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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, 2021
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Balance at January 1, $ 103 $ (37) $ 64
Total realized and unrealized gains (losses):      
Included in regulatory assets/liabilities 119 140 (90)
Settlements (10) 33 17
Purchases 0 0 (10)
Sales 0 (1) 6
Transfers out of Level 3 9 0 (3)
Balance at December 31, 222 103 (37)
Virginia Electric and Power Company      
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]      
Balance at January 1, 103 (37) 60
Total realized and unrealized gains (losses):      
Included in regulatory assets/liabilities (1) 140 (88)
Settlements (4) 33 13
Balance at December 31, 102 103 (37)
Operating Revenue      
Total realized and unrealized gains (losses):      
Included in earnings (9) 0 (1)
Electric Fuel and Other Energy-Related Purchases      
Total realized and unrealized gains (losses):      
Included in earnings 10 (33) (22)
Electric Fuel and Other Energy-Related Purchases | Virginia Electric and Power Company      
Total realized and unrealized gains (losses):      
Included in earnings 4 (33) (22)
Purchased Gas      
Total realized and unrealized gains (losses):      
Included in earnings 0 0 2
Discontinued Operations      
Total realized and unrealized gains (losses):      
Included in earnings $ 0 $ 1 $ 0