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Derivatives and Hedge Accounting Activities (Selected Information Related to Losses on Cash Flow Hedges Included in AOCI) (Detail)
$ in Millions
6 Months Ended
Jun. 30, 2021
USD ($)
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ (371)
Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax (43)
Virginia Electric and Power Company  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax (39)
Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax (2)
Interest rate contracts  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax (371)
Interest rate, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ (43)
Maximum Term 390 months
Interest rate contracts | Virginia Electric and Power Company  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ (39)
Interest rate, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ (2)
Maximum Term 390 months