Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 3,084	$ 2,692
Non Accrual	958	2,336
Total Past Due	4,042	5,028
Current Loans	1,837,051	1,726,508
Total loans, net	1,841,093	1,731,536	$ 1,640,830
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual	95	99
Total Past Due	95	99
Current Loans	19,427	27,322
Total loans, net	19,522	27,421	27,308
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	318
Non Accrual	49	60
Total Past Due	367	60
Current Loans	548,923	452,175
Total loans, net	549,290	452,235	378,154
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	145,063	85,685
Total loans, net	145,063	85,685	87,016
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,313	1,462
Non Accrual	156	174
Total Past Due	1,469	1,636
Current Loans	691,309	719,870
Total loans, net	692,778	721,506	701,523
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	866	596
Non Accrual	656	1,559
Total Past Due	1,522	2,155
Current Loans	221,359	253,191
Total loans, net	222,881	255,346	264,105
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	24	6
Non Accrual	2
Total Past Due	26	6
Current Loans	11,448	11,317
Total loans, net	11,474	11,323	10,633
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	563	628
Non Accrual		444
Total Past Due	563	1,072
Current Loans	199,522	176,948
Total loans, net	$ 200,085	$ 178,020	$ 172,091