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Document and Entity Information - shares	6 Months Ended
Document and Entity Information - shares	Jun. 30, 2013	Jul. 29, 2013
Document Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Jun. 30, 2013
Document Fiscal Year Focus	2013
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		46,336,769

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Utility plant:
Gas plant	$ 5,112,488	$ 5,019,500
Less: accumulated depreciation	(1,809,514)	(1,750,795)
Acquisition adjustments, net	821	911
Construction work in progress	83,172	74,178
Net utility plant	3,386,967	3,343,794
Other property and investments	252,467	242,096
Current assets:
Cash and cash equivalents	17,720	25,530
Accounts receivable, net of allowances	160,264	196,913
Accrued utility revenue	30,400	72,000
Income taxes receivable, net	1,863	2,945
Deferred income taxes	10,476	47,088
Deferred purchased gas costs	11	6,031
Prepaids and other current assets	85,110	107,910
Total current assets	305,844	458,417
Deferred charges and other assets	436,776	443,750
Total assets	4,382,054	4,488,057
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 46,334,769 and 46,147,788 shares)	47,965	47,778
Additional paid-in capital	834,129	828,777
Accumulated other comprehensive income (loss), net	(48,407)	(50,745)
Retained earnings	544,300	484,369
Total Southwest Gas Corporation equity	1,377,987	1,310,179
Noncontrolling interest	(1,821)	(1,681)
Total equity	1,376,166	1,308,498
Long-term debt, less current maturities	1,256,338	1,268,373
Total capitalization	2,632,504	2,576,871
Current liabilities:
Current maturities of long-term debt	11,000	50,137
Accounts payable	105,199	155,667
Customer deposits	76,381	77,858
Accrued general taxes	34,243	37,644
Accrued interest	15,377	16,080
Deferred purchased gas costs	28,561	98,957
Other current liabilities	103,940	98,786
Total current liabilities	374,701	535,129
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	626,786	616,184
Taxes payable	418	551
Accumulated removal costs	268,000	256,000
Other deferred credits	479,645	503,322
Total deferred income taxes and other credits	1,374,849	1,376,057
Total capitalization and liabilities	$ 4,382,054	$ 4,488,057

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares shares in Thousands	Jun. 30, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	46,334,769	46,147,788
Common stock, outstanding	46,334,769	46,147,788

CONDENSED CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Operating revenues:
Gas operating revenues	$ 238,869	$ 255,917	$ 732,469	$ 786,630	$ 1,267,567	$ 1,362,729
Construction revenues	172,705	153,851	292,610	280,783	617,877	574,927
Total operating revenues	411,574	409,768	1,025,079	1,067,413	1,885,444	1,937,656
Operating expenses:
Net cost of gas sold	69,388	91,959	269,996	334,706	414,892	547,334
Operations and maintenance	94,935	91,884	192,022	187,734	374,267	366,574
Depreciation and amortization	58,570	55,537	117,503	109,700	231,225	211,892
Taxes other than income taxes	11,073	9,734	22,868	20,465	44,131	41,249
Construction expenses	148,700	145,274	255,388	264,805	532,106	517,427
Total operating expenses	382,666	394,388	857,777	917,410	1,596,621	1,684,476
Operating income	28,908	15,380	167,302	150,003	288,823	253,180
Other income and (expenses):
Net interest deductions	(15,290)	(18,252)	(31,168)	(35,411)	(63,777)	(69,699)
Other income (deductions)	1,452	(2,490)	5,521	2,938	6,994	(3,747)
Total other income and (expenses)	(13,838)	(20,742)	(25,647)	(32,473)	(56,783)	(73,446)
Income (loss) before income taxes	15,070	(5,362)	141,655	117,530	232,040	179,734
Income tax expense (benefit)	5,003	(1,474)	50,914	42,583	83,607	65,391
Net income (loss)	10,067	(3,888)	90,741	74,947	148,433	114,343
Net income (loss) attributable to noncontrolling interest	(41)	(212)	(140)	(296)	(536)	(583)
Net income (loss) attributable to Southwest Gas Corporation	$ 10,108	$ (3,676)	$ 90,881	$ 75,243	$ 148,969	$ 114,926
Basic earnings (loss) per share	$ 0.22	$ (0.08)	$ 1.96	$ 1.63	$ 3.22	$ 2.50
Diluted earnings (loss) per share	0.22	(0.08)	1.95	1.62	3.19	2.48
Dividends declared per share	$ 0.330	$ 0.295	$ 0.660	$ 0.590	$ 1.250	$ 1.120
Average number of common shares outstanding	46,331	46,114	46,291	46,091	46,214	45,996
Average shares outstanding (assuming dilution)	46,757		46,704	46,504	46,654	46,423

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ 10,067	$ (3,888)	$ 90,741	$ 74,947	$ 148,433	$ 114,343
Defined benefit pension plans:
Net actuarial gain (loss)					(46,409)	(84,005)
Amortization of transition obligation		134		268	270	536
Amortization of net actuarial loss	5,298	3,968	10,596	7,936	18,530	12,763
Prior service cost	54		109		(1,393)
Regulatory adjustment	(4,702)	(3,626)	(9,404)	(7,252)	24,366	62,880
Net defined benefit pension plans	650	476	1,301	952	(4,636)	(7,826)
Forward-starting interest rate swaps:
Unrealized/realized gain (loss)				1,834		(7,350)
Amounts reclassified into net income	519	519	1,037	700	2,074	1,062
Net forward-starting interest rate swaps	519	519	1,037	2,534	2,074	(6,288)
Total other comprehensive income (loss), net of tax	1,169	995	2,338	3,486	(2,562)	(14,114)
Comprehensive income (loss)	11,236	(2,893)	93,079	78,433	145,871	100,229
Comprehensive income (loss) attributable to noncontrolling interest	(41)	(212)	(140)	(296)	(536)	(583)
Comprehensive income (loss) attributable to Southwest Gas Corporation	$ 11,277	$ (2,681)	$ 93,219	$ 78,729	$ 146,407	$ 100,812

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 90,741	$ 74,947	$ 148,433	$ 114,343
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	117,503	109,700	231,225	211,892
Deferred income taxes	45,781	38,237	73,824	59,538
Changes in current assets and liabilities:
Accounts receivable, net of allowances	36,649	54,418	(5,436)	(32,963)
Accrued utility revenue	41,600	41,100	(1,200)	2,500
Deferred purchased gas costs	(64,376)	46,771	(88,324)	18,061
Accounts payable	(53,924)	(102,668)	22,746	(16,766)
Accrued taxes	(2,452)	(6,439)	4,100	(1,421)
Other current assets and liabilities	24,088	28,344	(23,204)	11,455
Gains on sale	(2,285)	(3,294)	(7,031)	(5,297)
Changes in undistributed stock compensation	3,855	3,153	5,839	5,618
AFUDC and property-related changes	(1,002)	(714)	(2,231)	(1,550)
Changes in other assets and deferred charges	(16,393)	(9,178)	(22,582)	16,116
Changes in other liabilities and deferred credits	(4,564)	(12,922)	3,931	(56,315)
Net cash provided by operating activities	215,221	261,455	340,090	325,211
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(151,877)	(176,382)	(371,207)	(418,270)
Restricted cash		12,785		37,782
Changes in customer advances	3,127	(13,798)	13,900	(19,754)
Miscellaneous inflows	4,905	5,102	13,766	10,171
Miscellaneous outflows		(1,035)	(969)	(1,035)
Net cash used in investing activities	(143,845)	(173,328)	(344,510)	(391,106)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	1,357	1,312	1,626	3,986
Dividends paid	(28,947)	(25,818)	(56,169)	(50,145)
Interest rate swap settlement		(21,754)		(21,754)
Issuance of long-term debt, net	41,673	400,804	130,387	510,518
Retirement of long-term debt	(101,269)	(323,043)	(205,269)	(430,109)
Change in credit facility and commercial paper	8,000	(109,000)	119,000
Net cash provided by (used in) financing activities	(79,186)	(77,499)	(10,425)	12,496
Change in cash and cash equivalents	(7,810)	10,628	(14,845)	(53,399)
Cash and cash equivalents at beginning of period	25,530	21,937	32,565	85,964
Cash and cash equivalents at end of period	17,720	32,565	17,720	32,565
Supplemental information:
Interest paid, net of amounts capitalized	29,116	56,452	60,103	89,275
Income taxes paid	$ 3,531	$ 1,766	$ 4,608	$ 2,722

Nature of Operations and Basis of Presentation

6 Months Ended

Jun. 30, 2013

Accounting Policies [Abstract]

Nature of Operations and Basis of Presentation

Note 1 – Nature of Operations and Basis of Presentation

Nature of Operations. Southwest Gas Corporation and its subsidiaries (the “Company”) consist of two segments: natural gas operations (“Southwest” or the “natural gas operations” segment) and construction services. Southwest is engaged in the business of purchasing, distributing, and transporting natural gas for customers in portions of Arizona, Nevada, and California. Public utility rates, practices, facilities, and service territories of Southwest are subject to regulatory oversight. The timing and amount of rate relief can materially impact results of operations. Natural gas sales are seasonal, peaking during the winter months; therefore, results of operations for interim periods are not necessarily indicative of results for a full year. Natural gas purchases and the timing of related recoveries can materially impact liquidity. NPL Construction Co. (“NPL” or the “construction services” segment), a wholly owned subsidiary, is a full-service underground piping contractor that primarily provides utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems. Typically, NPL revenues are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating revenues typically improve as more favorable weather conditions occur during the summer and fall months.

