Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					$ 2,419,363
Beginning balance, attributable to parent			$ 2,505,914
Other comprehensive income (loss) before reclassifications, net of tax			(2,211)
Other comprehensive income (loss), before tax	$ 3,981	$ 2,437	2,165	$ 4,878	(8,705)	$ 9,077
Other comprehensive income (loss), tax	(525)	(395)	(1,050)	(791)	1,669	(2,138)
Total other comprehensive income (loss), net of tax	3,456	2,042	1,115	4,087	(7,036)	6,939
Ending balance, attributable to parent	2,611,295		2,611,295		2,611,295
Ending balance	2,611,295	2,419,363	$ 2,611,295	2,419,363	$ 2,611,295	2,419,363
Effective income tax rate			24.00%		24.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			$ (66,601)
Beginning balance, tax			15,985
Beginning balance			(50,616)
Other comprehensive income (loss), before tax	1,351	814	2,703	1,628	$ (10,299)	5,564
Other comprehensive income (loss), tax	(324)	(195)	(648)	(390)	2,473	(1,335)
Total other comprehensive income (loss), net of tax	1,027	619	2,055	1,238	(7,826)	4,229
Ending balance, before tax	(63,898)		(63,898)		(63,898)
Ending balance, tax	15,337		15,337		15,337
Ending balance	(48,561)		(48,561)		(48,561)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	289	318	578	635	1,214	1,302
Reclassification from AOCI, tax	(69)	(76)	(138)	(152)	(291)	(312)
Reclassification from AOCI, net of tax	220	242	440	483	923	990
Other comprehensive income (loss), before tax			578
Other comprehensive income (loss), tax			(138)
Total other comprehensive income (loss), net of tax			440
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax					(71,087)	(20,426)
Other comprehensive income (loss) before reclassifications, net of tax					(54,026)	(15,524)
Reclassification from AOCI, before Tax	9,457	5,844	18,915	11,688	30,603	28,497
Reclassification from AOCI, tax	(2,270)	(1,403)	(4,540)	(2,805)	(7,345)	(6,839)
Reclassification from AOCI, net of tax	7,187	4,441	14,375	8,883	23,258	21,658
Other comprehensive income (loss), before tax			18,915
Other comprehensive income (loss), tax			(4,540)
Total other comprehensive income (loss), net of tax			14,375
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(8,395)	(5,348)	(16,790)	(10,695)	30,849	(3,809)
Reclassification from AOCI, tax	2,015	1,284	4,030	2,567	(7,404)	914
Reclassification from AOCI, net of tax	(6,380)	(4,064)	(12,760)	(8,128)	23,445	(2,895)
Other comprehensive income (loss), before tax			(16,790)
Other comprehensive income (loss), tax			4,030
Total other comprehensive income (loss), net of tax			(12,760)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(5,966)
Beginning balance, tax			1,431
Beginning balance			(4,535)
Reclassification from AOCI, before Tax	836	836	1,673	1,672	3,345	3,344
Reclassification from AOCI, tax	(201)	(200)	(402)	(401)	(804)	(803)
Reclassification from AOCI, net of tax	635	636	1,271	1,271	2,541	2,541
Other comprehensive income (loss), before tax	836	836	1,673	1,672	3,345	3,344
Other comprehensive income (loss), tax	(201)	(200)	(402)	(401)	(804)	(803)
Total other comprehensive income (loss), net of tax	635	636	1,271	1,271	2,541	2,541
Ending balance, before tax	(4,293)		(4,293)		(4,293)
Ending balance, tax	1,029		1,029		1,029
Ending balance	(3,264)		(3,264)		(3,264)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(1,581)
Beginning balance			(1,581)
Other comprehensive income (loss) before reclassifications, before tax	1,794	787	(2,211)	1,578	(1,751)	169
Other comprehensive income (loss) before reclassifications, net of tax	1,794	787	(2,211)	1,578	(1,751)	169
Other comprehensive income (loss), before tax			(2,211)
Total other comprehensive income (loss), net of tax			(2,211)
Ending balance, before tax	(3,792)		(3,792)		(3,792)
Ending balance	(3,792)		(3,792)		(3,792)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(59,073)	(50,623)	(56,732)	(52,668)	(48,581)
Ending balance	(55,617)	(48,581)	(55,617)	(48,581)	(55,617)	(48,581)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					1,948,222
Beginning balance, attributable to parent			2,005,152
Other comprehensive income (loss), before tax	2,187	1,650	4,376	3,300	(6,954)	8,908
Other comprehensive income (loss), tax	(525)	(395)	(1,050)	(791)	1,669	(2,138)
Total other comprehensive income (loss), net of tax	1,662	1,255	3,326	2,509	(5,285)	6,770
Ending balance, attributable to parent	2,151,977		2,151,977		2,151,977
Ending balance	2,151,977	1,948,222	2,151,977	1,948,222	2,151,977	1,948,222
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(66,601)
Beginning balance, attributable to parent, tax			15,985
Beginning balance, attributable to parent			(50,616)
Other comprehensive income (loss), before tax			2,703
Other comprehensive income (loss), tax			(648)
Total other comprehensive income (loss), net of tax			2,055
Ending balance, attributable to parent, before tax	(63,898)		(63,898)		(63,898)
Ending balance, attributable to parent, tax	15,337		15,337		15,337
Ending balance, attributable to parent	(48,561)		(48,561)		(48,561)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			578
Other comprehensive income (loss), tax			(138)
Total other comprehensive income (loss), net of tax			440
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			18,915
Other comprehensive income (loss), tax			(4,540)
Total other comprehensive income (loss), net of tax			14,375
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			(16,790)
Other comprehensive income (loss), tax			4,030
Total other comprehensive income (loss), net of tax			(12,760)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(5,966)
Beginning balance, attributable to parent, tax			1,431
Beginning balance, attributable to parent			(4,535)
Reclassification from AOCI, before Tax			1,673
Reclassification from AOCI, tax			(402)
Reclassification from AOCI, net of tax			1,271
Other comprehensive income (loss), before tax			1,673
Other comprehensive income (loss), tax			(402)
Total other comprehensive income (loss), net of tax			1,271
Ending balance, attributable to parent, before tax	(4,293)		(4,293)		(4,293)
Ending balance, attributable to parent, tax	1,029		1,029		1,029
Ending balance, attributable to parent	(3,264)		(3,264)		(3,264)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(53,487)	(47,795)	(55,151)	(49,049)	(46,540)
Beginning balance, attributable to parent			(55,151)
Ending balance, attributable to parent	(51,825)		(51,825)		(51,825)
Ending balance	$ (51,825)	$ (46,540)	$ (51,825)	$ (46,540)	$ (51,825)	$ (46,540)