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Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2012	Apr. 27, 2012
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2012
Document Fiscal Year Focus	2012
Document Fiscal Period Focus	Q1
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS CORP
Entity Central Index Key	0000092416
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		46,112,072

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2012	Dec. 31, 2011
Utility plant:
Gas plant	$ 4,842,972	$ 4,811,050
Less: accumulated depreciation	(1,667,904)	(1,638,091)
Acquisition adjustments, net	1,045	1,091
Construction work in progress	58,817	44,894
Net utility plant	3,234,930	3,218,944
Other property and investments	220,847	192,004
Restricted cash		12,785
Current assets:
Cash and cash equivalents	218,060	21,937
Accounts receivable, net of allowances	210,735	209,246
Accrued utility revenue	43,700	70,300
Income taxes receivable, net	2,299	7,793
Deferred income taxes	33,815	53,435
Deferred purchased gas costs		2,323
Prepaids and other current assets	77,330	96,598
Total current assets	585,939	461,632
Deferred charges and other assets	395,381	390,642
Total assets	4,437,097	4,276,007
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 46,110,072 and 45,956,088 shares)	47,740	47,586
Additional paid-in capital	823,460	821,640
Accumulated other comprehensive income (loss), net	(46,840)	(49,331)
Retained earnings	471,265	406,125
Total Southwest Gas Corporation equity	1,295,625	1,226,020
Noncontrolling interest	(1,073)	(989)
Total equity	1,294,552	1,225,031
Long-term debt, less current maturities	1,188,076	930,858
Total capitalization	2,482,628	2,155,889
Current liabilities:
Current maturities of long-term debt	205,055	322,618
Accounts payable	132,544	186,755
Customer deposits	83,138	83,839
Accrued general taxes	58,565	42,102
Accrued interest	19,305	16,699
Deferred purchased gas costs	75,715	72,426
Other current liabilities	115,012	123,129
Total current liabilities	689,334	847,568
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits	578,563	557,118
Taxes payable	759	828
Accumulated removal costs	239,000	233,000
Other deferred credits	446,813	481,604
Total deferred income taxes and other credits	1,265,135	1,272,550
Total capitalization and liabilities	$ 4,437,097	$ 4,276,007

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2012	Dec. 31, 2011
Condensed Consolidated Balance Sheets [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	46,110,072	45,956,088
Common stock, outstanding	46,110,072	45,956,088

Condensed Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Operating revenues:
Gas operating revenues	$ 530,713	$ 553,853	$ 1,380,226	$ 1,451,251
Construction revenues	126,932	74,587	536,167	338,809
Total operating revenues	657,645	628,440	1,916,393	1,790,060
Operating expenses:
Net cost of gas sold	242,747	283,806	572,430	667,726
Operations and maintenance	95,850	90,950	363,398	359,188
Depreciation and amortization	54,163	48,862	205,770	191,629
Taxes other than income taxes	10,731	9,869	41,811	38,972
Construction expenses	119,531	68,618	474,616	295,825
Total operating expenses	523,022	502,105	1,658,025	1,553,340
Operating income	134,623	126,335	258,368	236,720
Other income and (expenses):
Net interest deductions	(17,159)	(17,959)	(68,802)	(75,461)
Other income (deductions)	5,428	(278)	294	4,095
Total other income and (expenses)	(11,731)	(18,237)	(68,508)	(71,366)
Income before income taxes	122,892	108,098	189,860	165,354
Income tax expense	44,057	39,744	67,616	58,007
Net income	78,835	68,354	122,244	107,347
Net income (loss) attributable to noncontrolling interest	(84)	(195)	(413)	(431)
Net income attributable to Southwest Gas Corporation	$ 78,919	$ 68,549	$ 122,657	$ 107,778
Basic earnings per share	$ 1.71	$ 1.50	$ 2.67	$ 2.37
Diluted earnings per share	1.70	1.48	2.65	2.34
Dividends declared per share	$ 0.295	$ 0.265	$ 1.090	$ 1.015
Average number of common shares outstanding	46,068	45,763	45,934	45,538
Average shares outstanding (assuming dilution)	46,467	46,178	46,363	45,966

Condensed Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Condensed Consolidated Statements Of Comprehensive Income [Abstract]
Net Income	$ 78,835	$ 68,354	$ 122,244	$ 107,347
Other comprehensive income (loss), net of tax
Net actuarial gain (loss)	(476)		(84,005)	(5,616)
Amortization of transition obligation	134	134	537	537
Amortization of net loss	3,968	2,412	11,209	8,048
Regulatory adjustment	(3,626)	(2,226)	64,277	(148)
Net defined benefit pension plans	476	320	(7,982)	2,821
Forward-starting interest rate swaps:
Unrealized/realized gain (loss)	1,834	416	(9,716)	(10,408)
Amounts reclassified into net income	181	181	725	241
Net forward-starting interest rate swaps	2,015	597	(8,991)	(10,167)
Total other comprehensive income (loss), net of tax	2,491	917	(16,973)	(7,346)
Comprehensive income	81,326	69,271	105,271	100,001
Comprehensive income (loss) attributable to noncontrolling interest	(84)	(195)	(413)	(431)
Comprehensive income attributable to Southwest Gas Corporation	$ 81,410	$ 69,466	$ 105,684	$ 100,432

Condensed Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
CASH FLOW FROM OPERATING ACTIVITIES:
Net income	$ 78,835	$ 68,354	$ 122,244	$ 107,347
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	54,163	48,862	205,770	191,629
Deferred income taxes	39,538	13,949	82,056	62,862
Changes in current assets and liabilities:
Accounts receivable, net of allowances	(1,489)	(21,432)	(41,698)	7,170
Accrued utility revenue	26,600	22,900	(2,200)	2,200
Deferred purchased gas costs	5,612	(32,152)	(15,121)	(2,508)
Accounts payable	(56,085)	(51,078)	10,819	(5,870)
Accrued taxes	21,888	52,754	(15,887)	(20,368)
Other current assets and liabilities	36,413	29,281	3,785	16,636
Gains on sale	(1,414)	(885)	(3,836)	(2,200)
Changes in undistributed stock compensation	1,101	2,843	4,383	4,585
AFUDC and property-related changes	(322)	(124)	(1,352)	(791)
Changes in other assets and deferred charges	(16,058)	(6,633)	1,600	(15,328)
Changes in other liabilities and deferred credits	(22,409)	4,152	(62,939)	(6,200)
Net cash provided by operating activities	166,373	130,791	287,624	339,164
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(84,965)	(51,817)	(414,139)	(233,104)
Change in restricted cash	12,785		37,781	11,995
Changes in customer advances	(6,553)	(880)	(13,444)	(446)
Miscellaneous inflows	2,264	1,371	8,579	4,489
Miscellaneous outflows		(2,500)	(219)	(5,300)
Net cash used in investing activities	(76,469)	(53,826)	(381,442)	(222,366)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	732	4,003	4,131	11,095
Dividends paid	(12,215)	(11,446)	(48,698)	(45,550)
Interest rate swap settlement	(21,754)		(21,754)	(11,691)
Issuance of long-term debt	325,878	125,000	475,476	248,960
Retirement of long-term debt	(77,422)	(202,243)	(205,652)	(205,242)
Change in credit facility	(109,000)			(45,000)
Net cash provided by (used in) financing activities	106,219	(84,686)	203,503	(47,428)
Change in cash and cash equivalents	196,123	(7,721)	109,685	69,370
Cash and cash equivalents at beginning of period	21,937	116,096	108,375	39,005
Cash and cash equivalents at end of period	218,060	108,375	218,060	108,375
Supplemental information:
Interest paid, net of amounts capitalized	37,246	17,715	89,373	84,039
Income taxes paid (received)	$ (1,189)	$ (16,859)	$ 2,035	$ 5,996

Nature Of Operations And Basis Of Presentation

3 Months Ended

Mar. 31, 2012

Nature Of Operations And Basis Of Presentation [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. The 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 and profit are lowest during the first quarter of the year due to unfavorable winter weather conditions. Operating results 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 revenue and expense 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 2011 Annual Report to Shareholders, which is incorporated by reference into the 2011 Form 10-K.

