Loans - Allowance for Loan Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	$ 19,693	$ 16,330
Charge-offs	(726)	(931)
Recoveries	158	131
Provision (benefit) for loan losses	1,631	1,016
Allowance for loan losses, Ending Balance	20,756	16,546
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	6,160	5,428
Charge-offs	(31)	(1)
Recoveries	68	12
Provision (benefit) for loan losses	85	(654)
Allowance for loan losses, Ending Balance	6,282	4,785
Commercial Mortgages [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	9,007	6,753
Charge-offs	0	0
Recoveries	0	2
Provision (benefit) for loan losses	1,013	602
Allowance for loan losses, Ending Balance	10,020	7,357
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	2,033	1,653
Charge-offs	0	(68)
Recoveries	3	71
Provision (benefit) for loan losses	16	366
Allowance for loan losses, Ending Balance	2,052	2,022
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	2,179	2,215
Charge-offs	(590)	(735)
Recoveries	49	2
Provision (benefit) for loan losses	427	607
Allowance for loan losses, Ending Balance	2,065	2,089
Credit Cards [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	120	93
Charge-offs	(19)	(58)
Recoveries	7	11
Provision (benefit) for loan losses	15	59
Allowance for loan losses, Ending Balance	123	105
Overdrafts [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses, Beginning Balance	194	188
Charge-offs	(86)	(69)
Recoveries	31	33
Provision (benefit) for loan losses	75	36
Allowance for loan losses, Ending Balance	$ 214	$ 188