Related-Party Transactions (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2017	Dec. 31, 2016	[1]
Related Party Transaction [Line Items]
Receivable from affiliates		$ 98.6		$ 98.6			$ 87.8
Pension expense (credit)				(4.4)	$ 2.8
Payables to affiliates		12.8		12.8			13.0
Affiliated current borrowings		$ 10.0		$ 10.0			$ 48.0
Dominion Energy | IRCA | Revolving Credit Facility
Related Party Transaction [Line Items]
Weighted-average interest rate percentage		1.36%		1.36%
Related party interest expense			$ 0.2		0.5
Dominion Energy | Maximum | IRCA | Revolving Credit Facility
Related Party Transaction [Line Items]
Related party interest expense		$ 0.1		$ 0.1
Affiliated operator service fee
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	79.7	78.9	155.6	158.7
Transportation and storage services from affiliates
Related Party Transaction [Line Items]
Related party expenses in cost of sales	[2]	17.3	17.3	36.9	36.5
Services provided by/to related parties
Related Party Transaction [Line Items]
Related party expenses		11.1	12.6	21.9	26.1
Related party revenues		1.4	0.5	2.9	1.0
Merger and restructuring costs | Dominion Energy Questar Corporation | Dominion Energy Questar Corporation
Related Party Transaction [Line Items]
Transaction costs		$ 6.4	$ 0.0	$ 6.7	$ 0.0
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.