Southwest Gas 10-Q 6 Aug 14 2014 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Jun. 30, 2014	Jul. 29, 2014
Document And Entity Information [Abstract]	'	'
Document Type	'10-Q	'
Amendment Flag	'false	'
Document Period End Date	30-Jun-14	'
Document Fiscal Year Focus	'2014	'
Document Fiscal Period Focus	'Q2	'
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	'	46,512,597

Condensed_Consolidated_Balance

Condensed Consolidated Balance Sheets (Unaudited) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Utility plant:	'	'
Gas plant	$5,398,729	$5,252,469
Less: accumulated depreciation	-1,928,978	-1,868,504
Acquisition adjustments, net	640	730
Construction work in progress	74,884	101,413
Net utility plant	3,545,275	3,486,108
Other property and investments	267,763	260,871
Restricted cash	16,649	'
Current assets:	'	'
Cash and cash equivalents	26,890	41,077
Accounts receivable, net of allowances	183,472	219,469
Accrued utility revenue	31,600	72,700
Income taxes receivable, net	4,283	3,790
Deferred income taxes	'	31,130
Deferred purchased gas costs	80,441	18,217
Prepaids and other current assets	83,805	108,289
Total current assets	410,491	494,672
Deferred charges and other assets	322,111	323,523
Total assets	4,562,289	4,565,174
Capitalization:	'	'
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 46,503,111 and 46,356,125 shares)	48,133	47,986
Additional paid-in capital	844,851	840,521
Accumulated other comprehensive income (loss), net	-39,638	-41,698
Retained earnings	613,760	567,714
Total Southwest Gas Corporation equity	1,467,106	1,414,523
Noncontrolling interest	-2,214	-2,128
Total equity	1,464,892	1,412,395
Long-term debt, less current maturities	1,365,926	1,381,327
Total capitalization	2,830,818	2,793,722
Current liabilities:	'	'
Current maturities of long-term debt	23,741	11,105
Accounts payable	94,400	183,511
Customer deposits	72,896	73,367
Accrued general taxes	35,657	39,681
Accrued interest	18,187	17,920
Deferred income taxes	5,566	'
Other current liabilities	132,165	108,580
Total current liabilities	382,612	434,164
Deferred income taxes and other credits:	'	'
Deferred income taxes and investment tax credits	663,470	674,411
Taxes payable	'	284
Accumulated removal costs	291,000	279,000
Other deferred credits	394,389	383,593
Total deferred income taxes and other credits	1,348,859	1,337,288
Total capitalization and liabilities	$4,562,289	$4,565,174

Condensed_Consolidated_Balance1

Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]	'	'
Common stock, par	$1	$1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	46,503,111	46,356,125
Common stock, outstanding	46,503,111	46,356,125

Condensed_Consolidated_Stateme

Condensed Consolidated Statements of Income (Unaudited) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Operating revenues:	'	'	'	'	'	'
Gas operating revenues	$271,479	$238,869	$757,972	$732,469	$1,325,657	$1,267,567
Construction revenues	181,674	172,705	303,577	292,610	661,595	617,877
Total operating revenues	453,153	411,574	1,061,549	1,025,079	1,987,252	1,885,444
Operating expenses:	'	'	'	'	'	'
Net cost of gas sold	97,985	69,388	289,362	269,996	455,367	414,892
Operations and maintenance	97,620	94,935	200,028	192,022	392,920	374,267
Depreciation and amortization	62,186	58,570	125,077	117,503	244,391	231,225
Taxes other than income taxes	10,965	11,073	22,421	22,868	45,104	44,131
Construction expenses	157,642	148,700	270,841	255,388	588,737	532,106
Total operating expenses	426,398	382,666	907,729	857,777	1,726,519	1,596,621
Operating income	26,755	28,908	153,820	167,302	260,733	288,823
Other income and (expenses):	'	'	'	'	'	'
Net interest deductions	-17,305	-15,290	-34,824	-31,168	-67,356	-63,777
Other income (deductions)	2,863	1,452	4,475	5,521	11,254	6,994
Total other income and (expenses)	-14,442	-13,838	-30,349	-25,647	-56,102	-56,783
Income before income taxes	12,313	15,070	123,471	141,655	204,631	232,040
Income tax expense	2,686	5,003	43,147	50,914	70,175	83,607
Net income	9,627	10,067	80,324	90,741	134,456	148,433
Net income (loss) attributable to noncontrolling interest	0	-41	-86	-140	-393	-536
Net income attributable to Southwest Gas Corporation	$9,627	$10,108	$80,410	$90,881	$134,849	$148,969
Basic earnings per share	$0.21	$0.22	$1.73	$1.96	$2.91	$3.22
Diluted earnings per share	$0.21	$0.22	$1.71	$1.95	$2.88	$3.19
Dividends declared per share	$0.37	$0.33	$0.73	$0.66	$1.39	$1.25
Average number of common shares outstanding	46,502	46,331	46,471	46,291	46,407	46,214
Average shares outstanding (assuming dilution)	46,948	46,757	46,910	46,704	46,860	46,654

Condensed_Consolidated_Stateme1

Condensed Consolidated Statements of Comprehensive Income (Unaudited) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Income And Comprehensive Income [Abstract]	'	'	'	'	'	'
Net income	$9,627	$10,067	$80,324	$90,741	$134,456	$148,433
Defined benefit pension plans:	'	'	'	'	'	'
Net actuarial gain (loss)	'	'	'	'	62,214	-46,409
Amortization of prior service cost	55	54	110	109	221	109
Amortization of transition obligation	'	'	'	'	'	270
Amortization of net actuarial loss	3,666	5,298	7,333	10,596	17,927	18,530
Prior service cost	'	'	'	'	'	-1,502
Regulatory adjustment	-3,210	-4,702	-6,420	-9,404	-73,667	24,366
Net defined benefit pension plans	511	650	1,023	1,301	6,695	-4,636
Forward-starting interest rate swaps:	'	'	'	'	'	'
Amounts reclassified into net income	519	519	1,037	1,037	2,074	2,074
Net forward-starting interest rate swaps	519	519	1,037	1,037	2,074	2,074
Total other comprehensive income (loss), net of tax	1,030	1,169	2,060	2,338	8,769	-2,562
Comprehensive income	10,657	11,236	82,384	93,079	143,225	145,871
Comprehensive income (loss) attributable to noncontrolling interest	'	-41	-86	-140	-393	-536
Comprehensive income attributable to Southwest Gas Corporation	$10,657	$11,277	$82,470	$93,219	$143,618	$146,407

Condensed_Consolidated_Stateme2

Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $)	6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
CASH FLOW FROM OPERATING ACTIVITIES:	'	'	'	'
Net income	$80,324	$90,741	$134,456	$148,433
Adjustments to reconcile net income to net cash provided by operating activities:	'	'	'	'
Depreciation and amortization	125,077	117,503	244,391	231,225
Deferred income taxes	24,493	45,781	47,351	73,824
Changes in current assets and liabilities:	'	'	'	'
Accounts receivable, net of allowances	35,997	36,649	-23,208	-5,436
Accrued utility revenue	41,100	41,600	-1,200	-1,200
Deferred purchased gas costs	-62,224	-64,376	-108,991	-88,324
Accounts payable	-91,674	-53,924	-10,082	22,746
Accrued taxes	-4,801	-2,452	-1,424	4,100
Other current assets and liabilities	46,238	24,088	27,234	-23,204
Gains on sale	-4,137	-2,285	-5,964	-7,031
Changes in undistributed stock compensation	3,766	3,855	6,869	5,839
AFUDC and property-related changes	-1,024	-1,002	-2,296	-2,231
Changes in other assets and deferred charges	-15,150	-16,393	-20,476	-22,582
Changes in other liabilities and deferred credits	19,462	-4,564	41,775	3,931
Net cash provided by operating activities	197,447	215,221	328,435	340,090
CASH FLOW FROM INVESTING ACTIVITIES:	'	'	'	'
Construction expenditures and property additions	-175,444	-151,877	-387,843	-371,207
Restricted cash	-16,649	'	-16,649	'
Changes in customer advances	8,947	3,127	13,593	13,900
Miscellaneous inflows	7,060	4,905	10,620	13,766
Miscellaneous outflows	'	'	'	-969
Net cash used in investing activities	-176,086	-143,845	-380,279	-344,510
CASH FLOW FROM FINANCING ACTIVITIES:	'	'	'	'
Issuance of common stock, net	237	1,357	515	1,626
Dividends paid	-32,316	-28,947	-62,904	-56,169
Issuance of long-term debt, net	17,719	41,673	287,336	130,387
Retirement of long-term debt	-10,716	-101,269	-46,460	-205,269
Change in credit facility and commercial paper	-10,000	8,000	-119,000	119,000
Other	-472	'	1,527	'
Net cash provided by (used in) financing activities	-35,548	-79,186	61,014	-10,425
Change in cash and cash equivalents	-14,187	-7,810	9,170	-14,845
Cash and cash equivalents at beginning of period	41,077	25,530	17,720	32,565
Cash and cash equivalents at end of period	26,890	17,720	26,890	17,720
Supplemental information:	'	'	'	'
Interest paid, net of amounts capitalized	31,787	29,116	61,401	60,103
Income taxes paid	$12,246	$3,531	$15,565	$4,608

