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Investment Securities (Details 2) (USD $)
In Millions, except Share data, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Estimated Fair Value, Less than 12 months $ 239 $ 15
Estimated Fair Value, 12 months or more 468 1,096
Estimated Fair Value, Total 707 1,111
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]    
Gross Unrealized Losses, Less than 12 months (6) (2)
Gross Unrealized Losses, 12 months or more (36) (73)
Gross Unrealized Losses, Total (42) (75)
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]    
Number of securities, less than 12 months 33 1
Number of securities, 12 months or more 51 136
Number of securities, total 84 137
Ratio Of Fair Value To Amortized Cost Between Ninety And One Hundred Percent [Member]
    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Estimated Fair Value, Less than 12 months 239 0
Estimated Fair Value, 12 months or more 344 884
Estimated Fair Value, Total 583 884
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]    
Gross Unrealized Losses, Less than 12 months (6) 0
Gross Unrealized Losses, 12 months or more (18) (35)
Gross Unrealized Losses, Total (24) (35)
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]    
Number of securities, less than 12 months 33 0
Number of securities, 12 months or more 41 114
Number of securities, total 74 114
Ratio Of Fair Value To Amortized Cost Less Than Ninety Percent [Member]
    
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]    
Estimated Fair Value, Less than 12 months 0 15
Estimated Fair Value, 12 months or more 124 212
Estimated Fair Value, Total 124 227
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses [Abstract]    
Gross Unrealized Losses, Less than 12 months 0 (2)
Gross Unrealized Losses, 12 months or more (18) (38)
Gross Unrealized Losses, Total $ (18) $ (40)
Available For Sale Securities Continuous Unrealized Loss Position Qualitative Disclosure [Abstract]    
Number of securities, less than 12 months 0 1
Number of securities, 12 months or more 10 22
Number of securities, total 10 23