Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - AOCI Rollforward (Details) - 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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 1,812,403
Beginning balance, attributable to parent			1,814,768
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
Reclassification of excess deferred taxes			(9,300)
Other	(9,300)		(9,300)		(9,300)
Ending balance, attributable to parent	1,946,233		1,946,233		1,946,233	$ 1,814,768
Ending balance	1,945,781		$ 1,945,781		1,945,781	$ 1,812,403
Effective income tax rate			24.00%			38.00%
Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			$ (61,520)
Beginning balance, tax			22,293
Beginning balance			(39,227)
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)
Ending balance, before tax	(57,985)		(57,985)		(57,985)	$ (61,520)
Ending balance, tax	21,445		21,445		21,445	22,293
Ending balance	(36,540)		(36,540)		(36,540)	(39,227)
Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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 net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss) before reclassifications, before tax					(43,027)		(22,770)
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
Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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)
FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(12,655)
Beginning balance, tax			4,809
Beginning balance			(7,846)
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
Ending balance, before tax	(10,146)		(10,146)		(10,146)	(12,655)
Ending balance, tax	4,207		4,207		4,207	4,809
Ending balance	(5,939)		(5,939)		(5,939)	(7,846)
Foreign Currency Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, before tax			(609)
Beginning balance			(609)
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, 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)
Ending balance, before tax	(1,611)		(1,611)		(1,611)	(609)
Ending balance	(1,611)		(1,611)		(1,611)	(609)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(47,682)
Ending balance	(53,390)		(53,390)		(53,390)	(47,682)
Southwest Gas Corporation
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			1,609,999
Beginning balance, attributable to parent			1,609,999
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
Reclassification of excess deferred taxes			(9,300)
Other	(9,300)		(9,300)		(9,300)
Ending balance, attributable to parent	1,719,927		1,719,927		1,719,927	1,609,999
Ending balance	1,719,927		$ 1,719,927		1,719,927	$ 1,609,999
Effective income tax rate			24.00%			38.00%
Southwest Gas Corporation | Defined Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			$ (61,520)
Beginning balance, attributable to parent, tax			22,293
Beginning balance, attributable to parent			(39,227)
Other comprehensive income (loss), before tax			3,535
Other comprehensive income (loss), tax			(848)
Total other comprehensive income (loss), net of tax			2,687
Ending balance, attributable to parent, before tax	(57,985)		(57,985)		(57,985)	$ (61,520)
Ending balance, attributable to parent, tax	21,445		21,445		21,445	22,293
Ending balance, attributable to parent	(36,540)		(36,540)		(36,540)	(39,227)
Southwest Gas Corporation | Amortization of prior service cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Other comprehensive income (loss), before tax			1,002
Other comprehensive income (loss), tax			(240)
Total other comprehensive income (loss), net of tax			762
Southwest Gas Corporation | Amortization of net actuarial loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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
Southwest Gas Corporation | Regulatory adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
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)
Southwest Gas Corporation | FSIRS
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, attributable to parent, before tax			(12,655)
Beginning balance, attributable to parent, tax			4,809
Beginning balance, attributable to parent			(7,846)
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
Ending balance, attributable to parent, before tax	(10,146)		(10,146)		(10,146)	(12,655)
Ending balance, attributable to parent, tax	4,207		4,207		4,207	4,809
Ending balance, attributable to parent	(5,939)		(5,939)		(5,939)	(7,846)
Southwest Gas Corporation | AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(47,073)
Beginning balance, attributable to parent			(47,073)
Ending balance, attributable to parent	(51,779)		(51,779)		(51,779)	(47,073)
Ending balance	$ (51,779)		$ (51,779)		$ (51,779)	$ (47,073)