Basis of Presentation. The condensed consolidated financial statements included herein have been prepared by the Company, without audit, pursuant to the rules and regulations of the Securities and Exchange Commission (the “SEC”). Certain information and footnote disclosures normally included in financial statements prepared in accordance with generally accepted accounting principles (“GAAP”) have been condensed or omitted pursuant to such rules and regulations. The preparation of the condensed consolidated financial statements requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. In the opinion of management, all adjustments, consisting of normal recurring items and estimates necessary for a fair presentation of results for the interim periods, have been made. It is suggested that these condensed consolidated financial statements be read in conjunction with the consolidated financial statements and the notes thereto included in the 2012 Annual Report to Shareholders, which is incorporated by reference into the 2012 Form 10-K, and the first quarter 2013 Form 10-Q.

Prepaids and other current assets. Prepaids and other current assets includes plant materials and operating supplies of $25 million at both June 30, 2013 and December 31, 2012.

Cash and Cash Equivalents. For purposes of reporting consolidated cash flows, cash and cash equivalents include cash on hand and financial instruments with a purchase-date maturity of three months or less. Cash and cash equivalents fall within Level 1 (quoted prices for identical financial instruments) of the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. During the six months ended June 30, 2013, approximately $4.8 million of customer advances, upon contract expiration, were applied as contributions toward utility construction activity and represent a non-cash investing activity.

Intercompany Transactions. NPL recognizes revenues generated from contracts with Southwest (see Note 3 - Segment Information below). Accounts receivable for these services are presented in the table below (thousands of dollars):


	June 30, 2013		December 31, 2012

Accounts receivable for NPL services	$	9,136	$	8,179

The accounts receivable balance, revenues, and associated profits are included in the condensed consolidated financial statements of the Company and were not eliminated during consolidation in accordance with accounting treatment for rate-regulated entities.

Other Property and Investments. Other property and investments includes (millions of dollars):


	June 30, 2013		December 31, 2012

NPL property and equipment	$	305	$	287
NPL accumulated provision for depreciation and amortization		(149)		(136)
Net cash surrender value of COLI policies		86		80
Other property		10		11

Total	$	252	$	242

Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):


	Three Months Ended				Six Months Ended				Twelve Months Ended
	June 30				June 30				June 30
	2013		2012		2013		2012		2013		2012

Change in COLI policies	$	1,800	$	(1,900)	$	5,600	$	3,300	$	8,900	$	(800)
Interest income		144		279		264		505		683		800
Pipe replacement costs		(36)		(1,033)		(121)		(1,042)		(1,759)		(3,956)
Miscellaneous income and (expense)		(456)		164		(222)		175		(830)		209

Total other income (deductions)	$	1,452	$	(2,490)	$	5,521	$	2,938	$	6,994	$	(3,747)

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.

- Definition

The entire disclosure for organization, consolidation and basis of presentation of financial statements disclosure.

+ References

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+ Details

Components of Net Periodic Benefit Cost

6 Months Ended

Jun. 30, 2013

Text Block [Abstract]

Components of Net Periodic Benefit Cost

Note 2 – Components of Net Periodic Benefit Cost

Southwest has a noncontributory qualified retirement plan with defined benefits covering substantially all employees and a separate unfunded supplemental retirement plan (“SERP”) which is limited to officers. Southwest also provides postretirement benefits other than pensions (“PBOP”) to its qualified retirees for health care, dental, and life insurance.

Net periodic benefit costs included in the table below are components of an overhead loading process associated with the cost of labor. The overhead process ultimately results in allocation of net periodic benefit costs to the same accounts to which productive labor is charged. As a result, net periodic benefit costs become components of various accounts, primarily operations and maintenance expense, net utility plant, and deferred charges and other assets.


	Qualified Retirement Plan
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	5,764	$	5,080	$	11,528	$	10,159	$	21,688	$	19,022
Interest cost		9,402		9,566		18,803		19,133		37,936		37,771
Expected return on plan assets		(12,460)		(11,445)		(24,920)		(22,890)		(47,810)		(42,947)
Amortization of net actuarial loss		8,065		5,971		16,131		11,942		28,072		19,116

Net periodic benefit cost	$	10,771	$	9,172	$	21,542	$	18,344	$	39,886	$	32,962


	SERP
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	93	$	68	$	187	$	137	$	324	$	245
Interest cost		384		407		767		814		1,582		1,697
Amortization of net actuarial loss		243		171		486		342		827		658

Net periodic benefit cost	$	720	$	646	$	1,440	$	1,293	$	2,733	$	2,600


	PBOP
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	305	$	244	$	610	$	489	$	1,098	$	918
Interest cost		621		637		1,241		1,273		2,515		2,588
Expected return on plan assets		(706)		(601)		(1,412)		(1,202)		(2,614)		(2,391)
Amortization of prior service cost		88		-		177		-		177		-
Amortization of transition obligation		-		216		-		433		434		867
Amortization of net actuarial loss		237		258		473		516		988		811

Net periodic benefit cost	$	545	$	754	$	1,089	$	1,509	$	2,598	$	2,793

Segment Information

6 Months Ended

Jun. 30, 2013

Segment Reporting [Abstract]

Segment Information

Note 3 – Segment Information

The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):


	Natural Gas Operations		Construction Services		Total
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


Three months ended June 30, 2012
Revenues from external customers	$	255,917	$	134,402	$	390,319
Intersegment revenues		-		19,449		19,449

Total	$	255,917	$	153,851	$	409,768

Segment net income (loss)	$	(3,368)	$	(308)	$	(3,676)


	Natural Gas Operations		Construction Services		Total
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


Six months ended June 30, 2012
Revenues from external customers	$	786,630	$	245,289	$	1,031,919
Intersegment revenues		-		35,494		35,494

Total	$	786,630	$	280,783	$	1,067,413

Segment net income (loss)	$	75,998	$	(755)	$	75,243


	Natural Gas Operations		Construction Services		Total
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


Twelve months ended June 30, 2012
Revenues from external customers	$	1,362,729	$	479,051	$	1,841,780
Intersegment revenues		-		95,876		95,876

Total	$	1,362,729	$	574,927	$	1,937,656

Segment net income	$	99,204	$	15,722	$	114,926

- Definition

The entire disclosure for reporting segments including data and tables. Reportable segments include those that meet any of the following quantitative thresholds a) it's reported revenue, including sales to external customers and intersegment sales or transfers is 10 percent or more of the combined revenue, internal and external, of all operating segments b) the absolute amount of its reported profit or loss is 10 percent or more of the greater, in absolute amount of 1) the combined reported profit of all operating segments that did not report a loss or 2) the combined reported loss of all operating segments that did report a loss c) its assets are 10 percent or more of the combined assets of all operating segments.

+ References

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-Publisher FASB
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+ Details

Derivatives and Fair Value Measurements

6 Months Ended

Jun. 30, 2013

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Derivatives and Fair Value Measurements

Note 4 – Derivatives and Fair Value Measurements

Derivatives. In managing its natural gas supply portfolios, Southwest has historically entered into fixed- and variable-price contracts, which qualify as derivatives. Additionally, Southwest utilizes fixed-for-floating swap contracts (“Swaps”) to supplement its fixed-price contracts. The fixed-price contracts, firm commitments to purchase a fixed amount of gas in the future at a fixed price, qualify for the normal purchases and normal sales exception that is allowed for contracts that are probable of delivery in the normal course of business, and are exempt from fair value reporting. The variable-price contracts have no significant market value. The Swaps are recorded at fair value.

The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (currently ranging from 25% to 35%, depending on the jurisdiction) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from July 2013 through March 2015. Under such contracts, Southwest pays the counterparty at a fixed rate and receives from the counterparty a floating rate per MMBtu (“dekatherm”) of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):


	June 30, 2013		December 31, 2012

Contract notional amounts		15,736		14,579

Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations.

The following table sets forth the gains and (losses) recognized on the Company’s Swaps (derivatives) for the three-, six-, and twelve-month periods ended June 30, 2013 and 2012 and their location in the Condensed Consolidated Statements of Income (thousands of dollars):


Gains (losses) recognized in income for derivatives not designated as hedging instruments:
(Thousands of dollars)
	Location of Gain or (Loss) Recognized in Income on Derivative	Three Months Ended						Six Months Ended						Twelve Months Ended
		June 30						June 30						June 30
Instrument		2013			2012			2013			2012			2013			2012

Swaps	Net cost of gas sold	$	(7,669)		$	2,374		$	(2,593)		$	(4,562)		$	(2,885)		$	(20,794)
Swaps	Net cost of gas sold		7,669	*		(2,374)	*		2,593	*		4,562	*		2,885	*		20,794	*

Total		$	-		$	-		$	-		$	-		$	-		$	-

* Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.

