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Credit Risk (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Counterparty
Dec. 31, 2012
Gross credit exposure $ 552  
Our credit exposure after the application of collateral 551  
Number of counterparties 1  
Additional collateral to be posted if the credit related contingent features were triggered 58 110
Collateral already posted, aggregate fair value 9 4
Fair value of all derivative instruments not fully collateralized $ 108 $ 163
Investment Grade | Internal Credit Rating
   
Concentration risk, percentage (percentage) 80.00%  
Investment Grade | Counterparty, A
   
Concentration risk, percentage (percentage) 10.00%