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Fair Value Measurements (Narrative) (Detail) - USD ($)
1 Months Ended 3 Months Ended 12 Months Ended
Mar. 31, 2020
Sep. 30, 2019
May 31, 2019
Mar. 31, 2019
Jan. 31, 2019
Mar. 31, 2021
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Impairment of assets and other charges (benefits)             $ 195,000,000 $ 2,105,000,000 $ 1,520,000,000
Unrealized gains or losses included in earnings in Level 3 fair value category             0 0 0
Virginia Electric and Power Company                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Unrealized gains or losses included in earnings in Level 3 fair value category             0 0 $ 0
Corporate Office Building | Level 3                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Long-lived assets, estimated fair value             26,000,000    
Nonregulated Retail Software Development | Level 2 | Maximum                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Long-lived assets, estimated fair value             1,000,000    
Impairment of Assets and Other Charges                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Impairment of assets and other charges (benefits)   $ 26,000,000              
Asset impairment charges after tax   19,000,000              
Impairment of Assets and Other Charges | Virginia Electric and Power Company                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Impairment of assets and other charges (benefits) $ 751,000,000 17,000,000 $ 62,000,000 $ 346,000,000 $ 160,000,000 $ 76,000,000   127,000,000  
Asset impairment charges after tax $ 559,000,000 $ 12,000,000 $ 46,000,000 $ 257,000,000 $ 119,000,000 $ 56,000,000   $ 94,000,000  
Impairment of Assets and Other Charges | Corporate Office Building                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Impairment of assets and other charges (benefits)             16,000,000    
Asset impairment charges after tax             12,000,000    
Impairment of Assets and Other Charges | Nonregulated Retail Software Development                  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                  
Impairment of assets and other charges (benefits)             20,000,000    
Asset impairment charges after tax             $ 15,000,000