Loans - Loans Individually Evaluated for Impairment Including Loans Modified in Troubled Debt Restructurings by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Total	$ 20,565	$ 24,657
Recorded Investment, Total	18,883	23,203
Allowance for Loan Losses Allocated	3,941	3,415
Commercial, Industrial, and Agricultural [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	6,520	5,737
Recorded Investment, Related Allowance	6,520	5,737
Allowance for Loan Losses Allocated	1,331	411
Unpaid Principal Balance, No Related Allowance	2,305	2,530
Recorded Investment, No Related Allowance	1,414	1,577
Allowance for Loan Losses Allocated, No Related Allowance	0	0
Commercial Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	6,716	10,651
Recorded Investment, Related Allowance	5,926	10,212
Allowance for Loan Losses Allocated	2,424	2,807
Unpaid Principal Balance, No Related Allowance	4,624	5,020
Recorded Investment, No Related Allowance	4,623	5,020
Allowance for Loan Losses Allocated, No Related Allowance	0	0
Residential Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Unpaid Principal Balance, Related Allowance	400	400
Recorded Investment, Related Allowance	400	400
Allowance for Loan Losses Allocated	186	197
Unpaid Principal Balance, No Related Allowance	0	319
Recorded Investment, No Related Allowance	0	257
Allowance for Loan Losses Allocated, No Related Allowance	$ 0	$ 0