Fair value measurements (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 544,938	$ 561,834
Investment securities	5,534,054	6,228,560
Real estate loans held for sale	442,664	371,437
Other assets	185,496	156,853
Total assets	6,707,152	7,318,684
Trading account liabilities	430,101	434,559
Other liabilities	14,355	6,126
Total liabilities	444,456	440,685
U.S. Treasury and federal agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	56,599	70,723
Obligations of states and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	34,568	40,269
Government issued or guaranteed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	4,058,500	4,521,233
Privately issued residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,068,392	1,136,256
Privately issued commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	12,127	15,029
Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	55,098	52,500
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	128,929	176,845
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	119,841	215,705
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	53,472	53,165
Investment securities	108,172	205,587
Total assets	161,644	258,752
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	108,172	205,587
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	491,466	508,669
Investment securities	4,290,265	4,819,188
Real estate loans held for sale	442,664	371,437
Other assets	151,385	148,862
Total assets	5,375,780	5,848,156
Trading account liabilities	430,101	434,559
Other liabilities	14,295	5,058
Total liabilities	444,396	439,617
Significant Observable Inputs (Level 2) [Member] | U.S. Treasury and federal agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	56,599	70,723
Significant Observable Inputs (Level 2) [Member] | Obligations of states and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	34,568	40,269
Significant Observable Inputs (Level 2) [Member] | Government issued or guaranteed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	4,058,500	4,521,233
Significant Observable Inputs (Level 2) [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	128,929	176,845
Significant Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	11,669	10,118
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,135,617	1,203,785
Other assets	34,111	7,991
Total assets	1,169,728	1,211,776
Other liabilities	60	1,068
Total liabilities	60	1,068
Significant Unobservable Inputs (Level 3) [Member] | Privately issued residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,068,392	1,136,256
Significant Unobservable Inputs (Level 3) [Member] | Privately issued commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	12,127	15,029
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	$ 55,098	$ 52,500