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Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]      
Total other-than-temporary impairment losses $ 3 $ 30 $ 44
Losses recorded to the nuclear decommissioning trust regulatory liability 0 0 (16)
Losses recognized in other comprehensive income (before taxes) (3) (30) (5)
Net impairment losses recognized in earnings 0 0 23
Virginia Electric and Power Company      
Schedule of Available-for-sale Securities [Line Items]      
Total other-than-temporary impairment losses 2 15 20
Losses recorded to the nuclear decommissioning trust regulatory liability 0 0 (16)
Losses recognized in other comprehensive income (before taxes) (2) (15) (2)
Net impairment losses recognized in earnings $ 0 $ 0 $ 2