Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	$ 467,987	$ 273,783
Investment securities	12,200,647	12,242,671
Real estate loans held for sale	396,764	392,036
Other assets	59,028	56,551
Total assets	13,124,426	12,965,041
Trading account liabilities	295,869	160,745
Other liabilities	5,700	1,905
Total liabilities	301,569	162,650
U.S. Treasury and Federal Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	202,192	299,997
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	5,448	6,028
Government Issued or Guaranteed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	11,750,062	11,686,628
Privately Issued Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	65	74
Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	45,040	47,393
Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	112,708	118,880
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	85,132	83,671
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	69,689	56,763
Investment securities	67,150	65,178
Total assets	136,839	121,941
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	67,150	65,178
Significant Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account assets	398,298	217,020
Investment securities	12,088,392	12,130,026
Real estate loans held for sale	396,764	392,036
Other assets	42,099	46,269
Total assets	12,925,553	12,785,351
Trading account liabilities	295,869	160,745
Other liabilities	5,656	1,502
Total liabilities	301,525	162,247
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	202,192	299,997
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	5,448	6,028
Significant Observable Inputs (Level 2) [Member] | Government Issued or Guaranteed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	11,750,062	11,686,628
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	112,708	118,880
Significant Observable Inputs (Level 2) [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	17,982	18,493
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	45,105	47,467
Other assets	16,929	10,282
Total assets	62,034	57,749
Other liabilities	44	403
Total liabilities	44	403
Significant Unobservable Inputs (Level 3) [Member] | Privately Issued Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities	65	74
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	$ 45,040	$ 47,393