Cash and Cash Equivalents. 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.

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


	March 31, 2012		December 31, 2011

Accounts receivable for NPL services	$	5,185	$	6,205

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 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						Twelve Months Ended
	March 31						March 31
	2012			2011			2012			2011

Change in COLI policies	$	5,200		$	2,200		$	3,700		$	10,480
Interest income		226			92			619			265
Pipe replacement costs		(9	)		(886	)		(3,884	)		(4,339	)
Miscellaneous income and (expense)		11			(1,684	)		(141	)		(2,311	)

Total other income (deductions)	$	5,428		$	(278	)	$	294		$	4,095

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, the change in the cash surrender value components of COLI policies, as they progress towards the ultimate death benefits, is also recorded without tax consequences. Pipe replacement costs include amounts associated with certain Arizona non-recoverable pipe replacement work.

Recently Issued Accounting Standards Updates. In December 2011, the Financial Accounting Standards Board ("FASB") issued the update "Balance Sheet (Topic 210)." The update requires an entity to disclose information about financial instruments and derivative instruments that are either offset or subject to an enforceable master netting arrangement or similar agreement. This information is intended to enable users of an entity's financial statements to understand the effect of those arrangements on the entity's financial position. The Company will adopt this update, as required, on January 1, 2013 for interim and annual reporting periods. All disclosures are required to be provided retrospectively for all periods presented. This update is not expected to have a material impact on the Company's disclosures.

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 51
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Paragraph 10
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
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+ Details

Components Of Net Periodic Benefit Cost

3 Months Ended

Mar. 31, 2012

Components Of Net Periodic Benefit Cost [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.


	Qualified Retirement Plan
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	5,079		$	4,431		$	18,373		$	17,130
Interest cost		9,567			9,319			37,524			36,029
Expected return on plan assets		(11,445	)		(10,029	)		(41,530	)		(37,432	)
Amortization of net loss		5,971			3,587			16,732			11,445

Net periodic benefit cost	$	9,172		$	7,308		$	31,099		$	27,172


	SERP
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	69		$	55		$	231		$	334
Interest cost		407			441			1,732			1,975
Amortization of net loss		171			157			645			1,023

Net periodic benefit cost	$	647		$	653		$	2,608		$	3,332


	PBOP
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	245		$	215		$	888		$	857
Interest cost		636			658			2,609			2,527
Expected return on plan assets		(601	)		(595	)		(2,385	)		(2,165	)
Amortization of transition obligation		217			216			868			866
Amortization of net loss		258			147			701			513

Net periodic benefit cost	$	755		$	641		$	2,681		$	2,598

Segment Information

3 Months Ended

Mar. 31, 2012

Segment Information [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 March 31, 2012
Revenues from external customers	$	530,713	$	110,887		$	641,600
Intersegment revenues		-		16,045			16,045

Total	$	530,713	$	126,932		$	657,645

Segment net income (loss)	$	79,366	$	(447	)	$	78,919


Three months ended March 31, 2011
Revenues from external customers	$	553,853	$	61,777		$	615,630
Intersegment revenues		-		12,810			12,810

Total	$	553,853	$	74,587		$	628,440

Segment net income	$	68,015	$	534		$	68,549


Twelve months ended March 31, 2012
Revenues from external customers	$	1,380,226	$	440,811		$	1,821,037
Intersegment revenues		-		95,356			95,356

Total	$	1,380,226	$	536,167		$	1,916,393

Segment net income	$	102,771	$	19,886		$	122,657


Twelve months ended March 31, 2011
Revenues from external customers	$	1,451,251	$	276,833		$	1,728,084
Intersegment revenues		-		61,976			61,976

Total	$	1,451,251	$	338,809		$	1,790,060

Segment net income	$	94,080	$	13,698		$	107,778

- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8971-108599

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 26
-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8844-108599

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8380-108599

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8595-108599

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8933-108599

+ Details

Derivatives And Fair Value Measurements

3 Months Ended

Mar. 31, 2012

Derivatives And Fair Value Measurements [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 April 2012 through March 2014. Under such contracts, Southwest pays the counterparty at a fixed rate and receives from the counterparty a floating rate per MMBtu ("dekatherm") of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms):


	March 31, 2012		December 31, 2011

Swaps contracts		10,072		10,827

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

Gains (losses) recognized in income for derivatives not designated as hedging instruments:

(Thousands of dollars)


		Three Months Ended						Twelve Months Ended
Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	March 31						March 31
Instrument		2012			2011			2012			2011

Swaps	Net cost of gas sold	$	(6,936	)	$	289		$	(25,426	)	$	(11,059	)
Swaps	Net cost of gas sold		6,936	^*		(289	)^*		25,426	^*		11,059	^*

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 maturing in May 2012. The counterparties to each agreement were four major banking institutions. The first FSIRS was a designated cash flow hedge and terminated in December 2010 concurrent with the related issuance of $125 million 4.45% 10-year Senior Notes. The second FSIRS was also a designated cash flow hedge and had a notional amount of $100 million. It terminated in March 2012 concurrent with the related issuance of $250 million 3.875% 10-year Senior Notes. At settlement of the second FSIRS, Southwest paid an aggregate $21.8 million to the counterparties. 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				Twelve Months Ended
	March 31, 2012		March 31, 2011		March 31, 2012			March 31, 2011
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	$	2,959	$	671	$	(15,670	)	$	(16,788	)

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


Fair values of derivatives not designated as hedging instruments:

March 31, 2012		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
Swaps	Other current liabilities	$	-	$	(9,421	)	$	(9,421	)
Swaps	Other deferred credits		-		(1,712	)		(1,712	)

Total		$	-	$	(11,133	)	$	(11,133	)


December 31, 2011		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
Swaps	Other current liabilities	$	-	$	(11,122	)	$	(11,122	)
Swaps	Other deferred credits		-		(621	)		(621	)

Total		$	-	$	(11,743	)	$	(11,743	)


Fair values of derivatives designated as hedging instruments:

December 31, 2011		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
FSIRS	Other current liabilities	$	-	$	(24,713	)	$	(24,713	)

As noted above, the FSIRS that remained at December 31, 2011 terminated in March 2012.

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.

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 counterparties for settlements of matured Swaps.


(Thousands of dollars)	Three Months Ended March 31, 2012		Twelve Months Ended March 31, 2012

Paid to counterparties	$	7,547	$	20,851

No amounts were received from counterparties for settlements of matured Swaps for the three months and twelve months ended March 31, 2012.

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


March 31, 2012
Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	9,421
Swaps	Deferred charges and other assets		1,712

December 31, 2011
Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	11,122
Swaps	Deferred charges and other assets		621

Fair Value Measurements. The estimated fair values of Southwest's Swaps were determined at March 31, 2012 and December 31, 2011 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 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 estimated fair value of Southwest's FSIRS at December 31, 2011 was determined using a discounted cash flow model that utilized forward interest rate curves. The inputs to the model were the terms of the FSIRS. These Level 2 inputs were observable in the marketplace throughout the full term of the FSIRS, but were credit-risk adjusted with no significant impact to the overall fair value measure. See Note 6 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income for more information on the FSIRS.

