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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities |  | $ 425 | $ 363 | $ 844 | $ 1,119
Realized gains | [1] | 36 | 23 | 72 | 117
Realized losses | [1] | 23 | 16 | 42 | 36
Virginia Electric and Power Company |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities |  | 196 | 168 | 414 | 498
Realized gains | [1] | 15 | 10 | 33 | 55
Realized losses | [1] | $ 7 | $ 8 | $ 12 | $ 18
[1] | Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.

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