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Investment Securities (Contractual maturities) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2012
Amortized Cost [Member]
 
Investment [Line Items]  
2011 $ 2,717
2012-2015 7,832
2016-2020 4,373
2021 and later 17,766
Estimated Fair Value [Member]
 
Investment [Line Items]  
2011 2,748
2012-2015 7,925
2016-2020 4,730
2021 and later $ 20,691