Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 5,063	$ 9,627	$ 76,942	$ 80,324	$ 137,766	$ 134,456
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(107,661)	62,214
Amortization of prior service cost	208	55	414	110	524	221
Amortization of net actuarial loss	5,328	3,666	10,658	7,333	17,992	17,927
Prior service cost	0	0	0	0	(4,130)	0
Regulatory adjustment	(4,828)	(3,210)	(9,656)	(6,420)	83,755	(73,667)
Net defined benefit pension plans	708	511	1,416	1,023	(9,520)	6,695
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	519	1,037	1,037	2,073	2,074
Net forward-starting interest rate swaps	518	519	1,037	1,037	2,073	2,074
Foreign currency translation adjustments	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), net of tax	1,435	1,030	1,390	2,060	(9,169)	8,769
Comprehensive income	6,498	10,657	78,332	82,384	128,597	143,225
Comprehensive income (loss) attributable to noncontrolling interests	122	0	(25)	(86)	61	(393)
Comprehensive income attributable to Southwest Gas Corporation	$ 6,376	$ 10,657	$ 78,357	$ 82,470	$ 128,536	$ 143,618