Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 2,646	$ 2,692
Non Accrual	1,486	2,336
Total Past Due	4,132	5,028
Current Loans	1,797,772	1,726,508
Total loans, net	1,801,904	1,731,536	$ 1,544,694
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual	96	99
Total Past Due	96	99
Current Loans	31,082	27,322
Total loans, net	31,178	27,421	26,521
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	470
Non Accrual	52	60
Total Past Due	522	60
Current Loans	492,511	452,175
Total loans, net	493,033	452,235	291,142
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	134,386	85,685
Total loans, net	134,386	85,685	87,241
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,080	1,462
Non Accrual	162	174
Total Past Due	1,242	1,636
Current Loans	727,416	719,870
Total loans, net	728,658	721,506	713,770
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	736	596
Non Accrual	844	1,559
Total Past Due	1,580	2,155
Current Loans	217,914	253,191
Total loans, net	219,494	255,346	248,103
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1	6
Non Accrual	3
Total Past Due	4	6
Current Loans	11,335	11,317
Total loans, net	11,339	11,323	10,646
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	359	628
Non Accrual	329	444
Total Past Due	688	1,072
Current Loans	183,128	176,948
Total loans, net	$ 183,816	$ 178,020	$ 167,271