Other Comprehensive Income and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (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
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Other comprehensive income (loss) before reclassifications, net of tax			$ (324)
Effective income tax rate			24.00%	24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
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 prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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 (gain)/loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Other comprehensive income (loss) before reclassifications, before tax					(57,539)	(71,087)
Other comprehensive income (loss) before reclassifications, tax					13,809	17,061
Other comprehensive income (loss) before reclassifications, net of tax					(43,730)	(54,026)
Amortization of net actuarial (gain)/loss | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax					0	(1,878)
Other comprehensive income (loss) before reclassifications, tax					0	452
Other comprehensive income (loss) before reclassifications, net of tax					0	(1,426)
Pension plans other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Pension plans other comprehensive income (loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Foreign currency items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	(2,056)	1,024	(324)	(1,187)	2,576	(280)
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0	0	0
Other comprehensive income (loss) before reclassifications, net of tax	$ (2,056)	$ 1,024	$ (324)	$ (1,187)	$ 2,576	$ (280)