Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 1,614	$ 2,692
Non Accrual	1,084	2,336	$ 4,146
Total Past Due	2,698	5,028
Current Loans	1,921,235	1,726,508
Total loans, net	1,923,933	1,731,536
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual	94	99
Total Past Due	94	99
Current Loans	14,834	27,322
Total loans, net	14,928	27,421
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	37
Non Accrual	65	60
Total Past Due	102	60
Current Loans	612,401	452,175
Total loans, net	612,503	452,235
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	135,418	85,685
Total loans, net	135,418	85,685
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	597	1,462
Non Accrual	150	174
Total Past Due	747	1,636
Current Loans	695,426	719,870
Total loans, net	696,173	721,506
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	245	596
Non Accrual	656	1,559
Total Past Due	901	2,155
Current Loans	240,456	253,191
Total loans, net	241,357	255,346
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due		6
Non Accrual	11
Total Past Due	11	6
Current Loans	11,686	11,317
Total loans, net	11,697	11,323
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	735	628
Non Accrual	108	444
Total Past Due	843	1,072
Current Loans	211,014	176,948
Total loans, net	$ 211,857	$ 178,020