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Credit Risk (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Counterparty
Dec. 31, 2012
Concentration Risk and Guarantor Obligations [Line Items]    
Gross credit exposure $ 545  
Credit exposure after application of collateral 544  
Number of counterparties 1  
Additional collateral to be posted if the credit related contingent features were triggered 50 110
Collateral posted 1 4
Fair value of all derivative instruments not fully collateralized 86 163
Investment Grade | Internal Credit Rating
   
Concentration Risk and Guarantor Obligations [Line Items]    
Concentration risk, percentage (percentage) 80.00%  
Investment Grade | Counterparty, A
   
Concentration Risk and Guarantor Obligations [Line Items]    
Concentration risk, percentage (percentage) 10.00%  
Virginia Electric and Power Company
   
Concentration Risk and Guarantor Obligations [Line Items]    
Gross credit exposure 14  
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure Per Counterparty $ 5  
Virginia Electric and Power Company | Investment Grade | Internal Credit Rating
   
Concentration Risk and Guarantor Obligations [Line Items]    
Concentration risk, percentage (percentage) 52.00%