Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 371,370	$ 488,966
Investment securities	4,692,180	4,739,437
Real estate loans held for sale	819,029	1,387,491
Other assets	141,625	194,331
Total assets	6,024,204	6,810,225
Trading account liabilities	261,449	374,274
Other liabilities	24,210	10,592
Total liabilities	285,659	384,866
U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	37,887	39,344
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	17,963	20,901
Government Issued or Guaranteed [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	4,330,399	3,371,041
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	3,816	1,023,886
Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	59,303	61,869
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	119,404	111,950
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	123,408	110,446
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	48,344	56,106
Investment securities	91,075	98,364
Real estate loans held for sale
Other assets
Total assets	139,419	154,470
Trading account liabilities
Other liabilities
Total liabilities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Quoted Prices in Active Markets for Identical Assets (Level 1) [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
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
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	91,075	98,364
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	323,026	432,860
Investment securities	4,537,986	3,555,318
Real estate loans held for sale	819,029	1,387,491
Other assets	123,440	146,275
Total assets	5,803,481	5,521,944
Trading account liabilities	261,449	374,274
Other liabilities	19,100	10,395
Total liabilities	280,549	384,669
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	37,887	39,344
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	17,963	20,901
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	4,330,399	3,371,041
Significant Observable Inputs (Level 2) [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Significant Observable Inputs (Level 2) [Member] | Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
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	119,404	111,950
Significant Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	32,333	12,082
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets
Investment securities	63,119	1,085,755
Real estate loans held for sale
Other assets	18,185	48,056
Total assets	81,304	1,133,811
Trading account liabilities
Other liabilities	5,110	197
Total liabilities	5,110	197
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Significant Unobservable Inputs (Level 3) [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Significant Unobservable Inputs (Level 3) [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
Significant Unobservable Inputs (Level 3) [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	3,816	1,023,886
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	59,303	61,869
Significant Unobservable Inputs (Level 3) [Member] | Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities
Significant Unobservable Inputs (Level 3) [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities