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Credit Risk (Narrative) (Detail)
3 Months Ended
Mar. 31, 2023
USD ($)
Counterparty
Dec. 31, 2022
USD ($)
Concentration Risk and Guarantor Obligations [Line Items]    
Credit exposure $ 217,000,000  
Additional collateral to be posted if the credit related contingent features were triggered 106,000,000 $ 140,000,000
Collateral derivatives with credit-related contingent provision in a liability position 1,000,000 72,000,000
Letter of credit as collateral posted for derivatives in liability position 6,000,000 20,000,000
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position 107,000,000 212,000,000
Virginia Electric and Power Company    
Concentration Risk and Guarantor Obligations [Line Items]    
Credit exposure 34,000,000  
Additional collateral to be posted if the credit related contingent features were triggered 45,000,000 28,000,000
Collateral derivatives with credit-related contingent provision in a liability position 0 72,000,000
Letter of credit as collateral posted for derivatives in liability position 6,000,000 20,000,000
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position $ 45,000,000 $ 99,000,000
Credit Concentration Risk    
Concentration Risk and Guarantor Obligations [Line Items]    
Number of counterparties | Counterparty 0  
Amount of exposure for single counterparty $ 48,000,000  
Credit Concentration Risk | Virginia Electric and Power Company    
Concentration Risk and Guarantor Obligations [Line Items]    
Number of counterparties | Counterparty 0  
Amount of exposure for single counterparty $ 13,000,000  
Credit Concentration Risk | Investment Grade Counterparty | Virginia Electric and Power Company    
Concentration Risk and Guarantor Obligations [Line Items]    
Concentration risk, percentage (percentage) 54.00%  
Credit Concentration Risk | Investment Grade | Investment Grade Counterparty    
Concentration Risk and Guarantor Obligations [Line Items]    
Concentration risk, percentage (percentage) 89.00%