Allocation of Allowance for Loan and Lease Losses by Portfolio Segment Based on Methodology Used to Evaluate Loans and Leases for Impairment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Jun. 30, 2013	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 15,027		$ 14,444	$ 14,425		$ 13,638	$ 13,140	$ 12,753
Commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	4,482		4,481	3,907		3,623	3,384	3,165
Home equity lines and loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2,186		2,109	1,857		1,678	1,749	1,707
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	2,098		1,773	2,024		1,621	1,636	1,592
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	668		653	1,019		1,024	1,112	1,384
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	4,430		4,295	4,637		4,580	3,789	3,816
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	258		218	189		200	180	119
Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	571		551	493		467	535	532
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	334		364	299		445	755	438
Allowance for Loan and Lease Losses
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	1,347			2,354
Collectively evaluated for impairment	13,672			12,033
Purchased credit-impaired	8	[1]		38	[1]
Total	15,027			14,425
Allowance for Loan and Lease Losses | Commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	4,474			3,894
Purchased credit-impaired	8	[1]		13	[1]
Total	4,482			3,907
Allowance for Loan and Lease Losses | Home equity lines and loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	119			217
Collectively evaluated for impairment	2,067			1,640
Total	2,186			1,857
Allowance for Loan and Lease Losses | Residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	628			667
Collectively evaluated for impairment	1,470			1,357
Total	2,098			2,024
Allowance for Loan and Lease Losses | Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment				543
Collectively evaluated for impairment	668			451
Purchased credit-impaired				25	[1]
Total	668			1,019
Allowance for Loan and Lease Losses | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	544			919
Collectively evaluated for impairment	3,886			3,718
Total	4,430			4,637
Allowance for Loan and Lease Losses | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	56			8
Collectively evaluated for impairment	202			181
Total	258			189
Allowance for Loan and Lease Losses | Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	571			493
Total	571			493
Allowance for Loan and Lease Losses | Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	334			299
Total	$ 334			$ 299

[1] Purchased credit-impaired loans are evaluated for impairment on an individual basis.