Equity, 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, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax	$ 791
Other comprehensive income (loss), before tax	2,441	$ 1,104	$ 7,965		$ 296
Other comprehensive income (loss), tax	(396)	(485)	(2,225)		(673)
Total other comprehensive income (loss), net of tax	$ 2,045	619	5,740		(377)
Effective income tax rate	24.00%			38.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	$ 1,650	2,015	9,273		(344)
Other comprehensive income (loss), tax	(396)	(485)	(2,225)		(673)
Total other comprehensive income (loss), net of tax	1,254	1,530	7,048		(1,017)
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	814	1,178	5,929		(3,689)
Other comprehensive income (loss), tax	(195)	(283)	(1,422)		482
Total other comprehensive income (loss), net of tax	619	895	4,507		(3,207)
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	814
Other comprehensive income (loss), tax	(195)
Total other comprehensive income (loss), net of tax	619
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	317	334	1,318		1,335
Reclassification from AOCI, tax	(76)	(80)	(316)		(460)
Reclassification from AOCI, net of tax	241	254	1,002		875
Other comprehensive income (loss), before tax	317
Other comprehensive income (loss), tax	(76)
Total other comprehensive income (loss), net of tax	241
Amortization of prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	317
Other comprehensive income (loss), tax	(76)
Total other comprehensive income (loss), net of tax	241
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax			(20,426)		(43,027)
Other comprehensive income (loss) before reclassifications, tax			4,902		10,326
Other comprehensive income (loss) before reclassifications, net of tax			(15,524)		(32,701)
Reclassification from AOCI, before Tax	5,844	8,404	31,057		27,488
Reclassification from AOCI, tax	(1,403)	(2,017)	(7,454)		(9,269)
Reclassification from AOCI, net of tax	4,441	6,387	23,603		18,219
Other comprehensive income (loss), before tax	5,844
Other comprehensive income (loss), tax	(1,403)
Total other comprehensive income (loss), net of tax	4,441
Net actuarial gain/(loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	5,844
Other comprehensive income (loss), tax	(1,403)
Total other comprehensive income (loss), net of tax	4,441
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	(5,347)	(7,560)	(6,020)		10,515
Reclassification from AOCI, tax	1,284	1,814	1,446		(115)
Reclassification from AOCI, net of tax	(4,063)	(5,746)	(4,574)		10,400
Other comprehensive income (loss), before tax	(5,347)
Other comprehensive income (loss), tax	1,284
Total other comprehensive income (loss), net of tax	(4,063)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	(5,347)
Other comprehensive income (loss), tax	1,284
Total other comprehensive income (loss), net of tax	(4,063)
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	836	837	3,344		3,345
Reclassification from AOCI, tax	(201)	(202)	(803)		(1,155)
Reclassification from AOCI, net of tax	635	635	2,541		2,190
Other comprehensive income (loss), before tax	836	837	3,344		3,345
Other comprehensive income (loss), tax	(201)	(202)	(803)		(1,155)
Total other comprehensive income (loss), net of tax	635	635	2,541		2,190
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	836
Reclassification from AOCI, tax	(201)
Reclassification from AOCI, net of tax	635
Other comprehensive income (loss), before tax	836
Other comprehensive income (loss), tax	(201)
Total other comprehensive income (loss), net of tax	635
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	791	(911)	(1,308)		640
Other comprehensive income (loss) before reclassifications, tax	0	0	0		0
Other comprehensive income (loss) before reclassifications, net of tax	791	$ (911)	$ (1,308)		$ 640
Other comprehensive income (loss), before tax	791
Total other comprehensive income (loss), net of tax	$ 791