The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, the Company's financial assets and liabilities that were accounted for at fair value:

		september 300			september 300
Level 2 – Significant other observable inputs

(Thousands of dollars)	March 31, 2012			December 31, 2011

Liabilities at fair value:
Other current liabilities – Swaps	$	(9,421	)	$	(11,122	)
Other deferred credits – Swaps		(1,712	)		(621	)
Other current liabilities – FSIRS		-			(24,713	)

Net Assets (Liabilities)	$	(11,133	)	$	(36,456	)

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

3 Months Ended

Mar. 31, 2012

Long-Term Debt [Abstract]

Long-Term Debt

Note 5 – Long-Term Debt

Carrying amounts of the Company's long-term debt and their related estimated fair values as of March 31, 2012 and December 31, 2011 are disclosed in the following table. The fair values of the revolving credit facility and the variable-rate Industrial Development Revenue Bonds ("IDRBs") approximate carrying value 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 4.45% Notes) and fixed-rate IDRBs are categorized as Level 2. The 4.45% Notes and NPL 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 debt obligations were determined through a market-based valuation approach, where fair market values are determined based on evaluated pricing data, such as broker quotes and yields for similar securities adjusted for observable differences. Significant inputs used in the valuation generally include benchmark yield curves and issuer spreads. The external credit rating, coupon rate, and maturity of each security are considered in the valuation, as applicable.


	March 31, 2012					December 31, 2011
	Carrying			Market		Carrying			Market
	Amount			Value		Amount			Value
(Thousands of dollars)
Debentures:
Notes, 7.625%, due 2012	$	200,000		$	201,480	$	200,000		$	204,312
Notes, 4.45%, due 2020		125,000			134,863		125,000			128,673
Notes, 6.1%, due 2041		125,000			142,840		125,000			143,074
Notes, 3.875%, due 2022		250,000			256,418
8% Series, due 2026		75,000			104,583		75,000			96,340
Medium-term notes, 7.59% series, due 2017		25,000			30,729		25,000			30,199
Medium-term notes, 7.78% series, due 2022		25,000			33,208		25,000			31,932
Medium-term notes, 7.92% series, due 2027		25,000			34,548		25,000			31,648
Medium-term notes, 6.76% series, due 2027		7,500			9,353		7,500			8,510
Unamortized discount		(3,632	)				(2,087	)

		853,868					605,413

Revolving credit facility and commercial paper		-			-		109,000			109,000

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:
6.10% 1999 Series A, due 2038		-			-		12,410			12,410
5.95% 1999 Series C, due 2038		14,320			14,403		14,320			14,449
5.55% 1999 Series D, due 2038		8,270			8,330		8,270			8,253
5.45% 2003 Series C, due 2038 (rate resets in 2013)		30,000			31,144		30,000			31,332
5.25% 2003 Series D, due 2038		20,000			19,912		20,000			19,583
5.80% 2003 Series E, due 2038 (rate resets in 2013)		15,000			15,544		15,000			15,634
5.25% 2004 Series A, due 2034		65,000			65,002		65,000			64,291
5.00% 2004 Series B, due 2033		31,200			30,728		31,200			30,283
4.85% 2005 Series A, due 2035		100,000			96,888		100,000			94,836
4.75% 2006 Series A, due 2036		24,855			23,766		24,855			23,179
Unamortized discount		(3,325	)				(3,360	)

		505,320					517,695

NPL debt obligaitons		33,943			33,981		21,368			21,380

		1,393,131					1,253,476
Less: current maturities		(205,055	)				(322,618	)

Long-term debt, less current maturities	$	1,188,076				$	930,858

In March 2012, the Company replaced the existing $300 million revolving credit facility that was to expire in May 2012 with a $300 million facility that is scheduled to expire in March 2017. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate ("LIBOR") or the "alternate base rate," plus, in each case, an applicable margin that is determined based on the Company's senior unsecured debt rating. At the Company's current unsecured debt rating, the applicable margin is 1.125% for loans bearing interest with reference to LIBOR and 0.125% for loans bearing interest with reference to the alternative base rate. Southwest has designated $150 million of the $300 million facility for long-term purposes and the remaining $150 million for working capital purposes.

In March 2012, the Company issued $250 million in 3.875% Senior Notes at a 0.034% discount. The notes will mature on April 1, 2022. Management intends to use approximately $200 million of the net proceeds in connection with the repayment of the $200 million 7.625% Senior Notes maturing in May 2012. The remaining net proceeds are intended for general corporate purposes. Pending the repayment of the $200 million 7.625% Senior Notes as described above, the corresponding net proceeds of the sale of the 3.875% Notes are in temporary cash investments.

In January 2012, the Company redeemed at par its $12.4 million 1999 6.1% Series A fixed-rate IDRBs (originally due in 2038).

Equity, Other Comprehensive Income, And Accumulated Other Comprehensive Income

3 Months Ended

Mar. 31, 2012

Equity Other Comprehensive Income And Accumulated Other Comprehensive Income [Abstract]

Equity, Other Comprehensive Income, And Accumulated Other Comprehensive Income

Note 6 – Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income

The table below provides details of activity in equity during the three months ended March 31, 2012.


	Southwest Gas Corporation Equity
	Common Stock				Additional Paid-in Capital		Accumulated Other Comprehensive Income (Loss)			Retained Earnings			Non- controlling Interest			Total
(In thousands, except per share amounts)	Shares		Amount		Additional Paid-in Capital		Accumulated Other Comprehensive Income (Loss)			Retained Earnings			Non- controlling Interest			Total
DECEMBER 31, 2011		45,956	$	47,586	$	821,640	$	(49,331	)	$	406,125		$	(989	)	$	1,225,031
Common stock issuances		154		154		1,820											1,974
Net income (loss)											78,919			(84	)		78,835
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								476									476
FSIRS realized and unrealized loss, net of tax (Note 4)								1,834									1,834
Amounts reclassified to net income, net of tax (Note 4)								181									181
Dividends declared
Common: $0.295 per share											(13,779	)					(13,779	)

MARCH 31, 2012		46,110	$	47,740	$	823,460	$	(46,840	)	$	471,265		$	(1,073	)	$	1,294,552

The following information provides insight into amounts impacting Other Comprehensive Income (Loss), before and after- tax impacts, within the Condensed Consolidated Statements of Comprehensive Income, which also impact Accumulated Other Comprehensive Income in the Company's Condensed Consolidated Balance Sheets. 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 March 31, 2012									Three Months Ended March 31, 2011

	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	-		$	-		$	-		$	-		$	-		$	-
Amortization of transition obligation		217			(83	)		134			216			(82	)		134
Amortization of net loss		6,400			(2,432	)		3,968			3,891			(1,479	)		2,412
Regulatory adjustment		(5,849	)		2,223			(3,626	)		(3,591	)		1,365			(2,226	)

Pension plans other comprehensive income (loss)		768			(292	)		476			516			(196	)		320

FSIRS (designated hedging activities):
Unrealized/realized gain		2,959			(1,125	)		1,834			671			(255	)		416
Amounts reclassifed into net income		292			(111	)		181			292			(111	)		181

FSIRS other comprehensive income		3,251			(1,236	)		2,015			963			(366	)		597

Total other comprehensive income (loss)	$	4,019		$	(1,528	)	$	2,491		$	1,479		$	(562	)	$	917


	Twelve Months Ended March 31, 2012									Twelve Months Ended March 31, 2011

	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(135,492	)	$	51,487		$	(84,005	)	$	(9,058	)	$	3,442		$	(5,616	)
Amortization of transition obligation		868			(331	)		537			866			(329	)		537
Amortization of net loss		18,078			(6,869	)		11,209			12,981			(4,933	)		8,048
Regulatory adjustment		103,673			(39,396	)		64,277			(241	)		93			(148	)

Pension plans other comprehensive income (loss)		(12,873	)		4,891			(7,982	)		4,548			(1,727	)		2,821

FSIRS (designated hedging activities):
Unrealized/realized loss		(15,670	)		5,954			(9,716	)		(16,788	)		6,380			(10,408	)
Amounts reclassifed into net income		1,169			(444	)		725			389			(148	)		241

FSIRS other comprehensive income (loss)		(14,501	)		5,510			(8,991	)		(16,399	)		6,232			(10,167	)

Total other comprehensive income (loss)	$	(27,374	)	$	10,401		$	(16,973	)	$	(11,851	)	$	4,505		$	(7,346	)

(1)	Tax amounts are calculated using a 38% rate.

