Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	$ 488,588	$ 273,783
Investment securities	11,862,567	12,242,671
U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	726,401	299,997
Obligations of States and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	5,015	6,028
Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	10,930,662	11,686,628
Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities		47,393
Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	118,226	118,880
Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	82,212	83,671
Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	488,588	273,783
Investment securities	11,862,567	12,242,671
Real estate loans held for sale	704,984	392,036
Other assets	51,919	56,551
Total assets	13,108,058	12,965,041
Trading account liabilities	292,713	160,745
Other liabilities	12,434	1,905
Total liabilities	305,147	162,650
Fair Value Measurements, Recurring [Member] | U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	726,401	299,997
Fair Value Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	5,015	6,028
Fair Value Measurements, Recurring [Member] | Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	10,930,662	11,686,628
Fair Value Measurements, Recurring [Member] | Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	51	74
Fair Value Measurements, Recurring [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities		47,393
Fair Value Measurements, Recurring [Member] | Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	118,226	118,880
Fair Value Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	82,212	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	50,699	56,763
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	50,699	56,763
Investment securities	62,546	65,178
Total assets	113,245	121,941
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	62,546	65,178
Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	437,889	217,020
Trading account liabilities	292,713	160,745
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	437,889	217,020
Investment securities	11,799,970	12,130,026
Real estate loans held for sale	704,984	392,036
Other assets	22,068	46,269
Total assets	12,964,911	12,785,351
Trading account liabilities	292,713	160,745
Other liabilities	12,383	1,502
Total liabilities	305,096	162,247
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member] | U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	726,401	299,997
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	5,015	6,028
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member] | Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	10,930,662	11,686,628
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member] | Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	118,226	118,880
Significant Observable Inputs (Level 2) [Member] | Fair Value Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	19,666	18,493
Significant Unobservable Inputs (Level 3) [Member] | Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	51	47,467
Other assets	29,851	10,282
Total assets	29,902	57,749
Other liabilities	51	403
Total liabilities	51	403
Significant Unobservable Inputs (Level 3) [Member] | Fair Value Measurements, Recurring [Member] | Privately Issued Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	$ 51	74
Significant Unobservable Inputs (Level 3) [Member] | Fair Value Measurements, Recurring [Member] | Collateralized Debt Obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities		$ 47,393