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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Effective income tax rate			24.00%		24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	2,382	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
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			4,055
Other comprehensive income (loss), tax			(973)
Total other comprehensive income (loss), net of tax			3,082
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax					(71,087)	(20,426)
Other comprehensive income (loss) before reclassifications, tax					17,061	4,902
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
Net actuarial gain/(loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
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 prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			867
Other comprehensive income (loss), tax			(208)
Total other comprehensive income (loss), net of tax			659
Prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax					(1,878)	0
Other comprehensive income (loss) before reclassifications, tax					452	0
Other comprehensive income (loss) before reclassifications, net of tax					(1,426)	0
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
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)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	1,024	(447)	(1,187)	1,131	(280)	(877)
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0	0	0
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)