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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
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Balance at January 1, $ (37) $ 64 $ 150
Total realized and unrealized gains (losses):      
Included in other comprehensive income 0 0 1
Included in regulatory assets/liabilities 140 (90) (44)
Settlements 33 17 (27)
Purchases 0 (10) 0
Sales (1) 6 0
Transfers out of Level 3 0 (3) 1
Balance at December 31, 103 (37) 64
Virginia Electric and Power Company      
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Balance at January 1, (37) 60 147
Total realized and unrealized gains (losses):      
Included in regulatory assets/liabilities 140 (88) (45)
Settlements 33 13 (25)
Balance at December 31, 103 (37) 60
Operating Revenue      
Total realized and unrealized gains (losses):      
Included in earnings 0 (1) (2)
Electric Fuel and Other Energy-Related Purchases      
Total realized and unrealized gains (losses):      
Included in earnings (33) (22) (15)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company      
Total realized and unrealized gains (losses):      
Included in earnings (33) (22) (17)
Purchased Gas      
Total realized and unrealized gains (losses):      
Included in earnings 0 2 0
Discontinued Operations      
Total realized and unrealized gains (losses):      
Included in earnings $ 1 $ 0 $ 0