Approximately $2.1 million of realized/unrealized losses (net of tax) related to the FSIRS reported in Accumulated other comprehensive income ("AOCI") at March 31, 2012 will be reclassified into expense within the next 12 months as interest payments on the related 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, 2011	$	(44,429	)	$	16,883		$	(27,546	)	$	(35,138	)	$	13,353		$	(21,785	)	$	(49,331	)
Current period change		768			(292	)		476			3,251			(1,236	)		2,015			2,491

Ending Balance AOCI March 31, 2012	$	(43,661	)	$	16,591		$	(27,070	)	$	(31,887	)	$	12,117		$	(19,770	)	$	(46,840	)

Nature Of Operations And Basis Of Presentation (Policy)

3 Months Ended

Mar. 31, 2012

Nature Of Operations And Basis Of Presentation [Abstract]

Nature Of Operations

Basis Of Presentation

Cash And Cash Equivalents

Intercompany Transactions


	March 31, 2012		December 31, 2011

Accounts receivable for NPL services	$	5,185	$	6,205

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						Twelve Months Ended
	March 31						March 31
	2012			2011			2012			2011

Change in COLI policies	$	5,200		$	2,200		$	3,700		$	10,480
Interest income		226			92			619			265
Pipe replacement costs		(9	)		(886	)		(3,884	)		(4,339	)
Miscellaneous income and (expense)		11			(1,684	)		(141	)		(2,311	)

Total other income (deductions)	$	5,428		$	(278	)	$	294		$	4,095

Recently Issued Accounting Standards Updates

- 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 1
-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 95
-Paragraph 7, 8, 9, 10
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Technical Practice Aid (TPA)
-Number 2110
-Paragraph 6
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Financial Reporting Release (FRR)
-Number 203
-Paragraph 02-03

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash
-URI http://asc.fasb.org/extlink&oid=6506951

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6367646&loc=d3e18780-107790

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4273-108586

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 305
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2122427

+ Details

Nature Of Operations And Basis Of Presentation (Tables)

3 Months Ended

Mar. 31, 2012

Nature Of Operations And Basis Of Presentation [Abstract]

Intercompany Transactions


	March 31, 2012		December 31, 2011

Accounts receivable for NPL services	$	5,185	$	6,205

Other Income (Deductions)


	Three Months Ended						Twelve Months Ended
	March 31						March 31
	2012			2011			2012			2011

Change in COLI policies	$	5,200		$	2,200		$	3,700		$	10,480
Interest income		226			92			619			265
Pipe replacement costs		(9	)		(886	)		(3,884	)		(4,339	)
Miscellaneous income and (expense)		11			(1,684	)		(141	)		(2,311	)

Total other income (deductions)	$	5,428		$	(278	)	$	294		$	4,095

Components Of Net Periodic Benefit Cost (Tables)

3 Months Ended

Mar. 31, 2012

Components Of Net Periodic Benefit Cost [Abstract]

Schedule Of Net Periodic Benefit Cost Of Defined Benefit Plans


	Qualified Retirement Plan
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	5,079		$	4,431		$	18,373		$	17,130
Interest cost		9,567			9,319			37,524			36,029
Expected return on plan assets		(11,445	)		(10,029	)		(41,530	)		(37,432	)
Amortization of net loss		5,971			3,587			16,732			11,445

Net periodic benefit cost	$	9,172		$	7,308		$	31,099		$	27,172


	SERP
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	69		$	55		$	231		$	334
Interest cost		407			441			1,732			1,975
Amortization of net loss		171			157			645			1,023

Net periodic benefit cost	$	647		$	653		$	2,608		$	3,332


	PBOP
	Period Ended March 31,
	Three Months						Twelve Months
	2012			2011			2012			2011
(Thousands of dollars)
Service cost	$	245		$	215		$	888		$	857
Interest cost		636			658			2,609			2,527
Expected return on plan assets		(601	)		(595	)		(2,385	)		(2,165	)
Amortization of transition obligation		217			216			868			866
Amortization of net loss		258			147			701			513

Net periodic benefit cost	$	755		$	641		$	2,681		$	2,598

Segment Information (Tables)

3 Months Ended

Mar. 31, 2012

Segment Information [Abstract]

List Of Revenues From External Customers, Intersegment Revenues, And Segment Net Income


	Natural Gas Operations		Construction Services			Total
Three months ended March 31, 2012
Revenues from external customers	$	530,713	$	110,887		$	641,600
Intersegment revenues		-		16,045			16,045

Total	$	530,713	$	126,932		$	657,645

Segment net income (loss)	$	79,366	$	(447	)	$	78,919


Three months ended March 31, 2011
Revenues from external customers	$	553,853	$	61,777		$	615,630
Intersegment revenues		-		12,810			12,810

Total	$	553,853	$	74,587		$	628,440

Segment net income	$	68,015	$	534		$	68,549


Twelve months ended March 31, 2012
Revenues from external customers	$	1,380,226	$	440,811		$	1,821,037
Intersegment revenues		-		95,356			95,356

Total	$	1,380,226	$	536,167		$	1,916,393

Segment net income	$	102,771	$	19,886		$	122,657


Twelve months ended March 31, 2011
Revenues from external customers	$	1,451,251	$	276,833		$	1,728,084
Intersegment revenues		-		61,976			61,976

Total	$	1,451,251	$	338,809		$	1,790,060

Segment net income	$	94,080	$	13,698		$	107,778

- Definition

Tabular disclosure of the profit or loss and total assets for each reportable segment. An entity discloses certain information on each reportable segment if the amounts (a) are included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) are otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss.

+ References + Details

Derivatives And Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2012

Derivatives And Fair Value Measurements [Abstract]

Notional Amounts Under Swaps Contracts


	March 31, 2012		December 31, 2011

Swaps contracts		10,072		10,827

Amount Of Gain Or (Loss) Recognized In Income On Derivative


		Three Months Ended						Twelve Months Ended
Instrument	Location of Gain or (Loss) Recognized in Income on Derivative	March 31						March 31
Instrument		2012			2011			2012			2011

Swaps	Net cost of gas sold	$	(6,936	)	$	289		$	(25,426	)	$	(11,059	)
Swaps	Net cost of gas sold		6,936	^*		(289	)^*		25,426	^*		11,059	^*

Total		$	-		$	-		$	-		$	-

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

Gains (Losses) On Derivatives In Cash Flow Hedging



	Three Months Ended				Twelve Months Ended
	March 31, 2012		March 31, 2011		March 31, 2012			March 31, 2011
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	$	2,959	$	671	$	(15,670	)	$	(16,788	)

Fair Values Of Swaps And FSIRS In Balance Sheet Location


Fair values of derivatives not designated as hedging instruments:

March 31, 2012		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
Swaps	Other current liabilities	$	-	$	(9,421	)	$	(9,421	)
Swaps	Other deferred credits		-		(1,712	)		(1,712	)

Total		$	-	$	(11,133	)	$	(11,133	)


December 31, 2011		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
Swaps	Other current liabilities	$	-	$	(11,122	)	$	(11,122	)
Swaps	Other deferred credits		-		(621	)		(621	)

Total		$	-	$	(11,743	)	$	(11,743	)


Fair values of derivatives designated as hedging instruments:

December 31, 2011		Asset		Liability
Instrument	Balance Sheet Location	Derivatives		Derivatives			Net Total
FSIRS	Other current liabilities	$	-	$	(24,713	)	$	(24,713	)

Paid To And Received From Counterparties For Settlements Of Matured Swaps


(Thousands of dollars)	Three Months Ended March 31, 2012		Twelve Months Ended March 31, 2012

Paid to counterparties	$	7,547	$	20,851

Regulatory Assets/Liabilities Offsetting The Derivatives At Fair Value In The Balance Sheet


March 31, 2012
Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	9,421
Swaps	Deferred charges and other assets		1,712

December 31, 2011
Instrument	Balance Sheet Location	Net Total
Swaps	Prepaids and other current assets	$	11,122
Swaps	Deferred charges and other assets		621

Significant Other Observable Inputs

		september 300			september 300
Level 2 – Significant other observable inputs

(Thousands of dollars)	March 31, 2012			December 31, 2011

Liabilities at fair value:
Other current liabilities – Swaps	$	(9,421	)	$	(11,122	)
Other deferred credits – Swaps		(1,712	)		(621	)
Other current liabilities – FSIRS		-			(24,713	)

Net Assets (Liabilities)	$	(11,133	)	$	(36,456	)

- Definition

Tabular disclosure of the location and amount of gains and losses reported in the statement of financial performance, or when applicable, the statement of financial position. For example, (a) gains and losses recognized in the income statement on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges and (b) gains and losses initially recognized in other comprehensive income on derivative instruments designated and qualifying as cash flow hedges.

