Allowance for Loan Losses - Loans by Risk Rating (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Loans by risk rating
Financing Receivable, Net	$ 2,310,660	$ 2,285,578	$ 2,187,147
Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	13,305	13,628	17,583
Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	767,436	761,625	758,621
Municipal [Member]
Loans by risk rating
Financing Receivable, Net	105,288	97,290	104,044
Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	746,703	750,362	$ 726,440
1-3 (Pass) [Member] | Construction and Land Development [Member]
Loans by risk rating
Financing Receivable, Net	13,305	13,628
1-3 (Pass) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	762,991	757,089
1-3 (Pass) [Member] | Municipal [Member]
Loans by risk rating
Financing Receivable, Net	105,288	97,290
1-3 (Pass) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	719,367	723,170
4 (Monitor) [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	4,125	4,135
4 (Monitor) [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	24,390	24,542
Impaired [Member] | Commercial and Industrial [Member]
Loans by risk rating
Financing Receivable, Net	320	401
Impaired [Member] | Commercial Real Estate [Member]
Loans by risk rating
Financing Receivable, Net	$ 2,946	$ 2,650