Operating Segment - Narrative (Details) - Corporate and Other - Merger and restructuring costs associated with Dominion Questar combination - Dominion Energy Questar Corporation - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Transaction costs	$ 13.9	$ 11.9
Merger and restructuring costs, net of tax	$ 8.6	$ 7.7