Related-Party Transactions (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Affiliated current borrowings		$ 23,600,000		$ 23,600,000		$ 75,000,000	[1]
Dominion | Pension Plan
Related Party Transaction [Line Items]
Amounts due from related party		109,800,000		109,800,000		107,100,000
Dominion | Other Postretirement Benefit Plan
Related Party Transaction [Line Items]
Amounts due to related party		12,000,000		12,000,000		14,900,000
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	61,900,000	$ 79,700,000	131,500,000	$ 155,600,000
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	17,600,000	17,300,000	37,600,000	36,900,000
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		14,200,000	11,100,000	28,800,000	21,900,000
Related party revenues		1,300,000	1,400,000	2,700,000	2,900,000
Merger and restructuring costs | Dominion Energy Questar Corporation
Related Party Transaction [Line Items]
Transaction costs			6,400,000		6,700,000
IRCA | Dominion | Revolving Credit Facility
Related Party Transaction [Line Items]
Related party interest expense				200,000
IRCA | Dominion | Revolving Credit Facility | Maximum
Related Party Transaction [Line Items]
Related party interest expense		100,000	$ 100,000		$ 100,000
Dominion Energy Questar Pipeline | Other Current Assets
Related Party Transaction [Line Items]
Natural gas imbalance receivable		$ 800,000		$ 800,000		$ 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.