Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,251,590	$ 1,812,403	$ 1,876,376	$ 1,812,403
Beginning balance, attributable to parent	2,252,042
Other comprehensive income (loss) before reclassifications, net of tax	791
Other comprehensive income (loss), before tax	2,441	1,104	7,965		$ 296
Other comprehensive income (loss), tax	(396)	(485)	(2,225)		(673)
Total other comprehensive income (loss), net of tax	2,045	619	5,740		(377)
Ending balance, attributable to parent	2,347,000		2,347,000	2,252,042
Ending balance	$ 2,346,548	1,876,376	2,346,548	$ 2,251,590	1,876,376
Effective income tax rate	24.00%			38.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	$ (55,227)
Beginning balance, tax	13,254
Beginning balance	(41,973)
Other comprehensive income (loss), before tax	814	1,178	5,929		(3,689)
Other comprehensive income (loss), tax	(195)	(283)	(1,422)		482
Total other comprehensive income (loss), net of tax	619	895	4,507		(3,207)
Ending balance, before tax	(54,413)		(54,413)	$ (55,227)
Ending balance, tax	13,059		13,059	13,254
Ending balance	(41,354)		(41,354)	(41,973)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	317	334	1,318		1,335
Reclassification from AOCI, tax	(76)	(80)	(316)		(460)
Reclassification from AOCI, net of tax	241	254	1,002		875
Other comprehensive income (loss), before tax	317
Other comprehensive income (loss), tax	(76)
Total other comprehensive income (loss), net of tax	241
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax			(20,426)		(43,027)
Other comprehensive income (loss) before reclassifications, net of tax			(15,524)		(32,701)
Reclassification from AOCI, before Tax	5,844	8,404	31,057		27,488
Reclassification from AOCI, tax	(1,403)	(2,017)	(7,454)		(9,269)
Reclassification from AOCI, net of tax	4,441	6,387	23,603		18,219
Other comprehensive income (loss), before tax	5,844
Other comprehensive income (loss), tax	(1,403)
Total other comprehensive income (loss), net of tax	4,441
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(5,347)	(7,560)	(6,020)		10,515
Reclassification from AOCI, tax	1,284	1,814	1,446		(115)
Reclassification from AOCI, net of tax	(4,063)	(5,746)	(4,574)		10,400
Other comprehensive income (loss), before tax	(5,347)
Other comprehensive income (loss), tax	1,284
Total other comprehensive income (loss), net of tax	(4,063)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(9,310)
Beginning balance, tax	2,234
Beginning balance	(7,076)
Reclassification from AOCI, before Tax	836	837	3,344		3,345
Reclassification from AOCI, tax	(201)	(202)	(803)		(1,155)
Reclassification from AOCI, net of tax	635	635	2,541		2,190
Other comprehensive income (loss), before tax	836	837	3,344		3,345
Other comprehensive income (loss), tax	(201)	(202)	(803)		(1,155)
Total other comprehensive income (loss), net of tax	635	635	2,541		2,190
Ending balance, before tax	(8,474)		(8,474)	(9,310)
Ending balance, tax	2,033		2,033	2,234
Ending balance	(6,441)		(6,441)	(7,076)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(3,619)
Beginning balance	(3,619)
Other comprehensive income (loss) before reclassifications, before tax	791	(911)	(1,308)		640
Other comprehensive income (loss) before reclassifications, net of tax	791	(911)	(1,308)		640
Other comprehensive income (loss), before tax	791
Total other comprehensive income (loss), net of tax	791
Ending balance, before tax	(2,828)		(2,828)	(3,619)
Ending balance	(2,828)		(2,828)	(3,619)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(52,668)	(47,682)	(56,363)	(47,682)
Ending balance	(50,623)	(56,363)	(50,623)	(52,668)	(56,363)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	1,782,460	1,609,999	1,679,144	1,609,999
Beginning balance, attributable to parent	1,782,460
Other comprehensive income (loss), before tax	1,650	2,015	9,273		(344)
Other comprehensive income (loss), tax	(396)	(485)	(2,225)		(673)
Total other comprehensive income (loss), net of tax	1,254	1,530	7,048		(1,017)
Ending balance, attributable to parent	1,887,903		1,887,903	1,782,460
Ending balance	1,887,903	1,679,144	1,887,903	1,782,460	1,679,144
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(55,227)
Beginning balance, attributable to parent, tax	13,254
Beginning balance, attributable to parent	(41,973)
Other comprehensive income (loss), before tax	814
Other comprehensive income (loss), tax	(195)
Total other comprehensive income (loss), net of tax	619
Ending balance, attributable to parent, before tax	(54,413)		(54,413)	(55,227)
Ending balance, attributable to parent, tax	13,059		13,059	13,254
Ending balance, attributable to parent	(41,354)		(41,354)	(41,973)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	317
Other comprehensive income (loss), tax	(76)
Total other comprehensive income (loss), net of tax	241
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	5,844
Other comprehensive income (loss), tax	(1,403)
Total other comprehensive income (loss), net of tax	4,441
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	(5,347)
Other comprehensive income (loss), tax	1,284
Total other comprehensive income (loss), net of tax	(4,063)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(9,310)
Beginning balance, attributable to parent, tax	2,234
Beginning balance, attributable to parent	(7,076)
Reclassification from AOCI, before Tax	836
Reclassification from AOCI, tax	(201)
Reclassification from AOCI, net of tax	635
Other comprehensive income (loss), before tax	836
Other comprehensive income (loss), tax	(201)
Total other comprehensive income (loss), net of tax	635
Ending balance, attributable to parent, before tax	(8,474)		(8,474)	(9,310)
Ending balance, attributable to parent, tax	2,033		2,033	2,234
Ending balance, attributable to parent	(6,441)		(6,441)	(7,076)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(49,049)	(47,073)	(54,843)	(47,073)
Beginning balance, attributable to parent	(49,049)
Ending balance, attributable to parent	(47,795)		(47,795)	(49,049)
Ending balance	$ (47,795)	$ (54,843)	$ (47,795)	$ (49,049)	$ (54,843)