Investments (Selected Information Regarding Equity and Fixed Income Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		$ 614	$ 1,208	$ 3,324	$ 2,868
Realized gains	[1]	25	48	405	188
Realized losses	[1]	7	29	81	159
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		216	164	1,465	694
Realized gains	[1]	17	18	213	73
Realized losses	[1]	$ 2	$ 10	$ 28	$ 58

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