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Investments (Marketable Equity and Debt Securities and Cash Equivalents (Classified as Available-for-sale) and Cost Method Investments in Decommissioning Trust Funds) (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities | $ 5 | $ 9
Fair value of securities in an unrealized loss position | 565 | 576
Virginia Electric and Power Company |  |
Investment Holdings [Line Items] |  |
Net assets related to pending sales and purchases of securities | 6 | 7
Fair value of securities in an unrealized loss position | $ 234 | $ 287

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