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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					$ 2,423,710
Beginning balance, attributable to parent			$ 2,505,914
Other comprehensive income (loss) before reclassifications, net of tax			(1,187)
Other comprehensive income (loss), before tax	$ 3,406	$ 1,203	5,571	$ 6,081	(6,502)	$ 7,667
Other comprehensive income (loss), tax	(572)	(397)	(1,622)	(1,188)	1,494	(2,052)
Total other comprehensive income (loss), net of tax	2,834	806	3,949	4,893	(5,008)	5,615
Ending balance, attributable to parent	2,619,238		2,619,238		2,619,238
Ending balance	2,619,238	2,423,710	$ 2,619,238	2,423,710	$ 2,619,238	2,423,710
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	1,352	814	4,055	2,442	$ (9,761)	5,200
Other comprehensive income (loss), tax	(325)	(196)	(973)	(586)	2,344	(1,248)
Total other comprehensive income (loss), net of tax	1,027	618	3,082	1,856	(7,417)	3,952
Ending balance, before tax	(62,546)		(62,546)		(62,546)
Ending balance, tax	15,012		15,012		15,012
Ending balance	(47,534)		(47,534)		(47,534)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	289	318	867	953	1,185	1,286
Reclassification from AOCI, tax	(69)	(77)	(208)	(229)	(284)	(309)
Reclassification from AOCI, net of tax	220	241	659	724	901	977
Other comprehensive income (loss), before tax			867
Other comprehensive income (loss), tax			(208)
Total other comprehensive income (loss), net of tax			659
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	28,372	17,532	34,216	25,937
Reclassification from AOCI, tax	(2,270)	(1,402)	(6,809)	(4,207)	(8,212)	(6,224)
Reclassification from AOCI, net of tax	7,187	4,442	21,563	13,325	26,004	19,713
Other comprehensive income (loss), before tax			28,372
Other comprehensive income (loss), tax			(6,809)
Total other comprehensive income (loss), net of tax			21,563
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(8,394)	(5,348)	(25,184)	(16,043)	27,803	(1,597)
Reclassification from AOCI, tax	2,014	1,283	6,044	3,850	(6,673)	383
Reclassification from AOCI, net of tax	(6,380)	(4,065)	(19,140)	(12,193)	21,130	(1,214)
Other comprehensive income (loss), before tax			(25,184)
Other comprehensive income (loss), tax			6,044
Total other comprehensive income (loss), net of tax			(19,140)
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	1,030	836	2,703	2,508	3,539	3,344
Reclassification from AOCI, tax	(247)	(201)	(649)	(602)	(850)	(804)
Reclassification from AOCI, net of tax	783	635	2,054	1,906	2,689	2,540
Other comprehensive income (loss), before tax	1,030	836	2,703	2,508	3,539	3,344
Other comprehensive income (loss), tax	(247)	(201)	(649)	(602)	(850)	(804)
Total other comprehensive income (loss), net of tax	783	635	2,054	1,906	2,689	2,540
Ending balance, before tax	(3,263)		(3,263)		(3,263)
Ending balance, tax	782		782		782
Ending balance	(2,481)		(2,481)		(2,481)
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	1,024	(447)	(1,187)	1,131	(280)	(877)
Other comprehensive income (loss) before reclassifications, net of tax	1,024	(447)	(1,187)	1,131	(280)	(877)
Other comprehensive income (loss), before tax			(1,187)
Total other comprehensive income (loss), net of tax			(1,187)
Ending balance, before tax	(2,768)		(2,768)		(2,768)
Ending balance	(2,768)		(2,768)		(2,768)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55,617)	(48,581)	(56,732)	(52,668)	(47,775)
Ending balance	(52,783)	(47,775)	(52,783)	(47,775)	(52,783)	(47,775)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					1,929,606
Beginning balance, attributable to parent			2,005,152
Other comprehensive income (loss), before tax	2,382	1,650	6,758	4,950	(6,222)	8,544
Other comprehensive income (loss), tax	(572)	(397)	(1,622)	(1,188)	1,494	(2,052)
Total other comprehensive income (loss), net of tax	1,810	1,253	5,136	3,762	(4,728)	6,492
Ending balance, attributable to parent	2,144,273		2,144,273		2,144,273
Ending balance	2,144,273	1,929,606	2,144,273	1,929,606	2,144,273	1,929,606
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			4,055
Other comprehensive income (loss), tax			(973)
Total other comprehensive income (loss), net of tax			3,082
Ending balance, attributable to parent, before tax	(62,546)		(62,546)		(62,546)
Ending balance, attributable to parent, tax	15,012		15,012		15,012
Ending balance, attributable to parent	(47,534)		(47,534)		(47,534)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			867
Other comprehensive income (loss), tax			(208)
Total other comprehensive income (loss), net of tax			659
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			28,372
Other comprehensive income (loss), tax			(6,809)
Total other comprehensive income (loss), net of tax			21,563
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			(25,184)
Other comprehensive income (loss), tax			6,044
Total other comprehensive income (loss), net of tax			(19,140)
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			2,703
Reclassification from AOCI, tax			(649)
Reclassification from AOCI, net of tax			2,054
Other comprehensive income (loss), before tax			2,703
Other comprehensive income (loss), tax			(649)
Total other comprehensive income (loss), net of tax			2,054
Ending balance, attributable to parent, before tax	(3,263)		(3,263)		(3,263)
Ending balance, attributable to parent, tax	782		782		782
Ending balance, attributable to parent	(2,481)		(2,481)		(2,481)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(51,825)	(46,540)	(55,151)	(49,049)	(45,287)
Beginning balance, attributable to parent			(55,151)
Ending balance, attributable to parent	(50,015)		(50,015)		(50,015)
Ending balance	$ (50,015)	$ (45,287)	$ (50,015)	$ (45,287)	$ (50,015)	$ (45,287)