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Fair Value Measurements (Assets and Liabilities that are Measured at Fair Value on a Recurring Basis) (Parenthetical) (Detail) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value using NAV | $ 211 | $ 88
Alternative Investment, Fair Value by Fair Value Hierarchy Level and NAV [Extensible List] | us-gaap:Assets | us-gaap:Assets
Virginia Electric and Power Company |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets measured at fair value using NAV | $ 171 | $ 27
Alternative Investment, Fair Value by Fair Value Hierarchy Level and NAV [Extensible List] | us-gaap:Assets | us-gaap:Assets

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- Definition Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 6A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 35 -Paragraph 54B -URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257 + Details
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- Definition Indicates level, within fair value hierarchy and fair value measured at net asset value per share as practical expedient, of alternative investment. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + 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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