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, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Transaction costs	$ 0.2	$ 13.9
Merger and restructuring costs, net of tax	$ 0.2	$ 8.6