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Investments (Selected Information Regarding Equity and Fixed Income Securities) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sales $ 4,278 $ 1,712 $ 1,804
Realized gains [1] 340 195 140
Realized losses [1] 297 96 91
Virginia Electric and Power Company      
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from sales 884 858 887
Realized gains [1] 88 58 60
Realized losses [1] $ 68 $ 22 $ 27
[1] Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability as discussed in Note 2.