Related-Party Transactions (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Affiliated current borrowings		$ 48,400,000		$ 48,400,000		$ 75,000,000	[1]
Dominion | Pension Plan
Related Party Transaction [Line Items]
Amounts due from related party		111,100,000		111,100,000		107,100,000
Dominion | Other Postretirement Benefit Plan
Related Party Transaction [Line Items]
Amounts due to related party		12,200,000		12,200,000		14,900,000
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	60,500,000	$ 74,800,000	192,000,000	$ 230,400,000
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	17,500,000	17,400,000	55,100,000	54,300,000
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		13,100,000	17,100,000	41,900,000	39,000,000
Related party revenues		1,300,000	1,600,000	4,000,000	4,500,000
Merger and restructuring costs | Dominion Energy Questar Corporation
Related Party Transaction [Line Items]
Transaction costs			2,500,000		9,200,000
IRCA | Dominion | Revolving Credit Facility
Related Party Transaction [Line Items]
Related party interest expense		300,000		500,000	$ 100,000
IRCA | Dominion | Revolving Credit Facility | Maximum
Related Party Transaction [Line Items]
Related party interest expense			$ 100,000
Dominion Energy Questar Pipeline | Other Current Assets
Related Party Transaction [Line Items]
Natural gas imbalance receivable		$ 1,100,000		$ 1,100,000		$ 900,000

[1] Questar Gas’ Balance Sheet at December 31, 2017 has been derived from the audited Balance Sheet at that date.

[2] The costs of these services were included in purchased gas in Questar Gas’ Statements of Income.