Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 3,251	$ 4,647
Non Accrual	2,014	1,313	$ 1,684
Total Past Due	5,265	5,960
Current Loans	2,420,854	2,279,618
Total loans, net	2,426,119	2,285,578
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	8,992	13,628
Total loans, net	8,992	13,628
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	227	187
Non Accrual	400	115
Total Past Due	627	302
Current Loans	811,790	761,323
Total loans, net	812,417	761,625
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	120,455	97,290
Total loans, net	120,455	97,290
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	840	774
Non Accrual	492	190
Total Past Due	1,332	964
Current Loans	784,770	749,398
Total loans, net	786,102	750,362
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,563	2,554
Non Accrual	683	569
Total Past Due	2,246	3,123
Current Loans	369,651	345,127
Total loans, net	371,897	348,250
Consumer and overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	18	24
Non Accrual	4	14
Total Past Due	22	38
Current Loans	21,871	22,045
Total loans, net	21,893	22,083
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	603	1,108
Non Accrual	435	425
Total Past Due	1,038	1,533
Current Loans	303,325	290,807
Total loans, net	$ 304,363	$ 292,340