Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	$ 16,738	$ 17,682	$ 16,737	$ 17,373
Charge-offs	(1,065)	(724)	(2,408)	(1,486)
Recoveries	95	60	243	188
Provision (benefit) for loan losses	220	486	1,416	1,429
Allowances for loan losses, Ending Balance	15,988	17,504	15,988	17,504
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	5,627	7,223	6,035	7,114
Charge-offs	(162)	(65)	(433)	(139)
Recoveries	39	12	47	27
Provision (benefit) for loan losses	(286)	(572)	(431)	(404)
Allowances for loan losses, Ending Balance	5,218	6,598	5,218	6,598
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	6,044	5,296	5,605	5,310
Charge-offs	(20)	0	(20)	0
Recoveries	0	0	5	50
Provision (benefit) for loan losses	183	632	617	568
Allowances for loan losses, Ending Balance	6,207	5,928	6,207	5,928
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,574	2,786	2,475	2,479
Charge-offs	(124)	(90)	(149)	(156)
Recoveries	3	0	62	1
Provision (benefit) for loan losses	(154)	(84)	(89)	288
Allowances for loan losses, Ending Balance	2,299	2,612	2,299	2,612
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,274	2,129	2,371	2,205
Charge-offs	(701)	(477)	(1,688)	(1,000)
Recoveries	30	26	74	55
Provision (benefit) for loan losses	463	440	1,309	858
Allowances for loan losses, Ending Balance	2,066	2,118	2,066	2,118
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	81	74	90	71
Charge-offs	(28)	(44)	(37)	(86)
Recoveries	3	2	15	5
Provision (benefit) for loan losses	(6)	66	(18)	108
Allowances for loan losses, Ending Balance	50	98	50	98
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	138	174	161	194
Charge-offs	(30)	(48)	(81)	(105)
Recoveries	20	20	40	50
Provision (benefit) for loan losses	20	4	28	11
Allowances for loan losses, Ending Balance	$ 148	$ 150	$ 148	$ 150