+ References + Details

Long-Term Debt (Tables)

3 Months Ended

Mar. 31, 2012

Long-Term Debt [Abstract]

Schedule Of Estimated Fair Values Of Long-Term Debt


	March 31, 2012					December 31, 2011
	Carrying			Market		Carrying			Market
	Amount			Value		Amount			Value
(Thousands of dollars)
Debentures:
Notes, 7.625%, due 2012	$	200,000		$	201,480	$	200,000		$	204,312
Notes, 4.45%, due 2020		125,000			134,863		125,000			128,673
Notes, 6.1%, due 2041		125,000			142,840		125,000			143,074
Notes, 3.875%, due 2022		250,000			256,418
8% Series, due 2026		75,000			104,583		75,000			96,340
Medium-term notes, 7.59% series, due 2017		25,000			30,729		25,000			30,199
Medium-term notes, 7.78% series, due 2022		25,000			33,208		25,000			31,932
Medium-term notes, 7.92% series, due 2027		25,000			34,548		25,000			31,648
Medium-term notes, 6.76% series, due 2027		7,500			9,353		7,500			8,510
Unamortized discount		(3,632	)				(2,087	)

		853,868					605,413

Revolving credit facility and commercial paper		-			-		109,000			109,000

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:
6.10% 1999 Series A, due 2038		-			-		12,410			12,410
5.95% 1999 Series C, due 2038		14,320			14,403		14,320			14,449
5.55% 1999 Series D, due 2038		8,270			8,330		8,270			8,253
5.45% 2003 Series C, due 2038 (rate resets in 2013)		30,000			31,144		30,000			31,332
5.25% 2003 Series D, due 2038		20,000			19,912		20,000			19,583
5.80% 2003 Series E, due 2038 (rate resets in 2013)		15,000			15,544		15,000			15,634
5.25% 2004 Series A, due 2034		65,000			65,002		65,000			64,291
5.00% 2004 Series B, due 2033		31,200			30,728		31,200			30,283
4.85% 2005 Series A, due 2035		100,000			96,888		100,000			94,836
4.75% 2006 Series A, due 2036		24,855			23,766		24,855			23,179
Unamortized discount		(3,325	)				(3,360	)

		505,320					517,695

NPL debt obligaitons		33,943			33,981		21,368			21,380

		1,393,131					1,253,476
Less: current maturities		(205,055	)				(322,618	)

Long-term debt, less current maturities	$	1,188,076				$	930,858

- 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 SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

Reference 2: 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 3: 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 4: 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 5: 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 6: 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 7: 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 8: 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 9: 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 10: 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 11: 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=6881521&loc=d3e23780-122690

+ Details

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

3 Months Ended

Mar. 31, 2012

Equity Other Comprehensive Income And Accumulated Other Comprehensive Income [Abstract]

Detailed Activities Of Equity


	Southwest Gas Corporation Equity
	Common Stock				Additional Paid-in Capital		Accumulated Other Comprehensive Income (Loss)			Retained Earnings			Non- controlling Interest			Total
(In thousands, except per share amounts)	Shares		Amount		Additional Paid-in Capital		Accumulated Other Comprehensive Income (Loss)			Retained Earnings			Non- controlling Interest			Total
DECEMBER 31, 2011		45,956	$	47,586	$	821,640	$	(49,331	)	$	406,125		$	(989	)	$	1,225,031
Common stock issuances		154		154		1,820											1,974
Net income (loss)											78,919			(84	)		78,835
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax								476									476
FSIRS realized and unrealized loss, net of tax (Note 4)								1,834									1,834
Amounts reclassified to net income, net of tax (Note 4)								181									181
Dividends declared
Common: $0.295 per share											(13,779	)					(13,779	)

MARCH 31, 2012		46,110	$	47,740	$	823,460	$	(46,840	)	$	471,265		$	(1,073	)	$	1,294,552


	Three Months Ended March 31, 2012									Three Months Ended March 31, 2011

	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	-		$	-		$	-		$	-		$	-		$	-
Amortization of transition obligation		217			(83	)		134			216			(82	)		134
Amortization of net loss		6,400			(2,432	)		3,968			3,891			(1,479	)		2,412
Regulatory adjustment		(5,849	)		2,223			(3,626	)		(3,591	)		1,365			(2,226	)

Pension plans other comprehensive income (loss)		768			(292	)		476			516			(196	)		320

FSIRS (designated hedging activities):
Unrealized/realized gain		2,959			(1,125	)		1,834			671			(255	)		416
Amounts reclassifed into net income		292			(111	)		181			292			(111	)		181

FSIRS other comprehensive income		3,251			(1,236	)		2,015			963			(366	)		597

Total other comprehensive income (loss)	$	4,019		$	(1,528	)	$	2,491		$	1,479		$	(562	)	$	917


	Twelve Months Ended March 31, 2012									Twelve Months Ended March 31, 2011

	Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount			Before- Tax Amount			Tax (Expense) or Benefit (1)			Net-of- Tax Amount
Defined benefit pension plans:
Net actuarial gain/(loss)	$	(135,492	)	$	51,487		$	(84,005	)	$	(9,058	)	$	3,442		$	(5,616	)
Amortization of transition obligation		868			(331	)		537			866			(329	)		537
Amortization of net loss		18,078			(6,869	)		11,209			12,981			(4,933	)		8,048
Regulatory adjustment		103,673			(39,396	)		64,277			(241	)		93			(148	)

Pension plans other comprehensive income (loss)		(12,873	)		4,891			(7,982	)		4,548			(1,727	)		2,821

FSIRS (designated hedging activities):
Unrealized/realized loss		(15,670	)		5,954			(9,716	)		(16,788	)		6,380			(10,408	)
Amounts reclassifed into net income		1,169			(444	)		725			389			(148	)		241

FSIRS other comprehensive income (loss)		(14,501	)		5,510			(8,991	)		(16,399	)		6,232			(10,167	)

Total other comprehensive income (loss)	$	(27,374	)	$	10,401		$	(16,973	)	$	(11,851	)	$	4,505		$	(7,346	)

(1)	Tax amounts are calculated using a 38% rate.

