Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
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
Unfunded commitments related to nonperforming loans	$ 100,000		$ 100,000		$ 1,700,000
Off balance sheet reserve to nonperforming loans	23,000		23,000		12,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 0	0	$ 0
Total gains or losses included in earnings	0	245,000	0	198,000
Loans and Leases Receivable, Net of Deferred Income	6,949,716,000	6,099,561,000	6,949,716,000	6,099,561,000	6,189,148,000
Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs	4,705,000	$ 2,702,000	13,259,000	6,555,000
Recorded investment	47,673,000		47,673,000		29,698,000
Loans and Leases Receivable, Gross	6,575,840,000		6,575,840,000		5,714,373,000
Loans and Leases Receivable, Net of Deferred Income	6,575,840,000		6,575,840,000		5,714,373,000
Commercial, financial, agricultural and other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income					1,241,853,000
Commercial, financial, agricultural and other [Member] | Originated Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Net of Deferred Income	1,711,614,000		1,711,614,000		1,212,026,000
Loans And Leases Receivable, Paycheck Protection Program	573,500,000		573,500,000
Total Deferrals Still in Forbearance [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	65,400,000		65,400,000
Total Second Deferrals Granted [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 244,100,000		$ 244,100,000
Loans and Leases Receivable, Number of Contracts	136		136
Total Deferrals Granted [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 1,400,000,000		$ 1,400,000,000
Loans and Leases Receivable, Number of Contracts	6,500		6,500
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
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans with modifications to rate and payment due to reamortization			$ 766,000	149,000
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount	$ 0		0		$ 0
Loans and Leases Receivable, Gain (Loss) on Sales, Net			$ 0	$ 400,000