Components of Net Periodic Benefit Cost - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) Participant	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance	$ 105,508	$ 97,422	$ 313,294	$ 299,920	$ 406,137	$ 394,725
Other income (deductions)	$ 889	(1,980)	(6,151)	(5,780)	(6,401)	(8,007)
Percentage of eligible participants accepted pension payout offer	25.00%
Number of eligible participants for pension payout offer | Participant	385
Payment from pension assets	$ 6,800
Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance	104,657	97,359	312,055	298,827	404,549	393,632
Other income (deductions)	$ 836	(1,775)	$ (5,861)	(5,824)	$ (6,425)	(9,200)
Operating Expense
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		97,400		299,900		394,700
Operating Expense | Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		97,400		298,800		393,600
Other Income
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)		(2,000)		(5,800)		(8,000)
Other Income | Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)		(1,800)		(5,800)		(9,200)
Previously Reported | Operating Expense
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		102,300		314,500		414,200
Previously Reported | Operating Expense | Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		102,200		313,400		413,100
Previously Reported | Other Income
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)		2,900		8,800		11,500
Previously Reported | Other Income | Southwest Gas Corporation
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)		$ 3,100		$ 8,700		$ 10,300