Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (4,210)	$ 1,927	$ 72,732	$ 82,251	$ 131,629	$ 139,440
Defined benefit pension plans:
Net actuarial gain (loss)	0	0	0	0	(107,661)	62,214
Amortization of prior service cost	207	55	621	165	676	221
Amortization of net actuarial loss	5,329	3,667	15,987	11,000	19,654	16,297
Prior service cost	0	0	0	0	(4,130)	0
Regulatory adjustment	(4,828)	(3,210)	(14,484)	(9,630)	82,137	(72,176)
Net defined benefit pension plans	708	512	2,124	1,535	(9,324)	6,556
Forward-starting interest rate swaps:
Amounts reclassified into net income	519	518	1,556	1,555	2,074	2,074
Net forward-starting interest rate swaps	519	518	1,556	1,555	2,074	2,074
Foreign currency translation adjustments	(510)	0	(1,573)	0	(2,232)	0
Total other comprehensive income (loss), net of tax	717	1,030	2,107	3,090	(9,482)	8,630
Comprehensive income (loss)	(3,493)	2,957	74,839	85,341	122,147	148,070
Comprehensive income (loss) attributable to noncontrolling interests	504	(43)	479	(129)	608	(243)
Comprehensive income (loss) attributable to Southwest Gas Corporation	$ (3,997)	$ 3,000	$ 74,360	$ 85,470	$ 121,539	$ 148,313