Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		$ 419	$ 756
Realized gains	[1]	36	94
Realized losses	[1]	19	20
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		218	330
Realized gains	[1]	18	45
Realized losses	[1]	$ 5	$ 10

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