Other Comprehensive Income and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Effective income tax rate	24.00%		24.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	$ 2,189	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
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,352
Other comprehensive income (loss), tax	(324)
Total other comprehensive income (loss), net of tax	1,028
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
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 prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	289
Other comprehensive income (loss), tax	(69)
Total other comprehensive income (loss), net of tax	220
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	9,458	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
Net actuarial gain/(loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
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	(8,395)	(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)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
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
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	(4,005)	791	(2,758)	(1,308)
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0
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)