Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - 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
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 334	$ 668	$ 668	$ 1,335	$ 1,335
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,404	6,361	16,808	12,722	29,531	26,255
Regulatory adjustment, Before-Tax Amount	(7,559)	(5,735)	(15,119)	(11,470)	8,691	(4,808)
Pension plans other comprehensive income (loss)	1,179	960	2,357	1,920	(3,470)	12
Amounts reclassified into net income, Before-Tax Amount	836	836	1,673	1,672	3,345	3,344
FS IRS other comprehensive income (loss), Before-Tax Amount	836	836	1,673	1,672	3,345	3,344
Total other comprehensive income (loss), Before-Tax Amount	1,325	2,425	2,429	4,441	(804)	3,514
Translation adjustments, Before-Tax Amount	(690)	629	(1,601)	849	(679)	158
Foreign currency other comprehensive income (loss), Before-Tax Amount	(690)	629	(1,601)	849	(679)	158
Amortization of prior service cost, Tax (Expense) or Benefit	(80)	(127)	(160)	(254)	(413)	(507)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,017)	(2,417)	(4,034)	(4,834)	(8,869)	(9,976)
Regulatory adjustment, Tax (Expense) or Benefit	1,815	2,179	3,629	4,358	(479)	1,826
Net pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(282)	(365)	(565)	(730)	565	(5)
Amounts reclassified into net income, Tax (Expense) or Benefit	(200)	(318)	(402)	(636)	(1,037)	(1,271)
FSIRS other comprehensive income, Tax (Expense) or Benefit	(200)	(318)	(402)	(636)	1,037	1,271
Total other comprehensive income (loss), Tax (Expense) or Benefit	(482)	(683)	(967)	(1,366)	(472)	(1,276)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Amortization of prior service cost, Net-of-Tax Amount	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, Net-of-Tax Amount	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Pension plans other comprehensive income (loss)	897	595	1,792	1,190	(2,905)	7
Amounts reclassified into net income, Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	636	518	1,271	1,036	2,308	2,073
Net other comprehensive income	843	1,742	1,462	3,075	(1,276)	2,238
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(690)	629	(1,601)	849	(679)	158
Net actuarial gain/(loss), Before-Tax Amount					(43,027)	(22,770)
Net actuarial gain/(loss), Tax (Expense) or Benefit					10,326	8,652
Net actuarial gain/(loss), Net-of-Tax Amount					(32,701)	(14,118)
Southwest Gas Corporation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount			668
Amortization of net actuarial (gain)/loss, Before-Tax Amount			16,808
Regulatory adjustment, Before-Tax Amount			(15,119)
Total other comprehensive income (loss), Before-Tax Amount	2,015	1,796	4,030	3,592	(125)	3,356
Amortization of prior service cost, Tax (Expense) or Benefit			(160)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit			(4,034)
Regulatory adjustment, Tax (Expense) or Benefit			3,629
Amounts reclassified into net income, Tax (Expense) or Benefit			(402)
Total other comprehensive income (loss), Tax (Expense) or Benefit	(482)	(683)	(967)	(1,366)	(472)	(1,276)
Amortization of prior service cost, Net-of-Tax Amount	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, Net-of-Tax Amount	(5,744)	(3,556)	(11,490)	(7,112)	8,212	(2,982)
Amounts reclassified into net income, Net-of-Tax Amount	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
Net actuarial gain/(loss), Net-of-Tax Amount					$ (32,701)	$ (14,118)