Rollforward Of Accumulated Other Comprehensive Income


	Defined Benefit Plans									FSIRS
	Before- Tax			Tax (Expense) Benefit			After- Tax			Before- Tax			Tax (Expense) Benefit			After- Tax			AOCI
Beginning Balance AOCI December 31, 2011	$	(44,429	)	$	16,883		$	(27,546	)	$	(35,138	)	$	13,353		$	(21,785	)	$	(49,331	)
Current period change		768			(292	)		476			3,251			(1,236	)		2,015			2,491

Ending Balance AOCI March 31, 2012	$	(43,661	)	$	16,591		$	(27,070	)	$	(31,887	)	$	12,117		$	(19,770	)	$	(46,840	)

Nature Of Operations And Basis Of Presentation (Intercompany Transactions) (Details) - USD ($) $ in Thousands	Mar. 31, 2012	Dec. 31, 2011
NPL Construction Co [Member]
Accounts receivable for NPL services	$ 5,185	$ 6,205

Nature Of Operations And Basis Of Presentation (Other Income (Deductions)) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Nature Of Operations And Basis Of Presentation [Abstract]
Change in COLI policies	$ 5,200	$ 2,200	$ 3,700	$ 10,480
Interest income	226	92	619	265
Pipe replacement costs	(9)	(886)	(3,884)	(4,339)
Miscellaneous income and (expense)	11	(1,684)	(141)	(2,311)
Total other income (deductions)	$ 5,428	$ (278)	$ 294	$ 4,095

Components Of Net Periodic Benefit Cost (Schedule Of Net Periodic Benefit Cost Of Defined Benefit Plans) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,079	$ 4,431	$ 18,373	$ 17,130
Interest cost	9,567	9,319	37,524	36,029
Expected return on plan assets	(11,445)	(10,029)	(41,530)	(37,432)
Amortization of net loss	5,971	3,587	16,732	11,445
Net periodic benefit cost	9,172	7,308	31,099	27,172
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	69	55	231	334
Interest cost	407	441	1,732	1,975
Amortization of net loss	171	157	645	1,023
Net periodic benefit cost	647	653	2,608	3,332
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	245	215	888	857
Interest cost	636	658	2,609	2,527
Expected return on plan assets	(601)	(595)	(2,385)	(2,165)
Amortization of transition obligation	217	216	868	866
Amortization of net loss	258	147	701	513
Net periodic benefit cost	$ 755	$ 641	$ 2,681	$ 2,598

Segment Information (List Of Revenues From External Customers, Intersegment Revenues, And Segment Net Income) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting Information [Line Items]
Revenues from external customers	$ 641,600	$ 615,630	$ 1,821,037	$ 1,728,084
Intersegment revenues	16,045	12,810	95,356	61,976
Total	657,645	628,440	1,916,393	1,790,060
Segment net income (loss)	78,919	68,549	122,657	107,778
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	530,713	553,853	1,380,226	1,451,251
Total	530,713	553,853	1,380,226	1,451,251
Segment net income (loss)	79,366	68,015	102,771	94,080
Construction Services [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	110,887	61,777	440,811	276,833
Intersegment revenues	16,045	12,810	95,356	61,976
Total	126,932	74,587	536,167	338,809
Segment net income (loss)	$ (447)	$ 534	$ 19,886	$ 13,698

Derivatives And Fair Value Measurements (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2012 USD ($) yr
Derivative [Line Items]
Natural gas portfolios, minimum % rate	25.00%
Natural gas portfolios, maximum % rate	35.00%
Maturities of natural gas swaps, minimum	April 2012
Maturities of natural gas swaps, maximum	March 2014
Notional amount	$ 100.0
Settlement paid to the counterparties	$ 21.8
February 2011 [Member]
Derivative [Line Items]
Debt maturing date	February 2011
Senior debt repayment	$ 200.0
May 2012 [Member]
Derivative [Line Items]
Debt maturing date	May 2012
Retirement of long-term debt	$ 200.0
Notes, 4.45%, Due 2020 [Member]
Derivative [Line Items]
Notes payable	$ 125.0
Debt instrument interest rate	4.45%
Issuance of fixed rate debt (in years) \| yr	10
Notes, 3.875%, Due 2022 [Member]
Derivative [Line Items]
Notes payable	$ 250.0
Debt instrument interest rate	3.875%
Issuance of fixed rate debt (in years) \| yr	10

Derivatives And Fair Value Measurements (Notional Amounts Under Swaps Contracts) (Details) Unit12 in Thousands	Mar. 31, 2012	Dec. 31, 2011
Derivatives And Fair Value Measurements [Abstract]
Swaps contracts	10,072	10,827

Derivatives And Fair Value Measurements (Amount Of Gain Or (Loss) Recognized In Income On Derivative) (Details) - Swaps Contracts [Member] - Net Cost Of Gas Sold [Member] - USD ($)
$ in Thousands

3 Months Ended

12 Months Ended

Mar. 31, 2012

Mar. 31, 2011

Mar. 31, 2012

Mar. 31, 2011

Derivative Instruments, Gain (Loss) [Line Items]

Amount of (loss) recognized in income on derivative

$ (6,936)

$ 289

$ (25,426)

$ (11,059)

Amount of gain or (loss) recognized in income on derivative

^[1]

$ 6,936

(289)

$ 25,426

$ 11,059

Amount of gain or (loss) recognized in income on derivative, total

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

- Definition

The amount of net gains and losses recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments.

+ References + Details

- Definition

The amount of gain recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments.

+ References + Details

- Definition

The amount of loss recognized in income during the period on derivative instruments designated and qualifying as hedging instruments in fair value hedges and related hedged items designated and qualifying in fair value hedges, on derivative instruments designated and qualifying as hedging instruments in cash flow hedges, and on derivative instruments not designated as hedging instruments.

+ References + Details

Derivatives And Fair Value Measurements (Gains (Losses) On Derivatives In Cash Flow Hedging) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Derivatives And Fair Value Measurements [Abstract]
Amount of gain/(loss) realized/ unrealized on FSIRS recognized in other comprehensive income on derivative	$ 2,959	$ 671	$ (15,670)	$ (16,788)

Derivatives And Fair Value Measurements (Fair Values Of Swaps And FSIRS In Balance Sheet Location) (Details) - USD ($) $ in Thousands	Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Asset Derivatives not designated as hedging instruments
Liability derivatives not designated as hedging instruments	(11,133)	(11,743)
Net Total not designated as hedging instruments	(11,133)	(11,743)
Swaps [Member] \| Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives not designated as hedging instruments
Liability derivatives not designated as hedging instruments	(9,421)	(11,122)
Net Total not designated as hedging instruments	(9,421)	(11,122)
Swaps [Member] \| Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives not designated as hedging instruments
Liability derivatives not designated as hedging instruments	(1,712)	(621)
Net Total not designated as hedging instruments	$ (1,712)	(621)
FSIRS [Member] \| Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Assets Derivatives designated as hedging instruments
Liability Derivatives designated as hedging instruments		(24,713)
Net Total designated as hedging instruments		$ (24,713)

Derivatives And Fair Value Measurements (Paid To And Received From Counterparties For Settlements Of Matured Swaps) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2012
Derivatives And Fair Value Measurements [Abstract]
Paid to counterparties	$ 7,547	$ 20,851

Derivatives And Fair Value Measurements (Regulatory Assets/Liabilities Offsetting The Derivatives At Fair Value In The Balance Sheet) (Details) - Swaps [Member] - USD ($) $ in Thousands	Mar. 31, 2012	Dec. 31, 2011
Prepaids And Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 9,421	$ 11,122
Deferred Charges And Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ 1,712	$ 621

Derivatives And Fair Value Measurements (Significant Other Observable Inputs) (Details) - Level 2-Significant Other Observable Inputs [Member] - USD ($) $ in Thousands	Mar. 31, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ (11,133)	$ (36,456)
Swaps [Member] \| Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	(9,421)	(11,122)
Swaps [Member] \| Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ (1,712)	(621)
FSIRS [Member] \| Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities		$ (24,713)

- Definition

Fair values as of the balance sheet date of all liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References + Details

Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	Mar. 31, 2012	Jan. 31, 2012	Mar. 31, 2012
Debt Instrument [Line Items]
Interest rate description			that is determined based on the Company's senior unsecured debt rating. At the Company's current unsecured debt rating, the applicable margin is <font class="_mt">1.125</font>% for loans bearing interest with reference to LIBOR and <font class="_mt">0.125</font>%
Line of credit designated as long-term debt	$ 150.0		$ 150.0
Line of credit designated for working capital purposes	150.0		150.0
March 2017 [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 300.0		300.0
Credit facility expiration date	March 2017
May 2012 [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity	$ 300.0		$ 300.0
Credit facility expiration date	May 2012
Notes, 4.45%, Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	4.45%		4.45%
Debt instrument due date			2020
6.10% 1999 Series A, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%	6.10%
Senior Notes		$ 12.4
Debt instrument due date		2038	2038
Notes, 3.875%, Due 2022 [Member]
Debt Instrument [Line Items]
Senior Notes issued	$ 250.0
Debt instrument interest rate	3.875%		3.875%
Discount on issuance of debt	0.034%
Senior Notes issued maturity date	April 1, 2022
Debt instrument due date			2022
Notes, 7.625%, Due 2012 [Member]
Debt Instrument [Line Items]
Senior debt repayment	$ 200.0
Debt instrument interest rate	7.625%		7.625%
Senior Notes issued maturity date	May 2012
Debt instrument due date			2012
LIBOR [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin	1.125%		1.125%
Alternative Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Applicable margin	0.125%		0.125%

