, 2013, AOCI	-47,238	'	-47,238	'	-47,238	'
Reclassification out of Accumulated Other Comprehensive Income [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
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	'	'	2,509	'	'	'
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit	'	'	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	'	'	-954	'	'	'
Other comprehensive income before reclassifications, FSIRS - After-Tax	'	'	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	'	'	1,555	'	'	'
Other comprehensive income before reclassifications, AOCI	'	'	'	'	'	'
Amounts reclassified into net income	'	'	$1,555	'	'	'

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 $)	3 Months Ended	9 Months Ended	12 Months Ended			9 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013	Dec. 31, 2012
						Defined Benefit Plans [Member]	Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'	'	'	'
Net actuarial (loss) gain	'	'	$74,853	$135,492	'	($398,028)	($423,662)
Prior service cost	-89	-266	2,157	'	'	-2,157	-2,423
Less: amount recognized in regulatory assets	'	'	'	'	'	350,864	373,615
Recognized in AOCI	($49,321)	($49,321)	($49,321)	'	($52,470)	($49,321)	($52,470)