Changes in Allowance for Loan Losses by Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2011	Mar. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	$ 7,803	$ 7,764	$ 6,999	$ 6,934
Provision	260	220	175	339
Charge-offs	(47)	(199)	(257)	(359)
Recoveries	49	18	156	85
Ending Balance	8,065	7,803	7,073	6,999
Residential real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	1,636	1,531	1,005	877
Provision	201	104	184	127
Charge-offs	(40)		(2)
Recoveries	3	1	3	1
Ending Balance	1,800	1,636	1,190	1,005
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	3,343	3,504	3,177	3,182
Provision	137	20	(93)	(9)
Charge-offs		(195)	(175)
Recoveries	37	14	132	4
Ending Balance	3,517	3,343	3,041	3,177
Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	2,808	2,712	2,797	2,849
Provision	(77)	95	84	234
Charge-offs			(77)	(355)
Recoveries	3	1	20	69
Ending Balance	2,734	2,808	2,824	2,797
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning Balance	16	17	20	26
Provision	(1)	1		(13)
Charge-offs	(7)	(4)	(3)	(4)
Recoveries	6	2	1	11
Ending Balance	$ 14	$ 16	$ 18	$ 20