Long-Term Debt (Schedule Of Estimated Fair Values Of Long-Term Debt) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Jan. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Less: current maturities		$ (205,055)	$ (322,618)
Long-term debt, less current maturities		1,188,076	930,858
Carrying Amount [Member]
Debt Instrument [Line Items]
Unamortized discount		(3,325)	(3,360)
Long-term debt, gross		1,393,131	1,253,476
Less: current maturities		(205,055)	(322,618)
Long-term debt, less current maturities		$ 1,188,076	930,858
Notes, 7.625%, Due 2012 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.625%
Debt instrument due date		2012
Notes, 4.45%, Due 2020 [Member]
Debt Instrument [Line Items]
Notes payable		$ 125,000
Debt instrument interest rate		4.45%
Debt instrument due date		2020
Notes, 6.1%, Due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.10%
Debt instrument due date		2041
Notes, 3.875%, Due 2022 [Member]
Debt Instrument [Line Items]
Notes payable		$ 250,000
Debt instrument interest rate		3.875%
Debt instrument due date		2022
8% Series, Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		8.00%
Debt instrument due date		2026
Medium-Term Notes, 7.59% Series, Due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.59%
Debt instrument due date		2017
Medium-Term Notes, 7.78% Series, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.78%
Debt instrument due date		2022
Medium-Term Notes, 7.92% Series, Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.92%
Debt instrument due date		2027
Medium-Term Notes, 6.76% Series, Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.76%
Debt instrument due date		2027
Notes Payable [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		$ 853,868	605,413
Revolving Credit Facility And Commercial Paper [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding			109,000
Revolving Credit Facility And Commercial Paper [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value			109,000
Tax-Exempt Series A, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument due date		2028
2003 Series A, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		2038
2008 Series A, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		2038
2009 Series A, Due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date		2039
6.10% 1999 Series A, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	6.10%	6.10%
Debt instrument due date	2038	2038
5.95% 1999 Series C, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.95%
Debt instrument due date		2038
5.55% 1999 Series D, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.55%
Debt instrument due date		2038
5.45% 2003 Series C, Due 2038 (Rate Resets In 2013) [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.45%
Debt instrument due date		2038
Debt instrument rate reset		2013
5.25% 2003 Series D, Due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.25%
Debt instrument due date		2038
5.80% 2003 Series E, Due 2038 (Rate Resets In 2013) [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.80%
Debt instrument due date		2038
Debt instrument rate reset		2013
5.25% 2004 Series A, Due 2034 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.25%
Debt instrument due date		2034
5.00% 2004 Series B, Due 2033 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		5.00%
Debt instrument due date		2033
4.85% 2005 Series A, Due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		4.85%
Debt instrument due date		2035
4.75% 2006 Series A, Due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		4.75%
Debt instrument due date		2036
Bonds [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		$ 505,320	517,695
NPL debt obligations [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
NPL debt obligations		33,943	21,368
NPL debt obligations [Member] \| Market Value [Member]
Debt Instrument [Line Items]
NPL debt obligations fair value		33,981	21,380
Debentures [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unamortized discount		(3,632)	(2,087)
Debentures [Member] \| Notes, 7.625%, Due 2012 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		200,000	200,000
Debentures [Member] \| Notes, 7.625%, Due 2012 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		201,480	204,312
Debentures [Member] \| Notes, 4.45%, Due 2020 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		125,000	125,000
Debentures [Member] \| Notes, 4.45%, Due 2020 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		134,863	128,673
Debentures [Member] \| Notes, 6.1%, Due 2041 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		125,000	125,000
Debentures [Member] \| Notes, 6.1%, Due 2041 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		142,840	143,074
Debentures [Member] \| Notes, 3.875%, Due 2022 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		250,000
Debentures [Member] \| Notes, 3.875%, Due 2022 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		256,418
Debentures [Member] \| 8% Series, Due 2026 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		75,000	75,000
Debentures [Member] \| 8% Series, Due 2026 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		104,583	96,340
Debentures [Member] \| Medium-Term Notes, 7.59% Series, Due 2017 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		25,000	25,000
Debentures [Member] \| Medium-Term Notes, 7.59% Series, Due 2017 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		30,729	30,199
Debentures [Member] \| Medium-Term Notes, 7.78% Series, Due 2022 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		25,000	25,000
Debentures [Member] \| Medium-Term Notes, 7.78% Series, Due 2022 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		33,208	31,932
Debentures [Member] \| Medium-Term Notes, 7.92% Series, Due 2027 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		25,000	25,000
Debentures [Member] \| Medium-Term Notes, 7.92% Series, Due 2027 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		34,548	31,648
Debentures [Member] \| Medium-Term Notes, 6.76% Series, Due 2027 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		7,500	7,500
Debentures [Member] \| Medium-Term Notes, 6.76% Series, Due 2027 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure		9,353	8,510
Variable-Rate Bonds [Member] \| Tax-Exempt Series A, Due 2028 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		50,000	50,000
Variable-Rate Bonds [Member] \| Tax-Exempt Series A, Due 2028 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		50,000	50,000
Variable-Rate Bonds [Member] \| 2003 Series A, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		50,000	50,000
Variable-Rate Bonds [Member] \| 2003 Series A, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		50,000	50,000
Variable-Rate Bonds [Member] \| 2008 Series A, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		50,000	50,000
Variable-Rate Bonds [Member] \| 2008 Series A, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		50,000	50,000
Variable-Rate Bonds [Member] \| 2009 Series A, Due 2039 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		50,000	50,000
Variable-Rate Bonds [Member] \| 2009 Series A, Due 2039 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		50,000	50,000
Fixed-Rate Bonds [Member] \| 6.10% 1999 Series A, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt			12,410
Fixed-Rate Bonds [Member] \| 6.10% 1999 Series A, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value			12,410
Fixed-Rate Bonds [Member] \| 5.95% 1999 Series C, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		14,320	14,320
Fixed-Rate Bonds [Member] \| 5.95% 1999 Series C, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		14,403	14,449
Fixed-Rate Bonds [Member] \| 5.55% 1999 Series D, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		8,270	8,270
Fixed-Rate Bonds [Member] \| 5.55% 1999 Series D, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		8,330	8,253
Fixed-Rate Bonds [Member] \| 5.45% 2003 Series C, Due 2038 (Rate Resets In 2013) [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		30,000	30,000
Fixed-Rate Bonds [Member] \| 5.45% 2003 Series C, Due 2038 (Rate Resets In 2013) [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		31,144	31,332
Fixed-Rate Bonds [Member] \| 5.25% 2003 Series D, Due 2038 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		20,000	20,000
Fixed-Rate Bonds [Member] \| 5.25% 2003 Series D, Due 2038 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		19,912	19,583
Fixed-Rate Bonds [Member] \| 5.80% 2003 Series E, Due 2038 (Rate Resets In 2013) [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		15,000	15,000
Fixed-Rate Bonds [Member] \| 5.80% 2003 Series E, Due 2038 (Rate Resets In 2013) [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		15,544	15,634
Fixed-Rate Bonds [Member] \| 5.25% 2004 Series A, Due 2034 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		65,000	65,000
Fixed-Rate Bonds [Member] \| 5.25% 2004 Series A, Due 2034 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		65,002	64,291
Fixed-Rate Bonds [Member] \| 5.00% 2004 Series B, Due 2033 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		31,200	31,200
Fixed-Rate Bonds [Member] \| 5.00% 2004 Series B, Due 2033 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		30,728	30,283
Fixed-Rate Bonds [Member] \| 4.85% 2005 Series A, Due 2035 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		100,000	100,000
Fixed-Rate Bonds [Member] \| 4.85% 2005 Series A, Due 2035 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		96,888	94,836
Fixed-Rate Bonds [Member] \| 4.75% 2006 Series A, Due 2036 [Member] \| Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt		24,855	24,855
Fixed-Rate Bonds [Member] \| 4.75% 2006 Series A, Due 2036 [Member] \| Market Value [Member]
Debt Instrument [Line Items]
Unsecured Debt Fair Value		$ 23,766	$ 23,179

