Fair Value Measurements - Assets and Liabilities Measured at Estimated Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	$ 4,364,437	$ 4,822,606
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	49,115	50,060
Significant Observable Inputs (Level 2) [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	638,244	1,018,521
Trading account liabilities	95,239	116,902
U.S. Treasury and Federal Agencies [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	7,013	9,338
Government Issued or Guaranteed [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	4,225,089	4,683,438
Other Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Investment securities	132,319	129,814
Fair Value Measurements, Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Trading account assets	687,359	1,068,581
Investment securities	4,364,437	4,822,606
Equity securities	81,792	92,985
Real estate loans held for sale	871,026	1,054,676
Other assets	65,542	49,464
Total assets	6,070,156	7,088,312
Trading account liabilities	95,239	116,902
Other liabilities	18,281	23,583
Total liabilities	113,520	140,485
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	49,115	50,060
Equity securities	64,150	63,129
Total assets	113,265	113,189
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	638,244	1,018,521
Investment securities	4,364,421	4,822,590
Equity securities	17,642	29,856
Real estate loans held for sale	871,026	1,054,676
Other assets	42,621	5,865
Total assets	5,933,954	6,931,508
Trading account liabilities	95,239	116,902
Other liabilities	9,117	23,218
Total liabilities	104,356	140,120
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	22,921	43,599
Total assets	22,937	43,615
Other liabilities	9,164	365
Total liabilities	9,164	365
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	7,013	9,338
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	7,013	9,338
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	4,225,089	4,683,438
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	4,225,089	4,683,438
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	132,319	129,814
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	$ 132,319	$ 129,814