Short-Term Debt - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Short term borrowings outstanding	$ 22,500,000	$ 214,500,000
Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	$ 400,000,000
Credit facility expiration date	Mar. 01, 2022
Short-term Debt [Member] | Southwest Gas Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	$ 100,000,000
Credit facility expiration date	Mar. 01, 2022
Short-term Debt [Member] | Southwest Gas Credit Facility [Member] | Credit Facility [Member]
Short-term Debt [Line Items]
Short term borrowings outstanding	$ 22,500,000