Nature_of_Operations_and_Basis

Nature of Operations and Basis of Presentation	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Nature of Operations and Basis of Presentation	'
	Note 1 – Nature of Operations and Basis of Presentation
	Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. All of Southwest’s service territories have decoupled rate structures, which are designed to mitigate the impacts of weather variability and conservation on operating margin. The timing and amount of rate relief can materially impact results of operations. Natural gas sales are seasonal, peaking during the winter months; therefore, results of operations for interim periods are not necessarily indicative of results for a full year. Natural gas purchases and the timing of related recoveries can materially impact liquidity. NPL Construction Co. (“NPL” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems. Typically, NPL revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.
	Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2013 Annual Report to Shareholders, which is incorporated by reference into the 2013 Form 10-K, and the first quarter 2014 Form 10-Q.
	Prepaids and other current assets. Prepaids and other current assets includes gas pipe inventory and operating supplies of $23 million at June 30, 2014 and $21 million at December 31, 2013.
	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. Cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. Upon contract expiration, customer advances of approximately $5.6 million and $4.8 million, during the first six months of 2014 and 2013, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.
	Intercompany Transactions. NPL recognizes revenues generated from contracts with Southwest (see Note 3 - Segment Information below). Accounts receivable for these services are presented in the table below (thousands of dollars):
		June 30,			December 31,
		2014			2013
	Accounts receivable for NPL services	$	10,786		$	10,787
	The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.
	Other Property and Investments. Other property and investments includes (millions of dollars):
		June 30,			December 31,
		2014			2013
	NPL property and equipment	$	332		$	320
	NPL accumulated provision for depreciation and amortization		(172	)		(163	)
	Net cash surrender value of COLI policies		96			93
	Other property		12			11
	Total	$	268		$	261
	Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars):
		Three Months Ended						Six Months Ended					Twelve Months Ended
		June 30						June 30					June 30
		2014			2013			2014		2013			2014			2013
	Change in COLI policies	$	2,300		$	1,800		$	3,200	$	5,600		$	10,000		$	8,900
	Interest income		612			144			1,109		264			1,306			683
	Pipe replacement costs		—			(36	)		—		(121	)		(11	)		(1,759	)
	Miscellaneous income and (expense)		(49	)		(456	)		166		(222	)		(41	)		(830	)
	Total other income (deductions)	$	2,863		$	1,452		$	4,475	$	5,521		$	11,254		$	6,994
	Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by Southwest to indemnify itself against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Current tax regulations provide for tax-free treatment of life insurance (death benefit) proceeds. Therefore, changes in the cash surrender value components of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work. The replacement program work subject to non-recoverability was substantially completed in 2012.
	Reclassifications. A reclassification was made to the prior year’s financial information between two categories on the Condensed Consolidated Statements of Comprehensive Income to present it on a basis comparable with the current year’s presentation, with no impact on comprehensive income overall.
	Recently Issued Accounting Standards Update. In May 2014, the Financial Accounting Standards Board (“FASB”) issued the update “Revenue from Contracts with Customers (Topic 606).” The update replaces much of the current guidance regarding revenue recognition including most industry-specific guidance. The core principle of the update is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. An entity will be required to identify the contract with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. In addition to the new revenue recognition requirements, entities will be required to disclose sufficient information to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities may choose between two retrospective transition methods when applying the update. The Company plans to adopt this update, as required, on January 1, 2017 for interim and annual reporting periods. Early adoption is not permitted. The Company is evaluating what impact this standard might have on its consolidated financial statements and disclosures.

Components_of_Net_Periodic_Ben

Components of Net Periodic Benefit Cost	6 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Components of Net Periodic Benefit Cost	'
	Note 2 – Components of Net Periodic Benefit Cost
	Southwest has a noncontributory qualified retirement plan with defined benefits covering substantially all employees and a separate unfunded supplemental retirement plan (“SERP”) which is limited to officers. Southwest also provides postretirement benefits other than pensions (“PBOP”) to its qualified retirees for health care, dental, and life insurance.
	Net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets.
		Qualified Retirement Plan
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	5,340		$	5,764		$	10,680		$	11,528		$	22,208		$	21,688
	Interest cost		10,860			9,402			21,721			18,803			40,525			37,936
	Expected return on plan assets		(13,335	)		(12,460	)		(26,671	)		(24,920	)		(51,591	)		(47,810	)
	Amortization of net actuarial loss		5,718			8,065			11,436			16,131			27,566			28,072
	Net periodic benefit cost	$	8,583		$	10,771		$	17,166		$	21,542		$	38,708		$	39,886
		SERP
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	73		$	93		$	146		$	187		$	332		$	324
	Interest cost		436			384			872			767			1,640			1,582
	Amortization of net actuarial loss		196			243			392			486			877			827
	Net periodic benefit cost	$	705		$	720		$	1,410		$	1,440		$	2,849		$	2,733
		PBOP
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	275		$	305		$	551		$	610		$	1,161		$	1,098
	Interest cost		708			621			1,415			1,241			2,656			2,515
	Expected return on plan assets		(816	)		(706	)		(1,632	)		(1,412	)		(3,044	)		(2,614	)
	Amortization of prior service cost		88			88			177			177			355			177
	Amortization of transition obligation		—			—			—			—			—			434
	Amortization of net actuarial loss		—			237			—			473			472			988
	Net periodic benefit cost	$	255		$	545		$	511		$	1,089		$	1,600		$	2,598

Segment_Information

Segment Information	6 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Segment Information	'
	Note 3 – Segment Information
	The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):
		Natural Gas		Construction		Total
		Operations		Services
	Three months ended June 30, 2014
	Revenues from external customers	$	271,479	$	156,966	$	428,445
	Intersegment revenues		—		24,708		24,708
	Total	$	271,479	$	181,674	$	453,153
	Segment net income	$	1,798	$	7,829	$	9,627
	Three months ended June 30, 2013
	Revenues from external customers	$	238,869	$	150,434	$	389,303
	Intersegment revenues		—		22,271		22,271
	Total	$	238,869	$	172,705	$	411,574
	Segment net income	$	1,964	$	8,144	$	10,108
		Natural Gas		Construction
		Operations		Services		Total
	Six months ended June 30, 2014
	Revenues from external customers	$	757,972	$	255,119	$	1,013,091
	Intersegment revenues		—		48,458		48,458
	Total	$	757,972	$	303,577	$	1,061,549
	Segment net income	$	74,397	$	6,013	$	80,410
	Six months ended June 30, 2013
	Revenues from external customers	$	732,469	$	255,300	$	987,769
	Intersegment revenues		—		37,310		37,310
	Total	$	732,469	$	292,610	$	1,025,079
	Segment net income	$	81,256	$	9,625	$	90,881
		Natural Gas		Construction		Total
		Operations		Services
	Twelve months ended June 30, 2014
	Revenues from external customers	$	1,325,657	$	562,294	$	1,887,951
	Intersegment revenues		—		99,301		99,301
	Total	$	1,325,657	$	661,595	$	1,987,252
	Segment net income	$	117,310	$	17,539	$	134,849
	Twelve months ended June 30, 2013
	Revenues from external customers	$	1,267,567	$	532,687	$	1,800,254
	Intersegment revenues		—		85,190		85,190
	Total	$	1,267,567	$	617,877	$	1,885,444
	Segment net income	$	121,877	$	27,092	$	148,969

Derivatives_and_Fair_Value_Mea

Derivatives and Fair Value Measurements	6 Months Ended
	Jun. 30, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Derivatives and Fair Value Measurements	'
	Note 4 – Derivatives and Fair Value Measurements
	Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts have no significant market value. The Swaps are recorded at fair value.
	As part of the June 2013 Nevada Annual Rate Adjustment and associated stipulation, the Company agreed to suspend further Swaps and fixed-price purchases pursuant to the Volatility Mitigation Program for its Nevada service territories. The decision will not impact previously executed purchase agreements. The Company, along with its regulators, will continue to evaluate this strategy in light of prevailing or anticipated changing market conditions.
	The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to approximately 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 July 2014 through March 2016. Under such contracts, Southwest pays the counterparty at a fixed rate and receives from the counterparty a floating rate per MMBtu (“dekatherm”) of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):
			June 30, 2014				December 31, 2013
	Contract notional amounts			7,658				13,571
	Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.
	The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three-, six-, and twelve-month periods ended June 30, 2014 and 2013 and their location in the Condensed Consolidated Statements of Income (thousands of dollars):
	Gains (losses) recognized in income for derivatives not designated as hedging instruments:
	(Thousands of dollars)
					Three Months Ended								Six Months Ended						Twelve Months Ended
	Instrument		Location of Gain or (Loss)		June 30								June 30						June 30
			Recognized in Income on Derivative		2014				2013				2014			2013			2014			2013
	Swaps		Net cost of gas sold		$	(83	)		$	(7,669	)		$	5,907		$	(2,593	)	$	9,476		$	(2,885	)
	Swaps		Net cost of gas sold			83	*			7,669	*			(5,907	)*		2,593	*		(9,476	)*		2,885	*
	Total				$	—			$	—			$	—		$	—		$	—		$	—
	*	Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
	No gains (losses) were recognized in income or other comprehensive income during the periods presented for derivatives designated as cash flow hedging instruments. Previously, Southwest entered into two forward-starting interest rate swaps (“FSIRS”) to hedge the risk of interest rate variability during the period leading up to the issuance of fixed-rate debt. At settlement of the first FSIRS in December 2010, Southwest paid an aggregate $11.7 million to the counterparties. The second FSIRS terminated in March 2012 at which time Southwest paid counterparties an aggregate $21.8 million. The losses on both FSIRS are being amortized over ten-year periods from Accumulated other comprehensive income (loss) and into interest expense.
	The following table sets forth the fair values of the Company’s Swaps and their location in the Condensed Consolidated Balance Sheets (thousands of dollars):
	Fair values of derivatives not designated as hedging instruments:
	June 30, 2014				Asset				Liability				Net Total
	Instrument		Balance Sheet Location		Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	483			$	—			$	483
	Swaps		Prepaids and other current assets			1,884				(140	)			1,744
	Total				$	2,367			$	(140	)		$	2,227
	December 31, 2013				Asset				Liability				Net Total
	Instrument		Balance Sheet Location		Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	257			$	(77	)		$	180
	Swaps		Prepaids and other current assets			1,054				(253	)			801
	Swaps		Other current liabilities			126				(282	)			(156	)
	Swaps		Other deferred credits			7				(11	)			(4	)
	Total				$	1,444			$	(623	)		$	821
	The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). The Company has master netting arrangements with each counterparty that provide for the net settlement of all contracts through a single payment. As applicable, the Company has elected to reflect the net amounts in its balance sheets. The Company had no outstanding collateral associated with the Swaps during either period shown in the above table.
	Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income.
	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.
	(Thousands of dollars)		Three Months Ended				Six Months Ended				Twelve Months Ended
			30-Jun-14				30-Jun-14				30-Jun-14
	Paid to counterparties		$	—			$	15			$	2,291
	Received from counterparties		$	196			$	4,515			$	4,522
	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars).
	June 30, 2014		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(483	)
	Swaps		Other current liabilities			(1,744	)
	December 31, 2013		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(180	)
	Swaps		Other current liabilities			(801	)
	Swaps		Prepaids and other current assets			156
	Swaps		Deferred charges and other assets			4
	Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at June 30, 2014 and December 31, 2013 using New York Mercantile Exchange (“NYMEX”) futures settlement prices for delivery of natural gas at Henry Hub adjusted by the price of NYMEX ClearPort basis Swaps, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs (inputs, other than quoted prices, for similar assets or liabilities) are observable in the marketplace throughout the full term of the Swaps, but have been credit-risk adjusted with no significant impact to the overall fair value measure.
	The following table sets forth by level, within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the Company’s financial assets and liabilities recorded at fair value:
	Level 2 - Significant other observable inputs
	(Thousands of dollars)		June 30, 2014				December 31, 2013
	Assets at fair value:
	Prepaids and other current assets - Swaps		$	1,744			$	801
	Deferred charges and other assets - Swaps			483				180
	Liabilities at fair value:
	Other current liabilities - Swaps			—				(156	)
	Other deferred credits - Swaps			—				(4	)
	Net Assets (Liabilities)		$	2,227			$	821
	No financial assets or liabilities accounted for at fair value fell within Level 1 (quoted prices in active markets for identical financial assets) or Level 3 (significant unobservable inputs) of the fair value hierarchy.

