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Credit Risk (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
6 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Concentration risk, cedit rsk, fnancial istrument, mximum exposure $ 464  
Concentration risk credit risk financial instrument maximum exposure net 424  
Additional collateral, aggregate fair value 32 88
Collateral already posted, aggregate fair value 5 110
Pledged financial instruments, not separately reported, securities for letter of credit facilities 3 4
Derivative liability, fair value, amount not offset against collateral $ 146 $ 259
Investment Grade | Credit Concentration Risk [Member]
   
Concentration risk, percentage (percentage) 62.00%  
Investment Grade | Counterparty, A
   
Concentration risk, percentage (percentage) 10.00%