Allowance for Loan Losses - Aging of Past Due Loan Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	$ 4,119	$ 3,165
Non Accrual	1,760	4,146
Total Past Due	5,879	7,311
Current Loans	1,634,951	1,324,055
Total loans, net	1,640,830	1,331,366	$ 1,336,633
Construction and Land Development [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Non Accrual	100	103
Total Past Due	100	103
Current Loans	27,208	22,641
Total loans, net	27,308	22,744	25,339
Commercial and Industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	174	905
Non Accrual	75	157
Total Past Due	249	1,062
Current Loans	377,905	148,670
Total loans, net	378,154	149,732	152,823
Municipal [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Current Loans	87,016	41,850
Total loans, net	87,016	41,850	36,624
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	2,569	1,046
Non Accrual	180	2,781
Total Past Due	2,749	3,827
Current Loans	698,774	692,445
Total loans, net	701,523	696,272	695,074
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	1,089	632
Non Accrual	981	846
Total Past Due	2,070	1,478
Current Loans	262,035	255,827
Total loans, net	264,105	257,305	268,927
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	14	6
Non Accrual	7	5
Total Past Due	21	11
Current Loans	10,612	12,177
Total loans, net	10,633	12,188	10,253
Home Equity [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Accruing 30-89 Days Past Due	273	576
Non Accrual	417	254
Total Past Due	690	830
Current Loans	171,401	150,445
Total loans, net	$ 172,091	$ 151,275	$ 147,593