Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) - USD ($) $ / shares in Units, $ 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	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 1,812,403
Beginning balance (in shares)			48,090,470
Common stock issuances			$ 94,551
Net income (loss)	$ 12,331	$ 10,420	112,176	$ 97,546	$ 208,572	$ 163,240
Foreign currency exchange translation adjustment			(1,002)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			2,687
Amounts reclassified to net income, net of tax (FSIRS)	636	$ 518	1,907	$ 1,554	2,426	$ 2,073
Ending balance	$ 1,945,781		$ 1,945,781		$ 1,945,781
Ending balance (in shares)	49,422,938		49,422,938		49,422,938
Dividends declared per share (in USD per share)	$ 0.52	$ 0.495	$ 1.56	$ 1.485	$ 2.055	$ 1.935
IntelliChoice Energy LLC
Other comprehensive income (loss):
Historical ownership interest percentage	65.00%		65.00%		65.00%
Residual ownership percentage by third party	35.00%		35.00%		35.00%
Increased ownership percentage	95.00%
Additional capital contributed	$ 1,000
Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 1,609,999
Net income (loss)			79,301	$ 82,436	$ 153,683	$ 148,130
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			2,687
Amounts reclassified to net income, net of tax (FSIRS)	636	$ 518	1,907	$ 1,554	2,426	$ 2,073
Stock-based compensation			1,389
Dividends declared			(66,000)
Contributions from Southwest Gas Holdings, Inc.			90,644
Ending balance	1,719,927		1,719,927		1,719,927
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 49,720
Beginning balance (in shares)			48,090,000
Common stock issuances			$ 1,333
Common stock issuances (in shares)			1,333,000
Other comprehensive income (loss):
Ending balance	$ 51,053		$ 51,053		$ 51,053
Ending balance (in shares)	49,423,000		49,423,000		49,423,000
Common Stock | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 49,112
Beginning balance (in shares)			47,482,000
Other comprehensive income (loss):
Ending balance	$ 49,112		$ 49,112		$ 49,112
Ending balance (in shares)	47,482,000		47,482,000		47,482,000
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			$ 955,332
Common stock issuances			93,218
Other comprehensive income (loss):
Change in ownership of noncontrolling interest			(2,710)
Ending balance	$ 1,045,840		1,045,840		$ 1,045,840
Additional Paid-in Capital | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			948,767
Other comprehensive income (loss):
Stock-based compensation			1,899
Contributions from Southwest Gas Holdings, Inc.			90,644
Ending balance	1,041,310		1,041,310		1,041,310
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(47,682)
Foreign currency exchange translation adjustment			(1,002)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			2,687
Amounts reclassified to net income, net of tax (FSIRS)			1,907
Reclassification of excess deferred taxes			(9,300)
Ending balance	(53,390)		(53,390)		(53,390)
Accumulated Other Comprehensive Income (Loss) | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(47,073)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax			2,687
Amounts reclassified to net income, net of tax (FSIRS)			1,907
Reclassification of excess deferred taxes			(9,300)
Ending balance	(51,779)		(51,779)		(51,779)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			857,398
Net income (loss)			112,973
Other comprehensive income (loss):
Reclassification of excess deferred taxes			9,300
Dividends declared			(76,941)
Ending balance	902,730		902,730		902,730
Retained Earnings | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			659,193
Net income (loss)			79,301
Other comprehensive income (loss):
Reclassification of excess deferred taxes			9,300
Stock-based compensation			(510)
Dividends declared			(66,000)
Ending balance	681,284		681,284		681,284
Non-controlling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			(2,365)
Net income (loss)			(797)
Other comprehensive income (loss):
Change in ownership of noncontrolling interest			2,710
Ending balance	$ (452)		$ (452)		$ (452)