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Credit Risk (Narrative) (Details)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
counterparty
Jun. 30, 2016
USD ($)
Jun. 30, 2017
USD ($)
counterparty
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Concentration Risk and Guarantor Obligations [Line Items]          
Gross credit exposure     $ 69    
Operating Revenue $ 2,813 $ 2,598 6,197 $ 5,519  
Additional collateral to be posted if the credit related contingent features were triggered         $ 3
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position         $ 9
Maximum          
Concentration Risk and Guarantor Obligations [Line Items]          
Additional collateral to be posted if the credit related contingent features were triggered 1   1    
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position $ 1   $ 1    
Credit Concentration Risk          
Concentration Risk and Guarantor Obligations [Line Items]          
Number of counterparties | counterparty 0   0    
Amount of exposure for single counterparty     $ 17    
Credit Concentration Risk | Investment Grade | Investment Grade Counterparty          
Concentration Risk and Guarantor Obligations [Line Items]          
Concentration risk, percentage (percentage)     74.00%    
Virginia Electric and Power Company          
Concentration Risk and Guarantor Obligations [Line Items]          
Electric distribution poles renting space, settlement charge $ 16        
Electric distribution poles renting space, settlement charge, net of tax 10        
Operating Revenue [1] $ 1,747 $ 1,776 $ 3,578 $ 3,666  
Virginia Electric and Power Company | Credit Concentration Risk | Wholesale Customers | Sales to wholesale customers          
Concentration Risk and Guarantor Obligations [Line Items]          
Number of counterparties | counterparty 0   0    
Operating Revenue     $ 17    
Amount of exposure related to sales to wholesale customers for single counterparty     $ 4    
Virginia Electric and Power Company | Credit Concentration Risk | Investment Grade | Investment Grade Counterparty          
Concentration Risk and Guarantor Obligations [Line Items]          
Concentration risk, percentage (percentage)     47.00%    
[1] See Note 17 for amounts attributable to affiliates.