CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	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