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Derivatives and Hedge Accounting Activities (Selected Information Related to Gains (Losses) on Cash Flow Hedges Included in AOCI) (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2020
USD ($)
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ (446)
Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax (44)
Virginia Electric and Power Company  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax (72)
Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax (1)
Commodity | Natural Gas  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax (2)
Commodities, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ (2)
Maximum Term 15 months
Commodity | Electricity  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ 2
Commodities, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ 2
Maximum Term 3 months
Interest rate  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ (446)
Interest rate, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ (44)
Maximum Term 399 months
Interest rate | Virginia Electric and Power Company  
Derivative Instruments Gain Loss [Line Items]  
AOCI After-Tax $ (72)
Interest rate, Amounts Expected to be Reclassified to Earnings During the Next 12 Months After-Tax $ (1)
Maximum Term 399 months