In January 2010, Southwest entered into two forward-starting interest rate swaps (“FSIRS”) to hedge the risk of interest rate variability during the period leading up to the planned issuance of fixed-rate debt to replace $200 million of debt that matured in February 2011 and $200 million that matured in May 2012. Both were designated cash flow hedges. The first FSIRS terminated in December 2010 concurrent with the related issuance of $125 million 4.45% 10-year Senior Notes. The second FSIRS had a notional amount of $100 million, and terminated in March 2012 concurrent with the related issuance of $250 million 3.875% 10-year Senior Notes. No gain or loss was recognized in income (ineffective portion) for either FSIRS during any period, including the periods presented in the following table. See Note 6 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income for additional information on both FSIRS contracts.


Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:
(Thousands of dollars)
	Three Months Ended				Six Months Ended				Twelve Months Ended
	June 30				June 30				June 30
	2013		2012		2013		2012		2013		2012
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	$	-	$	-	$	-	$	2,959	$	-	$	(11,854)

The following table sets forth the fair values of the Company’s Swaps and their location in the balance sheets (thousands of dollars):


Fair values of derivatives not designated as hedging instruments:

June 30, 2013 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives		Net Total
Swaps	Other current liabilities	$	42	$	(4,340)	$	(4,298)
Swaps	Other deferred credits		-		(719)		(719)

Total		$	42	$	(5,059)	$	(5,017)


December 31, 2012 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives		Net Total
Swaps	Deferred charges and other assets	$	132	$	(126)	$	6
Swaps	Other current liabilities		391		(2,467)		(2,076)
Swaps	Other deferred credits		233		(552)		(319)

Total		$	756	$	(3,145)	$	(2,389)

The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). The Company has master netting arrangements with each counterparty that provide for the net settlement of all contracts through a single payment. As applicable, the Company has elected to reflect the net amounts in its balance sheets. The Company had no outstanding collateral associated with the Swaps during either period shown in the above table.

Pursuant to regulatory deferral accounting treatment for rate-regulated entities, Southwest records the unrealized gains and losses in fair value of the Swaps as a regulatory asset and/or liability. When the Swaps mature, Southwest reverses any prior positions held and records the settled position as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income.

The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.


(Thousands of dollars)	Three Months Ended June 30, 2013		Six Months Ended June 30, 2013		Twelve Months Ended June 30, 2013
Paid to counterparties	$	3	$	872	$	4,546

Received from counterparties	$	543	$	908	$	1,542

The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the balance sheets (thousands of dollars).


June 30, 2013 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	4,298
Swaps	Deferred charges and other assets		719

December 31, 2012 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(6)
Swaps	Prepaids and other current assets		2,076
Swaps	Deferred charges and other assets		319

Fair Value Measurements. The estimated fair values of Southwest’s Swaps were determined at June 30, 2013 and December 31, 2012 using New York Mercantile Exchange (“NYMEX”) futures settlement prices for delivery of natural gas at Henry Hub adjusted by the price of NYMEX ClearPort basis Swaps, which reflect the difference between the price of natural gas at a given delivery basin and the Henry Hub pricing points. These Level 2 inputs (inputs, other than quoted prices, for similar assets or liabilities) are observable in the marketplace throughout the full term of the Swaps, but have been credit-risk adjusted with no significant impact to the overall fair value measure.

The following table sets forth the Company’s Level 2 financial assets and liabilities recorded at fair value:

Level 2 - Significant other observable inputs


(Thousands of dollars)	June 30, 2013		December 31, 2012

Assets at fair value:
Deferred charges and other assets - Swaps	$	-		6

Liabilities at fair value:
Other current liabilities - Swaps		(4,298)		(2,076)
Other deferred credits - Swaps		(719)		(319)


Net Assets (Liabilities)	$	(5,017)	$	(2,389)

No financial assets or liabilities accounted for at fair value fell within Level 1 or Level 3 of the fair value hierarchy.

Long-Term Debt

6 Months Ended

Jun. 30, 2013

Debt Disclosure [Abstract]

Long-Term Debt

Note 5 – Long-Term Debt

Carrying amounts of the Company’s long-term debt and their related estimated fair values as of June 30, 2013 and December 31, 2012 are disclosed in the following table. The fair values of the revolving credit facility including commercial paper, the NPL revolving credit facility, and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, and are categorized as Level 1 (quoted prices for identical financial instruments) within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The market values of debentures (except the 6.1% Notes) and fixed-rate IDRBs are categorized as Level 2. The 6.1% Notes and NPL other debt obligations are categorized as Level 3 (based on significant unobservable inputs to their fair values). Fair values for the debentures, fixed-rate IDRBs, and NPL other debt obligations were determined through a market-based valuation approach, where fair market values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.


	June 30, 2013				December 31, 2012
	Carrying Amount		Market Value		Carrying Amount		Market Value
	Carrying Amount		Market Value		Carrying Amount		Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000	$	134,581	$	125,000	$	141,771
Notes, 6.1%, due 2041		125,000		143,655		125,000		165,779
Notes, 3.875%, due 2022		250,000		261,148		250,000		277,950
8% Series, due 2026		75,000		102,116		75,000		111,501
Medium-term notes, 7.59% series, due 2017		25,000		29,442		25,000		30,710
Medium-term notes, 7.78% series, due 2022		25,000		31,998		25,000		34,637
Medium-term notes, 7.92% series, due 2027		25,000		33,398		25,000		36,953
Medium-term notes, 6.76% series, due 2027		7,500		8,976		7,500		10,058
Unamortized discount		(3,320)				(3,403)

		654,180				654,097

Revolving credit facility and commercial paper		119,000		119,000		111,000		111,000

Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028		50,000		50,000		50,000		50,000
2003 Series A, due 2038		50,000		50,000		50,000		50,000
2008 Series A, due 2038		50,000		50,000		50,000		50,000
2009 Series A, due 2039		50,000		50,000		50,000		50,000
Fixed-rate bonds:
5.55% 1999 Series D, due 2038		8,270		8,314		8,270		8,375
5.45% 2003 Series C, due 2038		-		-		30,000		30,152
5.25% 2003 Series D, due 2038		20,000		20,436		20,000		20,571
5.80% 2003 Series E, due 2038		-		-		15,000		15,102
5.25% 2004 Series A, due 2034		65,000		63,500		65,000		66,955
5.00% 2004 Series B, due 2033		31,200		30,300		31,200		31,655
4.85% 2005 Series A, due 2035		100,000		94,660		100,000		101,184
4.75% 2006 Series A, due 2036		24,855		23,588		24,855		25,189
Unamortized discount		(2,854)				(3,195)

		446,471				491,130

NPL credit facility		-		-		41,562		41,562
NPL other debt obligations		47,687		47,454		20,721		20,991

		1,267,338				1,318,510
Less: current maturities		(11,000)				(50,137)

Long-term debt, less current maturities	$	1,256,338			$	1,268,373

In February 2013, a notice of mandatory tender was sent to holders of the Clark County, Nevada 5.45% Series 2003C and 5.80% Series 2003E IDRBs. These IDRBs (totaling $45 million) were subject to mandatory tender on March 1, 2013 at a price of 100% plus accrued interest, and the Company tendered these IDRBs to the trustee for cancellation immediately following the mandatory tender, thereby extinguishing this debt. The Company facilitated the redemption primarily from borrowings under its $300 million credit facility.

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income

6 Months Ended

Jun. 30, 2013

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, 2013.


