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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 334	$ 668	$ 668	$ 1,335	$ 1,336
Translation adjustments, Before-Tax Amount	629	70	849	852	158	(39)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Amortization of net actuarial (gain)/loss, Before-Tax Amount	6,361	6,766	12,722	13,533	26,255	30,723
Foreign currency other comprehensive income (loss), Before-Tax Amount	629	70	849	852	158	(39)
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Net actuarial gain/(loss), Before-Tax Amount					(22,770)	(30,519)
Total other comprehensive income (loss), Before-Tax Amount	2,425	1,884	4,441	4,480	3,514	2,528
Total other comprehensive income (loss), Tax (Expense) or Benefit	(683)	(690)	(1,366)	(1,378)	(1,276)	(975)
Regulatory adjustment, Before-Tax Amount	(5,735)	(6,123)	(11,470)	(12,246)	(4,808)	(2,318)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	960	977	1,920	1,955	12	(778)
Amounts reclassified into net income, Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
Net actuarial gain/(loss), Tax (Expense) or Benefit					8,652	11,597
Amortization of prior service cost, Tax (Expense) or Benefit	(127)	(127)	(254)	(254)	(507)	(508)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,417)	(2,572)	(4,834)	(5,143)	(9,976)	(11,675)
Regulatory adjustment, Tax (Expense) or Benefit	2,179	2,327	4,358	4,654	1,826	882
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(365)	(372)	(730)	(743)	(5)	296
Amounts reclassified into net income, Tax (Expense) or Benefit	(318)	(318)	(636)	(635)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(318)	(318)	(636)	(635)	(1,271)	(1,271)
Net actuarial gain/(loss), Net-of-Tax Amount					(14,118)	(18,922)
Amortization of prior service cost, Net-of-Tax Amount	207	207	414	414	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,194	7,888	8,390	16,279	19,048
Regulatory adjustment, Net-of-Tax Amount	(3,556)	(3,796)	(7,112)	(7,592)	(2,982)	(1,436)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	595	605	1,190	1,212	7	(482)
Amounts reclassified into net income	518	519	1,036	1,038	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	519	1,036	1,038	2,073	2,074
Translation adjustments, Net-of-Tax Amount	629	70	849	852	158	(39)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	629	70	849	852	158	(39)
Total other comprehensive income (loss), net of tax	1,742	1,194	3,075	3,102	2,238	1,553
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			12,722
Total other comprehensive income (loss), Before-Tax Amount	1,796	1,814	3,592	3,628	3,356	2,567
Total other comprehensive income (loss), Tax (Expense) or Benefit	(683)	(690)	(1,366)	(1,378)	(1,276)	(975)
Regulatory adjustment, Before-Tax Amount			(11,470)
Amounts reclassified into net income, Before-Tax Amount			1,672
Amortization of prior service cost, Tax (Expense) or Benefit			(254)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit			(4,834)
Regulatory adjustment, Tax (Expense) or Benefit			4,358
Amounts reclassified into net income, Tax (Expense) or Benefit			(636)
Net actuarial gain/(loss), Net-of-Tax Amount					(14,118)	(18,922)
Amortization of prior service cost, Net-of-Tax Amount	207	207	414	414	828	828
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	3,944	4,194	7,888	8,390	16,279	19,048
Regulatory adjustment, Net-of-Tax Amount	(3,556)	(3,796)	(7,112)	(7,592)	(2,982)	(1,436)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	595	605	1,190	1,212	7	(482)
Amounts reclassified into net income	518	519	1,036	1,038	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	519	1,036	1,038	2,073	2,074
Total other comprehensive income (loss), net of tax	$ 1,113	$ 1,124	$ 2,226	$ 2,250	$ 2,080	$ 1,592