Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Loans by risk rating
Financing Receivable, Net	$ 2,426,119	$ 2,285,578
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	8,992	13,628
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	812,417	761,625
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	120,455	97,290
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	786,102	750,362
1-3 (Pass) [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	8,992	13,628
1-3 (Pass) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	807,486	757,089
1-3 (Pass) [Member] | Municipal [Member]
Loans by risk rating
Financing Receivable, Net	120,455	97,290
1-3 (Pass) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	759,402	723,170
4 (Monitor) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	4,025	4,135
4 (Monitor) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	24,354	24,542
Impaired [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	906	401
Impaired [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 2,346	$ 2,650