	Southwest Gas Corporation Equity
							Accumulated
					Additional		Other				Non-
(In thousands, except per share amounts)	Common Stock				Paid-in		Comprehensive		Retained		controlling
(In thousands, except per share amounts)	Shares		Amount		Capital		Income (Loss)		Earnings		Interest		Total

DECEMBER 31, 2012		46,148	$	47,778	$	828,777	$	(50,745)	$	484,369	$	(1,681)	$	1,308,498
Common stock issuances		187		187		5,352								5,539
Net income (loss)										90,881		(140)		90,741
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								1,301						1,301
Amounts reclassified to net income, net of tax (FSIRS)								1,037						1,037
Dividends declared
Common: $0.66 per share										(30,950)				(30,950)

JUNE 30, 2013		46,335	$	47,965	$	834,129	$	(48,407)	$	544,300	$	(1,821)	$	1,376,166

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, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Amortization of transition obligation	$	-	$	-	$	-	$	216	$	(82)	$	134
Amortization of net actuarial (gain)/loss		8,545		(3,247)		5,298		6,400		(2,432)		3,968
Prior service cost		88		(34)		54		-		-		-
Regulatory adjustment		(7,584)		2,882		(4,702)		(5,848)		2,222		(3,626)

Pension plans other comprehensive income (loss)		1,049		(399)		650		768		(292)		476

FSIRS (designated hedging activities):
Unrealized/realized gain		-		-		-		-		-		-
Amounts reclassified into net income		837		(318)		519		837		(318)		519

FSIRS other comprehensive income		837		(318)		519		837		(318)		519


Total other comprehensive income (loss)	$	1,886	$	(717)	$	1,169	$	1,605	$	(610)	$	995


	Six Months Ended						Six Months Ended
	June 30, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Amortization of transition obligation	$	-	$	-	$	-	$	433	$	(165)	$	268
Amortization of net actuarial (gain)/loss		17,090		(6,494)		10,596		12,800		(4,864)		7,936
Prior service cost		177		(68)		109		-		-		-
Regulatory adjustment		(15,168)		5,764		(9,404)		(11,697)		4,445		(7,252)

Pension plans other comprehensive income (loss)		2,099		(798)		1,301		1,536		(584)		952

FSIRS (designated hedging activities):
Unrealized/realized gain		-		-		-		2,959		(1,125)		1,834
Amounts reclassified into net income		1,673		(636)		1,037		1,129		(429)		700

FSIRS other comprehensive income		1,673		(636)		1,037		4,088		(1,554)		2,534


Total other comprehensive income (loss)	$	3,772	$	(1,434)	$	2,338	$	5,624	$	(2,138)	$	3,486


	Twelve Months Ended						Twelve Months Ended
	June 30, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(74,853)	$	28,444	$	(46,409)	$	(135,492)	$	51,487	$	(84,005)
Amortization of transition obligation		434		(164)		270		867		(331)		536
Amortization of net actuarial (gain)/loss		29,887		(11,357)		18,530		20,585		(7,822)		12,763
Prior service cost		(2,246)		853		(1,393)		-		-		-
Regulatory adjustment		39,300		(14,934)		24,366		101,419		(38,539)		62,880

Pension plans other comprehensive income (loss)		(7,478)		2,842		(4,636)		(12,621)		4,795		(7,826)

FSIRS (designated hedging activities):
Unrealized/realized loss		-		-		-		(11,854)		4,504		(7,350)
Amounts reclassified into net income		3,345		(1,271)		2,074		1,713		(651)		1,062

FSIRS other comprehensive income (loss)		3,345		(1,271)		2,074		(10,141)		3,853		(6,288)


Total other comprehensive income (loss)	$	(4,133)	$	1,571	$	(2,562)	$	(22,762)	$	8,648	$	(14,114)

(1)

Tax amounts are calculated using a 38% rate.

Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, reported in Accumulated other comprehensive income (“AOCI”) at June 30, 2013, will be reclassified into interest expense within the next 12 months, as the related interest payments on long-term debt occur.

The following represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets:


AOCI - Rollforward
(Thousands of dollars)
	Defined Benefit Plans						FSIRS
	Before-Tax		Tax (Expense) Benefit		After- Tax		Before-Tax		Tax (Expense) Benefit		After- Tax		AOCI
Beginning Balance AOCI December 31, 2012	$	(52,470)	$	19,939	$	(32,531)	$	(29,378)	$	11,164	$	(18,214)	$	(50,745)

Other comprehensive income before reclassifications		-		-		-		-		-		-		-
FSIRS amounts reclassified from AOCI (1)		-		-		-		1,673		(636)		1,037		1,037
Amortization of prior service costs (2)		177		(68)		109		-		-		-		109
Amortization of net actuarial loss (2)		17,090		(6,494)		10,596		-		-		-		10,596
Regulatory adjustment (3)		(15,168)		5,764		(9,404)		-		-		-		(9,404)

Net current period other comprehensive income (loss)		2,099		(798)		1,301		1,673		(636)		1,037		2,338

Ending Balance AOCI June 30, 2013	$	(50,371)	$	19,141	$	(31,230)	$	(27,705)	$	10,528	$	(17,177)	$	(48,407)

(1)

The FSIRS reclassification amounts are included in the Net interest deductions line item on the Condensed Consolidated Statements of Income.

(2)

These AOCI components are included in the computation of net periodic benefit cost (see Note 2 – Components of Net Periodic Benefit Cost for additional details).

(3)

The regulatory adjustment represents the portion of the defined benefit plan reclassifications above that relates to the regulatory asset included in the Deferred charges and other assets line item on the Condensed Consolidated Balance Sheets.

The following table represents amounts (before income tax impacts) included in AOCI (in the table above), that have not yet been recognized in net periodic benefit cost:


Amounts Recognized in AOCI (Before Tax)
(Thousands of dollars)
	June 30, 2013		December 31, 2012
Net actuarial (loss) gain	$	(406,572)	$	(423,662)
Prior service cost		(2,246)		(2,423)
Less: amount recognized in regulatory assets		358,447		373,615

Recognized in AOCI	$	(50,371)	$	(52,470)

Nature of Operations and Basis of Presentation (Policies)

6 Months Ended

Jun. 30, 2013

Accounting Policies [Abstract]

Nature of Operations

Basis of Presentation

Prepaids and Other Current Assets

Prepaids and other current assets. Prepaids and other current assets includes plant materials and operating supplies of $25 million at both June 30, 2013 and December 31, 2012.

Cash and Cash Equivalents

Intercompany Transactions


	June 30, 2013		December 31, 2012

Accounts receivable for NPL services	$	9,136	$	8,179

Other Property and Investments

Other Property and Investments. Other property and investments includes (millions of dollars):


	June 30, 2013		December 31, 2012

NPL property and equipment	$	305	$	287
NPL accumulated provision for depreciation and amortization		(149)		(136)
Net cash surrender value of COLI policies		86		80
Other property		10		11

Total	$	252	$	242

Other Income (Deductions)

Other Income (Deductions). The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):


	Three Months Ended				Six Months Ended				Twelve Months Ended
	June 30				June 30				June 30
	2013		2012		2013		2012		2013		2012

Change in COLI policies	$	1,800	$	(1,900)	$	5,600	$	3,300	$	8,900	$	(800)
Interest income		144		279		264		505		683		800
Pipe replacement costs		(36)		(1,033)		(121)		(1,042)		(1,759)		(3,956)
Miscellaneous income and (expense)		(456)		164		(222)		175		(830)		209

Total other income (deductions)	$	1,452	$	(2,490)	$	5,521	$	2,938	$	6,994	$	(3,747)

- Definition

Disclosure of accounting policy for cash and cash equivalents, including the policy for determining which items are treated as cash equivalents. Other information that may be disclosed includes (1) the nature of any restrictions on the entity's use of its cash and cash equivalents, (2) whether the entity's cash and cash equivalents are insured or expose the entity to credit risk, (3) the classification of any negative balance accounts (overdrafts), and (4) the carrying basis of cash equivalents (for example, at cost) and whether the carrying amount of cash equivalents approximates fair value.

+ References + Details

Nature of Operations and Basis of Presentation (Tables)

6 Months Ended

Jun. 30, 2013

Accounting Policies [Abstract]

Intercompany Transactions

Accounts receivable for these services are presented in the table below (thousands of dollars):


	June 30, 2013		December 31, 2012

Accounts receivable for NPL services	$	9,136	$	8,179

Schedule of Other Property and Investments

Other property and investments includes (millions of dollars):


	June 30, 2013		December 31, 2012

NPL property and equipment	$	305	$	287
NPL accumulated provision for depreciation and amortization		(149)		(136)
Net cash surrender value of COLI policies		86		80
Other property		10		11

Total	$	252	$	242

Other Income (Deductions)

The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars):


	Three Months Ended				Six Months Ended				Twelve Months Ended
	June 30				June 30				June 30
	2013		2012		2013		2012		2013		2012

Change in COLI policies	$	1,800	$	(1,900)	$	5,600	$	3,300	$	8,900	$	(800)
Interest income		144		279		264		505		683		800
Pipe replacement costs		(36)		(1,033)		(121)		(1,042)		(1,759)		(3,956)
Miscellaneous income and (expense)		(456)		164		(222)		175		(830)		209

Total other income (deductions)	$	1,452	$	(2,490)	$	5,521	$	2,938	$	6,994	$	(3,747)

Components of Net Periodic Benefit Cost (Tables)

6 Months Ended

Jun. 30, 2013

Text Block [Abstract]

Schedule of Components of Net Periodic Benefit Costs


	Qualified Retirement Plan
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	5,764	$	5,080	$	11,528	$	10,159	$	21,688	$	19,022
Interest cost		9,402		9,566		18,803		19,133		37,936		37,771
Expected return on plan assets		(12,460)		(11,445)		(24,920)		(22,890)		(47,810)		(42,947)
Amortization of net actuarial loss		8,065		5,971		16,131		11,942		28,072		19,116

