Investments (Selected Information Regarding Equity and Fixed Income Securities) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		$ 1,765	$ 602
Realized gains	[1]	232	66
Realized losses	[1]	59	69
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		789	294
Realized gains	[1]	106	31
Realized losses	[1]	$ 23	$ 31

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