Related-Party Transactions (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Capital expenditures			$ 3,111	$ 4,122
Virginia Electric and Power Company
Related Party Transaction [Line Items]
Capital expenditures			1,696	1,901
Virginia Electric and Power Company | Services provided by affiliates | Affiliated Entity
Related Party Transaction [Line Items]
Capital expenditures	$ 34	$ 33	109	104
Dominion Energy Gas Holdings, LLC
Related Party Transaction [Line Items]
Capital expenditures			550	535
Dominion Energy Gas Holdings, LLC | Services provided by affiliates | Affiliated Entity
Related Party Transaction [Line Items]
Capital expenditures	$ 10	$ 13	$ 27	$ 33