Net periodic benefit cost	$	10,771	$	9,172	$	21,542	$	18,344	$	39,886	$	32,962


	SERP
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	93	$	68	$	187	$	137	$	324	$	245
Interest cost		384		407		767		814		1,582		1,697
Amortization of net actuarial loss		243		171		486		342		827		658

Net periodic benefit cost	$	720	$	646	$	1,440	$	1,293	$	2,733	$	2,600


	PBOP
	Period Ended June 30,
	Three Months				Six Months				Twelve Months
	2013		2012		2013		2012		2013		2012
(Thousands of dollars)
Service cost	$	305	$	244	$	610	$	489	$	1,098	$	918
Interest cost		621		637		1,241		1,273		2,515		2,588
Expected return on plan assets		(706)		(601)		(1,412)		(1,202)		(2,614)		(2,391)
Amortization of prior service cost		88		-		177		-		177		-
Amortization of transition obligation		-		216		-		433		434		867
Amortization of net actuarial loss		237		258		473		516		988		811

Net periodic benefit cost	$	545	$	754	$	1,089	$	1,509	$	2,598	$	2,793

Segment Information (Tables)

6 Months Ended

Jun. 30, 2013

Segment Reporting [Abstract]

Schedule of Segment Reporting Information

The following tables present revenues from external customers, intersegment revenues, and segment net income (thousands of dollars):


	Natural Gas Operations		Construction Services		Total
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


Three months ended June 30, 2012
Revenues from external customers	$	255,917	$	134,402	$	390,319
Intersegment revenues		-		19,449		19,449

Total	$	255,917	$	153,851	$	409,768

Segment net income (loss)	$	(3,368)	$	(308)	$	(3,676)


	Natural Gas Operations		Construction Services		Total
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


Six months ended June 30, 2012
Revenues from external customers	$	786,630	$	245,289	$	1,031,919
Intersegment revenues		-		35,494		35,494

Total	$	786,630	$	280,783	$	1,067,413

Segment net income (loss)	$	75,998	$	(755)	$	75,243


	Natural Gas Operations		Construction Services		Total
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


Twelve months ended June 30, 2012
Revenues from external customers	$	1,362,729	$	479,051	$	1,841,780
Intersegment revenues		-		95,876		95,876

Total	$	1,362,729	$	574,927	$	1,937,656

Segment net income	$	99,204	$	15,722	$	114,926

Derivatives and Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2013

Derivative Instruments And Hedging Activities Disclosure [Abstract]

Notional Amounts Under Swaps Contracts

The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):


	June 30, 2013		December 31, 2012

Contract notional amounts		15,736		14,579

Amount of Gain or Losses Recognized in Income on Derivatives


Gains (losses) recognized in income for derivatives not designated as hedging instruments:
(Thousands of dollars)
	Location of Gain or (Loss) Recognized in Income on Derivative	Three Months Ended						Six Months Ended						Twelve Months Ended
		June 30						June 30						June 30
Instrument		2013			2012			2013			2012			2013			2012

Swaps	Net cost of gas sold	$	(7,669)		$	2,374		$	(2,593)		$	(4,562)		$	(2,885)		$	(20,794)
Swaps	Net cost of gas sold		7,669	*		(2,374)	*		2,593	*		4,562	*		2,885	*		20,794	*

Total		$	-		$	-		$	-		$	-		$	-		$	-

* Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.

Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments


Gains (losses) recognized in other comprehensive income for derivatives designated as cash flow hedging instruments:
(Thousands of dollars)
	Three Months Ended				Six Months Ended				Twelve Months Ended
	June 30				June 30				June 30
	2013		2012		2013		2012		2013		2012
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	$	-	$	-	$	-	$	2,959	$	-	$	(11,854)

Fair Values of Swaps in Balance Sheet Location

The following table sets forth the fair values of the Company’s Swaps and their location in the balance sheets (thousands of dollars):


Fair values of derivatives not designated as hedging instruments:

June 30, 2013 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives		Net Total
Swaps	Other current liabilities	$	42	$	(4,340)	$	(4,298)
Swaps	Other deferred credits		-		(719)		(719)

Total		$	42	$	(5,059)	$	(5,017)


December 31, 2012 Instrument	Balance Sheet Location	Asset Derivatives		Liability Derivatives		Net Total
Swaps	Deferred charges and other assets	$	132	$	(126)	$	6
Swaps	Other current liabilities		391		(2,467)		(2,076)
Swaps	Other deferred credits		233		(552)		(319)

Total		$	756	$	(3,145)	$	(2,389)

Paid to and Received from Counterparties for Settlements of Matured Swaps

The following table shows the amounts Southwest paid to and received from counterparties for settlements of matured Swaps.


(Thousands of dollars)	Three Months Ended June 30, 2013		Six Months Ended June 30, 2013		Twelve Months Ended June 30, 2013
Paid to counterparties	$	3	$	872	$	4,546

Received from counterparties	$	543	$	908	$	1,542

Regulatory Assets/Liabilities Offsetting the Derivatives at Fair Value in the Balance Sheet

The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the balance sheets (thousands of dollars).


June 30, 2013 Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	4,298
Swaps	Deferred charges and other assets		719

December 31, 2012 Instrument	Balance Sheet Location	Net Total
Swaps	Other deferred credits	$	(6)
Swaps	Prepaids and other current assets		2,076
Swaps	Deferred charges and other assets		319

Significant Other Observable Inputs

The following table sets forth the Company’s Level 2 financial assets and liabilities recorded at fair value:

Level 2 - Significant other observable inputs


(Thousands of dollars)	June 30, 2013		December 31, 2012

Assets at fair value:
Deferred charges and other assets - Swaps	$	-		6

Liabilities at fair value:
Other current liabilities - Swaps		(4,298)		(2,076)
Other deferred credits - Swaps		(719)		(319)


Net Assets (Liabilities)	$	(5,017)	$	(2,389)

Long-Term Debt (Tables)

6 Months Ended

Jun. 30, 2013

Debt Disclosure [Abstract]

Schedule of Estimated Fair Values of Long-Term Debt

The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.


	June 30, 2013				December 31, 2012
	Carrying Amount		Market Value		Carrying Amount		Market Value
	Carrying Amount		Market Value		Carrying Amount		Market Value
(Thousands of dollars)
Debentures:
Notes, 4.45%, due 2020	$	125,000	$	134,581	$	125,000	$	141,771
Notes, 6.1%, due 2041		125,000		143,655		125,000		165,779
Notes, 3.875%, due 2022		250,000		261,148		250,000		277,950
8% Series, due 2026		75,000		102,116		75,000		111,501
Medium-term notes, 7.59% series, due 2017		25,000		29,442		25,000		30,710
Medium-term notes, 7.78% series, due 2022		25,000		31,998		25,000		34,637
Medium-term notes, 7.92% series, due 2027		25,000		33,398		25,000		36,953
Medium-term notes, 6.76% series, due 2027		7,500		8,976		7,500		10,058
Unamortized discount		(3,320)				(3,403)

		654,180				654,097

Revolving credit facility and commercial paper		119,000		119,000		111,000		111,000

Industrial development revenue bonds:
Variable-rate bonds:
Tax-exempt Series A, due 2028		50,000		50,000		50,000		50,000
2003 Series A, due 2038		50,000		50,000		50,000		50,000
2008 Series A, due 2038		50,000		50,000		50,000		50,000
2009 Series A, due 2039		50,000		50,000		50,000		50,000
Fixed-rate bonds:
5.55% 1999 Series D, due 2038		8,270		8,314		8,270		8,375
5.45% 2003 Series C, due 2038		-		-		30,000		30,152
5.25% 2003 Series D, due 2038		20,000		20,436		20,000		20,571
5.80% 2003 Series E, due 2038		-		-		15,000		15,102
5.25% 2004 Series A, due 2034		65,000		63,500		65,000		66,955
5.00% 2004 Series B, due 2033		31,200		30,300		31,200		31,655
4.85% 2005 Series A, due 2035		100,000		94,660		100,000		101,184
4.75% 2006 Series A, due 2036		24,855		23,588		24,855		25,189
Unamortized discount		(2,854)				(3,195)

		446,471				491,130

NPL credit facility		-		-		41,562		41,562
NPL other debt obligations		47,687		47,454		20,721		20,991

		1,267,338				1,318,510
Less: current maturities		(11,000)				(50,137)

Long-term debt, less current maturities	$	1,256,338			$	1,268,373

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

+ Details

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income (Tables)

6 Months Ended

Jun. 30, 2013

Text Block [Abstract]

Detailed Activities of Equity

The table below provides details of activity in equity during the six months ended June 30, 2013.


