Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 9,099	$ 5,063	$ 84,454	$ 76,942	$ 146,942	$ 137,766
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(18,922)	(107,661)
Amortization of prior service cost	207	208	414	414	828	524
Amortization of net actuarial loss	4,194	5,328	8,390	10,658	19,048	17,992
Prior service cost	0	0	0	0	0	(4,130)
Regulatory adjustment	(3,796)	(4,828)	(7,592)	(9,656)	(1,436)	83,755
Net defined benefit pension plans	605	708	1,212	1,416	(482)	(9,520)
Forward-starting interest rate swaps:
Amounts reclassified into net income	519	518	1,038	1,037	2,074	2,073
Net forward-starting interest rate swaps	519	518	1,038	1,037	2,074	2,073
Foreign currency translation adjustments	70	209	852	(1,063)	(39)	(1,722)
Total other comprehensive income (loss), net of tax	1,194	1,435	3,102	1,390	1,553	(9,169)
Comprehensive income	10,293	6,498	87,556	78,332	148,495	128,597
Comprehensive income (loss) attributable to noncontrolling interests	159	122	94	(25)	1,166	61
Comprehensive income attributable to Southwest Gas Corporation	$ 10,134	$ 6,376	$ 87,462	$ 78,357	$ 147,329	$ 128,536