Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 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	$ 4,839,000		$ 8,604,000
Unfunded commitments related to nonperforming loans	200,000		200,000		$ 200,000
Off balance sheet reserve to nonperforming loans					26,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 0	0	$ 0
Total gains or losses included in earnings	0	0	0	0
Recorded investment	52,793,000		52,793,000		54,053,000
Loans with modifications to rate and payment due to reamortization	29,000	53,000	198,000	71,000
Loans and Leases Receivable, Gross	6,740,535,000		6,740,535,000		6,761,183,000
Loans and Leases Receivable, Net of Deferred Income	6,740,535,000	6,922,075,000	6,740,535,000	6,922,075,000	6,761,183,000
Unearned Deferred (Fees) Costs on Loans	7,100,000		7,100,000		6,000,000.0
Unearned Premiums	6,500,000		$ 6,500,000		7,000,000.0
Forecasted Value, National Unemployment, Percent			5.76%
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery			$ 7,200,000	8,000,000.0
Weighted average [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent			5.37%
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent			5.76%
Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,360,065,000		$ 1,360,065,000		1,541,382,000
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs		4,768,000		8,554,000
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	3,887,000		4,456,000
Loans and Leases Receivable, Gross	1,374,177,000		1,374,177,000		1,555,986,000
Loans and Leases Receivable, Net of Deferred Income	1,374,177,000	$ 1,773,099,000	1,374,177,000	1,773,099,000	1,555,986,000
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	3,878,000		4,338,000
Loans and Leases Receivable, Gross	1,360,065,000		1,360,065,000		1,541,382,000
Loans and Leases Receivable, Net of Deferred Income	1,360,065,000		1,360,065,000		1,541,382,000
Loans And Leases Receivable, Paycheck Protection Program	292,400,000		292,400,000		478,900,000
Commercial, financial, agricultural and other [Member] | Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs				1,771,000
Total Deferrals Still in Forbearance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	59,500,000		$ 59,500,000
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		$ 0		$ 13,000
Loans and Leases Receivable, Gain (Loss) on Sales, Net			400,000	$ 0
Nonaccrual Commercial Loans Transferred to HFS
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount	$ 5,000,000.0		$ 5,000,000.0