	Southwest Gas Corporation Equity
							Accumulated
					Additional		Other				Non-
(In thousands, except per share amounts)	Common Stock				Paid-in		Comprehensive		Retained		controlling
(In thousands, except per share amounts)	Shares		Amount		Capital		Income (Loss)		Earnings		Interest		Total

DECEMBER 31, 2012		46,148	$	47,778	$	828,777	$	(50,745)	$	484,369	$	(1,681)	$	1,308,498
Common stock issuances		187		187		5,352								5,539
Net income (loss)										90,881		(140)		90,741
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								1,301						1,301
Amounts reclassified to net income, net of tax (FSIRS)								1,037						1,037
Dividends declared
Common: $0.66 per share										(30,950)				(30,950)

JUNE 30, 2013		46,335	$	47,965	$	834,129	$	(48,407)	$	544,300	$	(1,821)	$	1,376,166


Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)
(Thousands of dollars)
	Three Months Ended						Three Months Ended
	June 30, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Amortization of transition obligation	$	-	$	-	$	-	$	216	$	(82)	$	134
Amortization of net actuarial (gain)/loss		8,545		(3,247)		5,298		6,400		(2,432)		3,968
Prior service cost		88		(34)		54		-		-		-
Regulatory adjustment		(7,584)		2,882		(4,702)		(5,848)		2,222		(3,626)

Pension plans other comprehensive income (loss)		1,049		(399)		650		768		(292)		476

FSIRS (designated hedging activities):
Unrealized/realized gain		-		-		-		-		-		-
Amounts reclassified into net income		837		(318)		519		837		(318)		519

FSIRS other comprehensive income		837		(318)		519		837		(318)		519


Total other comprehensive income (loss)	$	1,886	$	(717)	$	1,169	$	1,605	$	(610)	$	995


	Six Months Ended						Six Months Ended
	June 30, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Amortization of transition obligation	$	-	$	-	$	-	$	433	$	(165)	$	268
Amortization of net actuarial (gain)/loss		17,090		(6,494)		10,596		12,800		(4,864)		7,936
Prior service cost		177		(68)		109		-		-		-
Regulatory adjustment		(15,168)		5,764		(9,404)		(11,697)		4,445		(7,252)

Pension plans other comprehensive income (loss)		2,099		(798)		1,301		1,536		(584)		952

FSIRS (designated hedging activities):
Unrealized/realized gain		-		-		-		2,959		(1,125)		1,834
Amounts reclassified into net income		1,673		(636)		1,037		1,129		(429)		700

FSIRS other comprehensive income		1,673		(636)		1,037		4,088		(1,554)		2,534


Total other comprehensive income (loss)	$	3,772	$	(1,434)	$	2,338	$	5,624	$	(2,138)	$	3,486


	Twelve Months Ended						Twelve Months Ended
	June 30, 2013						June 30, 2012
	Before-		Tax		Net-of-		Before-		Tax		Net-of-
	Tax		(Expense)		Tax		Tax		(Expense)		Tax
	Amount		or Benefit (1)		Amount		Amount		or Benefit (1)		Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(74,853)	$	28,444	$	(46,409)	$	(135,492)	$	51,487	$	(84,005)
Amortization of transition obligation		434		(164)		270		867		(331)		536
Amortization of net actuarial (gain)/loss		29,887		(11,357)		18,530		20,585		(7,822)		12,763
Prior service cost		(2,246)		853		(1,393)		-		-		-
Regulatory adjustment		39,300		(14,934)		24,366		101,419		(38,539)		62,880

Pension plans other comprehensive income (loss)		(7,478)		2,842		(4,636)		(12,621)		4,795		(7,826)

FSIRS (designated hedging activities):
Unrealized/realized loss		-		-		-		(11,854)		4,504		(7,350)
Amounts reclassified into net income		3,345		(1,271)		2,074		1,713		(651)		1,062

FSIRS other comprehensive income (loss)		3,345		(1,271)		2,074		(10,141)		3,853		(6,288)


Total other comprehensive income (loss)	$	(4,133)	$	1,571	$	(2,562)	$	(22,762)	$	8,648	$	(14,114)

(1)

Tax amounts are calculated using a 38% rate.

Rollforward of Accumulated Other Comprehensive Income

The following represents a rollforward of AOCI, presented on the Company’s Condensed Consolidated Balance Sheets:


AOCI - Rollforward
(Thousands of dollars)
	Defined Benefit Plans						FSIRS
	Before-Tax		Tax (Expense) Benefit		After- Tax		Before-Tax		Tax (Expense) Benefit		After- Tax		AOCI
Beginning Balance AOCI December 31, 2012	$	(52,470)	$	19,939	$	(32,531)	$	(29,378)	$	11,164	$	(18,214)	$	(50,745)

Other comprehensive income before reclassifications		-		-		-		-		-		-		-
FSIRS amounts reclassified from AOCI (1)		-		-		-		1,673		(636)		1,037		1,037
Amortization of prior service costs (2)		177		(68)		109		-		-		-		109
Amortization of net actuarial loss (2)		17,090		(6,494)		10,596		-		-		-		10,596
Regulatory adjustment (3)		(15,168)		5,764		(9,404)		-		-		-		(9,404)

Net current period other comprehensive income (loss)		2,099		(798)		1,301		1,673		(636)		1,037		2,338

Ending Balance AOCI June 30, 2013	$	(50,371)	$	19,141	$	(31,230)	$	(27,705)	$	10,528	$	(17,177)	$	(48,407)

(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)

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, 2013		December 31, 2012
Net actuarial (loss) gain	$	(406,572)	$	(423,662)
Prior service cost		(2,246)		(2,423)
Less: amount recognized in regulatory assets		358,447		373,615

Recognized in AOCI	$	(50,371)	$	(52,470)

Nature of Operations and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Noncash construction advances - noncash investing item	$ 4.8
Plant materials and operating supplies [Member]
Plant materials and operating supplies	$ 25.0	$ 25.0

Nature of Operations and Basis of Presentation - Intercompany Transactions (Detail) - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
NPL Construction Co [Member]
Accounts receivable for NPL services	$ 9,136	$ 8,179

Nature of Operations and Basis of Presentation - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Net cash surrender value of COLI policies	$ 86,000	$ 80,000
Other property	10,000	11,000
Total	252,467	242,096
NPL Construction Co [Member]
NPL property and equipment	305,000	287,000
NPL accumulated provision for depreciation and amortization	$ (149,000)	$ (136,000)

Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other Income And Expenses [Abstract]
Change in COLI policies	$ 1,800	$ (1,900)	$ 5,600	$ 3,300	$ 8,900	$ (800)
Interest income	144	279	264	505	683	800
Pipe replacement costs	(36)	(1,033)	(121)	(1,042)	(1,759)	(3,956)
Miscellaneous income and (expense)	(456)	164	(222)	175	(830)	209
Total other income (deductions)	$ 1,452	$ (2,490)	$ 5,521	$ 2,938	$ 6,994	$ (3,747)

Components of Net Periodic Benefit Cost - Schedule of Components of Net Periodic Benefit Costs (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Amortization of prior service cost			$ (177)
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,764	$ 5,080	11,528	$ 10,159	$ 21,688	$ 19,022
Interest cost	9,402	9,566	18,803	19,133	37,936	37,771
Expected return on plan assets	(12,460)	(11,445)	(24,920)	(22,890)	(47,810)	(42,947)
Amortization of net actuarial loss	8,065	5,971	16,131	11,942	28,072	19,116
Net periodic benefit cost	10,771	9,172	21,542	18,344	39,886	32,962
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	93	68	187	137	324	245
Interest cost	384	407	767	814	1,582	1,697
Amortization of net actuarial loss	243	171	486	342	827	658
Net periodic benefit cost	720	646	1,440	1,293	2,733	2,600
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	305	244	610	489	1,098	918
Interest cost	621	637	1,241	1,273	2,515	2,588
Expected return on plan assets	(706)	(601)	(1,412)	(1,202)	(2,614)	(2,391)
Amortization of prior service cost	88		177		177
Amortization of transition obligation		216		433	434	867
Amortization of net actuarial loss	237	258	473	516	988	811
Net periodic benefit cost	$ 545	$ 754	$ 1,089	$ 1,509	$ 2,598	$ 2,793

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Total operating revenues	$ 411,574	$ 409,768	$ 1,025,079	$ 1,067,413	$ 1,885,444	$ 1,937,656
Segment net income (loss)	10,108	(3,676)	90,881	75,243	148,969	114,926
Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	389,303	390,319	987,769	1,031,919	1,800,254	1,841,780
Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	22,271	19,449	37,310	35,494	85,190	95,876
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues	238,869	255,917	732,469	786,630	1,267,567	1,362,729
Segment net income (loss)	1,964	(3,368)	81,256	75,998	121,877	99,204
Natural Gas Operations [Member] \| Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	238,869	255,917	732,469	786,630	1,267,567	1,362,729
Natural Gas Operations [Member] \| Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues	172,705	153,851	292,610	280,783	617,877	574,927
Segment net income (loss)	8,144	(308)	9,625	(755)	27,092	15,722
Construction Services [Member] \| Revenues from external customers [Member]
Segment Reporting Information [Line Items]
Revenues	150,434	134,402	255,300	245,289	532,687	479,051
Construction Services [Member] \| Intersegment revenues [Member]
Segment Reporting Information [Line Items]
Revenues	$ 22,271	$ 19,449	$ 37,310	$ 35,494	$ 85,190	$ 95,876

