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Allowance for Credit Losses (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Allowance for Credit Losses        
Troubled debt restructuring charge-offs at date of modification $ 12 $ 6 $ 21 $ 29