Investments (Selected Information Regarding Equity and Fixed Income Securities) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		$ 602	$ 506
Realized gains	[1]	66	43
Realized losses	[1]	69	23
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		294	253
Realized gains	[1]	31	10
Realized losses	[1]	$ 31	$ 9

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