Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule Of Available For Sale Securities [Line Items]
Proceeds from sales		$ 363	$ 341	$ 1,119	$ 709
Realized gains	[1]	23	37	117	62
Realized losses	[1]	16	15	36	34
Virginia Electric and Power Company
Schedule Of Available For Sale Securities [Line Items]
Proceeds from sales		168	154	498	347
Realized gains	[1]	10	18	55	30
Realized losses	[1]	$ 8	$ 7	$ 18	$ 17

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