Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 2,907	$ (4,210)	$ 87,361	$ 72,732	$ 154,059	$ 131,629
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(18,922)	(107,661)
Amortization of prior service cost	207	207	621	621	828	676
Amortization of net actuarial loss	4,196	5,329	12,586	15,987	17,915	19,654
Prior service cost	0	0	0	0	0	(4,130)
Regulatory adjustment	(3,796)	(4,828)	(11,388)	(14,484)	(404)	82,137
Net defined benefit pension plans	607	708	1,819	2,124	(583)	(9,324)
Forward-starting interest rate swaps:
Amounts reclassified into net income	518	519	1,556	1,556	2,073	2,074
Net forward-starting interest rate swaps	518	519	1,556	1,556	2,073	2,074
Foreign currency translation adjustments	(238)	(510)	614	(1,573)	233	(2,232)
Total other comprehensive income (loss), net of tax	887	717	3,989	2,107	1,723	(9,482)
Comprehensive income (loss)	3,794	(3,493)	91,350	74,839	155,782	122,147
Comprehensive income (loss) attributable to noncontrolling interests	427	504	521	479	1,089	608
Comprehensive income (loss) attributable to Southwest Gas Corporation	$ 3,367	$ (3,997)	$ 90,829	$ 74,360	$ 154,693	$ 121,539