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Accumulated Other Comprehensive Income (Loss) (Schedule of Changes in AOCI Net of Tax and Reclassifications out of AOCI by Component) (Parenthetical) (Detail) - USD ($)
$ in Millions
Sep. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Dec. 31, 2022
Total Derivative-Hedging Activities            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax $ 64 $ 64 $ 73 $ 70 $ 79 $ 83
Total Derivative-Hedging Activities | Virginia Electric and Power Company            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax (6) (8) (5) (11) (4) (5)
Investment Securities            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax (5) 3 (2) 16 9 13
Investment Securities | Virginia Electric and Power Company            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax (2) 0 0 3 1 2
Pension and Other Postretirement Benefit Costs            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax 522 522 456 461 453 445
Equity Method Investees            
Accumulated Other Comprehensive Income (Loss) [Line Items]            
Amount of tax $ 0 $ 0 $ 0 $ 0 $ 0 $ 1