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		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 334	$ 1,335	$ 1,335
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,404	6,361	27,488	26,660
Regulatory adjustment, Before-Tax Amount	(7,560)	(5,735)	10,515	(5,196)
Pension plans other comprehensive income (loss)	1,178	960	(3,689)	29
Amounts reclassified into net income, Before-Tax Amount	837	836	(3,345)	(3,345)
FSIRS other comprehensive income, Before-Tax Amount	837	836
Total other comprehensive income (loss), Before-Tax Amount	1,104	2,016	296	2,973
Translation adjustments, Before-Tax Amount	(911)	220	640	(401)
Foreign currency other comprehensive income (loss), Before-Tax Amount	(911)	220	640	(401)
Amortization of prior service cost, Tax (Expense) or Benefit	(80)	(127)	(460)	(507)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,017)	(2,417)	(9,269)	(10,131)
Regulatory adjustment, Tax (Expense) or Benefit	1,814	2,179	(115)	1,974
Net pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(283)	(365)	482	(12)
Amounts reclassified into net income, Tax (Expense) or Benefit	(202)	(318)	(1,155)	(1,271)
FSIRS other comprehensive income, Tax (Expense) or Benefit	(202)	(318)	1,155	1,271
Total other comprehensive income (loss), Tax (Expense) or Benefit	(485)	(683)	(673)	(1,283)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0
Amortization of prior service cost, Net-of-Tax Amount	254	207	875	828
Amortization of net actuarial loss	6,387	3,944	18,219	16,529
Regulatory adjustment, Net-of-Tax Amount	(5,746)	(3,556)	10,400	(3,222)
Pension plans other comprehensive income (loss)	895	595	(3,207)	17
Amounts reclassified into net income	635	518	2,190	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	635	518	2,190	2,074
Net other comprehensive income (loss)	619	1,333	(377)	1,690
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(911)	220	640	(401)
Net actuarial gain/(loss), Before-Tax Amount			(43,027)	(22,770)
FSIRS other comprehensive income (loss), Before-Tax Amount	837		3,345	3,345
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	334
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,404
Regulatory adjustment, Before-Tax Amount	(7,560)
Total other comprehensive income (loss), Before-Tax Amount	2,015	1,796	(344)	3,374
Amortization of prior service cost, Tax (Expense) or Benefit	80
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,017)
Regulatory adjustment, Tax (Expense) or Benefit	(1,814)
Amounts reclassified into net income, Tax (Expense) or Benefit	(202)
Total other comprehensive income (loss), Tax (Expense) or Benefit	(485)	(683)	(673)	(1,283)
Amortization of prior service cost, Net-of-Tax Amount	254	207	875	828
Amortization of net actuarial loss	6,387	3,944	18,219	16,529
Regulatory adjustment, Net-of-Tax Amount	(5,746)	(3,556)	10,400	(3,222)
Amounts reclassified into net income	635	518	2,190	2,074
Net other comprehensive income (loss)	$ 1,530	$ 1,113	(1,017)	2,091
Net actuarial gain/(loss), Net-of-Tax Amount			$ (32,701)	$ (14,118)