Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Detailed Activities of Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Schedule Of Stockholders Equity [Line Items]
Balance	$ 1,812,403
Balance	$ 1,814,768
Balance, shares	48,090,470
Common stock issuances	$ 10,395
Net income	78,294	$ 69,005	$ 203,231	$ 146,705
Net income	78,294
Foreign currency exchange translation adj.	(911)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	895
Amounts reclassified to net income, net of tax (FSIRS)	635	518	2,190	2,074
Dividends declared Common	(25,335)
Balance	1,876,376		1,876,376
Balance	$ 1,879,538		$ 1,879,538
Balance, shares	48,336,922		48,336,922
Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 1,609,999
Net income	90,349	76,938	$ 170,229	154,638
Net income	90,349
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	895
Amounts reclassified to net income, net of tax (FSIRS)	635	$ 518	2,190	$ 2,074
Stock-based compensation	(734)
Dividends declared Common	(22,000)
Balance	1,679,144		1,679,144
Common Stock [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 49,720
Balance, shares	48,090,000
Common stock issuances	$ 247
Common stock issuances, Shares	247,000
Other comprehensive income (loss):
Balance	$ 49,967		$ 49,967
Balance, shares	48,337,000		48,337,000
Common Stock [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 49,112
Balance, shares	47,482,000
Other comprehensive income (loss):
Balance	$ 49,112		$ 49,112
Balance, shares	47,482,000		47,482,000
Additional Paid-in Capital [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	$ 955,332
Common stock issuances	10,148
Other comprehensive income (loss):
Balance	965,480		$ 965,480
Additional Paid-in Capital [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	948,767
Other comprehensive income (loss):
Stock-based compensation	(568)
Balance	948,199		948,199
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(47,682)
Foreign currency exchange translation adj.	(911)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	895
Amounts reclassified to net income, net of tax (FSIRS)	635
Reclass of stranded deferred taxes due to TCJA	(9,300)
Balance	(56,363)		(56,363)
Accumulated Other Comprehensive Income (Loss) [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(47,073)
Other comprehensive income (loss):
Net actuarial gain (loss) arising during period, less amortization of unamortized benefit plan cost, net of tax	895
Amounts reclassified to net income, net of tax (FSIRS)	635
Reclass of stranded deferred taxes due to TCJA	(9,300)
Balance	(54,843)		(54,843)
Retained Earnings [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	857,398
Net income	79,091
Other comprehensive income (loss):
Reclass of stranded deferred taxes due to TCJA	9,300
Dividends declared Common	(25,335)
Balance	920,454		920,454
Retained Earnings [Member] | Southwest Gas Corporation [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	659,193
Net income	90,349
Other comprehensive income (loss):
Reclass of stranded deferred taxes due to TCJA	9,300
Stock-based compensation	(166)
Dividends declared Common	(22,000)
Balance	736,676		736,676
Non-controlling Interest [Member]
Schedule Of Stockholders Equity [Line Items]
Balance	(2,365)
Net income	(797)
Other comprehensive income (loss):
Balance	$ (3,162)		$ (3,162)