Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent	$ 2,953,820
Other comprehensive income (loss) before reclassifications, net of tax	1,247
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
Ending balance, attributable to parent	$ 3,489,082		3,489,082
Effective income tax rate	24.00%	24.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	$ (61,182)
Beginning balance, tax	14,685
Beginning balance	(46,497)
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)
Ending balance, before tax	(59,701)		(59,701)
Ending balance, tax	14,330		14,330
Ending balance	(45,371)		(45,371)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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 net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax			59,176	(57,539)
Other comprehensive income (loss) before reclassifications, net of tax			44,974	(43,730)
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
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	(545)
Beginning balance, tax	129
Beginning balance	(416)
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
Ending balance, before tax	0		0
Ending balance, tax	0		0
Ending balance	0		0
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax	152
Beginning balance	152
Other comprehensive income (loss) before reclassifications, before tax	1,247
Other comprehensive income (loss) before reclassifications, net of tax	1,247
Ending balance, before tax	1,399		1,399
Ending balance	1,399		1,399
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	2,789
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(46,761)	(61,003)	(58,388)
Ending balance	(43,972)	(58,388)	(43,972)	(58,388)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			2,374,124
Beginning balance, attributable to parent	2,527,937
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)
Ending balance, attributable to parent	2,611,534		2,611,534
Ending balance	2,611,534	2,374,124	2,611,534	2,374,124
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(61,182)
Beginning balance, attributable to parent, tax	14,685
Beginning balance, attributable to parent	(46,497)
Other comprehensive income (loss), before tax	1,481
Other comprehensive income (loss), tax	(355)
Total other comprehensive income, net of tax	1,126
Ending balance, attributable to parent, before tax	(59,701)		(59,701)
Ending balance, attributable to parent, tax	14,330		14,330
Ending balance, attributable to parent	(45,371)		(45,371)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	44
Other comprehensive income (loss), tax	(11)
Total other comprehensive income, net of tax	33
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	8,705
Other comprehensive income (loss), tax	(2,089)
Total other comprehensive income, net of tax	6,616
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	(7,268)
Other comprehensive income (loss), tax	1,745
Total other comprehensive income, net of tax	(5,523)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax	(545)
Beginning balance, attributable to parent, tax	129
Beginning balance, attributable to parent	(416)
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
Ending balance, attributable to parent, before tax	0		0
Ending balance, attributable to parent, tax	0		0
Ending balance, attributable to parent	0		0
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(46,913)	(61,135)	(59,343)
Beginning balance, attributable to parent	(46,913)
Ending balance, attributable to parent	(45,371)		(45,371)
Ending balance	$ (45,371)	$ (59,343)	$ (45,371)	$ (59,343)