Carrying Value for Loans and Leases by Portfolio Segment Based on Methodology Used to Evaluate Loans and Leases for Impairment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	$ 1,500,015		$ 1,398,456
Commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	622,771		546,358
Home equity lines and loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	187,634		194,861
Residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	291,645		288,212
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	39,055		26,908
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	303,259		291,620
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	17,572		17,666
Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Total	38,079		32,831
Carrying value of loans and leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	19,408		21,790
Collectively evaluated for impairment	1,469,985		1,364,538
Purchased credit-impaired	10,622	[1]	12,128	[1]
Total	1,500,015		1,398,456
Carrying value of loans and leases | Commercial mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	421		541
Collectively evaluated for impairment	612,393		535,506
Purchased credit-impaired	9,957	[1]	10,311	[1]
Total	622,771		546,358
Carrying value of loans and leases | Home equity lines and loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	2,066		3,403
Collectively evaluated for impairment	185,552		191,439
Purchased credit-impaired	16	[1]	19	[1]
Total	187,634		194,861
Carrying value of loans and leases | Residential mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	9,539		9,211
Collectively evaluated for impairment	282,065		278,951
Purchased credit-impaired	41	[1]	50	[1]
Total	291,645		288,212
Carrying value of loans and leases | Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	3,072		4,631
Collectively evaluated for impairment	35,655		20,785
Purchased credit-impaired	328	[1]	1,492	[1]
Total	39,055		26,908
Carrying value of loans and leases | Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	4,228		3,997
Collectively evaluated for impairment	298,751		287,367
Purchased credit-impaired	280	[1]	256	[1]
Total	303,259		291,620
Carrying value of loans and leases | Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Individually evaluated for impairment	82		7
Collectively evaluated for impairment	17,490		17,659
Total	17,572		17,666
Carrying value of loans and leases | Leases
Financing Receivable, Allowance for Credit Losses [Line Items]
Collectively evaluated for impairment	38,079		32,831
Total	$ 38,079		$ 32,831

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