Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Operating revenues:
Gas operating revenues	$ 494,313	$ 462,602	$ 1,334,019	$ 1,258,914
Construction revenues	260,017	192,135	1,314,366	1,125,065
Total operating revenues	754,330	654,737	2,648,385	2,383,979
Gas operating revenues	494,313	462,602	1,334,019	1,258,914
Operating expenses:
Net cost of gas sold	185,732	146,879	393,898	330,400
Operations and maintenance	102,351	104,295	390,819	390,402
Depreciation and amortization	62,478	72,478	240,951	286,250
Taxes other than income taxes	15,257	14,782	58,421	53,145
Construction expenses	258,952	191,956	1,215,959	1,022,997
Total operating expenses	624,770	530,390	2,300,048	2,083,194
Operating income	129,560	124,347	348,337	300,785
Other income and (expenses):
Net interest deductions	(22,631)	(18,714)	(81,981)	(74,653)
Other income (deductions)	(4,334)	(990)	(9,374)	(8,062)
Total other income and (expenses)	(26,965)	(19,704)	(91,355)	(82,715)
Income before income taxes	102,595	104,643	256,982	218,070
Income tax expense	24,301	35,638	53,751	71,365
Net income	78,294	69,005	203,231	146,705
Net income (loss) attributable to noncontrolling interests	(797)	(303)	(393)	802
Net income	$ 79,091	$ 69,308	$ 203,624	$ 145,903
Basic earnings per share	$ 1.63	$ 1.46	$ 4.23	$ 3.07
Diluted earnings per share	1.63	1.45	4.23	3.05
Dividends declared per share	$ 0.520	$ 0.495	$ 2.005	$ 1.845
Average number of common shares	48,416	47,530	48,105	47,492
Average shares (assuming dilution)	48,459	47,864	48,139	47,839
Southwest Gas Corporation [Member]
Operating revenues:
Gas operating revenues	$ 494,313	$ 462,602	$ 1,334,019	$ 1,258,914
Gas operating revenues	494,313	462,602	1,334,019	1,258,914
Operating expenses:
Net cost of gas sold	185,732	146,879	393,898	330,400
Operations and maintenance	102,190	103,824	389,687	389,931
Depreciation and amortization	49,961	61,195	190,688	233,913
Taxes other than income taxes	15,257	14,782	58,421	53,145
Total operating expenses	353,140	326,680	1,032,694	1,007,389
Operating income	141,173	135,922	301,325	251,525
Other income and (expenses):
Net interest deductions	(19,255)	(17,210)	(71,778)	(67,977)
Other income (deductions)	(4,603)	(1,244)	(9,747)	(9,543)
Total other income and (expenses)	(23,858)	(18,454)	(81,525)	(77,520)
Income before income taxes	117,315	117,468	219,800	174,005
Income tax expense	26,966	40,530	49,571	55,227
Net income	90,349	76,938	170,229	154,638
Income from continuing operations	90,349	76,938	170,229	118,778
Discontinued operations-construction services, income before income taxes				56,890
Discontinued operations-construction services, Income tax expense				21,030
Discontinued operations-construction services, Income				35,860
Discontinued operations-construction services - Noncontrolling interests				1,105
Income - discontinued operations				34,755
Net income	$ 90,349	$ 76,938	$ 170,229	$ 153,533