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Commitments and Contingencies (Schedule of Long Term Purchase Commitments) (Parenthetical) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Power Purchase Contracts [Abstract] |  |
Energy payments | $ 52 | $ 29

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- Definition Power purchase contracts. + References No definition available. + Details
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Namespace Prefix: | d_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount purchased during the period under an unrecorded unconditional purchase obligation (for example, under the take-or-pay or throughput contract). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 440 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=121559207&loc=d3e25336-109308 + Details
Name: | us-gaap_UnrecordedUnconditionalPurchaseObligationPurchases
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration