Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Loans by risk rating
Financing Receivable, Net	$ 2,925,477	$ 2,995,829	$ 2,990,133
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	6,358	10,909	9,116
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	1,321,907	1,314,245	1,315,407
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	138,945	137,607	130,047
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	729,384	789,836	$ 784,895
1-3 (Pass) [Member] \| Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	6,358	10,909
1-3 (Pass) [Member] \| Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	1,317,908	1,309,861
1-3 (Pass) [Member] \| Municipal [Member]
Loans by risk rating
Financing Receivable, Net	138,945	137,607
1-3 (Pass) [Member] \| Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	704,205	761,101
4 (Monitor) [Member] \| Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	3,922	3,945
4 (Monitor) [Member] \| Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	22,940	23,795
Impaired [Member] \| Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	77	439
Impaired [Member] \| Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 2,239	$ 4,940

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