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Investments (Recorded Other-Than-Temporary Impairment Losses on Investments) (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items] |  |
Other-than-temporary impairment losses for debt securities | $ 5 | $ 13
Virginia Electric and Power Company |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Other-than-temporary impairment losses for debt securities | $ 2 | $ 8

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- Definition Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 8A -URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284422-111562 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Details
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