Equity, Other Comprehensive Income, And Accumulated Other Comprehensive Income (Detailed Activities Of Equity) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011, shares	45,956,088
Balance, December 31, 2011	$ 1,225,031
Common stock issuances, value	1,974
Net income (loss)	78,835	$ 68,354	$ 122,244	$ 107,347
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	476		84,005	5,616
FSIRS realized and unrealized loss, net of tax (Note 4)	1,834
Amounts reclassified to net income, net of tax (Note 4)	181	$ 181	$ 725	$ 241
Dividends declared Common: $0.295 per share	$ (13,779)
Balance, March 31, 2012, shares	46,110,072		46,110,072
Balance, March 31, 2012	$ 1,294,552		$ 1,294,552
Dividends declared per share	$ 0.295	$ 0.265	$ 1.090	$ 1.015
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011, shares	45,956,000
Balance, December 31, 2011	$ 47,586
Common stock issuances, Shares	154,000
Common stock issuances, value	$ 154
Balance, March 31, 2012, shares	46,110,000		46,110,000
Balance, March 31, 2012	$ 47,740		$ 47,740
Additional Paid-In Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011	821,640
Common stock issuances, value	1,820
Balance, March 31, 2012	823,460		823,460
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011	(49,331)
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	476
FSIRS realized and unrealized loss, net of tax (Note 4)	1,834
Amounts reclassified to net income, net of tax (Note 4)	181
Balance, March 31, 2012	(46,840)		(46,840)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011	406,125
Net income (loss)	78,919
Dividends declared Common: $0.295 per share	(13,779)
Balance, March 31, 2012	471,265		471,265
Non-Controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance, December 31, 2011	(989)
Net income (loss)	(84)
Balance, March 31, 2012	$ (1,073)		$ (1,073)

Equity, Other Comprehensive Income, And Accumulated Other Comprehensive Income (Schedule Of Related Tax Effects Allocated To Each Component Of Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Thousands

3 Months Ended

12 Months Ended

Mar. 31, 2012

Mar. 31, 2011

Mar. 31, 2012

Mar. 31, 2011

Equity Other Comprehensive Income And Accumulated Other Comprehensive Income [Abstract]

Net actuarial gain/(loss), Before-Tax Amount

$ (135,492)

$ (9,058)

Net actuarial gain/(loss), Tax (Expense) or Benefit

^[1]

51,487

3,442

Net actuarial gain/(loss), Net-of-Tax Amount

(84,005)

(5,616)

Amortization of transition obligation, Before-Tax Amount

$ 217

$ 216

868

866

Amortization of transition obligation, Tax (Expense) or Benefit

^[1]

(83)

(82)

(331)

(329)

Amortization of transition obligation, Net-of-Tax Amount

134

537

Amortization of net loss, Before-Tax Amount

6,400

3,891

18,078

12,981

Amortization of net loss, Tax (Expense) or Benefit

^[1]

(2,432)

(1,479)

(6,869)

(4,933)

Amortization of net loss, Net-of-Tax Amount

3,968

2,412

11,209

8,048

Regulatory adjustment, Before-Tax Amount

(5,849)

(3,591)

103,673

(241)

Regulatory adjustment, Tax (Expense) or Benefit

^[1]

2,223

1,365

(39,396)

Regulatory adjustment, Net-of-Tax Amount

(3,626)

(2,226)

64,277

(148)

Pension plans other comprehensive income (loss), Before-Tax Amount

768

516

(12,873)

4,548

Pension plans other comprehensive income (loss), Tax (Expense) or Benefit

^[1]

(292)

(196)

4,891

(1,727)

Pension plans other comprehensive income (loss), Net-of-Tax Amount

476

320

(7,982)

2,821

Unrealized/realized loss, Before-Tax Amount

2,959

671

(15,670)

(16,788)

Unrealized/realized loss, Tax (Expense) or Benefit

^[1]

(1,125)

(255)

5,954

6,380

Unrealized/realized loss, Net-of-Tax Amount

1,834

416

(9,716)

(10,408)

Amounts reclassified into net income, Before-Tax Amount

292

1,169

389

Amounts reclassified into net income, Tax (Expense) or Benefit

^[1]

(111)

(444)

(148)

Amounts reclassified into net income, Net-of-Tax Amount

181

725

241

FSIRS other comprehensive income (loss), Before-Tax Amount

3,251

963

(14,501)

(16,399)

FSIRS other comprehensive income (loss), Tax (Expense) or Benefit

^[1]

(1,236)

(366)

5,510

6,232

FSIRS other comprehensive income (loss), Net-of-Tax Amount

2,015

597

(8,991)

(10,167)

Total other comprehensive income (loss), Before-Tax Amount

4,019

1,479

(27,374)

(11,851)

Total other comprehensive income (loss), Tax (Expense) or Benefit

^[1]

(1,528)

(562)

10,401

4,505

Total other comprehensive income (loss), Net-of-Tax Amount

$ 2,491

$ 917

(16,973)

$ (7,346)

Other comprehensive income loss, effective income tax rate

38.00%

Amount of FSIRS existing AOCI losses expected to reclassified income in next 12 months

$ 2,100

[1]	Tax amounts are calculated using a 38% rate.

Equity, Other Comprehensive Income, And Accumulated Other Comprehensive Income (Rollforward Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Equity Other Comprehensive Income And Accumulated Other Comprehensive Income [Abstract]
Beginning Balance AOCI December 31, 2011, Defined Benefit Plans - Before-Tax	$ (44,429)
Beginning Balance AOCI December 31, 2011, Defined Benefit Plans - Tax (Expense) Benefit	16,883
Beginning balance AOCI December 31, 2011 Defined Benefit Plans, After-Tax	(27,546)
Beginning Balance AOCI December 31, 2011, FSIRS - Before-Tax	(35,138)
Beginning Balance AOCI December 31, 2011, FSIRS - Tax (Expense) Benefit	13,353
Beginning Balance AOCI December 31, 2011, FSIRS - After-Tax	(21,785)
Beginning Balance AOCI December 31, 2011	(49,331)
Current period change, Defined Benefit Plans - Before-Tax	768
Current period change, Defined Benefit Plans - Tax (Expense) Benefit	(292)
Current period change, Defined Benefit Plans - After-Tax	476	$ 320	$ (7,982)	$ 2,821
Current period change, FSIRS - Before-Tax	3,251	963	(14,501)	(16,399)
Current period change, FSIRS - Tax (Expense) Benefit	(1,236)
Current period change, FSIRS - After-Tax	2,015	$ 597	(8,991)	$ (10,167)
Current period change, AOCI	2,491
Ending Balance AOCI March 31, 2012, Defined Benefit Plans - Before-Tax	(43,661)		(43,661)
Ending Balance AOCI March 31, 2012, Defined Benefit Plans - Tax (Expense) Benefit	16,591		16,591
Ending Balance AOCI March 31, 2012, Defined Benefit Plans - After-Tax	(27,070)		(27,070)
Ending Balance AOCI March 31, 2012, FSIRS - Before-Tax	(31,887)		(31,887)
Ending Balance AOCI March 31, 2012, FSIRS - Tax (Expense) Benefit	12,117		12,117
Ending Balance AOCI March 31, 2012, FSIRS - After-Tax	(19,770)		(19,770)
Ending Balance AOCI March 31, 2012	$ (46,840)		$ (46,840)