LongTerm_Debt

Long-Term Debt	6 Months Ended
	Jun. 30, 2014
Debt Disclosure [Abstract]	'
Long-Term Debt	'
	Note 5 – Long-Term Debt
	Carrying amounts of the Company’s long-term debt and their related estimated fair values as of June 30, 2014 and December 31, 2013 are disclosed in the following table. The fair values of the revolving credit facility (including commercial paper), the NPL revolving credit facility, and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, and are categorized as Level 1 (quoted prices for identical financial instruments or liabilities that can be accessed at the measurement date) within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The market values of debentures (except the 6.1% Notes) and fixed-rate IDRBs are categorized as Level 2. The 6.1% Notes (private placement, not actively traded) and NPL other debt obligations (not actively traded) are categorized as Level 3 (based on significant unobservable inputs to their fair values). Fair values for the debentures, fixed-rate IDRBs, and NPL other debt obligations were determined through a market-based valuation approach, where fair market values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.
		June 30, 2014					December 31, 2013
	(Thousands of dollars)	Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	136,964	$	125,000		$	130,953
	Notes, 6.1%, due 2041		125,000			157,689		125,000			141,873
	Notes, 3.875%, due 2022		250,000			266,398		250,000			252,485
	Notes, 4.875%, due 2043		250,000			277,840		250,000			257,280
	8% Series, due 2026		75,000			104,001		75,000			96,263
	Medium-term notes, 7.59% series, due 2017		25,000			28,539		25,000			28,741
	Medium-term notes, 7.78% series, due 2022		25,000			32,116		25,000			30,586
	Medium-term notes, 7.92% series, due 2027		25,000			34,194		25,000			31,497
	Medium-term notes, 6.76% series, due 2027		7,500			9,259		7,500			8,468
	Unamortized discount		(5,394	)				(5,560	)
			902,106					901,940
	Revolving credit facility and commercial paper		—			—		10,000			10,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,215		20,000			20,150
	5.25% 2004 Series A, due 2034		65,000			65,041		65,000			64,522
	5.00% 2004 Series B, due 2033		31,200			31,207		31,200			30,284
	4.85% 2005 Series A, due 2035		100,000			99,934		100,000			95,192
	4.75% 2006 Series A, due 2036		24,855			24,911		24,855			22,974
	Unamortized discount		(2,710	)				(2,776	)
			438,345					438,279
	NPL credit facility		12,529			12,529		—			—
	NPL other debt obligations		36,687			36,851		42,213			42,119
			1,389,667					1,392,432
	Less: current maturities		(23,741	)				(11,105	)
	Long-term debt, less current maturities	$	1,365,926				$	1,381,327
	In March 2014, the Company amended its $300 million credit facility. The facility was previously scheduled to expire in March 2017 and was extended to March 2019. The Company will continue to use $150 million of the facility as long-term debt and the remaining $150 million for working capital purposes. In addition to extending the credit facility, among other amendments, the applicable margins and unused commitment fees were reduced and the Pricing Level definitions were modified. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. At June 30, 2014, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate.
	Also in March 2014, the Company amended its 6.1% $125 million notes (“Notes”) note purchase agreement. The amendment modifies the Permitted Lien category permitting liens securing indebtedness not to exceed 10% of total capitalization as of the end of any quarter. This provision in the agreement prohibits liens on any property securing other indebtedness under bank facilities unless the Notes are secured in a similar manner. The provision was amended to clarify that it only applies to bank facilities of Southwest Gas Corporation.

Equity_Other_Comprehensive_Inc

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	6 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income	'
	Note 6 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income
	The table below provides details of activity in equity during the six months ended June 30, 2014.
			Southwest Gas Corporation Equity															Non-			Total
			Common Stock						Additional			Accumulated			Retained			controlling
									Paid-in			Other			Earnings			Interest
									Capital			Comprehensive
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-13			46,356		$	47,986		$	840,521		$	(41,698	)	$	567,714		$	(2,128	)	$	1,412,395
	Common stock issuances			147			147			4,330												4,477
	Net income (loss)															80,410			(86	)		80,324
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												1,023									1,023
	Amounts reclassified to net income, net of tax (FSIRS)												1,037									1,037
	Dividends declared
	Common: $0.73 per share															(34,364	)					(34,364	)
	30-Jun-14			46,503		$	48,133		$	844,851		$	(39,638	)	$	613,760		$	(2,214	)	$	1,464,892
	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
			June 30, 2014									June 30, 2013
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of net actuarial (gain)/loss		$	5,914		$	(2,248	)	$	3,666		$	8,545		$	(3,247	)	$	5,298
	Amortization of prior service cost			88			(33	)		55			88			(34	)		54
	Regulatory adjustment			(5,177	)		1,967			(3,210	)		(7,584	)		2,882			(4,702	)
	Pension plans other comprehensive income (loss)			825			(314	)		511			1,049			(399	)		650
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			837			(318	)		519			837			(318	)		519
	FSIRS other comprehensive income			837			(318	)		519			837			(318	)		519
	Total other comprehensive income (loss)		$	1,662		$	(632	)	$	1,030		$	1,886		$	(717	)	$	1,169
			Six Months Ended									Six Months Ended
			30-Jun-14									30-Jun-13
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of net actuarial (gain)/loss		$	11,828		$	(4,495	)	$	7,333		$	17,090		$	(6,494	)	$	10,596
	Amortization of prior service cost			177			(67	)		110			177			(68	)		109
	Regulatory adjustment			(10,355	)		3,935			(6,420	)		(15,168	)		5,764			(9,404	)
	Pension plans other comprehensive income (loss)			1,650			(627	)		1,023			2,099			(798	)		1,301
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			1,673			(636	)		1,037			1,673			(636	)		1,037
	FSIRS other comprehensive income			1,673			(636	)		1,037			1,673			(636	)		1,037
	Total other comprehensive income (loss)		$	3,323		$	(1,263	)	$	2,060		$	3,772		$	(1,434	)	$	2,338
			Twelve Months Ended									Twelve Months Ended
			30-Jun-14									30-Jun-13
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Net actuarial gain/(loss)		$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
	Amortization of prior service cost			355			(134	)		221			177			(68	)		109
	Amortization of transition obligation			—			—			—			434			(164	)		270
	Amortization of net actuarial (gain)/loss			28,915			(10,988	)		17,927			29,887			(11,357	)		18,530
	Prior service cost			—			—			—			(2,423	)		921			(1,502	)
	Regulatory adjustment			(118,817	)		45,150			(73,667	)		39,300			(14,934	)		24,366
	Pension plans other comprehensive income (loss)			10,798			(4,103	)		6,695			(7,478	)		2,842			(4,636	)
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
Total other comprehensive income (loss)		$	14,143		$	(5,374	)	$	8,769		$	(4,133	)	$	1,571		$	(2,562	)
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 June 30, 2014, will be reclassified into interest expense within the next 12 months, as the related interest payments on long-term debt occur.
The following represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets:
AOCI - Rollforward
(Thousands of dollars)
		Defined Benefit Plans									FSIRS
		Before-			Tax			After-			Before-			Tax			After-			AOCI
		Tax			(Expense)			Tax			Tax			(Expense)			Tax
					Benefit									Benefit
Beginning Balance AOCI December 31, 2013		$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	(41,698	)
Other comprehensive income before reclassifications			—			—			—			—			—			—			—
FSIRS amounts reclassified from AOCI (1)			—			—			—			1,673			(636	)		1,037			1,037
Amortization of prior service cost (2)			177			(67	)		110			—			—			—			110
Amortization of net actuarial loss (2)			11,828			(4,495	)		7,333			—			—			—			7,333
Regulatory adjustment (3)			(10,355	)		3,935			(6,420	)		—			—			—			(6,420	)
Net current period other comprehensive income (loss)			1,650			(627	)		1,023			1,673			(636	)		1,037			2,060
Ending Balance AOCI June 30, 2014		$	(39,573	)	$	15,038		$	(24,535	)	$	(24,360	)	$	9,257		$	(15,103	)	$	(39,638	)
-1	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Condensed Consolidated Statements of Income.
-2	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details).
-3	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Condensed Consolidated Balance Sheets).
The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:
Amounts Recognized in AOCI (Before Tax)
(Thousands of dollars)
		June 30, 2014			December 31, 2013
Net actuarial (loss) gain		$	(277,313	)	$	(289,141	)
Prior service cost			(1,890	)		(2,067	)
Less: amount recognized in regulatory assets			239,630			249,985
Recognized in AOCI		$	(39,573	)	$	(41,223	)

Purchase_of_Corporate_Headquar

Purchase of Corporate Headquarters Office Complex	6 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Purchase of Corporate Headquarters Office Complex	'
	Note 7 – Purchase of Corporate Headquarters Office Complex
	In July 2014, the Company purchased for $16.5 million a portion of its corporate headquarters office complex in Las Vegas that it had previously leased. With the closing of the purchase, the lease terminated. At June 30, 2014, the purchase price plus related closing costs and fees were being held in escrow until the purchase was consummated. This amount is shown as restricted cash on the condensed consolidated balance sheet at June 30, 2014.

