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Fair Value Measurements (Net Change in the Assets and Liabilities Measured at Fair Value on a Recurring Basis and Included in the Level 3 Fair Value Category) (Detail) - Commodity - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning balance $ (37) $ 64
Total realized and unrealized gains (losses):    
Included in regulatory assets/liabilities 80 7
Settlements 22 (1)
Purchases   (10)
Transfers out of Level 3   (2)
Ending balance 43 53
Virginia Electric and Power Company    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Beginning balance (37) 60
Total realized and unrealized gains (losses):    
Included in regulatory assets/liabilities 80 8
Settlements 22 (5)
Ending balance 43 59
Operating Revenue    
Total realized and unrealized gains (losses):    
Included in earnings   (1)
Electric Fuel and Other Energy-Related Purchases    
Total realized and unrealized gains (losses):    
Included in earnings (22) (4)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company    
Total realized and unrealized gains (losses):    
Included in earnings $ (22) $ (4)