Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	$ 17,504	$ 17,415	$ 17,373	$ 16,234
Charge-offs	(790)	(675)	(2,276)	(2,155)
Recoveries	59	65	247	206
Provision (benefit) for loan losses	463	1,038	1,892	3,558
Allowances for loan losses, Ending Balance	17,236	17,843	17,236	17,843
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	6,598	8,096	7,114	8,212
Charge-offs	(80)	(60)	(219)	(379)
Recoveries	12	0	39	1
Provision (benefit) for loan losses	89	690	(315)	892
Allowances for loan losses, Ending Balance	6,619	8,726	6,619	8,726
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	5,928	4,581	5,310	3,536
Charge-offs	0	(92)	0	(142)
Recoveries	1	0	51	10
Provision (benefit) for loan losses	(384)	(291)	184	794
Allowances for loan losses, Ending Balance	5,545	4,198	5,545	4,198
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,612	2,467	2,479	2,450
Charge-offs	(191)	(17)	(347)	(215)
Recoveries	4	18	5	37
Provision (benefit) for loan losses	115	93	403	289
Allowances for loan losses, Ending Balance	2,540	2,561	2,540	2,561
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	2,118	1,996	2,205	1,763
Charge-offs	(448)	(415)	(1,448)	(1,183)
Recoveries	21	25	76	78
Provision (benefit) for loan losses	585	467	1,443	1,415
Allowances for loan losses, Ending Balance	2,276	2,073	2,276	2,073
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	98	88	71	66
Charge-offs	(17)	(16)	(103)	(39)
Recoveries	3	4	8	7
Provision (benefit) for loan losses	14	3	122	45
Allowances for loan losses, Ending Balance	98	79	98	79
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowances for loan losses, Beginning Balance	150	187	194	207
Charge-offs	(54)	(75)	(159)	(197)
Recoveries	18	18	68	73
Provision (benefit) for loan losses	44	76	55	123
Allowances for loan losses, Ending Balance	$ 158	$ 206	$ 158	$ 206