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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Proceeds from sales |  | $ 1,804 | $ 1,831 | $ 1,422
Realized gains | [1] | 140 | 166 | 128
Realized losses | [1] | 91 | 71 | 55
Virginia Electric and Power Company |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |
Proceeds from sales |  | 887 | 849 | 733
Realized gains | [1] | 60 | 75 | 63
Realized losses | [1] | $ 27 | $ 30 | $ 27
[1] | Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability as discussed in Note 2.

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