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative [Line Items]
Natural gas portfolios, minimum % rate	25.00%
Natural gas portfolios, maximum % rate	35.00%
Retirement of long-term debt	$ 101,269	$ 323,043	$ 205,269	$ 430,109
February 2011 [Member]
Derivative [Line Items]
Debt maturing date	February 2011		February 2011
Retirement of long-term debt	$ 200,000
May 2012 [Member]
Derivative [Line Items]
Debt maturing date	May 2012		May 2012
Retirement of long-term debt	$ 200,000
Senior Notes, 4.45%, Due 2020 [Member]
Derivative [Line Items]
Notes payable	$ 125,000		$ 125,000
Debt instrument interest rate	4.45%		4.45%
Issuance of fixed rate debt (in years)	10 years
Notional amount	$ 100,000		$ 100,000
Senior Notes, 3.875%, Due 2022 [Member]
Derivative [Line Items]
Notes payable	$ 250,000		$ 250,000
Debt instrument interest rate	3.875%		3.875%
Issuance of fixed rate debt (in years)	10 years
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	July 2013
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	March 2015

Derivatives and Fair Value Measurements - Notional Amounts Under Swaps Contracts (Detail) - MMBTU MMBTU in Thousands	Jun. 30, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Contract notional amounts	15,736	14,579

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative
Swaps Contracts [Member] \| Net cost of gas sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	(7,669)	2,374	(2,593)	(4,562)	(2,885)	(20,794)
Regulatory Deferral Accounting Treatment [Member] \| Swaps Contracts [Member] \| Net cost of gas sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 7,669	$ (2,374)	$ 2,593	$ 4,562	$ 2,885	$ 20,794

Derivatives and Fair Value Measurements - Gains (Losses) Recognized in Other Comprehensive Income for Derivatives as Cash Flow Hedging Instruments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Offsetting [Abstract]
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative				$ 2,959		$ (11,854)

Derivatives and Fair Value Measurements - Fair Values of Swaps in Balance Sheet Location (Detail) - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 42	$ 756
Liability derivatives not designated as hedging instruments	(5,059)	(3,145)
Net total not designated as hedging instruments	(5,017)	(2,389)
Swaps [Member] \| Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		132
Liability derivatives not designated as hedging instruments		(126)
Net total not designated as hedging instruments		6
Swaps [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	42	391
Liability derivatives not designated as hedging instruments	(4,340)	(2,467)
Net total not designated as hedging instruments	(4,298)	(2,076)
Swaps [Member] \| Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		233
Liability derivatives not designated as hedging instruments	(719)	(552)
Net total not designated as hedging instruments	$ (719)	$ (319)

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jun. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 3	$ 872	$ 4,546
Received from counterparties	$ 543	$ 908	$ 1,542

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in the Balance Sheet (Detail) - Swaps [Member] - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Prepaids and other current assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 4,298	$ 2,076
Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 719	319
Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ (6)

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant other observable inputs [Member] - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (5,017)	$ (2,389)
Swaps [Member] \| Deferred charges and other assets [Member]
Derivatives, Fair Value [Line Items]
Derivative assets		6
Swaps [Member] \| Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	(4,298)	(2,076)
Swaps [Member] \| Other deferred credits [Member]
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ (719)	$ (319)

Long-Term Debt - Schedule of Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Less: current maturities	$ (11,000)	$ (50,137)
Long-term debt, less current maturities	1,256,338	1,268,373
Carrying Amount [Member]
Debt Instrument [Line Items]
NPL credit facility		41,562
Unamortized discount	(2,854)	(3,195)
Long-term debt, gross	1,267,338	1,318,510
Less: current maturities	(11,000)	(50,137)
Long-term debt, less current maturities	1,256,338	1,268,373
Carrying Amount [Member] \| Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	446,471	491,130
Carrying Amount [Member] \| NPL other debt obligations [Member]
Debt Instrument [Line Items]
NPL other debt obligations	47,687	20,721
Carrying Amount [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount	(3,320)	(3,403)
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041[Member]
Debt Instrument [Line Items]
Notes payable	125,000	125,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	250,000	250,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable	75,000	75,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	25,000	25,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	7,500	7,500
Carrying Amount [Member] \| Debentures [Member] \| Notes Payable [Member]
Debt Instrument [Line Items]
Notes payable	654,180	654,097
Carrying Amount [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	119,000	111,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000	50,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.55% 1999 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	8,270	8,270
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.45% 2003 Series C, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt		30,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	20,000	20,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.80% 2003 Series E, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt		15,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Unsecured debt	65,000	65,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured debt	31,200	31,200
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000	100,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855	24,855
Market Value [Member] \| NPL other debt obligations [Member]
Debt Instrument [Line Items]
NPL other debt obligations	47,454	20,991
Market Value [Member] \| NPL credit facility [Member]
Debt Instrument [Line Items]
NPL credit facility		41,562
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	134,581	141,771
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041[Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	143,655	165,779
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	261,148	277,950
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	102,116	111,501
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,442	30,710
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,998	34,637
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	33,398	36,953
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,976	10,058
Market Value [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	119,000	111,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	50,000	50,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.55% 1999 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	8,314	8,375
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.45% 2003 Series C, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		30,152
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	20,436	20,571
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.80% 2003 Series E, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		15,102
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	63,500	66,955
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	30,300	31,655
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	94,660	101,184
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value	$ 23,588	$ 25,189

Long-Term Debt - Schedule of Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
5.45% 2003 Series C, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.45%
5.80% 2003 Series E, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%
Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	Dec. 01, 2020	Dec. 01, 2020
Debentures [Member] \| Notes, 4.45%, due 2020 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%	4.45%
Debt instrument due date	Dec. 01, 2020	Dec. 01, 2020
Debentures [Member] \| Notes, 6.1%, due 2041[Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	Feb. 15, 2041	Feb. 15, 2041
Debentures [Member] \| Notes, 6.1%, due 2041[Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	Feb. 15, 2041	Feb. 15, 2041
Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%	3.875%
Debt instrument due date	Apr. 01, 2022	Apr. 01, 2022
Debentures [Member] \| Notes, 3.875%, due 2022 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.875%	3.875%
Debt instrument due date	Apr. 01, 2022	Apr. 01, 2022
Debentures [Member] \| 8% Series, due 2026 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 01, 2026	Aug. 01, 2026
Debentures [Member] \| 8% Series, due 2026 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	8.00%	8.00%
Debt instrument due date	Aug. 01, 2026	Aug. 01, 2026
Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	Jan. 17, 2017	Jan. 17, 2017
Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%	7.59%
Debt instrument due date	Jan. 17, 2017	Jan. 17, 2017
Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	Feb. 03, 2022	Feb. 03, 2022
Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.78%	7.78%
Debt instrument due date	Feb. 03, 2022	Feb. 03, 2022
Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	Jun. 04, 2027	Jun. 04, 2027
Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.92%	7.92%
Debt instrument due date	Jun. 04, 2027	Jun. 04, 2027
Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	Sep. 24, 2027	Sep. 24, 2027
Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.76%	6.76%
Debt instrument due date	Sep. 24, 2027	Sep. 24, 2027
Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2028	Dec. 01, 2028
Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2028	Dec. 01, 2028
Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2039	Dec. 01, 2039
Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument due date	Dec. 01, 2039	Dec. 01, 2039
Fixed-rate bonds [Member] \| 5.55% 1999 Series D, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.55%	5.55%
Debt instrument due date	Dec. 01, 2038	Dec. 01, 2038
Fixed-rate bonds [Member] \| 5.55% 1999 Series D, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.55%	5.55%
Debt instrument due date	Dec. 01, 2038	Dec. 01, 2038
Fixed-rate bonds [Member] \| 5.45% 2003 Series C, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.45%	5.45%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.45% 2003 Series C, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.45%	5.45%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.80% 2003 Series E, due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%	5.80%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.80% 2003 Series E, due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%	5.80%
Debt instrument due date	Mar. 01, 2038	Mar. 01, 2038
Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Jul. 01, 2034	Jul. 01, 2034
Fixed-rate bonds [Member] \| 5.25% 2004 Series A, due 2034 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.25%	5.25%
Debt instrument due date	Jul. 01, 2034	Jul. 01, 2034
Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	Dec. 01, 2033	Dec. 01, 2033
Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.00%	5.00%
Debt instrument due date	Dec. 01, 2033	Dec. 01, 2033
Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	Oct. 01, 2035	Oct. 01, 2035
Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.85%	4.85%
Debt instrument due date	Oct. 01, 2035	Oct. 01, 2035
Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	Sep. 01, 2036	Sep. 01, 2036
Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.75%	4.75%
Debt instrument due date	Sep. 01, 2036	Sep. 01, 2036

