Fair value measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 526,844	$ 561,834
Investment securities	5,183,878	6,228,560
Real estate loans held for sale	976,083	371,437
Other assets	226,554	156,853
Total assets	6,913,359	7,318,684
Trading account liabilities	424,312	434,559
Other liabilities	39,798	6,126
1Total liabilities	464,110	440,685
U.S. Treasury and federal agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	56,345	70,723
Obligations of states and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	33,980	40,269
Government issued or guaranteed [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	3,748,252	4,521,233
Privately issued residential [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,055,068	1,136,256
Privately issued commercial [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	10,568	15,029
Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	57,201	52,500
Other debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	109,405	176,845
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	113,059	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	55,391	53,165
Investment securities	106,789	205,587
Total assets	162,180	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	106,789	205,587
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	471,453	508,669
Investment securities	3,954,252	4,819,188
Real estate loans held for sale	976,083	371,437
Other assets	156,124	148,862
Total assets	5,557,912	5,848,156
Trading account liabilities	424,312	434,559
Other liabilities	39,713	5,058
1Total liabilities	464,025	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	56,345	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	33,980	40,269
Significant Observable Inputs (Level 2) [Member] | Government issued or guaranteed [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	3,748,252	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	109,405	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	6,270	10,118
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,122,837	1,203,785
Other assets	70,430	7,991
Total assets	1,193,267	1,211,776
Other liabilities	85	1,068
1Total liabilities	85	1,068
Significant Unobservable Inputs (Level 3) [Member] | Privately issued residential [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,055,068	1,136,256
Significant Unobservable Inputs (Level 3) [Member] | Privately issued commercial [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	10,568	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	$ 57,201	$ 52,500