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Investments (Unrealized Losses) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2012
Mar. 31, 2011
Schedule of Investments [Line Items]    
Investment securities available for sale, less than 12 months, fair value $ 2,752,137 $ 2,172,701
Investment securities available for sale, 12 months or more, fair value 1,123 472
Investment securities available for sale, total fair value 2,753,260 2,173,173
Investment securities available for sale, less than 12 months, unrealized losses 5,502 18,135
Investment securities available for sale, 12 months or more, unrealized losses 35 29
Investment securities available for sale, total unrealized losses 5,537 18,164
U. S. Treasury [Member]
   
Schedule of Investments [Line Items]    
Investment securities available for sale, less than 12 months, fair value 579,673 200,561
Investment securities available for sale, 12 months or more, fair value 0 0
Investment securities available for sale, total fair value 579,673 200,561
Investment securities available for sale, less than 12 months, unrealized losses 305 234
Investment securities available for sale, 12 months or more, unrealized losses 0 0
Investment securities available for sale, total unrealized losses 305 234
Government Agency [Member]
   
Schedule of Investments [Line Items]    
Investment securities available for sale, less than 12 months, fair value 2,143,742 1,937,968
Investment securities available for sale, 12 months or more, fair value 0 0
Investment securities available for sale, total fair value 2,143,742 1,937,968
Investment securities available for sale, less than 12 months, unrealized losses 5,040 17,394
Investment securities available for sale, 12 months or more, unrealized losses 0 0
Investment securities available for sale, total unrealized losses 5,040 17,394
Residential Mortgage-Backed Securities [Member]
   
Schedule of Investments [Line Items]    
Investment securities available for sale, less than 12 months, fair value 28,595 33,644
Investment securities available for sale, 12 months or more, fair value 1,113 462
Investment securities available for sale, total fair value 29,708 34,106
Investment securities available for sale, less than 12 months, unrealized losses 156 503
Investment securities available for sale, 12 months or more, unrealized losses 35 29
Investment securities available for sale, total unrealized losses 191 532
Investment securities held to maturity, less than 12 months, fair value 0 0
Investment securities held to maturity, 12 months or more, fair value 20 19
Investment securities held to maturity, total fair value 20 19
Investment securities held to maturity, less than 12 months, unrealized losses 0 0
Investment securities held to maturity, 12 months or more, unrealized losses 27 21
Investment securities held to maturity, total unrealized losses 27 21
State, County and Municipal [Member]
   
Schedule of Investments [Line Items]    
Investment securities available for sale, less than 12 months, fair value 127 528
Investment securities available for sale, 12 months or more, fair value 10 10
Investment securities available for sale, total fair value 137 538
Investment securities available for sale, less than 12 months, unrealized losses 1 4
Investment securities available for sale, 12 months or more, unrealized losses 0 0
Investment securities available for sale, total unrealized losses $ 1 $ 4