Fair value measurements (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Assets measured at estimated fair value on a recurring basis [Line Items]
Trading account assets	$ 502,986	$ 523,834
Investment securities	4,890,489	5,413,492
Real estate loans held for sale	414,643	544,567
Other assets	122,230	114,666
Total assets	5,930,348	6,596,559
Liabilities measured at estimated fair value on a recurring basis [Line Items]
Trading account liabilities	366,483	333,222
Other liabilities	7,021	3,607
Total liabilities	373,504	336,829
Quoted Prices in Active Markets for Identical Assets (Level 1) Member
Assets measured at estimated fair value on a recurring basis [Line Items]
Trading account assets	57,523	53,032
Investment securities	84,676	106,872
Real estate loans held for sale	0	0
Other assets	0	0
Total assets	142,199	159,904
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | U.S. Treasury and federal agencies [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Obligations of states and political subdivisions [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Government issued or guaranteed [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Privately issued residential [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Privately issued commercial [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Collateralized Debt Obligations [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Other Debt Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) Member | Equity Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	84,676	106,872
Quoted Prices in Active Markets for Identical Assets (Level 1) Member
Liabilities measured at estimated fair value on a recurring basis [Line Items]
Trading account liabilities	0	0
Other liabilities	0	0
Total liabilities	0	0
Significant Observable Inputs (Level 2) [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Trading account assets	445,463	470,802
Investment securities	3,420,777	3,737,896
Real estate loans held for sale	414,643	544,567
Other assets	108,672	111,839
Total assets	4,389,555	4,865,104
Significant Observable Inputs (Level 2) [Member] | U.S. Treasury and federal agencies [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	112,953	63,434
Significant Observable Inputs (Level 2) [Member] | Obligations of states and political subdivisions [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	67,491	60,425
Significant Observable Inputs (Level 2) [Member] | Government issued or guaranteed [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	3,023,018	3,306,241
Significant Observable Inputs (Level 2) [Member] | Privately issued residential [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Observable Inputs (Level 2) [Member] | Privately issued commercial [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Observable Inputs (Level 2) [Member] | Collateralized Debt Obligations [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Observable Inputs (Level 2) [Member] | Other Debt Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	195,536	298,900
Significant Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	21,779	8,896
Significant Observable Inputs (Level 2) [Member]
Liabilities measured at estimated fair value on a recurring basis [Line Items]
Trading account liabilities	366,483	333,222
Other liabilities	6,634	3,024
Total liabilities	373,117	336,246
Significant Unobservable Inputs (Level 3) [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Trading account assets	0	0
Investment securities	1,385,036	1,568,724
Real estate loans held for sale	0	0
Other assets	13,558	2,827
Total assets	1,398,594	1,571,551
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury and federal agencies [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Obligations of states and political subdivisions [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Government issued or guaranteed [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Privately issued residential [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	1,306,202	1,435,561
Significant Unobservable Inputs (Level 3) [Member] | Privately issued commercial [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	17,233	22,407
Significant Unobservable Inputs (Level 3) [Member] | Collateralized Debt Obligations [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	61,601	110,756
Significant Unobservable Inputs (Level 3) [Member] | Other Debt Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	0	0
Significant Unobservable Inputs (Level 3) [Member]
Liabilities measured at estimated fair value on a recurring basis [Line Items]
Trading account liabilities	0	0
Other liabilities	387	583
Total liabilities	387	583
U.S. Treasury and federal agencies [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	112,953	63,434
Obligations of states and political subdivisions [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	67,491	60,425
Government issued or guaranteed [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	3,023,018	3,306,241
Privately issued residential [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	1,306,202	1,435,561
Privately issued commercial [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	17,233	22,407
Collateralized Debt Obligations [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	61,601	110,756
Other Debt Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	195,536	298,900
Equity Securities [Member]
Assets measured at estimated fair value on a recurring basis [Line Items]
Investment securities	$ 106,455	$ 115,768