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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sales | $ 1,712 | $ 1,804 | $ 1,831
Realized gains | 195 | 140 | 166
Realized losses | 96 | 91 | 71
Virginia Electric and Power Company |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Proceeds from sales | 858 | 887 | 849
Realized gains | 58 | 60 | 75
Realized losses | $ 22 | $ 27 | $ 30

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