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Significant Accounting Policies (Narrative) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Mar. 31, 2021
Dec. 31, 2020
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2021
Jun. 30, 2020
Significant Accounting Policies [Line Items]              
Impairment of assets and other charges (benefit) $ 321     $ 44   $ 416 $ 812
Other Operations and Maintenance              
Significant Accounting Policies [Line Items]              
Asset retirement obligation, liabilities incurred 44            
Asset retirement obligation, liabilities incurred after tax 35            
Virginia Electric and Power Company              
Significant Accounting Policies [Line Items]              
Impairment of assets and other charges (benefit) $ 12     44   $ (39) $ 808
Virginia Electric and Power Company | Impairment of Assets and Other Charges              
Significant Accounting Policies [Line Items]              
Impairment of assets and other charges (benefit)   $ 76 $ 127   $ 754    
Asset impairment charges after tax   $ 56     $ 561    
Virginia Electric and Power Company | Electric Generation Facilities | Impairment of Assets and Other Charges              
Significant Accounting Policies [Line Items]              
Charges recorded with dismantling       30      
Charges recorded with dismantling after tax       $ 22      
Cove Point | Brookfield              
Significant Accounting Policies [Line Items]              
Percentage of equity interest sold to noncontrolling interest owners 25.00%         25.00%  
Four Brothers and Three Cedars              
Significant Accounting Policies [Line Items]              
Business Acquisition, Percentage of Voting Interests Acquired 50.00%         50.00%  
Four Brothers and Three Cedars | Dominion Energy Midstream Partners, LP | Terra Nova Renewable Partners | Nonregulated Solar Projects              
Significant Accounting Policies [Line Items]              
Percentage of equity interest sold to noncontrolling interest owners 33.00%         33.00%