Roll forward of Corporation's Allowance for Loan and Leases Losses by Loan Category (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	$ 14,444	$ 13,140	$ 14,425	$ 12,753
Charge-offs	(501)	(618)	(2,495)	(2,417)
Recoveries	125	116	334	299
Provision for loan and lease losses	959	1,000	2,763	3,003
Balance, end of period	15,027	13,638	15,027	13,638
Commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	4,481	3,384	3,907	3,165
Charge-offs	(19)		(19)	(235)
Recoveries		4		4
Provision for loan and lease losses	20	235	594	689
Balance, end of period	4,482	3,623	4,482	3,623
Home equity lines and loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	2,109	1,749	1,857	1,707
Charge-offs	(105)	(315)	(457)	(328)
Recoveries	29		29
Provision for loan and lease losses	153	244	757	299
Balance, end of period	2,186	1,678	2,186	1,678
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	1,773	1,636	2,024	1,592
Charge-offs	(203)	(18)	(203)	(188)
Recoveries	5		13
Provision for loan and lease losses	523	3	264	217
Balance, end of period	2,098	1,621	2,098	1,621
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	653	1,112	1,019	1,384
Charge-offs		(197)	(720)	(896)
Recoveries	6		24
Provision for loan and lease losses	9	109	345	536
Balance, end of period	668	1,024	668	1,024
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	4,295	3,789	4,637	3,816
Charge-offs	(19)		(737)	(409)
Recoveries	20	25	64	91
Provision for loan and lease losses	134	766	466	1,082
Balance, end of period	4,430	4,580	4,430	4,580
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	218	180	189	119
Charge-offs	(31)	(19)	(101)	(61)
Recoveries	3	1	7	5
Provision for loan and lease losses	68	38	163	137
Balance, end of period	258	200	258	200
Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	551	535	493	532
Charge-offs	(124)	(69)	(258)	(300)
Recoveries	62	86	197	199
Provision for loan and lease losses	82	(85)	139	36
Balance, end of period	571	467	571	467
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of period	364	755	299	438
Provision for loan and lease losses	(30)	(310)	35	7
Balance, end of period	$ 334	$ 445	$ 334	$ 445