Allowance for Loan Losses - Aging Analysis of Past Due Loans (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	$ 1,158	$ 4,070
60-89 Days Past Due	882	1,305
90 Days or Greater	5,714	8,072
Total Past Due	7,754	13,447
Current	879,264	847,794
Ending Balance	887,018	861,241
Past Due 90 Days and Accruing	0	0
Commercial and Agriculture [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	4	105
60-89 Days Past Due	0	0
90 Days or Greater	208	443
Total Past Due	212	548
Current	106,401	115,327
Ending Balance	106,613	115,875
Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	301	655
60-89 Days Past Due	68	201
90 Days or Greater	1,258	2,098
Total Past Due	1,627	2,954
Current	443,541	440,892
Ending Balance	445,168	443,846
Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	810	3,140
60-89 Days Past Due	768	1,084
90 Days or Greater	4,164	5,531
Total Past Due	5,742	9,755
Current	260,124	240,936
Ending Balance	265,866	250,691
Real Estate Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
90 Days or Greater	43	0
Total Past Due	43	0
Current	55,201	39,964
Ending Balance	55,244	39,964
Consumer and Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 Days Past Due	43	170
60-89 Days Past Due	46	20
90 Days or Greater	41
Total Past Due	130	190
Current	13,997	10,675
Ending Balance	$ 14,127	$ 10,865