Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 3,483	$ 4,647
Non Accrual	2,899	1,313
Total Past Due	6,382	5,960
Current Loans	2,306,189	2,279,618
Total loans, net	2,312,571	2,285,578	$ 2,221,613
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	13,751	13,628
Total loans, net	13,751	13,628	7,729
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	173	187
Non Accrual	258	115
Total Past Due	431	302
Current Loans	764,061	761,323
Total loans, net	764,492	761,625	761,467
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	95,682	97,290
Total loans, net	95,682	97,290	103,027
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	158	774
Non Accrual	1,140	190
Total Past Due	1,298	964
Current Loans	756,944	749,398
Total loans, net	758,242	750,362	735,083
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,906	2,554
Non Accrual	518	569
Total Past Due	2,424	3,123
Current Loans	344,161	345,127
Total loans, net	346,585	348,250	316,248
Consumer and overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	7	24
Non Accrual	2	14
Total Past Due	9	38
Current Loans	22,600	22,045
Total loans, net	22,609	22,083	21,662
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,239	1,108
Non Accrual	981	425
Total Past Due	2,220	1,533
Current Loans	308,990	290,807
Total loans, net	$ 311,210	$ 292,340	$ 276,397