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Allowance for Credit Losses (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Allowance for Credit Losses        
Troubled debt restructuring charge-offs at date of modification $ 5 $ 7 $ 9 $ 12