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