Other Comprehensive Income and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 2,346,548
Beginning balance, attributable to parent	$ 2,505,914
Other comprehensive income (loss) before reclassifications, net of tax	(4,005)
Other comprehensive income (loss), before tax	(1,816)	$ 2,441	(10,249)	$ 7,965
Other comprehensive income (loss), tax	(525)	(396)	1,799	(2,225)
Total other comprehensive income (loss), net of tax	(2,341)	2,045	(8,450)	5,740
Ending balance, attributable to parent	2,547,895		2,547,895
Ending balance	$ 2,547,895	2,346,548	$ 2,547,895	2,346,548
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	$ (10,836)	5,929
Other comprehensive income (loss), tax	(324)	(195)	2,602	(1,422)
Total other comprehensive income (loss), net of tax	1,028	619	(8,234)	4,507
Ending balance, before tax	(65,249)		(65,249)
Ending balance, tax	15,661		15,661
Ending balance	(49,588)		(49,588)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	289	317	1,243	1,318
Reclassification from AOCI, tax	(69)	(76)	(298)	(316)
Reclassification from AOCI, net of tax	220	241	945	1,002
Other comprehensive income (loss), before tax	289
Other comprehensive income (loss), tax	(69)
Total other comprehensive income (loss), net of tax	220
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	9,458	5,844	26,990	31,057
Reclassification from AOCI, tax	(2,270)	(1,403)	(6,477)	(7,454)
Reclassification from AOCI, net of tax	7,188	4,441	20,513	23,603
Other comprehensive income (loss), before tax	9,458
Other comprehensive income (loss), tax	(2,270)
Total other comprehensive income (loss), net of tax	7,188
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, before Tax	(8,395)	(5,347)	33,896	(6,020)
Reclassification from AOCI, tax	2,015	1,284	(8,136)	1,446
Reclassification from AOCI, net of tax	(6,380)	(4,063)	25,760	(4,574)
Other comprehensive income (loss), before tax	(8,395)
Other comprehensive income (loss), tax	2,015
Total other comprehensive income (loss), net of tax	(6,380)
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	837	836	3,345	3,344
Reclassification from AOCI, tax	(201)	(201)	(803)	(803)
Reclassification from AOCI, net of tax	636	635	2,542	2,541
Other comprehensive income (loss), before tax	837	836	3,345	3,344
Other comprehensive income (loss), tax	(201)	(201)	(803)	(803)
Total other comprehensive income (loss), net of tax	636	635	2,542	2,541
Ending balance, before tax	(5,129)		(5,129)
Ending balance, tax	1,230		1,230
Ending balance	(3,899)		(3,899)
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	(4,005)	791	(2,758)	(1,308)
Other comprehensive income (loss) before reclassifications, net of tax	(4,005)	791	(2,758)	(1,308)
Other comprehensive income (loss), before tax	(4,005)
Total other comprehensive income (loss), net of tax	(4,005)
Ending balance, before tax	(5,586)		(5,586)
Ending balance	(5,586)		(5,586)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(56,732)	(52,668)	(50,623)
Ending balance	(59,073)	(50,623)	(59,073)	(50,623)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			1,887,903
Beginning balance, attributable to parent	2,005,152
Other comprehensive income (loss), before tax	2,189	1,650	(7,491)	9,273
Other comprehensive income (loss), tax	(525)	(396)	1,799	(2,225)
Total other comprehensive income (loss), net of tax	1,664	1,254	(5,692)	7,048
Ending balance, attributable to parent	2,114,276		2,114,276
Ending balance	2,114,276	1,887,903	2,114,276	1,887,903
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	1,352
Other comprehensive income (loss), tax	(324)
Total other comprehensive income (loss), net of tax	1,028
Ending balance, attributable to parent, before tax	(65,249)		(65,249)
Ending balance, attributable to parent, tax	15,661		15,661
Ending balance, attributable to parent	(49,588)		(49,588)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	289
Other comprehensive income (loss), tax	(69)
Total other comprehensive income (loss), net of tax	220
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	9,458
Other comprehensive income (loss), tax	(2,270)
Total other comprehensive income (loss), net of tax	7,188
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax	(8,395)
Other comprehensive income (loss), tax	2,015
Total other comprehensive income (loss), net of tax	(6,380)
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	837
Reclassification from AOCI, tax	(201)
Reclassification from AOCI, net of tax	636
Other comprehensive income (loss), before tax	837
Other comprehensive income (loss), tax	(201)
Total other comprehensive income (loss), net of tax	636
Ending balance, attributable to parent, before tax	(5,129)		(5,129)
Ending balance, attributable to parent, tax	1,230		1,230
Ending balance, attributable to parent	(3,899)		(3,899)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55,151)	(49,049)	(47,795)
Beginning balance, attributable to parent	(55,151)
Ending balance, attributable to parent	(53,487)		(53,487)
Ending balance	$ (53,487)	$ (47,795)	$ (53,487)	$ (47,795)