Nature_of_Operations_and_Basis1

Nature of Operations and Basis of Presentation (Policies)	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Nature of Operations	'
	Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. All of Southwest’s service territories have decoupled rate structures, which are designed to mitigate the impacts of weather variability and conservation on operating margin. The timing and amount of rate relief can materially impact results of operations. Natural gas sales are seasonal, peaking during the winter months; therefore, results of operations for interim periods are not necessarily indicative of results for a full year. Natural gas purchases and the timing of related recoveries can materially impact liquidity. NPL Construction Co. (“NPL” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems. Typically, NPL revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.
Basis of Presentation	'
	Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2013 Annual Report to Shareholders, which is incorporated by reference into the 2013 Form 10-K, and the first quarter 2014 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 $23 million at June 30, 2014 and $21 million at December 31, 2013.
Cash and Cash Equivalents	'
	Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. Cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. Upon contract expiration, customer advances of approximately $5.6 million and $4.8 million, during the first six months of 2014 and 2013, respectively, were applied as contributions toward utility construction activity and represent non-cash investing activity.
Intercompany Transactions	'
	Intercompany Transactions. NPL recognizes revenues generated from contracts with Southwest (see Note 3 - Segment Information below). Accounts receivable for these services are presented in the table below (thousands of dollars):
		June 30, 2014			December 31, 2013
	Accounts receivable for NPL services	$	10,786		$	10,787
	The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.
Other Property and Investments	'
	Other Property and Investments. Other property and investments includes (millions of dollars):
		June 30, 2014			December 31, 2013
	NPL property and equipment	$	332		$	320
	NPL accumulated provision for depreciation and amortization		(172	)		(163	)
	Net cash surrender value of COLI policies		96			93
	Other property		12			11
	Total	$	268		$	261
Other Income (Deductions)	'
	Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars):
		Three Months Ended						Six Months Ended					Twelve Months Ended
		June 30						June 30					June 30
		2014			2013			2014		2013			2014			2013
	Change in COLI policies	$	2,300		$	1,800		$	3,200	$	5,600		$	10,000		$	8,900
	Interest income		612			144			1,109		264			1,306			683
	Pipe replacement costs		—			(36	)		—		(121	)		(11	)		(1,759	)
	Miscellaneous income and (expense)		(49	)		(456	)		166		(222	)		(41	)		(830	)
	Total other income (deductions)	$	2,863		$	1,452		$	4,475	$	5,521		$	11,254		$	6,994
	Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by Southwest to indemnify itself against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Current tax regulations provide for tax-free treatment of life insurance (death benefit) proceeds. Therefore, changes in the cash surrender value components of COLI policies, as they progress towards the ultimate death benefits, are also recorded without tax consequences. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work. The replacement program work subject to non-recoverability was substantially completed in 2012.
Reclassifications	'
	Reclassifications. A reclassification was made to the prior year’s financial information between two categories on the Condensed Consolidated Statements of Comprehensive Income to present it on a basis comparable with the current year’s presentation, with no impact on comprehensive income overall.
Recently Issued Accounting Standards Update	'
	Recently Issued Accounting Standards Update. In May 2014, the Financial Accounting Standards Board (“FASB”) issued the update “Revenue from Contracts with Customers (Topic 606).” The update replaces much of the current guidance regarding revenue recognition including most industry-specific guidance. The core principle of the update is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. An entity will be required to identify the contract with a customer, identify the performance obligations in the contract, determine the transaction price, allocate the transaction price to the performance obligations in the contract, and recognize revenue when (or as) the entity satisfies a performance obligation. In addition to the new revenue recognition requirements, entities will be required to disclose sufficient information to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities may choose between two retrospective transition methods when applying the update. The Company plans to adopt this update, as required, on January 1, 2017 for interim and annual reporting periods. Early adoption is not permitted. The Company is evaluating what impact this standard might have on its consolidated financial statements and disclosures.

Nature_of_Operations_and_Basis2

Nature of Operations and Basis of Presentation (Tables)	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Intercompany Transactions	'
	Accounts receivable for these services are presented in the table below (thousands of dollars):
		June 30, 2014			December 31, 2013
	Accounts receivable for NPL services	$	10,786		$	10,787
Schedule of Other Property and Investments	'
	Other property and investments includes (millions of dollars):
		June 30, 2014			December 31, 2013
	NPL property and equipment	$	332		$	320
	NPL accumulated provision for depreciation and amortization		(172	)		(163	)
	Net cash surrender value of COLI policies		96			93
	Other property		12			11
	Total	$	268		$	261
Other Income (Deductions)	'
	The following table provides the composition of significant items included in Other income (deductions) in the condensed consolidated statements of income (thousands of dollars):
		Three Months Ended						Six Months Ended					Twelve Months Ended
		June 30						June 30					June 30
		2014			2013			2014		2013			2014			2013
	Change in COLI policies	$	2,300		$	1,800		$	3,200	$	5,600		$	10,000		$	8,900
	Interest income		612			144			1,109		264			1,306			683
	Pipe replacement costs		—			(36	)		—		(121	)		(11	)		(1,759	)
	Miscellaneous income and (expense)		(49	)		(456	)		166		(222	)		(41	)		(830	)
	Total other income (deductions)	$	2,863		$	1,452		$	4,475	$	5,521		$	11,254		$	6,994

Components_of_Net_Periodic_Ben1

Components of Net Periodic Benefit Cost (Tables)	6 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Schedule of Components of Net Periodic Benefit Costs	'
	Net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets.
		Qualified Retirement Plan
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	5,340		$	5,764		$	10,680		$	11,528		$	22,208		$	21,688
	Interest cost		10,860			9,402			21,721			18,803			40,525			37,936
	Expected return on plan assets		(13,335	)		(12,460	)		(26,671	)		(24,920	)		(51,591	)		(47,810	)
	Amortization of net actuarial loss		5,718			8,065			11,436			16,131			27,566			28,072
	Net periodic benefit cost	$	8,583		$	10,771		$	17,166		$	21,542		$	38,708		$	39,886
		SERP
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	73		$	93		$	146		$	187		$	332		$	324
	Interest cost		436			384			872			767			1,640			1,582
	Amortization of net actuarial loss		196			243			392			486			877			827
	Net periodic benefit cost	$	705		$	720		$	1,410		$	1,440		$	2,849		$	2,733
		PBOP
		Period Ended June 30,
		Three Months						Six Months						Twelve Months
	(Thousands of dollars)	2014			2013			2014			2013			2014			2013
	Service cost	$	275		$	305		$	551		$	610		$	1,161		$	1,098
	Interest cost		708			621			1,415			1,241			2,656			2,515
	Expected return on plan assets		(816	)		(706	)		(1,632	)		(1,412	)		(3,044	)		(2,614	)
	Amortization of prior service cost		88			88			177			177			355			177
	Amortization of transition obligation		—			—			—			—			—			434
	Amortization of net actuarial loss		—			237			—			473			472			988
	Net periodic benefit cost	$	255		$	545		$	511		$	1,089		$	1,600		$	2,598

Segment_Information_Tables

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2014
Segment Reporting [Abstract]	'
Schedule of Segment Reporting Information	'
	The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):
		Natural Gas		Construction		Total
		Operations		Services
	Three months ended June 30, 2014
	Revenues from external customers	$	271,479	$	156,966	$	428,445
	Intersegment revenues		—		24,708		24,708
	Total	$	271,479	$	181,674	$	453,153
	Segment net income	$	1,798	$	7,829	$	9,627
	Three months ended June 30, 2013
	Revenues from external customers	$	238,869	$	150,434	$	389,303
	Intersegment revenues		—		22,271		22,271
	Total	$	238,869	$	172,705	$	411,574
	Segment net income	$	1,964	$	8,144	$	10,108
		Natural Gas		Construction
		Operations		Services		Total
	Six months ended June 30, 2014
	Revenues from external customers	$	757,972	$	255,119	$	1,013,091
	Intersegment revenues		—		48,458		48,458
	Total	$	757,972	$	303,577	$	1,061,549
	Segment net income	$	74,397	$	6,013	$	80,410
	Six months ended June 30, 2013
	Revenues from external customers	$	732,469	$	255,300	$	987,769
	Intersegment revenues		—		37,310		37,310
	Total	$	732,469	$	292,610	$	1,025,079
	Segment net income	$	81,256	$	9,625	$	90,881
		Natural Gas		Construction		Total
		Operations		Services
	Twelve months ended June 30, 2014
	Revenues from external customers	$	1,325,657	$	562,294	$	1,887,951
	Intersegment revenues		—		99,301		99,301
	Total	$	1,325,657	$	661,595	$	1,987,252
	Segment net income	$	117,310	$	17,539	$	134,849
	Twelve months ended June 30, 2013
	Revenues from external customers	$	1,267,567	$	532,687	$	1,800,254
	Intersegment revenues		—		85,190		85,190
	Total	$	1,267,567	$	617,877	$	1,885,444
	Segment net income	$	121,877	$	27,092	$	148,969

