Related-Party Transactions (Details) - USD ($)		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Related Party Transaction [Line Items]
Affiliated current borrowings		$ 33,900,000		$ 75,000,000	[1]
Dominion | Pension Plan
Related Party Transaction [Line Items]
Amounts due from related party		108,200,000		107,100,000
Dominion | Other Postretirement Benefit Plan
Related Party Transaction [Line Items]
Amounts due to related party		13,000,000		14,900,000
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	69,600,000	$ 75,900,000
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	20,000,000	19,600,000
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		14,600,000	10,800,000
Related party revenues		1,400,000	1,500,000
Merger and restructuring costs | Dominion Energy Questar Corporation
Related Party Transaction [Line Items]
Transaction costs			300,000
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
Dominion Energy Questar Pipeline | Other Current Assets
Related Party Transaction [Line Items]
Natural gas imbalance receivable		$ 900,000		$ 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.