Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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	$ 2,906,000		$ 11,510,000
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	38,086,000		38,086,000		54,053,000
Loans with modifications to rate and payment due to reamortization	86,000	694,000	284,000	766,000
Loans and Leases Receivable, Gross	6,732,580,000		6,732,580,000		6,761,183,000
Loans and Leases Receivable, Net of Deferred Income	6,732,580,000	6,949,716,000	6,732,580,000	6,949,716,000	6,761,183,000
Unearned Deferred (Fees) Costs on Loans	2,600,000		2,600,000		6,000,000
Unearned Premiums	6,300,000		$ 6,300,000		7,000,000
Forecasted Value, National Unemployment, Percent			5.13%
Financing Receivable, Allowance for Credit Loss, Writeoff, after Recovery			$ 9,500,000	12,400,000
Loan Pay Off	6,900,000		6,900,000
Specific Reserve Related to Loan Pay Off	300,000		$ 300,000
Weighted average [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent			5.00%
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Value, National Unemployment, Percent			5.13%
Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	1,210,694,000		$ 1,210,694,000		1,541,382,000
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs		4,705,000		13,259,000
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	2,066,000		6,522,000
Loans and Leases Receivable, Gross	1,224,591,000		1,224,591,000		1,555,986,000
Loans and Leases Receivable, Net of Deferred Income	1,224,591,000	1,736,736,000	1,224,591,000	1,736,736,000	1,555,986,000
Commercial, financial, agricultural and other [Member] | Time and Demand Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	2,059,000		6,397,000
Loans and Leases Receivable, Gross	1,210,694,000		1,210,694,000		1,541,382,000
Loans and Leases Receivable, Net of Deferred Income	1,210,694,000		1,210,694,000		1,541,382,000
Loans And Leases Receivable, Paycheck Protection Program	152,100,000		152,100,000		478,900,000
Commercial, financial, agricultural and other [Member] | Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs				5,166,000
Total Deferrals Still in Forbearance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	29,900,000	$ 65,400,000	$ 29,900,000	65,400,000	113,800,000
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		$ 5,000,000