Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 4,423	$ 4,647
Non Accrual	1,502	1,313
Total Past Due	5,925	5,960
Current Loans	2,304,735	2,279,618
Total loans, net	2,310,660	2,285,578	$ 2,187,147
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	13,305	13,628
Total loans, net	13,305	13,628	17,583
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	138	187
Non Accrual	31	115
Total Past Due	169	302
Current Loans	767,267	761,323
Total loans, net	767,436	761,625	758,621
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	105,288	97,290
Total loans, net	105,288	97,290	104,044
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	422	774
Non Accrual	182	190
Total Past Due	604	964
Current Loans	746,099	749,398
Total loans, net	746,703	750,362	726,440
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	2,202	2,554
Non Accrual	119	569
Total Past Due	2,321	3,123
Current Loans	347,645	345,127
Total loans, net	349,966	348,250	300,941
Consumer and overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	2	24
Non Accrual	6	14
Total Past Due	8	38
Current Loans	22,115	22,045
Total loans, net	22,123	22,083	19,339
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,659	1,108
Non Accrual	1,164	425
Total Past Due	2,823	1,533
Current Loans	303,016	290,807
Total loans, net	$ 305,839	$ 292,340	$ 260,179