Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	$ 276,322	$ 185,584
Investment securities	8,325,584	8,682,509
U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	1,139,428	1,336,931
Obligations of States and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	1,181	1,659
Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	7,053,907	7,216,991
Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	131,052	126,906
Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	276,322	185,584
Investment securities	8,325,584	8,682,509
Equity securities	76,586	93,917
Real estate loans held for sale	344,350	551,697
Other assets	13,158	19,626
Total assets	9,036,000	9,533,333
Trading account liabilities	98,771	178,125
Other liabilities	24,073	12,561
Total liabilities	122,844	190,686
Fair Value Measurements, Recurring [Member] | 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	46,521	46,018
Equity securities	43,301	71,989
Total assets	89,822	118,007
Fair Value Measurements, Recurring [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	229,801	139,566
Investment securities	8,325,568	8,682,487
Equity securities	33,285	21,928
Real estate loans held for sale	344,350	551,697
Other assets	5,099	10,322
Total assets	8,938,103	9,406,000
Trading account liabilities	98,771	178,125
Other liabilities	23,447	10,969
Total liabilities	122,218	189,094
Fair Value Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	16	22
Other assets	8,059	9,304
Total assets	8,075	9,326
Other liabilities	626	1,592
Total liabilities	626	1,592
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	1,139,428	1,336,931
Fair Value Measurements, Recurring [Member] | U.S. Treasury and Federal Agencies [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	1,139,428	1,336,931
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	1,181	1,659
Fair Value Measurements, Recurring [Member] | Obligations of States and Political Subdivisions [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	1,181	1,659
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	7,053,907	7,216,991
Fair Value Measurements, Recurring [Member] | Government Issued or Guaranteed [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	7,053,907	7,216,991
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	16	22
Fair Value Measurements, Recurring [Member] | Privately Issued Mortgage-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	16	22
Fair Value Measurements, Recurring [Member] | Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	131,052	126,906
Fair Value Measurements, Recurring [Member] | Other Debt Securities [Member] | Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	$ 131,052	$ 126,906