Derivatives_and_Fair_Value_Mea1

Derivatives and Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'
Notional Amounts under Swaps Contracts	'
	The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):
			June 30, 2014				December 31, 2013
	Contract notional amounts			7,658				13,571
Amount of Gain or Losses Recognized in Income on Derivatives	'
	The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three-, six-, and twelve-month periods ended June 30, 2014 and 2013 and their location in the Condensed Consolidated Statements of Income (thousands of dollars):
	Gains (losses) recognized in income for derivatives not designated as hedging instruments:
	(Thousands of dollars)
					Three Months Ended								Six Months Ended						Twelve Months Ended
	Instrument		Location of Gain or (Loss)		June 30								June 30						June 30
			Recognized in Income on Derivative		2014				2013				2014			2013			2014			2013
	Swaps		Net cost of gas sold		$	(83	)		$	(7,669	)		$	5,907		$	(2,593	)	$	9,476		$	(2,885	)
	Swaps		Net cost of gas sold			83	*			7,669	*			(5,907	)*		2,593	*		(9,476	)*		2,885	*
	Total				$	—			$	—			$	—		$	—		$	—		$	—
	*	Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
Fair Values of Swaps in Condensed Consolidated Balance Sheets	'
	The following table sets forth the fair values of the Company’s Swaps and their location in the Condensed Consolidated Balance Sheets (thousands of dollars):
	Fair values of derivatives not designated as hedging instruments:
	June 30, 2014				Asset				Liability				Net Total
	Instrument		Balance Sheet Location		Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	483			$	—			$	483
	Swaps		Prepaids and other current assets			1,884				(140	)			1,744
	Total				$	2,367			$	(140	)		$	2,227
	December 31, 2013				Asset				Liability				Net Total
	Instrument		Balance Sheet Location		Derivatives				Derivatives
	Swaps		Deferred charges and other assets		$	257			$	(77	)		$	180
	Swaps		Prepaids and other current assets			1,054				(253	)			801
	Swaps		Other current liabilities			126				(282	)			(156	)
	Swaps		Other deferred credits			7				(11	)			(4	)
	Total				$	1,444			$	(623	)		$	821
Paid to and Received from Counterparties for Settlements of Matured Swaps	'
	The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.
	(Thousands of dollars)		Three Months Ended				Six Months Ended				Twelve Months Ended
			30-Jun-14				30-Jun-14				30-Jun-14
	Paid to counterparties		$	—			$	15			$	2,291
	Received from counterparties		$	196			$	4,515			$	4,522
Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets	'
	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Condensed Consolidated Balance Sheets (thousands of dollars).
	June 30, 2014		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(483	)
	Swaps		Other current liabilities			(1,744	)
	December 31, 2013		Balance Sheet Location		Net Total
	Instrument
	Swaps		Other deferred credits		$	(180	)
	Swaps		Other current liabilities			(801	)
	Swaps		Prepaids and other current assets			156
	Swaps		Deferred charges and other assets			4
Significant Other Observable Inputs	'
	The following table sets forth by level, within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the Company’s financial assets and liabilities recorded at fair value:
	Level 2 - Significant other observable inputs
	(Thousands of dollars)		June 30, 2014				December 31, 2013
	Assets at fair value:
	Prepaids and other current assets - Swaps		$	1,744			$	801
	Deferred charges and other assets - Swaps			483				180
	Liabilities at fair value:
	Other current liabilities - Swaps			—				(156	)
	Other deferred credits - Swaps			—				(4	)
	Net Assets (Liabilities)		$	2,227			$	821

LongTerm_Debt_Tables

Long-Term Debt (Tables)	6 Months Ended
	Jun. 30, 2014
Debt Disclosure [Abstract]	'
Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt	'
	The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.
		June 30, 2014					December 31, 2013
	(Thousands of dollars)	Carrying			Market		Carrying			Market
		Amount			Value		Amount			Value
	Debentures:
	Notes, 4.45%, due 2020	$	125,000		$	136,964	$	125,000		$	130,953
	Notes, 6.1%, due 2041		125,000			157,689		125,000			141,873
	Notes, 3.875%, due 2022		250,000			266,398		250,000			252,485
	Notes, 4.875%, due 2043		250,000			277,840		250,000			257,280
	8% Series, due 2026		75,000			104,001		75,000			96,263
	Medium-term notes, 7.59% series, due 2017		25,000			28,539		25,000			28,741
	Medium-term notes, 7.78% series, due 2022		25,000			32,116		25,000			30,586
	Medium-term notes, 7.92% series, due 2027		25,000			34,194		25,000			31,497
	Medium-term notes, 6.76% series, due 2027		7,500			9,259		7,500			8,468
	Unamortized discount		(5,394	)				(5,560	)
			902,106					901,940
	Revolving credit facility and commercial paper		—			—		10,000			10,000
	Industrial development revenue bonds:
	Variable-rate bonds:
	Tax-exempt Series A, due 2028		50,000			50,000		50,000			50,000
	2003 Series A, due 2038		50,000			50,000		50,000			50,000
	2008 Series A, due 2038		50,000			50,000		50,000			50,000
	2009 Series A, due 2039		50,000			50,000		50,000			50,000
	Fixed-rate bonds:
	5.25% 2003 Series D, due 2038		20,000			20,215		20,000			20,150
	5.25% 2004 Series A, due 2034		65,000			65,041		65,000			64,522
	5.00% 2004 Series B, due 2033		31,200			31,207		31,200			30,284
	4.85% 2005 Series A, due 2035		100,000			99,934		100,000			95,192
	4.75% 2006 Series A, due 2036		24,855			24,911		24,855			22,974
	Unamortized discount		(2,710	)				(2,776	)
			438,345					438,279
	NPL credit facility		12,529			12,529		—			—
	NPL other debt obligations		36,687			36,851		42,213			42,119
			1,389,667					1,392,432
	Less: current maturities		(23,741	)				(11,105	)
	Long-term debt, less current maturities	$	1,365,926				$	1,381,327

Equity_Other_Comprehensive_Inc1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)	6 Months Ended
	Jun. 30, 2014
Text Block [Abstract]	'
Detailed Activities of Equity	'
	The table below provides details of activity in equity during the six months ended June 30, 2014.
			Southwest Gas Corporation Equity															Non-			Total
			Common Stock						Additional			Accumulated			Retained			controlling
									Paid-in			Other			Earnings			Interest
									Capital			Comprehensive
	(In thousands, except per share amounts)		Shares			Amount						Income (Loss)
	31-Dec-13			46,356		$	47,986		$	840,521		$	(41,698	)	$	567,714		$	(2,128	)	$	1,412,395
	Common stock issuances			147			147			4,330												4,477
	Net income (loss)															80,410			(86	)		80,324
	Other comprehensive income (loss):
	Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax												1,023									1,023
	Amounts reclassified to net income, net of tax (FSIRS)												1,037									1,037
	Dividends declared
	Common: $0.73 per share															(34,364	)					(34,364	)
	30-Jun-14			46,503		$	48,133		$	844,851		$	(39,638	)	$	613,760		$	(2,214	)	$	1,464,892
Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)	'
	Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)
	(Thousands of dollars)
			Three Months Ended									Three Months Ended
			June 30, 2014									June 30, 2013
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of net actuarial (gain)/loss		$	5,914		$	(2,248	)	$	3,666		$	8,545		$	(3,247	)	$	5,298
	Amortization of prior service cost			88			(33	)		55			88			(34	)		54
	Regulatory adjustment			(5,177	)		1,967			(3,210	)		(7,584	)		2,882			(4,702	)
	Pension plans other comprehensive income (loss)			825			(314	)		511			1,049			(399	)		650
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			837			(318	)		519			837			(318	)		519
	FSIRS other comprehensive income			837			(318	)		519			837			(318	)		519
	Total other comprehensive income (loss)		$	1,662		$	(632	)	$	1,030		$	1,886		$	(717	)	$	1,169
			Six Months Ended									Six Months Ended
			30-Jun-14									30-Jun-13
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Amortization of net actuarial (gain)/loss		$	11,828		$	(4,495	)	$	7,333		$	17,090		$	(6,494	)	$	10,596
	Amortization of prior service cost			177			(67	)		110			177			(68	)		109
	Regulatory adjustment			(10,355	)		3,935			(6,420	)		(15,168	)		5,764			(9,404	)
	Pension plans other comprehensive income (loss)			1,650			(627	)		1,023			2,099			(798	)		1,301
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			1,673			(636	)		1,037			1,673			(636	)		1,037
	FSIRS other comprehensive income			1,673			(636	)		1,037			1,673			(636	)		1,037
	Total other comprehensive income (loss)		$	3,323		$	(1,263	)	$	2,060		$	3,772		$	(1,434	)	$	2,338
			Twelve Months Ended									Twelve Months Ended
			30-Jun-14									30-Jun-13
			Before-			Tax			Net-of-			Before-			Tax			Net-of-
			Tax			(Expense)			Tax			Tax			(Expense)			Tax
			Amount			or Benefit (1)			Amount			Amount			or Benefit (1)			Amount
	Defined benefit pension plans:
	Net actuarial gain/(loss)		$	100,345		$	(38,131	)	$	62,214		$	(74,853	)	$	28,444		$	(46,409	)
	Amortization of prior service cost			355			(134	)		221			177			(68	)		109
	Amortization of transition obligation			—			—			—			434			(164	)		270
	Amortization of net actuarial (gain)/loss			28,915			(10,988	)		17,927			29,887			(11,357	)		18,530
	Prior service cost			—			—			—			(2,423	)		921			(1,502	)
	Regulatory adjustment			(118,817	)		45,150			(73,667	)		39,300			(14,934	)		24,366
	Pension plans other comprehensive income (loss)			10,798			(4,103	)		6,695			(7,478	)		2,842			(4,636	)
	FSIRS (designated hedging activities):
	Amounts reclassifed into net income			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	FSIRS other comprehensive income (loss)			3,345			(1,271	)		2,074			3,345			(1,271	)		2,074
	Total other comprehensive income (loss)		$	14,143		$	(5,374	)	$	8,769		$	(4,133	)	$	1,571		$	(2,562	)
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			Tax			After-			Before-			Tax			After-			AOCI
						(Expense)			Tax			Tax			(Expense)			Tax
						Benefit									Benefit
	Beginning Balance AOCI December 31, 2013		$	(41,223	)	$	15,665		$	(25,558	)	$	(26,033	)	$	9,893		$	(16,140	)	$	(41,698	)
	Other comprehensive income before reclassifications			—			—			—			—			—			—			—
	FSIRS amounts reclassified from AOCI (1)			—			—			—			1,673			(636	)		1,037			1,037
	Amortization of prior service cost (2)			177			(67	)		110			—			—			—			110
	Amortization of net actuarial loss (2)			11,828			(4,495	)		7,333			—			—			—			7,333
	Regulatory adjustment (3)			(10,355	)		3,935			(6,420	)		—			—			—			(6,420	)
	Net current period other comprehensive income (loss)			1,650			(627	)		1,023			1,673			(636	)		1,037			2,060
	Ending Balance AOCI June 30, 2014		$	(39,573	)	$	15,038		$	(24,535	)	$	(24,360	)	$	9,257		$	(15,103	)	$	(39,638	)
	-1	The FSIRS reclassification amounts are included in the Net interest deductions line item on the Condensed Consolidated Statements of Income.
	-2	These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details).
	-3	The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Condensed Consolidated Balance Sheets).
Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income	'
	The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:
	Amounts Recognized in AOCI (Before Tax)
	(Thousands of dollars)
			June 30, 2014			December 31, 2013
	Net actuarial (loss) gain		$	(277,313	)	$	(289,141	)
	Prior service cost			(1,890	)		(2,067	)
	Less: amount recognized in regulatory assets			239,630			249,985
	Recognized in AOCI		$	(39,573	)	$	(41,223	)

