Fair value measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 488,966	$ 561,834
Investment securities	4,739,437	6,228,560
Real estate loans held for sale	1,387,491	371,437
Other assets	194,331	156,853
Total assets	6,810,225	7,318,684
Trading account liabilities	374,274	434,559
Other liabilities	10,592	6,126
Total liabilities	384,866	440,685
U.S. Treasury and federal agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	39,344	70,723
Obligations of states and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	20,901	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	3,371,041	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	1,012,886	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	11,000	15,029
Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	61,869	52,500
Other debt securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	111,950	176,845
Equity securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	110,446	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	56,106	53,165
Investment securities	98,364	205,587
Total assets	154,470	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	98,364	205,587
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	432,860	508,669
Investment securities	3,555,318	4,819,188
Real estate loans held for sale	1,387,491	371,437
Other assets	146,275	148,862
Total assets	5,521,944	5,848,156
Trading account liabilities	374,274	434,559
Other liabilities	10,395	5,058
Total liabilities	384,669	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	39,344	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	20,901	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	3,371,041	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	111,950	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	12,082	10,118
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	1,085,755	1,203,785
Other assets	48,056	7,991
Total assets	1,133,811	1,211,776
Other liabilities	197	1,068
Total liabilities	197	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	1,012,886	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	11,000	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	$ 61,869	$ 52,500