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Investment securities (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Investment securities available for sale:    
Investment securities, available for sale, amortized cost $ 5,892,799 $ 6,312,423
Gross Unrealized Gains 207,573 210,001
Gross Unrealized Losses 262,350 293,864
Estimated fair value 5,838,022 6,228,560
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 1,000,294 1,077,708
Gross Unrealized Gains for Held-to-maturity 32,865 34,022
Gross Unrealized Losses for Held-to-maturity 103,681 99,168
Estimated fair value for Held-to-maturity 929,478 1,012,562
Amortized cost and estimated fair value of investment securities    
Other securities, Amortized Cost 356,980 366,886
Other securities, Estimated fair value 356,980 366,886
Total Amortized Cost 7,250,073 7,757,017
Total for Gross Unrealized Gains 240,438 244,023
Total for Gross Unrealized losses 366,031 393,032
Total for Estimated fair value 7,124,480 7,608,008
Obligations of states and political subdivisions [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 185,859 188,680
Gross Unrealized Gains for Held-to-maturity 7,776 9,141
Gross Unrealized Losses for Held-to-maturity 32 28
Estimated fair value for Held-to-maturity 193,603 197,793
Government issued or guaranteed [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 541,140 608,533
Gross Unrealized Gains for Held-to-maturity 25,089 24,881
Estimated fair value for Held-to-maturity 566,229 633,414
Privately issued [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 261,851 268,642
Gross Unrealized Losses for Held-to-maturity 103,649 99,140
Estimated fair value for Held-to-maturity 158,202 169,502
Other Debt Securities [Member]
   
Investment securities held to maturity:    
Amortized Cost for Held-to-maturity 11,444 11,853
Estimated fair value for Held-to-maturity 11,444 11,853
U.S. Treasury and federal agencies [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 52,010 69,468
Gross Unrealized Gains 1,130 1,255
Estimated fair value 53,140 70,723
Obligations of states and political subdivisions [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 37,431 39,518
Gross Unrealized Gains 630 771
Gross Unrealized Losses 21 20
Estimated fair value 38,040 40,269
Government issued or guaranteed [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 4,101,717 4,344,116
Gross Unrealized Gains 171,479 177,392
Gross Unrealized Losses 241 275
Estimated fair value 4,272,955 4,521,233
Privately issued residential [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 1,318,040 1,369,371
Gross Unrealized Gains 5,358 6,373
Gross Unrealized Losses 212,703 239,488
Estimated fair value 1,110,695 1,136,256
Privately issued commercial [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 15,193 17,679
Gross Unrealized Losses 1,758 2,650
Estimated fair value 13,435 15,029
Collateralized Debt Obligations [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 43,708 43,834
Gross Unrealized Gains 15,907 11,154
Gross Unrealized Losses 1,431 2,488
Estimated fair value 58,184 52,500
Other Debt Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 206,661 216,700
Gross Unrealized Gains 3,397 4,588
Gross Unrealized Losses 41,133 44,443
Estimated fair value 168,925 176,845
Equity Securities [Member]
   
Investment securities available for sale:    
Investment securities, available for sale, amortized cost 118,039 211,737
Gross Unrealized Gains 9,672 8,468
Gross Unrealized Losses 5,063 4,500
Estimated fair value $ 122,648 $ 215,705