Nature_of_Operations_and_Basis3

Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Dec. 31, 2013
	Segment		Gas pipe inventory and operating supplies [Member]	Gas pipe inventory and operating supplies [Member]
Significant Accounting Policies [Line Items]	'	'	'	'
Number of segments	2	'	'	'
Gas pipe inventory and operating supplies	'	'	$23	$21
Non-cash construction advances - non-cash investing item	$5.60	$4.80	'	'

Nature_of_Operations_and_Basis4

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) (NPL Construction Co. [Member], USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
NPL Construction Co. [Member]	'	'
Accounts, Notes, Loans and Financing Receivable [Line Items]	'	'
Accounts receivable for NPL services	$10,786	$10,787

Nature_of_Operations_and_Basis5

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Investment [Line Items]	'	'
Net cash surrender value of COLI policies	$96,000,000	$93,000,000
Other property	12,000,000	11,000,000
Total	267,763,000	260,871,000
NPL Construction Co. [Member]	'	'
Investment [Line Items]	'	'
NPL property and equipment	332,000,000	320,000,000
NPL accumulated provision for depreciation and amortization	($172,000,000)	($163,000,000)

Nature_of_Operations_and_Basis6

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Other Income And Expenses [Abstract]	'	'	'	'	'	'
Change in COLI policies	$2,300	$1,800	$3,200	$5,600	$10,000	$8,900
Interest income	612	144	1,109	264	1,306	683
Pipe replacement costs	'	-36	'	-121	-11	-1,759
Miscellaneous income and (expense)	-49	-456	166	-222	-41	-830
Total other income (deductions)	$2,863	$1,452	$4,475	$5,521	$11,254	$6,994

Components_of_Net_Periodic_Ben2

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Qualified Retirement Plan [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	$5,340	$5,764	$10,680	$11,528	$22,208	$21,688
Interest cost	10,860	9,402	21,721	18,803	40,525	37,936
Expected return on plan assets	-13,335	-12,460	-26,671	-24,920	-51,591	-47,810
Amortization of net actuarial loss	5,718	8,065	11,436	16,131	27,566	28,072
Net periodic benefit cost	8,583	10,771	17,166	21,542	38,708	39,886
SERP [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	73	93	146	187	332	324
Interest cost	436	384	872	767	1,640	1,582
Amortization of net actuarial loss	196	243	392	486	877	827
Net periodic benefit cost	705	720	1,410	1,440	2,849	2,733
PBOP [Member]	'	'	'	'	'	'
Defined Benefit Plan Disclosure [Line Items]	'	'	'	'	'	'
Service cost	275	305	551	610	1,161	1,098
Interest cost	708	621	1,415	1,241	2,656	2,515
Expected return on plan assets	-816	-706	-1,632	-1,412	-3,044	-2,614
Amortization of prior service cost	88	88	177	177	355	177
Amortization of transition obligation	'	'	'	'	'	434
Amortization of net actuarial loss	'	237	'	473	472	988
Net periodic benefit cost	$255	$545	$511	$1,089	$1,600	$2,598

Segment_Information_Schedule_o

Segment Information - Schedule of Segment Reporting Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	$453,153	$411,574	$1,061,549	$1,025,079	$1,987,252	$1,885,444
Segment net income	9,627	10,108	80,410	90,881	134,849	148,969
Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	428,445	389,303	1,013,091	987,769	1,887,951	1,800,254
Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	24,708	22,271	48,458	37,310	99,301	85,190
Natural Gas Operations [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	271,479	238,869	757,972	732,469	1,325,657	1,267,567
Segment net income	1,798	1,964	74,397	81,256	117,310	121,877
Natural Gas Operations [Member] | Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	271,479	238,869	757,972	732,469	1,325,657	1,267,567
Natural Gas Operations [Member] | Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	'	'	'	'	'	'
Construction Services [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Total operating revenues	181,674	172,705	303,577	292,610	661,595	617,877
Segment net income	7,829	8,144	6,013	9,625	17,539	27,092
Construction Services [Member] | Revenues from external customers [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	156,966	150,434	255,119	255,300	562,294	532,687
Construction Services [Member] | Intersegment revenues [Member]	'	'	'	'	'	'
Segment Reporting Information [Line Items]	'	'	'	'	'	'
Revenues	$24,708	$22,271	$48,458	$37,310	$99,301	$85,190

Derivatives_and_Fair_Value_Mea2

Derivatives and Fair Value Measurements - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative [Line Items]	'	'	'	'	'	'
Natural gas portfolios, maximum % rate	'	'	35.00%	'	'	'
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$519,000	$519,000	$1,037,000	$1,037,000	$2,074,000	$2,074,000
Derivative instrument loss at settlement amortization period	'	'	'10 years	'	'	'
Minimum [Member]	'	'	'	'	'	'
Derivative [Line Items]	'	'	'	'	'	'
Maturities of natural gas swaps	'	'	1-Jul-14	'	'	'
Maximum [Member]	'	'	'	'	'	'
Derivative [Line Items]	'	'	'	'	'	'
Maturities of natural gas swaps	'	'	1-Mar-16	'	'	'
Cash flow hedging [Member]	'	'	'	'	'	'
Derivative [Line Items]	'	'	'	'	'	'
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	'	'	0	'	'	'
Settlement at maturity of the first FSIRS [Member]	'	'	'	'	'	'
Derivative [Line Items]	'	'	'	'	'	'
Agreement settlement paid	'	'	11,700,000	'	'	'
Settlement at maturity of the second FSIRS [Member]	'	'	'	'	'	'
Derivative [Line Items]	'	'	'	'	'	'
Agreement settlement paid	'	'	$21,800,000	'	'	'

Derivatives_and_Fair_Value_Mea3

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail)	Jun. 30, 2014	Dec. 31, 2013
	MMBTU	MMBTU
Offsetting [Abstract]	'	'
Contract notional amounts	7,658,000	13,571,000

Derivatives_and_Fair_Value_Mea4

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	$0	$0	$0	$0	$0	$0
Cash flow hedging [Member] | Net cost of gas sold [Member]	'	'	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	-83	-7,669	5,907	-2,593	9,476	-2,885
Regulatory deferral accounting treatment [Member] | Cash flow hedging [Member] | Net cost of gas sold [Member]	'	'	'	'	'	'
Derivative Instruments, Gain (Loss) [Line Items]	'	'	'	'	'	'
Amount of gain or (loss) recognized in income on derivative	$83	$7,669	($5,907)	$2,593	($9,476)	$2,885

Derivatives_and_Fair_Value_Mea5

Derivatives and Fair Value Measurements - Fair Values of Swaps in Condensed Consolidated Balance Sheets (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	$2,367	$1,444
Liability derivatives not designated as hedging instruments	-140	-623
Net total not designated as hedging instruments	2,227	821
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	483	257
Liability derivatives not designated as hedging instruments	0	-77
Net total not designated as hedging instruments	483	180
Swaps [Member] | Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	1,884	1,054
Liability derivatives not designated as hedging instruments	-140	-253
Net total not designated as hedging instruments	1,744	801
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	'	126
Liability derivatives not designated as hedging instruments	'	-282
Net total not designated as hedging instruments	'	-156
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Asset derivatives not designated as hedging instruments	'	7
Liability derivatives not designated as hedging instruments	'	-11
Net total not designated as hedging instruments	'	($4)

Derivatives_and_Fair_Value_Mea6

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]	'	'	'
Paid to counterparties	'	$15	$2,291
Received from counterparties	$196	$4,515	$4,522

Derivatives_and_Fair_Value_Mea7

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) (Swaps [Member], USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	($483)	($180)
Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	-1,744	-801
Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	'	156
Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Net Total	'	$4

