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Investment securities (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Investment securities available for sale:    
Investment securities, available for sale, amortized cost $ 5,510,097 $ 6,312,423
Gross Unrealized Gains 251,094 210,001
Gross Unrealized Losses 227,137 293,864
Estimated fair value 5,534,054 6,228,560
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 1,188,465 1,077,708
Gross Unrealized Gains for Held-to-maturity 40,621 34,022
Gross Unrealized Losses for Held-to-maturity 97,250 99,168
Estimated fair value for Held-to-maturity 1,131,836 1,012,562
Amortized cost and estimated fair value of investment securities    
Other securities, Amortized Cost 334,781 366,886
Other securities, Estimated fair value 334,781 366,886
Total Amortized Cost 7,033,343 7,757,017
Total for Gross Unrealized Gains 291,715 244,023
Total for Gross Unrealized losses 324,387 393,032
Total for Estimated fair value 7,000,671 7,608,008
Obligations of states and political subdivisions [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 187,936 188,680
Gross Unrealized Gains for Held-to-maturity 8,375 9,141
Gross Unrealized Losses for Held-to-maturity 30 28
Estimated fair value for Held-to-maturity 196,281 197,793
Government issued or guaranteed [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 733,912 608,533
Gross Unrealized Gains for Held-to-maturity 31,929 24,881
Estimated fair value for Held-to-maturity 765,841 633,414
Privately issued [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 255,291 268,642
Gross Unrealized Gains for Held-to-maturity 317  
Gross Unrealized Losses for Held-to-maturity 97,220 99,140
Estimated fair value for Held-to-maturity 158,388 169,502
Other Debt Securities [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 11,326 11,853
Estimated fair value for Held-to-maturity 11,326 11,853
U.S. Treasury and federal agencies [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 55,484 69,468
Gross Unrealized Gains 1,115 1,255
Estimated fair value 56,599 70,723
Obligations of states and political subdivisions [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 34,007 39,518
Gross Unrealized Gains 569 771
Gross Unrealized Losses 8 20
Estimated fair value 34,568 40,269
Government issued or guaranteed [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 3,840,610 4,344,116
Gross Unrealized Gains 218,117 177,392
Gross Unrealized Losses 227 275
Estimated fair value 4,058,500 4,521,233
Privately issued [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 1,252,709 1,369,371
Gross Unrealized Gains 5,254 6,373
Gross Unrealized Losses 189,571 239,488
Estimated fair value 1,068,392 1,136,256
Privately issued commercial [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 13,048 17,679
Gross Unrealized Losses 921 2,650
Estimated fair value 12,127 15,029
Collateralized Debt Obligations [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 43,749 43,834
Gross Unrealized Gains 13,079 11,154
Gross Unrealized Losses 1,730 2,488
Estimated fair value 55,098 52,500
Other Debt Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 158,153 216,700
Gross Unrealized Gains 2,034 4,588
Gross Unrealized Losses 31,258 44,443
Estimated fair value 128,929 176,845
Equity Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 112,337 211,737
Gross Unrealized Gains 10,926 8,468
Gross Unrealized Losses 3,422 4,500
Estimated fair value $ 119,841 $ 215,705