Related-Party Transactions (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Related Party Transaction [Line Items]
Capital expenditures			$ 2,046	$ 2,748
Virginia Electric and Power Company
Related Party Transaction [Line Items]
Capital expenditures			1,170	1,216
Virginia Electric and Power Company | Services provided by affiliates | Affiliated Entity
Related Party Transaction [Line Items]
Capital expenditures	$ 38	$ 37	75	71
Dominion Energy Gas Holdings, LLC
Related Party Transaction [Line Items]
Capital expenditures			316	309
Dominion Energy Gas Holdings, LLC | Services provided by affiliates | Affiliated Entity
Related Party Transaction [Line Items]
Capital expenditures	$ 7	$ 12	$ 17	$ 20