Loans and Allowance for Credit Losses - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) Contract numberOfLoans	Dec. 31, 2019 USD ($) Contract	Dec. 31, 2018 USD ($) Contract
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	90 days
Loans Receivable Held-for-sale, Amount	$ 33,436,000	$ 15,989,000
Provision for credit losses	53,472,000
Charge-offs	19,250,000	$ 12,300,000
Recorded Investment	$ 54,053,000
Number of loans modified | Contract	41	38	58
Total amount of loan modified	$ 13,969,000	$ 1,928,000	$ 8,829,000
Total gains or losses included in earnings	0	198,000
Loans with modification to rate and payment due to amortization	$ 800,000	800,000	300,000
Minimum days required for conversion of trouble debt to be in default	90 days
Unearned Deferred (Fees) Costs on Loans	$ 6,000,000.0	2,800,000
Unearned Premiums	(7,000,000.0)	(8,500,000)
Loans	6,761,183,000	6,189,148,000	5,774,139,000
Loans and Leases Receivable, Gross	$ 6,761,183,000
Forecasted Value, National Unemployment, Percent	6.68%
Weighted Average [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent	7.25%
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent	7.48%
Total Deferrals Granted
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Number of Contracts | numberOfLoans	6,800
Loans and Leases Receivable, Gross	$ 1,400,000,000
Total Deferrals Still in Forbearance
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	113,800,000
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded commitments related to nonperforming loans	$ 200,000	1,700,000
Loans To Individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	150 days
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable Held-for-sale, Amount	$ 13,000	0
Gain (loss) on sale of impaired loans	0	400,000	$ 1,800,000
SEC Schedule, 12-09, Reserve, Off-balance Sheet Activity [Member] | Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Off balance sheet reserve to nonperforming loans	26,000	12,000
Commercial Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,555,986,000	$ 1,241,853,000
Loans and Leases Receivable, Gross	1,555,986,000
Commercial Loan [Member] | Payroll Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 478,900,000