Components of Net Periodic Benefit Cost - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 31, 2018 USD ($) Participant	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		$ 105,435	$ 98,203	$ 207,786	$ 202,498	$ 398,051	$ 394,802
Other income (deductions)		(2,706)	(2,810)	(7,040)	(3,800)	(9,270)	(8,401)
Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of eligible participants for pension payout offer | Participant	385
Payment from pension assets	$ 6,800
Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance		105,208	97,644	207,398	201,468	397,251	393,772
Other income (deductions)		$ (2,094)	(2,805)	$ (6,697)	(4,049)	$ (9,036)	(9,843)
Operating Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			103,100		212,200		414,400
Operating Expense [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			102,500		211,200		413,400
Other Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,100		5,900		11,200
Other Income [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,100		5,700		9,800
Revised [Member] | Operating Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			98,200		202,500		394,800
Revised [Member] | Operating Expense [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Operations and maintenance			97,600		201,500		393,800
Revised [Member] | Other Income [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			2,800		3,800		8,400
Revised [Member] | Other Income [Member] | Southwest Gas Corporation [Member]
Defined Benefit Plan Disclosure [Line Items]
Other income (deductions)			$ 2,800		$ 4,000		$ 9,800