Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 3,829	$ 1,468
Non Accrual	1,318	3,996
Accruing Greater Than 90 Days	0	90
Total Past Due	5,147	5,554
Current Loans	2,920,330	2,990,275
Total loans, net	2,925,477	2,995,829	$ 2,990,133
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	6,358	10,909
Total loans, net	6,358	10,909	9,116
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	0	56
Non Accrual	19	297
Accruing Greater Than 90 Days	0	90
Total Past Due	19	443
Current Loans	1,321,888	1,313,802
Total loans, net	1,321,907	1,314,245	1,315,407
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	138,945	137,607
Total loans, net	138,945	137,607	130,047
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,798
Non Accrual	240	2,881
Total Past Due	2,038	2,881
Current Loans	727,346	786,955
Total loans, net	729,384	789,836	784,895
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	773	390
Non Accrual	743	527
Total Past Due	1,516	917
Current Loans	464,593	447,519
Total loans, net	466,109	448,436	443,703
Consumer and overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1	21
Non Accrual	1	1
Total Past Due	2	22
Current Loans	19,547	20,417
Total loans, net	19,549	20,439	19,866
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,257	1,001
Non Accrual	315	290
Total Past Due	1,572	1,291
Current Loans	241,653	273,066
Total loans, net	$ 243,225	$ 274,357	$ 287,099