Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ 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
Operating revenues:
Gas operating revenues	$ 219,420	$ 226,027	$ 1,059,178	$ 983,999	$ 1,457,266	$ 1,356,653
Construction revenues	285,976	206,448	719,042	510,025	948,637	675,728
Total operating revenues	505,396	432,475	1,778,220	1,494,024	2,405,903	2,032,381
Operating expenses:
Net cost of gas sold	64,268	72,987	427,045	362,349	570,052	480,608
Operations and maintenance	100,102	93,389	294,956	293,417	385,271	390,328
Depreciation and amortization	66,476	62,037	200,898	187,114	266,811	247,684
Taxes other than income taxes	11,652	11,835	37,063	34,256	50,059	45,786
Construction expenses	246,755	173,937	647,512	444,778	850,591	595,093
Total operating expenses	489,253	414,185	1,607,474	1,321,914	2,122,784	1,759,499
Operating income	16,143	18,290	170,746	172,110	283,119	272,882
Other income and (expenses):
Net interest deductions	(18,401)	(17,421)	(54,095)	(52,245)	(73,919)	(69,680)
Other income (deductions)	(3,660)	440	(1,226)	4,915	966	9,026
Total other income and (expenses)	(22,061)	(16,981)	(55,321)	(47,330)	(72,953)	(60,654)
Income (loss) before income taxes	(5,918)	1,309	115,425	124,780	210,166	212,228
Income tax expense (benefit)	(1,708)	(618)	42,693	42,529	78,537	72,788
Net income (loss)	(4,210)	1,927	72,732	82,251	131,629	139,440
Net income (loss) attributable to noncontrolling interests	524	(43)	534	(129)	685	(243)
Net income (loss) attributable to Southwest Gas Corporation	$ (4,734)	$ 1,970	$ 72,198	$ 82,380	$ 130,944	$ 139,683
Basic earnings (loss) per share	$ (0.10)	$ 0.04	$ 1.54	$ 1.77	$ 2.80	$ 3.01
Diluted earnings (loss) per share	(0.10)	0.04	1.53	1.76	2.77	2.98
Dividends declared per share	$ 0.405	$ 0.365	$ 1.215	$ 1.095	$ 1.580	$ 1.425
Average number of common shares outstanding	47,102	46,513	46,863	46,485	46,777	46,451
Average shares outstanding (assuming dilution)	0	46,966	47,261	46,928	47,194	46,904