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Credit Risk (Narrative) (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
counterparty
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
counterparty
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Concentration Risk and Guarantor Obligations [Line Items]          
Gross credit exposure     $ 96    
Operating Revenue $ 3,132 $ 2,971 8,651 $ 9,127  
Additional collateral to be posted if the credit related contingent features were triggered 7   7   $ 12
Aggregate fair value of all derivative instruments with credit contingent provisions that are in a liability position 18   18   $ 49
Virginia Electric and Power Company          
Concentration Risk and Guarantor Obligations [Line Items]          
Operating Revenue [1] $ 2,211 $ 2,058 $ 5,877 $ 6,008  
Credit Concentration Risk          
Concentration Risk and Guarantor Obligations [Line Items]          
Number of counterparties | counterparty 0   0    
Amount of exposure for single counterparty     $ 21    
Credit Concentration Risk | Wholesale Customers | Sales to wholesale customers | Virginia Electric and Power Company          
Concentration Risk and Guarantor Obligations [Line Items]          
Number of counterparties | counterparty 0   0    
Operating Revenue     $ 23    
Amount of exposure related to sales to wholesale customers for single counterparty     $ 4    
Credit Concentration Risk | Investment Grade          
Concentration Risk and Guarantor Obligations [Line Items]          
Concentration risk, percentage (percentage)     70.00%    
Credit Concentration Risk | Investment Grade | Virginia Electric and Power Company          
Concentration Risk and Guarantor Obligations [Line Items]          
Concentration risk, percentage (percentage)     35.00%    
[1] See Note 17 for amounts attributable to affiliates.