Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	90 days
Default Status Of TDRs After Number Of Days Past Due	90 days
Charge-offs	$ 3,765,000
Unfunded commitments related to nonperforming loans	200,000		$ 200,000
Off balance sheet reserve to nonperforming loans	6,000		26,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 0
Total gains or losses included in earnings	0	0
Recorded investment	50,381,000		54,053,000
Loans with modifications to rate and payment due to reamortization	169,000	18,000
Loans and Leases Receivable, Gross	6,736,894,000		6,761,183,000
Loans and Leases Receivable, Net of Deferred Income	6,736,894,000	6,313,944,000	6,761,183,000
Unearned Deferred (Fees) Costs on Loans	8,300,000		6,000,000.0
Unearned Premiums	$ 6,800,000		7,000,000.0
Forecasted Value, National Unemployment, Percent	6.24%
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery	$ 3,300,000	3,500,000
Weighted average [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent	5.54%
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent	6.02%
Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 1,541,280,000		1,541,382,000
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs		3,786,000
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	569,000
Loans and Leases Receivable, Gross	1,555,671,000		1,555,986,000
Loans and Leases Receivable, Net of Deferred Income	1,555,671,000	1,272,240,000	1,555,986,000
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	460,000
Loans and Leases Receivable, Gross	1,541,280,000		1,541,382,000
Loans and Leases Receivable, Net of Deferred Income	1,541,280,000		1,541,382,000
Loans And Leases Receivable, Paycheck Protection Program	478,500,000		478,900,000
Commercial, financial, agricultural and other [Member] | Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs		486,000
Total Deferrals Still in Forbearance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 136,600,000
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 Held for sale, Amount	$ 0		13,000
Loans and Leases Receivable, Gain (Loss) on Sales, Net	$ 0	$ 400,000
Nonaccrual Commercial Loans Transferred to HFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount			$ 5,000,000.0