Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	$ 5,302,338	$ 6,318,776
U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	9,370	9,767
Obligations of States and Political Subdivisions [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities		775
Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	5,165,701	6,180,940
Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	127,251	127,278
Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	1,215,573	470,129
Investment securities	5,302,338	6,318,776
Equity securities	101,182	140,041
Real estate loans held for sale	909,335	442,079
Other assets	56,816	16,466
Total assets	7,585,244	7,387,491
Trading account liabilities	122,985	79,903
Other liabilities	14,946	7,188
Total liabilities	137,931	87,091
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	50,023	49,040
Equity securities	72,856	100,637
Total assets	122,879	149,677
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	1,165,550	421,089
Investment securities	5,302,322	6,318,760
Equity securities	28,326	39,404
Real estate loans held for sale	909,335	442,079
Other assets	5,030	4,501
Total assets	7,410,563	7,225,833
Trading account liabilities	122,985	79,903
Other liabilities	14,663	5,963
Total liabilities	137,648	85,866
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	16
Other assets	51,786	11,965
Total assets	51,802	11,981
Other liabilities	283	1,225
Total liabilities	283	1,225
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	9,370	9,767
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	9,370	9,767
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		775
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		775
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	5,165,701	6,180,940
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	5,165,701	6,180,940
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	16
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	16
Fair Value Measurements, Recurring [Member] | Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	127,251	127,278
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	$ 127,251	$ 127,278