Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of securities		$ 457	$ 377	$ 1,301	$ 1,496
Realized gains	[1]	24	25	96	142
Realized losses	[1]	18	16	60	52
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of securities		237	156	651	654
Realized gains	[1]	11	9	44	64
Realized losses	[1]	$ 5	$ 6	$ 17	$ 24

[1] Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.