Allowance for Loan Losses - Aging of Past Due Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 3,206	$ 1,614
Non Accrual	1,122	1,084
Accruing Greater than 90 Days	0	0
Total Past Due	4,328	2,698
Current Loans	2,029,471	1,921,235
Total loans, net	2,033,799	1,923,933	$ 1,801,904
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual		94
Accruing Greater than 90 Days	0	0
Total Past Due		94
Current Loans	14,159	14,834
Total loans, net	14,159	14,928	31,178
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	404	37
Non Accrual	49	65
Accruing Greater than 90 Days	0	0
Total Past Due	453	102
Current Loans	650,089	612,401
Total loans, net	650,542	612,503	493,033
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing Greater than 90 Days	0	0
Current Loans	127,259	135,418
Total loans, net	127,259	135,418	134,386
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	455	597
Non Accrual	230	150
Accruing Greater than 90 Days	0	0
Total Past Due	685	747
Current Loans	735,918	695,426
Total loans, net	736,603	696,173	728,658
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	984	245
Non Accrual	735	656
Accruing Greater than 90 Days	0	0
Total Past Due	1,719	901
Current Loans	263,025	240,456
Total loans, net	264,744	241,357	219,494
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	6
Non Accrual	1	11
Accruing Greater than 90 Days	0	0
Total Past Due	7	11
Current Loans	11,211	11,686
Total loans, net	11,218	11,697	11,339
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,357	735
Non Accrual	107	108
Accruing Greater than 90 Days	0	0
Total Past Due	1,464	843
Current Loans	227,810	211,014
Total loans, net	$ 229,274	$ 211,857	$ 183,816