Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	$ 17,682	$ 16,674	$ 17,373	$ 16,234
Charge-offs	(724)	(830)	(1,486)	(1,480)
Recoveries	60	70	188	141
Provision (benefit) for loan losses	486	1,501	1,429	2,520
Allowances for loan losses, Ending Balance	17,504	17,415	17,504	17,415
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	7,223	8,278	7,114	8,212
Charge-offs	(65)	(319)	(139)	(319)
Recoveries	12	1	27	1
Provision (benefit) for loan losses	(572)	136	(404)	202
Allowances for loan losses, Ending Balance	6,598	8,096	6,598	8,096
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	5,296	3,704	5,310	3,536
Charge-offs	0	0	0	(50)
Recoveries	0	10	50	10
Provision (benefit) for loan losses	632	867	568	1,085
Allowances for loan losses, Ending Balance	5,928	4,581	5,928	4,581
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,786	2,455	2,479	2,450
Charge-offs	(90)	(76)	(156)	(198)
Recoveries	0	0	1	19
Provision (benefit) for loan losses	(84)	88	288	196
Allowances for loan losses, Ending Balance	2,612	2,467	2,612	2,467
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,129	1,972	2,205	1,763
Charge-offs	(477)	(369)	(1,000)	(768)
Recoveries	26	31	55	53
Provision (benefit) for loan losses	440	362	858	948
Allowances for loan losses, Ending Balance	2,118	1,996	2,118	1,996
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	74	68	71	66
Charge-offs	(44)	(2)	(86)	(23)
Recoveries	2	1	5	3
Provision (benefit) for loan losses	66	21	108	42
Allowances for loan losses, Ending Balance	98	88	98	88
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	174	197	194	207
Charge-offs	(48)	(64)	(105)	(122)
Recoveries	20	27	50	55
Provision (benefit) for loan losses	4	27	11	47
Allowances for loan losses, Ending Balance	$ 150	$ 187	$ 150	$ 187