Long-Term Debt - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2013 USD ($)
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 300
IDRBs [Member]
Debt Instrument [Line Items]
IDRBs redeemed	$ 45
5.45% 2003 Series C, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.45%
5.80% 2003 Series E, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	5.80%

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Schedule Of Stockholders Equity [Line Items]
Balance			$ 1,308,498
Balance, shares			46,147,788
Common stock issuances			$ 5,539
Net income (loss)	$ 10,067	$ (3,888)	90,741	$ 74,947	$ 148,433	$ 114,343
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	650	476	1,301	952	(4,636)	(7,826)
Amounts reclassified into net income	519	$ 519	1,037	$ 700	2,074	$ 1,062
Dividends declared Common: $0.66 per share			(30,950)
Balance	$ 1,376,166		$ 1,376,166		$ 1,376,166
Balance, shares	46,334,769		46,334,769		46,334,769
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 47,778
Balance, shares			46,148
Common stock issuances			$ 187
Common stock issuances, shares			187
Other comprehensive income (loss):
Balance	$ 47,965		$ 47,965		$ 47,965
Balance, shares	46,335		46,335		46,335
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			$ 828,777
Common stock issuances			5,352
Other comprehensive income (loss):
Balance	$ 834,129		834,129		$ 834,129
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(50,745)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			1,301
Amounts reclassified into net income			1,037
Balance	(48,407)		(48,407)		(48,407)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			484,369
Net income (loss)			90,881
Other comprehensive income (loss):
Dividends declared Common: $0.66 per share			(30,950)
Balance	544,300		544,300		544,300
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance			(1,681)
Net income (loss)			(140)
Other comprehensive income (loss):
Balance	$ (1,821)		$ (1,821)		$ (1,821)

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Parenthetical) (Detail) - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Statement Of Stockholders Equity [Abstract]
Dividends declared per share	$ 0.330	$ 0.295	$ 0.660	$ 0.590	$ 1.250	$ 1.120

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined benefit pension plans:
Amortization of transition obligation, Before-Tax Amount		$ 216		$ 433	$ 434	$ 867
Amortization of net actuarial (gain)/loss, Before-Tax Amount	$ 8,545	6,400	$ 17,090	12,800	29,887	20,585
Prior service cost, Before-Tax Amount	88		177		(2,246)
Regulatory adjustment, Before-Tax Amount	(7,584)	(5,848)	(15,168)	(11,697)	39,300	101,419
Pension plans other comprehensive income (loss), Before-Tax Amount	1,049	768	2,099	1,536	(7,478)	(12,621)
Net actuarial gain/(loss), Before-Tax Amount					(74,853)	(135,492)
FSIRS (designated hedging activities):
Unrealized/realized gain (loss), Before-Tax Amount				2,959		(11,854)
Amounts reclassified into net income, Before-Tax Amount	837	837	1,673	1,129	3,345	1,713
FSIRS other comprehensive income (loss), Before-Tax Amount	837	837	1,673	4,088	3,345	(10,141)
Total other comprehensive income (loss), Before-Tax Amount	1,886	1,605	3,772	5,624	(4,133)	(22,762)
Amortization of transition obligation, Tax (Expense) or Benefit		(82)		(165)	(164)	(331)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(3,247)	(2,432)	(6,494)	(4,864)	(11,357)	(7,822)
Prior service cost, Tax (Expense) or Benefit	(34)		(68)		853
Regulatory adjustment, Tax (Expense) or Benefit	2,882	2,222	5,764	4,445	(14,934)	(38,539)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(399)	(292)	(798)	(584)	2,842	4,795
Unrealized/realized gain (loss), Tax (Expense) or Benefit				(1,125)		4,504
Amounts reclassified into net income, Tax (Expense) or Benefit	(318)	(318)	(636)	(429)	(1,271)	(651)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(318)	(318)	(636)	(1,554)	(1,271)	3,853
Total other comprehensive income (loss), Tax (Expense) or Benefit	(717)	(610)	(1,434)	(2,138)	1,571	8,648
Amortization of transition obligation, Net-of-Tax Amount		134		268	270	536
Amortization of net actuarial (gain)/loss, Net-of-Tax Amount	5,298	3,968	10,596	7,936	18,530	12,763
Prior service cost, Net-of-Tax Amount	54		109		(1,393)
Regulatory adjustment, Net-of-Tax Amount	(4,702)	(3,626)	(9,404)	(7,252)	24,366	62,880
Net defined benefit pension plans	650	476	1,301	952	(4,636)	(7,826)
Unrealized/realized gain (loss), Net-of-Tax Amount				1,834		(7,350)
Amounts reclassified into net income	519	519	1,037	700	2,074	1,062
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	519	1,037	2,534	2,074	(6,288)
Total other comprehensive income (loss), Net-of-Tax Amount	$ 1,169	$ 995	$ 2,338	$ 3,486	(2,562)	(14,114)
Net actuarial gain/(loss), Tax (Expense) or Benefit					28,444	51,487
Net actuarial gain/(loss), Net-of-Tax Amount					$ (46,409)	$ (84,005)

Equity, Other Comprehensive 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, 2013
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Additional Information (Detail) $ in Millions	Jun. 30, 2013 USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next 12 months	$ 2.1

Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Schedule Of Stockholders Equity [Line Items]
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Before-Tax			$ (52,470)
Amortization of prior service costs, Defined Benefit Plans Before-Tax			177
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	$ 8,545	$ 6,400	17,090	$ 12,800	$ 29,887	$ 20,585
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,584)	(5,848)	(15,168)	(11,697)	39,300	101,419
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax			2,099
Ending Balance AOCI June 30, 2013 Defined Benefit Plans Before-Tax	(50,371)		(50,371)		(50,371)
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Tax (Expense) Benefit			19,939
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit			(68)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(3,247)	(2,432)	(6,494)	(4,864)	(11,357)	(7,822)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,882	2,222	5,764	4,445	(14,934)	(38,539)
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	(399)	(292)	(798)	(584)	2,842	4,795
Ending Balance AOCI June 30, 2013 Defined Benefit Plans Tax (Expense) Benefit	19,141		19,141		19,141
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans After-Tax			(32,531)
Amortization of prior service costs, Defined Benefit Plans After-Tax			109
Amortization of net actuarial loss, Defined Benefit Plans After-Tax	5,298	3,968	10,596	7,936	18,530	12,763
Regulatory adjustment, AOCI	(4,702)	(3,626)	(9,404)	(7,252)	24,366	62,880
Net current period other comprehensive income (loss), Defined Benefit Plans After-Tax			1,301
Ending Balance AOCI June 30, 2013 Defined Benefit Plans After-Tax	(31,230)		(31,230)		(31,230)
Beginning Balance AOCI December 31, 2012, FSIRS - Before-Tax			(29,378)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	837	837	1,673	1,129	3,345	1,713
FSIRS other comprehensive income (loss), Before-Tax Amount	837	837	1,673	4,088	3,345	(10,141)
Ending Balance AOCI June 30, 2013, FSIRS - Before-Tax	(27,705)		(27,705)		(27,705)
Beginning Balance AOCI December 31, 2012, FSIRS - Tax (Expense) Benefit			11,164
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(318)	(318)	(636)	(429)	(1,271)	(651)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(318)	(318)	(636)	(1,554)	(1,271)	3,853
Ending Balance AOCI June 30, 2013, FSIRS - Tax (Expense) Benefit	10,528		10,528		10,528
Beginning Balance AOCI December 31, 2012, FSIRS - After-Tax			(18,214)
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	519	519	1,037	700	2,074	1,062
Net forward-starting interest rate swaps	519	519	1,037	2,534	2,074	(6,288)
Ending Balance AOCI June 30, 2013, FSIRS - After-Tax	(17,177)		(17,177)		(17,177)
Beginning Balance AOCI December 31, 2012, AOCI			(50,745)
Amounts reclassified into net income	519	519	1,037	700	2,074	1,062
Amortization of prior service costs, AOCI			109
Amortization of net actuarial loss, AOCI			10,596
Regulatory adjustment, Net-of-Tax Amount	(4,702)	$ (3,626)	(9,404)	$ (7,252)	24,366	$ 62,880
Net current period other comprehensive income (loss), AOCI			2,338
Ending Balance AOCI June 30, 2013, AOCI	$ (48,407)		(48,407)		$ (48,407)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Schedule Of Stockholders Equity [Line Items]
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax
Other comprehensive income before reclassifications, FSIRS - Before-Tax
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax			1,673
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit
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 Income, and Accumulated Other Comprehensive Income - Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain				$ 74,853		$ 135,492
Prior service cost	$ (88)		$ (177)	2,246
Recognized in AOCI	(50,371)		(50,371)	(50,371)	$ (52,470)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain			(406,572)		(423,662)
Prior service cost			(2,246)		(2,423)
Less: amount recognized in regulatory assets	358,447		358,447	358,447	373,615
Recognized in AOCI	$ (50,371)		$ (50,371)	$ (50,371)	$ (52,470)