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Credit Risk (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2012
Dec. 31, 2011
Gross credit exposure $ 529  
Our credit exposure after the application of collateral 524  
Additional collateral to be posted if the credit related contingent features were triggered 31 88
Collateral already posted, aggregate fair value 5 110
Letters of credit included in the collateral posted 4 4
Fair value of all derivative instruments not fully collateralized $ 144 $ 259
Investment Grade | Internal Credit Rating
   
Concentration risk, percentage (percentage) 81.00%  
Investment Grade | Counterparty, A
   
Concentration risk, percentage (percentage) 11.00%