Related-Party Transactions (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Mar. 31, 2017	Dec. 31, 2016	[1]
Related Party Transaction [Line Items]
Receivable from affiliates		$ 101.0		$ 101.0			$ 87.8
Pension expense (credit)				(6.6)	$ 4.4
Payables to affiliates		12.0		12.0			13.0
Affiliated current borrowings		0.0		0.0			$ 48.0
Dominion | IRCA | Revolving Credit Facility
Related Party Transaction [Line Items]
Related party interest expense		0.1	$ 0.3	0.1	0.8
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	74.8	77.7	230.4	236.4
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	17.4	17.4	54.3	53.9
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		17.1	22.9	39.0	49.0
Related party revenues		1.6	0.5	4.5	1.5
Merger and restructuring costs | Dominion Energy Questar Corporation
Related Party Transaction [Line Items]
Transaction costs		$ 2.5	$ 9.7	$ 9.2	$ 9.7
Transportation services | Dominion Energy Questar Pipeline
Related Party Transaction [Line Items]
Annual commitment						$ 55.0
Storage contract | Dominion Energy Questar Pipeline
Related Party Transaction [Line Items]
Annual commitment						$ 2.0

[1] Questar Gas' Balance Sheet at December 31, 2016 has been derived from the audited Balance Sheet at that date.

[2] The costs of these services were included in cost of sales in Questar Gas' Statements of Income.