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Investment securities (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Investment securities available for sale:    
Investment securities, available for sale, amortized cost $ 6,312,423 $ 5,494,377
Gross Unrealized Gains 210,001 230,852
Gross Unrealized Losses 293,864 311,737
Estimated fair value 6,228,560 5,413,492
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 1,077,708 1,324,339
Gross Unrealized Gains for Held-to-maturity 34,022 16,005
Gross Unrealized Losses for Held-to-maturity 99,168 115,091
Estimated fair value for Held-to-maturity 1,012,562 1,225,253
Amortized cost and estimated fair value of investment securities    
Other securities, Amortized Cost 366,886 412,709
Other securities, Estimated fair value 366,886 412,709
Total Amortized Cost 7,757,017 7,231,425
Total for Gross Unrealized Gains 244,023 246,857
Total for Gross Unrealized losses 393,032 426,828
Total for Estimated fair value 7,608,008 7,051,454
Obligations of states and political subdivisions [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 188,680 191,119
Gross Unrealized Gains for Held-to-maturity 9,141 1,944
Gross Unrealized Losses for Held-to-maturity 28 694
Estimated fair value for Held-to-maturity 197,793 192,369
Government issued or guaranteed [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 608,533 808,108
Gross Unrealized Gains for Held-to-maturity 24,881 14,061
Gross Unrealized Losses for Held-to-maturity 0 0
Estimated fair value for Held-to-maturity 633,414 822,169
Privately issued [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 268,642 312,537
Gross Unrealized Gains for Held-to-maturity 0 0
Gross Unrealized Losses for Held-to-maturity 99,140 114,397
Estimated fair value for Held-to-maturity 169,502 198,140
Other Debt Securities [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 11,853 12,575
Gross Unrealized Gains for Held-to-maturity 0 0
Gross Unrealized Losses for Held-to-maturity 0 0
Estimated fair value for Held-to-maturity 11,853 12,575
U.S. Treasury and federal agencies [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 69,468 61,772
Gross Unrealized Gains 1,255 1,680
Gross Unrealized Losses 0 18
Estimated fair value 70,723 63,434
Obligations of states and political subdivisions [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 39,518 59,921
Gross Unrealized Gains 771 561
Gross Unrealized Losses 20 57
Estimated fair value 40,269 60,425
Government issued or guaranteed [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 4,344,116 3,146,054
Gross Unrealized Gains 177,392 161,298
Gross Unrealized Losses 275 1,111
Estimated fair value 4,521,233 3,306,241
Privately issued residential [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 1,369,371 1,677,064
Gross Unrealized Gains 6,373 10,578
Gross Unrealized Losses 239,488 252,081
Estimated fair value 1,136,256 1,435,561
Privately issued commercial [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 17,679 25,357
Gross Unrealized Gains 0 0
Gross Unrealized Losses 2,650 2,950
Estimated fair value 15,029 22,407
Collateralized Debt Obligations [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 43,834 95,080
Gross Unrealized Gains 11,154 24,754
Gross Unrealized Losses 2,488 9,078
Estimated fair value 52,500 110,756
Other Debt Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 216,700 310,017
Gross Unrealized Gains 4,588 26,883
Gross Unrealized Losses 44,443 38,000
Estimated fair value 176,845 298,900
Equity Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 211,737 119,112
Gross Unrealized Gains 8,468 5,098
Gross Unrealized Losses 4,500 8,442
Estimated fair value $ 215,705 $ 115,768