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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 | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Investment Holdings [Line Items] |  |
Fair value of securities in an unrealized loss position | $ 900 | $ 565
Net assets related to pending sales and purchases of securities | 3 | 5
Virginia Electric and Power Company |  |
Investment Holdings [Line Items] |  |
Fair value of securities in an unrealized loss position | 414 | 234
Net assets related to pending sales and purchases of securities | $ 5 | $ 6

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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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