Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 378,235	$ 488,966
Investment securities	3,085,086	4,739,437
Real estate loans held for sale	2,014,505	1,387,491
Other assets	220,957	194,331
Total assets	5,698,783	6,810,225
Trading account liabilities	277,904	374,274
Other liabilities	23,242	10,592
Total liabilities	301,146	384,866
U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	38,108	39,344
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	19,231	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	2,694,893	3,371,041
Privately Issued [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	5,272	1,023,886
Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	59,916	61,869
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	117,951	111,950
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	149,715	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	48,001	56,106
Investment securities	121,254	98,364
Total assets	169,255	154,470
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	121,254	98,364
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	330,234	432,860
Investment securities	2,898,644	3,555,318
Real estate loans held for sale	2,014,505	1,387,491
Other assets	204,848	146,275
Total assets	5,448,231	5,521,944
Trading account liabilities	277,904	374,274
Other liabilities	14,541	10,395
Total liabilities	292,445	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	38,108	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	19,231	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	2,694,893	3,371,041
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	117,951	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	28,461	12,082
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	65,188	1,085,755
Other assets	16,109	48,056
Total assets	81,297	1,133,811
Other liabilities	8,701	197
Total liabilities	8,701	197
Significant Unobservable Inputs (Level 3) [Member] | Privately Issued [Member] | Mortgage-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	5,272	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	$ 59,916	$ 61,869