Derivatives_and_Fair_Value_Mea8

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) (Level 2 - Significant other observable inputs [Member], USD $)	Jun. 30, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Derivatives, Fair Value [Line Items]	'	'
Net Assets (Liabilities)	$2,227	$821
Swaps [Member] | Prepaids and other current assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Assets	1,744	801
Swaps [Member] | Deferred charges and other assets [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Assets	483	180
Swaps [Member] | Other current liabilities [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Liabilities	'	-156
Swaps [Member] | Other deferred credits [Member]	'	'
Derivatives, Fair Value [Line Items]	'	'
Derivative Liabilities	'	($4)

LongTerm_Debt_Schedule_of_Carr

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) (USD $)

Jun. 30, 2014

Dec. 31, 2013

Mar. 31, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

In Thousands, unless otherwise specified

Notes, 6.1%, due 2041 [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Bonds [Member]

Bonds [Member]

NPL credit facility [Member]

NPL other debt obligations [Member]

NPL other debt obligations [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

NPL credit facility [Member]

NPL other debt obligations [Member]

NPL other debt obligations [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 4.875%, due 2043 [Member]

Notes, 4.875%, due 2043 [Member]

8% Series, due 2026 [Member]

8% Series, due 2026 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Notes Payable [Member]

Notes Payable [Member]

Revolving credit facility and commercial paper [Member]

Tax-exempt Series A, due 2028 [Member]

Tax-exempt Series A, due 2028 [Member]

2003 Series A, due 2038 [Member]

2003 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2009 Series A, due 2039 [Member]

2009 Series A, due 2039 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.00% 2004 Series B, due 2033 [Member]

5.00% 2004 Series B, due 2033 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.75% 2006 Series A, due 2036 [Member]

4.75% 2006 Series A, due 2036 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 4.875%, due 2043 [Member]

Notes, 4.875%, due 2043 [Member]

8% Series, due 2026 [Member]

8% Series, due 2026 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Revolving credit facility and commercial paper [Member]

Tax-exempt Series A, due 2028 [Member]

Tax-exempt Series A, due 2028 [Member]

2003 Series A, due 2038 [Member]

2003 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2009 Series A, due 2039 [Member]

2009 Series A, due 2039 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.00% 2004 Series B, due 2033 [Member]

5.00% 2004 Series B, due 2033 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.75% 2006 Series A, due 2036 [Member]

4.75% 2006 Series A, due 2036 [Member]

Debt Instrument [Line Items]

'

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'

'

'

'

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'

'

Notes payable, fair value disclosure

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

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'

$136,964

$130,953

$157,689

$141,873

$266,398

$252,485

$277,840

$257,280

$104,001

$96,263

$28,539

$28,741

$32,116

$30,586

$34,194

$31,497

$9,259

$8,468

'

'

'

'

'

'

'

'

'

'

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'

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'

'

'

'

Line of credit facility, fair value

'

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'

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'

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10,000

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'

'

Unsecured debt, fair value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

50,000

50,000

50,000

50,000

50,000

50,000

50,000

50,000

20,215

20,150

65,041

64,522

31,207

30,284

99,934

95,192

24,911

22,974

NPL credit facility

'

'

'

'

'

'

'

12,529

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

12,529

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

NPL other debt obligations

'

'

'

'

'

'

'

'

36,687

42,213

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

36,851

42,119

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Notes payable

'

'

125,000

'

'

'

'

'

'

'

'

'

125,000

125,000

125,000

125,000

250,000

250,000

250,000

250,000

75,000

75,000

25,000

25,000

25,000

25,000

25,000

25,000

7,500

7,500

902,106

901,940

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Unamortized discount

'

'

'

-2,710

-2,776

'

'

'

'

'

-5,394

-5,560

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Line of credit facility

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Unsecured debt

'

'

'

'

'

438,345

438,279

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

50,000

50,000

50,000

50,000

50,000

50,000

50,000

50,000

20,000

20,000

65,000

65,000

31,200

31,200

100,000

100,000

24,855

24,855

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Long-term debt, gross

'

'

'

1,389,667

1,392,432

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Less: current maturities

-23,741

-11,105

'

-23,741

-11,105

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Long-term debt, less current maturities

1,365,926

1,381,327

'

1,365,926

1,381,327

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Long-term debt, gross

'

'

'

$1,389,667

$1,392,432

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

LongTerm_Debt_Schedule_of_Carr1

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)

Mar. 31, 2014

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Notes, 6.1%, due 2041 [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Debentures [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Variable-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Fixed-rate bonds [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Carrying Amount [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Market Value [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 4.875%, due 2043 [Member]

Notes, 4.875%, due 2043 [Member]

8% Series, due 2026 [Member]

8% Series, due 2026 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 4.45%, due 2020 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 6.1%, due 2041 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 3.875%, due 2022 [Member]

Notes, 4.875%, due 2043 [Member]

Notes, 4.875%, due 2043 [Member]

8% Series, due 2026 [Member]

8% Series, due 2026 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.59% series, due 2017 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.78% series, due 2022 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 7.92% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Medium-term notes, 6.76% series, due 2027 [Member]

Tax-exempt Series A, due 2028 [Member]

Tax-exempt Series A, due 2028 [Member]

2003 Series A, due 2038 [Member]

2003 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2009 Series A, due 2039 [Member]

2009 Series A, due 2039 [Member]

Tax-exempt Series A, due 2028 [Member]

Tax-exempt Series A, due 2028 [Member]

2003 Series A, due 2038 [Member]

2003 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2008 Series A, due 2038 [Member]

2009 Series A, due 2039 [Member]

2009 Series A, due 2039 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.00% 2004 Series B, due 2033 [Member]

5.00% 2004 Series B, due 2033 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.75% 2006 Series A, due 2036 [Member]

4.75% 2006 Series A, due 2036 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2003 Series D, due 2038 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.25% 2004 Series A, due 2034 [Member]

5.00% 2004 Series B, due 2033 [Member]

5.00% 2004 Series B, due 2033 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.85% 2005 Series A, due 2035 [Member]

4.75% 2006 Series A, due 2036 [Member]

4.75% 2006 Series A, due 2036 [Member]

Debt Instrument [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Debt instrument interest rate

6.10%

4.45%

4.45%

6.10%

6.10%

3.88%

3.88%

4.88%

4.88%

8.00%

8.00%

7.59%

7.59%

7.78%

7.78%

7.92%

7.92%

6.76%

6.76%

4.45%

4.45%

6.10%

6.10%

3.88%

3.88%

4.88%

4.88%

8.00%

8.00%

7.59%

7.59%

7.78%

7.78%

7.92%

7.92%

6.76%

6.76%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

5.25%

5.25%

5.25%

5.25%

5.00%

5.00%

4.85%

4.85%

4.75%

4.75%

5.25%

5.25%

5.25%

5.25%

5.00%

5.00%

4.85%

4.85%

4.75%

4.75%

Debt instrument due date

'

1-Dec-20

1-Dec-20

15-Feb-41

15-Feb-41

1-Apr-22

1-Apr-22

1-Oct-43

1-Oct-43

1-Aug-26

1-Aug-26

17-Jan-17

17-Jan-17

3-Feb-22

3-Feb-22

4-Jun-27

4-Jun-27

24-Sep-27

24-Sep-27

1-Dec-20

1-Dec-20

15-Feb-41

15-Feb-41

1-Apr-22

1-Apr-22

1-Oct-43

1-Oct-43

1-Aug-26

1-Aug-26

17-Jan-17

17-Jan-17

3-Feb-22

3-Feb-22

4-Jun-27

4-Jun-27

24-Sep-27

24-Sep-27

1-Dec-28

1-Dec-28

1-Mar-38

1-Mar-38

1-Mar-38

1-Mar-38

1-Dec-39

1-Dec-39

1-Dec-28

1-Dec-28

1-Mar-38

1-Mar-38

1-Mar-38

1-Mar-38

1-Dec-39

1-Dec-39

1-Dec-38

1-Dec-38

1-Jul-34

1-Jul-34

1-Dec-33

1-Dec-33

1-Oct-35

1-Oct-35

1-Sep-36

1-Sep-36

1-Dec-38

1-Dec-38

1-Jul-34

1-Jul-34

1-Dec-33

1-Dec-33

1-Oct-35

1-Oct-35

1-Sep-36

1-Sep-36

LongTerm_Debt_Additional_Infor

Long-Term Debt - Additional Information (Detail) (USD $)	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Mar. 31, 2014	Jun. 30, 2014	Jun. 30, 2014
		Two Thousand And Nineteen [Member]	Notes, 6.1%, due 2041 [Member]	Notes, 6.1%, due 2041 [Member]	LIBOR [Member]	Alternative base rate [Member]
				Maximum [Member]	Southwest Gas Credit Facility [Member]	Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]	'	'	'	'	'	'
Line of credit designated as long term debt	$150,000,000	'	'	'	'	'
Line of credit designated for working capital purposes	150,000,000	'	'	'	'	'
Credit facility maximum borrowing capacity	'	300,000,000	'	'	'	'
Credit facility expiration date	'	1-Mar-19	'	'	'	'
Applicable margin	'	'	'	'	1.00%	0.00%
Debt instrument interest rate	'	'	6.10%	'	'	'
Notes payable	'	'	$125,000,000	'	'	'
Percentage limit on liens securing indebtedness	'	'	'	10.00%	'	'

Equity_Other_Comprehensive_Inc2

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, except Share data, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	$1,412,395	'	'	'
Balance, shares	'	'	46,356,125	'	'	'
Common stock issuances	'	'	4,477	'	'	'
Net income (loss)	9,627	10,067	80,324	90,741	134,456	148,433
Other comprehensive income (loss):	'	'	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	511	650	1,023	1,301	6,695	-4,636
Amounts reclassified into net income	519	519	1,037	1,037	2,074	2,074
Dividends declared Common: $0.73 per share	'	'	-34,364	'	'	'
Balance	1,464,892	'	1,464,892	'	1,464,892	'
Balance, shares	46,503,111	'	46,503,111	'	46,503,111	'
Common Stock [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	47,986	'	'	'
Balance, shares	'	'	46,356,000	'	'	'
Common stock issuances	'	'	147	'	'	'
Common stock issuances, shares	'	'	147,000	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	48,133	'	48,133	'	48,133	'
Balance, shares	46,503,000	'	46,503,000	'	46,503,000	'
Additional Paid-in Capital [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	840,521	'	'	'
Common stock issuances	'	'	4,330	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	844,851	'	844,851	'	844,851	'
Accumulated Other Comprehensive Income (Loss) [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	-41,698	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	'	'	1,023	'	'	'
Amounts reclassified into net income	'	'	1,037	'	'	'
Balance	-39,638	'	-39,638	'	-39,638	'
Retained Earnings [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	567,714	'	'	'
Net income (loss)	'	'	80,410	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Dividends declared Common: $0.73 per share	'	'	-34,364	'	'	'
Balance	613,760	'	613,760	'	613,760	'
Non-controlling Interest [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Balance	'	'	-2,128	'	'	'
Net income (loss)	'	'	-86	'	'	'
Other comprehensive income (loss):	'	'	'	'	'	'
Balance	($2,214)	'	($2,214)	'	($2,214)	'

Equity_Other_Comprehensive_Inc3

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Parenthetical) (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement Of Stockholders Equity [Abstract]	'	'	'	'	'	'
Dividends declared per share	$0.37	$0.33	$0.73	$0.66	$1.39	$1.25

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		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Equity [Abstract]	'	'	'	'	'	'
Amortization of net actuarial (gain)/loss, Before-Tax Amount	$5,914	$8,545	$11,828	$17,090	$28,915	$29,887
Amortization of prior service cost, Before-Tax Amount	88	88	177	177	355	177
Regulatory adjustment, Before-Tax Amount	-5,177	-7,584	-10,355	-15,168	-118,817	39,300
Pension plans other comprehensive income (loss), Before-Tax Amount	825	1,049	1,650	2,099	10,798	-7,478
Amounts reclassified into net income, Before-Tax Amount	837	837	1,673	1,673	3,345	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	837	837	1,673	1,673	3,345	3,345
Total other comprehensive income (loss), Before-Tax Amount	1,662	1,886	3,323	3,772	14,143	-4,133
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	-2,248	-3,247	-4,495	-6,494	-10,988	-11,357
Amortization of prior service cost, Tax (Expense) or Benefit	-33	-34	-67	-68	-134	-68
Regulatory adjustment, Tax (Expense) or Benefit	1,967	2,882	3,935	5,764	45,150	-14,934
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	-314	-399	-627	-798	-4,103	2,842
Amounts reclassified into net income, Tax (Expense) or Benefit	-318	-318	-636	-636	-1,271	-1,271
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	-318	-318	-636	-636	-1,271	-1,271
Total other comprehensive income (loss), Tax (Expense) or Benefit	-632	-717	-1,263	-1,434	-5,374	1,571
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	3,666	5,298	7,333	10,596	17,927	18,530
Amortization of prior service cost, Net-of-Tax Amount	55	54	110	109	221	109
Regulatory adjustment, Net-of-Tax Amount	-3,210	-4,702	-6,420	-9,404	-73,667	24,366
Pension plans other comprehensive income (loss), Net-of-Tax Amount	511	650	1,023	1,301	6,695	-4,636
Amounts reclassified into net income	519	519	1,037	1,037	2,074	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	519	1,037	1,037	2,074	2,074
Total other comprehensive income (loss), Net-of-Tax Amount	1,030	1,169	2,060	2,338	8,769	-2,562
Net actuarial gain/(loss), Before-Tax Amount	'	'	'	'	100,345	-74,853
Amortization of transition obligation, Before-Tax Amount	'	'	'	'	'	434
Prior service cost, Before-Tax Amount	'	'	'	'	'	-2,423
Net actuarial gain/(loss), Tax (Expense) or Benefit	'	'	'	'	-38,131	28,444
Amortization of transition obligation, Tax (Expense) or Benefit	'	'	'	'	'	-164
Prior service cost, Tax (Expense) or Benefit	'	'	'	'	'	921
Net actuarial gain/(loss), Net-of-Tax Amount	'	'	'	'	62,214	-46,409
Amortization of transition obligation, Net-of-Tax Amount	'	'	'	'	'	270
Prior service cost, Net-of-Tax Amount	'	'	'	'	'	($1,502)

Equity_Other_Comprehensive_Inc5

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	6 Months Ended
	Jun. 30, 2014
Equity [Abstract]	'
Other comprehensive income loss, effective income tax rate	38.00%

Equity_Other_Comprehensive_Inc6

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) (USD $)	Jun. 30, 2014
In Millions, unless otherwise specified
Equity [Abstract]	'
Amount of FSIRS existing AOCI losses expected to reclassified income in next 12 months	$2.10

Equity_Other_Comprehensive_Inc7

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Before-Tax	'	'	($41,223)	'	'	'
Amortization of prior service cost, Defined Benefit Plans Before-Tax	88	88	177	177	355	177
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	5,914	8,545	11,828	17,090	28,915	29,887
Regulatory adjustment, Defined Benefit Plans Before-Tax	-5,177	-7,584	-10,355	-15,168	-118,817	39,300
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	825	1,049	1,650	2,099	10,798	-7,478
Ending Balance AOCI June 30, 2014, Defined Benefit Plans Before-Tax	-39,573	'	-39,573	'	-39,573	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans Tax (Expense) Benefit	'	'	15,665	'	'	'
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit	-33	-34	-67	-68	-134	-68
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	-2,248	-3,247	-4,495	-6,494	-10,988	-11,357
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	1,967	2,882	3,935	5,764	45,150	-14,934
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	-314	-399	-627	-798	-4,103	2,842
Ending Balance AOCI June 30, 2014 Defined Benefit Plans Tax (Expense) Benefit	15,038	'	15,038	'	15,038	'
Beginning Balance AOCI December 31, 2013, Defined Benefit Plans After-Tax	'	'	-25,558	'	'	'
Amortization of prior service costs, Defined Benefit Plans After-Tax	55	54	110	109	221	109
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	3,666	5,298	7,333	10,596	17,927	18,530
Regulatory adjustment, Defined Benefit Plans After-Tax	'	'	-6,420	'	'	'
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	511	650	1,023	1,301	6,695	-4,636
Ending Balance AOCI June 30, 2014, Defined Benefit Plans After-Tax	-24,535	'	-24,535	'	-24,535	'
Beginning Balance AOCI December 31, 2013, FSIRS - Before-Tax	'	'	-26,033	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	837	837	1,673	1,673	3,345	3,345
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	-318	-318	-636	-636	-1,271	-1,271
Ending Balance AOCI June 30, 2014, FSIRS - Before-Tax	-24,360	'	-24,360	'	-24,360	'
Beginning Balance AOCI December 31, 2013, FSIRS - Tax (Expense) Benefit	'	'	9,893	'	'	'
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	-318	-318	-636	-636	-1,271	-1,271
Ending Balance AOCI June 30, 2014, FSIRS - Tax (Expense) Benefit	9,257	'	9,257	'	9,257	'
Beginning Balance AOCI December 31, 2013, FSIRS - After-Tax	'	'	-16,140	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	519	519	1,037	1,037	2,074	2,074
Net forward-starting interest rate swaps	519	519	1,037	1,037	2,074	2,074
Ending Balance AOCI June 30, 2014, FSIRS - After-Tax	-15,103	'	-15,103	'	-15,103	'
Beginning Balance AOCI December 31, 2013, AOCI	'	'	-41,698	'	'	'
Amounts reclassified into net income	519	519	1,037	1,037	2,074	2,074
Amortization of prior service cost, AOCI	55	54	110	109	221	109
Amortization of net actuarial loss	3,666	5,298	7,333	10,596	17,927	18,530
Regulatory adjustment, AOCI	'	'	-6,420	'	'	'
Net current period other comprehensive income (loss), AOCI	'	'	2,060	'	'	'
Ending Balance AOCI June 30, 2014, AOCI	-39,638	'	-39,638	'	-39,638	'
Reclassification out of Accumulated Other Comprehensive Income [Member]	'	'	'	'	'	'
Schedule Of Stockholders Equity [Line Items]	'	'	'	'	'	'
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit	'	'	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax	'	'	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit	'	'	'	'	'	'
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax	'	'	'	'	'	'
Other comprehensive income before reclassifications, FSIRS - Before-Tax	'	'	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	'	'	1,673	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	'	'	-636	'	'	'
Other comprehensive income before reclassifications, FSIRS - After-Tax	'	'	'	'	'	'
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	'	'	1,037	'	'	'
Other comprehensive income before reclassifications, AOCI	'	'	'	'	'	'
Amounts reclassified into net income	'	'	$1,037	'	'	'

Equity_Other_Comprehensive_Inc8

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income (Detail) (USD $)	12 Months Ended			6 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Jun. 30, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Dec. 31, 2013
				Defined Benefit Plans [Member]	Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]	'	'	'	'	'
Net actuarial (loss) gain	'	'	'	($277,313)	($289,141)
Prior service cost	2,423	'	'	-1,890	-2,067
Less: amount recognized in regulatory assets	'	'	'	239,630	249,985
Recognized in AOCI	'	($39,573)	($41,223)	($39,573)	($41,223)

Purchase_of_Corporate_Headquar1

Purchase of Corporate Headquarters Office Complex - Additional Information (Detail) (Subsequent Event [Member], USD $)	1 Months Ended
In Millions, unless otherwise specified	Jul. 31, 2014
Subsequent Event [Member]	'
Operating Leased Assets [Line Items]	'
Purchase of Corporate Headquarters Office Complex	$16.50