Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	53,026,848
Beginning balance	$ 2,251,590	$ 1,812,403	$ 1,876,376
Common stock issuances	27,389	10,395
Net income (loss)	94,809	78,294
Foreign currency exchange translation adjustment	791	(911)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	619	895
FSIRS amounts reclassified to net income, net of tax	$ 635	635	$ 2,541	$ 2,190
Reclassification of excess deferred taxes		0
Ending balance (in shares)	53,391,731		53,391,731
Ending balance	$ 2,346,548	$ 1,876,376	$ 2,346,548	1,876,376
Increase (Decrease) in Temporary Equity [Roll Forward]
Beginning balance	81,831
Net income (loss)	575
Ending balance	$ 82,406		82,406
Dividends declared per share (in USD per share)	$ 0.545	$ 0.52
Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	$ 1,782,460	$ 1,609,999	1,679,144
Net income (loss)	103,389	90,349
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	619	895
FSIRS amounts reclassified to net income, net of tax	635	635	2,541	2,190
Reclassification of excess deferred taxes		0
Stock-based compensation	758	(734)
Dividends declared	(22,800)	(22,000)
Contributions from Southwest Gas Holdings, Inc.	22,842
Ending balance	$ 1,887,903	$ 1,679,144	$ 1,887,903	$ 1,679,144
Common Stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	53,026,000	48,090,000	48,337,000
Beginning balance	$ 54,656	$ 49,720	$ 49,967
Common stock issuances (in shares)	365,000	247,000
Common stock issuances	$ 365	$ 247
Other comprehensive income (loss):
Ending balance (in shares)	53,391,000	48,337,000	53,391,000	48,337,000
Ending balance	$ 55,021	$ 49,967	$ 55,021	$ 49,967
Common Stock | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	47,482,000	47,482,000	47,482,000
Beginning balance	$ 49,112	$ 49,112	$ 49,112
Other comprehensive income (loss):
Ending balance (in shares)	47,482,000	47,482,000	47,482,000	47,482,000
Ending balance	$ 49,112	$ 49,112	$ 49,112	$ 49,112
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,305,769	955,332	965,480
Common stock issuances	27,024	10,148
Other comprehensive income (loss):
Ending balance	1,332,793	965,480	1,332,793	965,480
Additional Paid-in Capital | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	1,065,242	948,767	948,199
Other comprehensive income (loss):
Stock-based compensation	918	(568)
Contributions from Southwest Gas Holdings, Inc.	22,842
Ending balance	1,089,002	948,199	1,089,002	948,199
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(52,668)	(47,682)	(56,363)
Foreign currency exchange translation adjustment	791	(911)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	619	895
FSIRS amounts reclassified to net income, net of tax	635	635
Reclassification of excess deferred taxes		(9,300)
Ending balance	(50,623)	(56,363)	(50,623)	(56,363)
Accumulated Other Comprehensive Income (Loss) | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(49,049)	(47,073)	(54,843)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	619	895
FSIRS amounts reclassified to net income, net of tax	635	635
Reclassification of excess deferred taxes		(9,300)
Ending balance	(47,795)	(54,843)	(47,795)	(54,843)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	944,285	857,398	920,454
Net income (loss)	94,809	79,091
Other comprehensive income (loss):
Reclassification of excess deferred taxes		9,300
Dividends declared	(29,285)	(25,335)
Ending balance	1,009,809	920,454	1,009,809	920,454
Retained Earnings | Southwest Gas Corporation
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	717,155	659,193	736,676
Net income (loss)	103,389	90,349
Other comprehensive income (loss):
Reclassification of excess deferred taxes		9,300
Stock-based compensation	(160)	(166)
Dividends declared	(22,800)	(22,000)
Ending balance	797,584	736,676	797,584	736,676
Non-controlling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	(452)	(2,365)	(3,162)
Net income (loss)		(797)
Other comprehensive income (loss):
Ending balance	$ (452)	$ (3,162)	$ (452)	$ (3,162)