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Investment Securities (Details 1) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Available-for-sale investment securities with gross unrealized losses and length of time    
Estimated Fair Value, Less than 12 months $ 93 $ 100
Estimated Fair Value, 12 months or more 75 73
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]    
Gross Unrealized Losses, Less than 12 months (2) (1)
Gross Unrealized Losses, 12 months or more (3) (4)
U.S. States and Political Subdivisions Debt Securities [Member]
   
Available-for-sale investment securities with gross unrealized losses and length of time    
Estimated Fair Value, Less than 12 months 93 100
Estimated Fair Value, 12 months or more 75 73
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]    
Gross Unrealized Losses, Less than 12 months (2) (1)
Gross Unrealized Losses, 12 months or more $ (3) $ (4)