Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent			$ 2,674,953
Other comprehensive income (loss) before reclassifications, net of tax			(324)
Other comprehensive income (loss), before tax	$ 303	$ 3,406	6,750	$ 5,571	$ (4,980)	$ (6,502)
Other comprehensive income (loss), tax	(566)	(572)	(1,698)	(1,622)	1,812	1,494
Total other comprehensive income (loss), net of tax	(263)	2,834	5,052	$ 3,949	(3,168)	(5,008)
Ending balance, attributable to parent	2,909,816		$ 2,909,816		2,909,816
Effective income tax rate			24.00%	24.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			$ (77,720)
Beginning balance, tax			18,653
Beginning balance			(59,067)
Other comprehensive income (loss), before tax	1,815	1,352	5,442	$ 4,055	(9,732)	(9,761)
Other comprehensive income (loss), tax	(435)	(325)	(1,306)	(973)	2,335	2,344
Total other comprehensive income (loss), net of tax	1,380	1,027	4,136	3,082	(7,397)	(7,417)
Ending balance, before tax	(72,278)		(72,278)		(72,278)
Ending balance, tax	17,347		17,347		17,347
Ending balance	(54,931)		(54,931)		(54,931)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	239	289	719	867	1,007	1,185
Reclassification from AOCI, tax	(56)	(69)	(172)	(208)	(241)	(284)
Reclassification from AOCI, net of tax	183	220	547	659	766	901
Other comprehensive income (loss), before tax			719
Other comprehensive income (loss), tax			(172)
Total other comprehensive income (loss), net of tax			547
Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax					(57,539)	(71,087)
Other comprehensive income (loss) before reclassifications, net of tax					(43,730)	(54,026)
Reclassification from AOCI, before Tax	11,151	9,457	33,448	28,372	42,906	34,216
Reclassification from AOCI, tax	(2,677)	(2,270)	(8,028)	(6,809)	(10,298)	(8,212)
Reclassification from AOCI, net of tax	8,474	7,187	25,420	21,563	32,608	26,004
Other comprehensive income (loss), before tax			33,448
Other comprehensive income (loss), tax			(8,028)
Total other comprehensive income (loss), net of tax			25,420
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(9,575)	(8,394)	(28,725)	(25,184)	3,894	27,803
Reclassification from AOCI, tax	2,298	2,014	6,894	6,044	(935)	(6,673)
Reclassification from AOCI, net of tax	(7,277)	(6,380)	(21,831)	(19,140)	2,959	21,130
Other comprehensive income (loss), before tax			(28,725)
Other comprehensive income (loss), tax			6,894
Total other comprehensive income (loss), net of tax			(21,831)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(2,719)
Beginning balance, tax			651
Beginning balance			(2,068)
Reclassification from AOCI, before Tax	544	1,030	1,632	2,703	2,176	3,539
Reclassification from AOCI, tax	(131)	(247)	(392)	(649)	(523)	(850)
Reclassification from AOCI, net of tax	413	783	1,240	2,054	1,653	2,689
Other comprehensive income (loss), before tax	544	1,030	1,632	2,703	2,176	3,539
Other comprehensive income (loss), tax	(131)	(247)	(392)	(649)	(523)	(850)
Total other comprehensive income (loss), net of tax	413	783	1,240	2,054	1,653	2,689
Ending balance, before tax	(1,087)		(1,087)		(1,087)
Ending balance, tax	259		259		259
Ending balance	(828)		(828)		(828)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			132
Beginning balance			132
Other comprehensive income (loss) before reclassifications, before tax			(324)
Other comprehensive income (loss) before reclassifications, net of tax			(324)
Ending balance, before tax	(192)		(192)		(192)
Ending balance	(192)		(192)		(192)
AOCI Including Portion Attributable to Noncontrolling Interest
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, net of tax			5,052
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55,688)	(55,617)	(61,003)	(56,732)	(52,783)
Ending balance	(55,951)	(52,783)	(55,951)	(52,783)	(55,951)	(52,783)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					2,144,273
Beginning balance, attributable to parent			2,233,468
Other comprehensive income (loss), before tax	2,359	2,382	7,074	6,758	(7,556)	(6,222)
Other comprehensive income (loss), tax	(566)	(572)	(1,698)	(1,622)	1,812	1,494
Total other comprehensive income (loss), net of tax	1,793	1,810	5,376	5,136	(5,744)	(4,728)
Ending balance, attributable to parent	2,462,794		2,462,794		2,462,794
Ending balance	2,462,794	2,144,273	2,462,794	2,144,273	2,462,794	2,144,273
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(77,720)
Beginning balance, attributable to parent, tax			18,653
Beginning balance, attributable to parent			(59,067)
Other comprehensive income (loss), before tax			5,442
Other comprehensive income (loss), tax			(1,306)
Total other comprehensive income (loss), net of tax			4,136
Ending balance, attributable to parent, before tax	(72,278)		(72,278)		(72,278)
Ending balance, attributable to parent, tax	17,347		17,347		17,347
Ending balance, attributable to parent	(54,931)		(54,931)		(54,931)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			719
Other comprehensive income (loss), tax			(172)
Total other comprehensive income (loss), net of tax			547
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			33,448
Other comprehensive income (loss), tax			(8,028)
Total other comprehensive income (loss), net of tax			25,420
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			(28,725)
Other comprehensive income (loss), tax			6,894
Total other comprehensive income (loss), net of tax			(21,831)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(2,719)
Beginning balance, attributable to parent, tax			651
Beginning balance, attributable to parent			(2,068)
Reclassification from AOCI, before Tax			1,632
Reclassification from AOCI, tax			(392)
Reclassification from AOCI, net of tax			1,240
Other comprehensive income (loss), before tax			1,632
Other comprehensive income (loss), tax			(392)
Total other comprehensive income (loss), net of tax			1,240
Ending balance, attributable to parent, before tax	(1,087)		(1,087)		(1,087)
Ending balance, attributable to parent, tax	259		259		259
Ending balance, attributable to parent	(828)		(828)		(828)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(57,552)	(51,825)	(61,135)	(55,151)	(50,015)
Beginning balance, attributable to parent			(61,135)
Ending balance, attributable to parent	(55,759)		(55,759)		(55,759)
Ending balance	$ (55,759)	$ (50,015)	$ (55,759)	$ (50,015)	$ (55,759)	$ (50,015)