Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	$ 15,988	$ 17,504	$ 16,737	$ 17,373
Charge-offs	(993)	(790)	(3,401)	(2,276)
Recoveries	86	59	329	247
Provision (benefit) for loan losses	622	463	2,038	1,892
Allowances for loan losses, Ending Balance	15,703	17,236	15,703	17,236
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	5,218	6,598	6,035	7,114
Charge-offs	(86)	(80)	(519)	(219)
Recoveries	37	12	84	39
Provision (benefit) for loan losses	(24)	89	(455)	(315)
Allowances for loan losses, Ending Balance	5,145	6,619	5,145	6,619
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	6,207	5,928	5,605	5,310
Charge-offs	0	0	(20)	0
Recoveries	2	1	7	51
Provision (benefit) for loan losses	27	(384)	644	184
Allowances for loan losses, Ending Balance	6,236	5,545	6,236	5,545
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,299	2,612	2,475	2,479
Charge-offs	(95)	(191)	(244)	(347)
Recoveries	10	4	72	5
Provision (benefit) for loan losses	(65)	115	(154)	403
Allowances for loan losses, Ending Balance	2,149	2,540	2,149	2,540
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,066	2,118	2,371	2,205
Charge-offs	(709)	(448)	(2,397)	(1,448)
Recoveries	20	21	94	76
Provision (benefit) for loan losses	545	585	1,854	1,443
Allowances for loan losses, Ending Balance	1,922	2,276	1,922	2,276
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	50	98	90	71
Charge-offs	(17)	(17)	(54)	(103)
Recoveries	3	3	18	8
Provision (benefit) for loan losses	36	14	18	122
Allowances for loan losses, Ending Balance	72	98	72	98
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	148	150	161	194
Charge-offs	(86)	(54)	(167)	(159)
Recoveries	14	18	54	68
Provision (benefit) for loan losses	103	44	131	55
Allowances for loan losses, Ending Balance	$ 179	$ 158	$ 179	$ 158