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Equity (Accumulated Other Comprehensive Income (Loss)) (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Deferred Gains and Losses on Derivatives-Hedging Activities |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax | $ 141 | $ 135
Unrealized Gains and Losses on Investment Securities |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax | (21) | (13)
Unrecognized Pension and Other Postretirement Benefit Costs |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax | 478 | 492
Other Comprehensive Loss From Equity Method Investees |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax |  | 1
Virginia Electric and Power Company | Deferred Gains and Losses on Derivatives-Hedging Activities |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax | 21 | 11
Virginia Electric and Power Company | Unrealized Gains and Losses on Investment Securities |  |
Accumulated Other Comprehensive Income Loss [Line Items] |  |
Amount of tax | $ (3) | $ (1)

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