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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, 2018 | Dec. 31, 2017
--- | --- | ---
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities |  | $ 5
Fair value of securities in an unrealized loss position | $ 833 | 565
Virginia Electric and Power Company |  |
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities | 6 | 6
Fair value of securities in an unrealized loss position | $ 404 | $ 234

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