Condensed Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenues:
Gas operating revenues	$ 275,679	$ 260,162	$ 769,992	$ 722,764	$ 1,349,536	$ 1,263,428
Construction revenues	395,204	300,307	655,221	492,442	1,409,263	1,133,272
Total operating revenues	670,883	560,469	1,425,213	1,215,206	2,758,799	2,396,700
Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Operating expenses:
Net cost of gas sold	83,466	69,421	269,198	216,300	407,943	328,405
Operations and maintenance	105,435	98,203	207,786	202,498	398,051	394,802
Depreciation and amortization	61,307	58,082	123,785	130,560	244,176	271,773
Taxes other than income taxes	14,666	14,497	29,923	29,279	58,590	54,655
Construction expenses	352,671	272,001	611,623	463,957	1,296,629	1,031,072
Total operating expenses	617,545	512,204	1,242,315	1,042,594	2,405,389	2,080,707
Operating income	53,338	48,265	182,898	172,612	353,410	315,993
Other income and (expenses):
Net interest deductions	(23,652)	(18,655)	(46,283)	(37,369)	(86,978)	(75,087)
Other income (deductions)	(2,706)	(2,810)	(7,040)	(3,800)	(9,270)	(8,401)
Total other income and (expenses)	(26,358)	(21,465)	(53,323)	(41,169)	(96,248)	(83,488)
Income before income taxes	26,980	26,800	129,575	131,443	257,162	232,505
Income tax expense	5,429	8,679	29,730	44,317	50,501	76,778
Net income (loss)	21,551	18,121	99,845	87,126	206,661	155,727
Net income (loss) attributable to noncontrolling interest		257	(797)	(46)	(650)	903
Net income (loss)	$ 21,551	$ 17,864	$ 100,642	$ 87,172	$ 207,311	$ 154,824
Basic earnings per share	$ 0.44	$ 0.38	$ 2.07	$ 1.83	$ 4.29	$ 3.26
Diluted earnings per share	0.44	0.37	2.07	1.82	4.28	3.24
Dividends declared per share	$ 0.520	$ 0.495	$ 1.040	$ 0.990	$ 2.030	$ 1.890
Average number of common shares	48,826	47,571	48,622	47,550	48,338	47,516
Average shares (assuming dilution)	48,880	47,884	48,671	47,874	48,387	47,857
Southwest Gas Corporation [Member]
Operating revenues:
Gas operating revenues	$ 275,679	$ 260,162	$ 769,992	$ 722,764	$ 1,349,536	$ 1,263,428
Gas operating revenues	275,679	260,162	769,992	722,764	1,349,536	1,263,428
Operating expenses:
Net cost of gas sold	83,466	69,421	269,198	216,300	407,943	328,405
Operations and maintenance	105,208	97,644	207,398	201,468	397,251	393,772
Depreciation and amortization	47,664	46,254	97,625	107,449	192,098	222,935
Taxes other than income taxes	14,666	14,497	29,923	29,279	58,590	54,655
Total operating expenses	251,004	227,816	604,144	554,496	1,055,882	999,767
Operating income	24,675	32,346	165,848	168,268	293,654	263,661
Other income and (expenses):
Net interest deductions	(20,149)	(16,991)	(39,404)	(34,201)	(74,936)	(68,407)
Other income (deductions)	(2,094)	(2,805)	(6,697)	(4,049)	(9,036)	(9,843)
Total other income and (expenses)	(22,243)	(19,796)	(46,101)	(38,250)	(83,972)	(78,250)
Income before income taxes	2,432	12,550	119,747	130,018	209,682	185,411
Income tax expense	(190)	3,028	26,776	43,558	46,353	59,469
Net income (loss)			92,971	86,460	163,329	155,061
Income from continuing operations	2,622	9,522	92,971	86,460	163,329	125,942
Discontinued operations-construction services, income before income taxes						45,669
Discontinued operations-construction services, Income tax expense						16,550
Discontinued operations-construction services, Income						29,119
Discontinued operations-construction services - Noncontrolling interests						949
Income - discontinued operations						28,170
Net income (loss)	$ 2,622	$ 9,522	$ 92,971	$ 86,460	$ 163,329	$ 154,112