Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to OCI (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, net of tax			$ (1,002)
Other comprehensive income (loss), before tax	$ 2,613	$ 2,808	5,042	$ 7,249	$ (999)		$ 4,747
Other comprehensive income (loss), tax	(483)	(682)	(1,450)	(2,048)	(273)		(1,270)
Total other comprehensive income (loss), net of tax	2,130	2,126	$ 3,592	5,201	(1,272)		3,477
Effective income tax rate			24.00%			38.00%
Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	2,014	1,796	$ 6,044	5,388	93		3,339
Other comprehensive income (loss), tax	(483)	(682)	(1,450)	(2,048)	(273)		(1,270)
Total other comprehensive income (loss), net of tax	1,531	1,114	$ 4,594	3,340	(180)		2,069
Effective income tax rate			24.00%			38.00%
Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax	1,178	961	$ 3,535	2,881	(3,253)		(5)
Other comprehensive income (loss), tax	(283)	(365)	(848)	(1,095)	647		1
Total other comprehensive income (loss), net of tax	895	596	2,687	1,786	(2,606)		(4)
Defined Benefit Plans | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			3,535
Other comprehensive income (loss), tax			(848)
Total other comprehensive income (loss), net of tax			2,687
Amortization of prior service cost
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	334	333	1,002	1,001	1,336		1,335
Reclassification from AOCI, tax	(80)	(126)	(240)	(380)	(367)		(507)
Reclassification from AOCI, net of tax	254	207	762	621	969		828
Other comprehensive income (loss), before tax			1,002
Other comprehensive income (loss), tax			(240)
Total other comprehensive income (loss), net of tax			762
Amortization of prior service cost | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			1,002
Other comprehensive income (loss), tax			(240)
Total other comprehensive income (loss), net of tax			762
Net actuarial gain/(loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax					(43,027)		(22,770)
Other comprehensive income (loss) before reclassifications, tax					10,326		8,652
Other comprehensive income (loss) before reclassifications, net of tax					(32,701)		(14,118)
Reclassification from AOCI, before Tax	8,404	6,362	25,212	19,084	31,573		25,850
Reclassification from AOCI, tax	(2,017)	(2,418)	(6,051)	(7,252)	(8,468)		(9,823)
Reclassification from AOCI, net of tax	6,387	3,944	19,161	11,832	23,105		16,027
Other comprehensive income (loss), before tax			25,212
Other comprehensive income (loss), tax			(6,051)
Total other comprehensive income (loss), net of tax			19,161
Net actuarial gain/(loss) | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			25,212
Other comprehensive income (loss), tax			(6,051)
Total other comprehensive income (loss), net of tax			19,161
Regulatory adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	(7,560)	(5,734)	(22,679)	(17,204)	6,865		(4,420)
Reclassification from AOCI, tax	1,814	2,179	5,443	6,537	(844)		1,679
Reclassification from AOCI, net of tax	(5,746)	(3,555)	(17,236)	(10,667)	6,021		(2,741)
Other comprehensive income (loss), before tax			(22,679)
Other comprehensive income (loss), tax			5,443
Total other comprehensive income (loss), net of tax			(17,236)
Regulatory adjustment | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax			(22,679)
Other comprehensive income (loss), tax			5,443
Total other comprehensive income (loss), net of tax			(17,236)
FSIRS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax	836	835	2,509	2,507	3,346		3,344
Reclassification from AOCI, tax	(200)	(317)	(602)	(953)	(920)		(1,271)
Reclassification from AOCI, net of tax	636	518	1,907	1,554	2,426		2,073
Other comprehensive income (loss), before tax	836	835	2,509	2,507	3,346		3,344
Other comprehensive income (loss), tax	(200)	(317)	(602)	(953)	(920)		(1,271)
Total other comprehensive income (loss), net of tax	636	518	1,907	1,554	2,426		2,073
FSIRS | Southwest Gas Corporation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from AOCI, before Tax			2,509
Reclassification from AOCI, tax			(602)
Reclassification from AOCI, net of tax			1,907
Other comprehensive income (loss), before tax			2,509
Other comprehensive income (loss), tax			(602)
Total other comprehensive income (loss), net of tax			1,907
Foreign Currency Items
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss) before reclassifications, before tax	599	1,012	(1,002)	1,861	(1,092)		1,408
Other comprehensive income (loss) before reclassifications, tax	0	0	0	0	0		0
Other comprehensive income (loss) before reclassifications, net of tax	$ 599	$ 1,012	(1,002)	$ 1,861	$ (1,092)		$ 1,408
Other comprehensive income (loss), before tax			(1,002)
Total other comprehensive income (loss), net of tax			$ (1,002)