Condensed Consolidated Statements of Comprehensive Income - USD ($) $ 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
Net income	$ 21,551	$ 18,121	$ 99,845	$ 87,126	$ 206,661	$ 155,727
Defined benefit pension plans:
Net actuarial gain (loss)					(32,701)	(14,118)
Amortization of prior service cost	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Net defined benefit pension plans	897	595	1,792	1,190	(2,905)	7
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Net forward-starting interest rate swaps	636	518	1,271	1,036	2,308	2,073
Foreign currency translation adjustments	(690)	629	(1,601)	849	(679)	158
Net other comprehensive income	843	1,742	1,462	3,075	(1,276)	2,238
Comprehensive income	22,394	19,863	101,307	90,201	205,385	157,965
Comprehensive income (loss) attributable to noncontrolling interests		279	(797)	(17)	(668)	908
Comprehensive income	22,394	19,584	102,104	90,218	206,053	157,057
Southwest Gas Corporation [Member]
Net income from continuing operations	2,622	9,522	92,971	86,460	163,329	125,942
Net income			92,971	86,460	163,329	155,061
Defined benefit pension plans:
Net actuarial gain (loss)					(32,701)	(14,118)
Amortization of prior service cost	254	207	508	414	922	828
Amortization of net actuarial loss	6,387	3,944	12,774	7,888	20,662	16,279
Regulatory adjustment	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Net defined benefit pension plans	897	595	1,792	1,190	(2,905)	7
Forward-starting interest rate swaps:
Amounts reclassified into net income	636	518	1,271	1,036	2,308	2,073
Net forward-starting interest rate swaps	636	518	1,271	1,036	2,308	2,073
Net other comprehensive income	1,533	1,113	3,063	2,226	(597)	2,080
Comprehensive income	4,155	10,635	96,034	88,686	162,732	128,022
Discontinued operations-construction services, net income						28,170
Discontinued operations-construction services, foreign currency translation adjustments						(691)
Discontinued operations-construction services, comprehensive income						27,479
Discontinued operations-construction services, comprehensive income (loss) attributable to noncontrolling interests						(24)
Comprehensive income attributable to discontinued operations - construction services						27,503
Comprehensive income	$ 4,155	$ 10,635	$ 96,034	$ 88,686	$ 162,732	$ 155,525