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Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities | $ 1 |
Fair value of securities in an unrealized loss position | 298 | $ 833
Virginia Electric and Power Company |  |
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities |  | 6
Fair value of securities in an unrealized loss position | $ 185 | $ 404

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