Condensed Consolidated Statements of Income (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Operating revenues:
Gas operating revenues	$ 553,115	$ 486,493	$ 1,448,709	$ 1,293,047
Construction revenues	181,105	121,903	798,822	652,626
Total operating revenues	734,220	608,396	2,247,531	1,945,673
Operating expenses:
Net cost of gas sold	253,762	191,377	567,741	426,770
Operations and maintenance	95,510	102,408	376,834	390,235
Depreciation and amortization	67,467	62,891	257,603	240,775
Taxes other than income taxes	12,997	11,456	48,793	45,212
Construction expenses	174,928	113,199	709,586	579,795
Total operating expenses	604,664	481,331	1,960,557	1,682,787
Operating income	129,556	127,065	286,974	262,886
Other income and (expenses):
Net interest deductions	(17,977)	(17,519)	(72,527)	(65,341)
Other income (deductions)	2,272	1,612	7,767	9,843
Total other income and (expenses)	(15,705)	(15,907)	(64,760)	(55,498)
Income before income taxes	113,851	111,158	222,214	207,388
Income tax expense	41,972	40,461	79,884	72,492
Net income	71,879	70,697	142,330	134,896
Net income (loss) attributable to noncontrolling interests	(104)	(86)	4	(434)
Net income attributable to Southwest Gas Corporation	$ 71,983	$ 70,783	$ 142,326	$ 135,330
Basic earnings per share	$ 1.54	$ 1.52	$ 3.06	$ 2.92
Diluted earnings per share	$ 1.53	$ 1.51	$ 3.03	$ 2.89
Dividends declared per share	$ 0.405	$ 0.365	$ 1.500	$ 1.355
Average number of common shares outstanding	46,612	46,440	46,537	46,364
Average shares outstanding (assuming dilution)	47,036	46,871	46,986	46,812