Investments (Selected Information Regarding Equity and Fixed Income Securities) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		$ 1,208	$ 429	$ 2,868	$ 1,311
Realized gains	[1]	48	53	188	152
Realized losses	[1]	29	25	159	75
Virginia Electric and Power Company
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales		164	230	694	677
Realized gains	[1]	18	21	73	46
Realized losses	[1]	$ 10	$ 6	$ 58	$ 18

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