Other Comprehensive Income and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	$ 3,273	$ 3,181	$ 18,824	$ (1,162)
Other comprehensive income (loss), tax	(484)	(566)	(4,408)	1,847
Total other comprehensive income, net of tax	2,789	$ 2,615	14,416	685
Other comprehensive income (loss) before reclassifications, net of tax	$ 1,247
Effective income tax rate	24.00%	24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	$ 2,026	$ 2,358	18,380	(7,703)
Other comprehensive income (loss), tax	(484)	(566)	(4,408)	1,847
Total other comprehensive income, net of tax	1,542	1,792	13,972	(5,856)
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	44	240	763	1,106
Reclassification from AOCI, tax	(11)	(58)	(183)	(266)
Reclassification from AOCI, net of tax	33	182	580	840
Other comprehensive income (loss), before tax	44
Other comprehensive income (loss), tax	(11)
Total other comprehensive income, net of tax	33
Amortization of prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	44
Other comprehensive income (loss), tax	(11)
Total other comprehensive income, net of tax	33
Amortization of net actuarial (gain)/loss
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	8,705	11,149	42,153	39,521
Reclassification from AOCI, tax	(2,089)	(2,675)	(10,117)	(9,484)
Reclassification from AOCI, net of tax	6,616	8,474	32,036	30,037
Other comprehensive income (loss), before tax	8,705
Other comprehensive income (loss), tax	(2,089)
Total other comprehensive income, net of tax	6,616
Other comprehensive income (loss) before reclassifications, before tax			59,176	(57,539)
Other comprehensive income (loss) before reclassifications, tax			(14,202)	13,809
Other comprehensive income (loss) before reclassifications, net of tax			44,974	(43,730)
Amortization of net actuarial (gain)/loss | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	8,705
Other comprehensive income (loss), tax	(2,089)
Total other comprehensive income, net of tax	6,616
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	(7,268)	(9,575)	(85,887)	6,255
Reclassification from AOCI, tax	1,745	2,298	20,614	(1,502)
Reclassification from AOCI, net of tax	(5,523)	(7,277)	(65,273)	4,753
Other comprehensive income (loss), before tax	(7,268)
Other comprehensive income (loss), tax	1,745
Total other comprehensive income, net of tax	(5,523)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	(7,268)
Other comprehensive income (loss), tax	1,745
Total other comprehensive income, net of tax	(5,523)
Pension plans other comprehensive income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,481	1,814	16,205	(10,657)
Other comprehensive income (loss), tax	(355)	(435)	(3,888)	2,557
Total other comprehensive income, net of tax	1,126	1,379	12,317	(8,100)
Pension plans other comprehensive income (loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,481
Other comprehensive income (loss), tax	(355)
Total other comprehensive income, net of tax	1,126
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	545	544	2,175	2,954
Reclassification from AOCI, tax	(129)	(131)	(520)	(710)
Reclassification from AOCI, net of tax	416	413	1,655	2,244
Other comprehensive income (loss), before tax	545	544	2,175	2,954
Other comprehensive income (loss), tax	(129)	(131)	(520)	(710)
Total other comprehensive income, net of tax	416	413	1,655	2,244
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	545
Reclassification from AOCI, tax	(129)
Reclassification from AOCI, net of tax	416
Other comprehensive income (loss), before tax	545
Other comprehensive income (loss), tax	(129)
Total other comprehensive income, net of tax	416
Foreign currency items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	1,247	823	444	6,541
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0
Other comprehensive income (loss) before reclassifications, net of tax	$ 1,247	$ 823	$ 444	$ 6,541