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Income Taxes (Unrecognized Tax Benefits) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Balance at January 1 | $ 175 | $ 44 |  | $ 38
Acquired unrecognized tax benefits | 0 | 129 | [1] | 0
Increases-prior period positions | 18 | 0 |  | 10
Decreases-prior period positions | (19) | 0 |  | 0
Increases-current period positions | 1 | 9 |  | 10
Settlements with tax authorities | 0 | (7) |  | (6)
Expiration of statutes of limitations | (8) | 0 |  | (8)
Balance at December 31 | 167 | 175 |  | 44
Virginia Electric and Power Company |  |  |  |
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] |  |  |  |
Balance at January 1 | 0 | 2 |  | 4
Acquired unrecognized tax benefits | 0 | 0 |  | 0
Increases-prior period positions | 0 | 0 |  | 0
Decreases-prior period positions | 0 | 0 |  | 0
Increases-current period positions | 0 | 0 |  | 0
Settlements with tax authorities | 0 | (2) |  | (1)
Expiration of statutes of limitations | 0 | 0 |  | (1)
Balance at December 31 | $ 0 | $ 0 |  | $ 2
[1] | Acquired unrecognized tax benefits reflect $106 million plus increases in prior period positions of $76 million and decreases in prior period positions of $53 million that were recorded through purchase accounting.

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- Definition Amount of unrecognized tax benefits. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 10B -URI http://asc.fasb.org/extlink&oid=120406818&loc=SL37586934-109318 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319 + Details
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