Activity in Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 23,010	$ 22,887	$ 23,011	$ 22,293	$ 22,293
Loans charged off	(861)	(1,170)	(1,950)	(2,017)	(5,025)
Recoveries	287	306	384	630	1,002
Provision (reduction)	826	966	1,817	2,083	4,741
Ending balance	23,262	22,989	23,262	22,989	23,011
Ending Balance: Individually evaluated for impairment	3,785	4,639	3,785	4,639	3,671
Ending Balance: Collectively evaluated for impairment	19,477	18,350	19,477	18,350	19,340
Ending Balance: Individually evaluated for impairment	27,485	24,962	27,485	24,962	27,386
Ending Balance: Collectively evaluated for impairment	1,508,979	1,526,494	1,508,979	1,526,494	1,486,642
Loans ending balance	1,536,464	1,551,456	1,536,464	1,551,456	1,514,028
Residential real estate loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,103	3,914	6,398	3,273	3,273
Loans charged off	(138)	(625)	(446)	(797)	(1,216)
Recoveries	63	63	68	113	120
Provision (reduction)	324	2,757	332	3,520	4,221
Ending balance	6,352	6,109	6,352	6,109	6,398
Ending Balance: Individually evaluated for impairment	1,903	2,828	1,903	2,828	1,364
Ending Balance: Collectively evaluated for impairment	4,449	3,281	4,449	3,281	5,034
Ending Balance: Individually evaluated for impairment	13,458	11,839	13,458	11,839	12,715
Ending Balance: Collectively evaluated for impairment	556,365	578,271	556,365	578,271	565,547
Loans ending balance	569,823	590,110	569,823	590,110	578,262
Commercial real estate loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,713	7,698	5,702	8,198	8,198
Loans charged off	(30)	(94)	(209)	(325)	(1,633)
Recoveries	145	174	166	183	374
Provision (reduction)	(991)	(1,454)	(822)	(1,732)	(1,237)
Ending balance	4,837	6,324	4,837	6,324	5,702
Ending Balance: Individually evaluated for impairment	707	933	707	933	961
Ending Balance: Collectively evaluated for impairment	4,130	5,391	4,130	5,391	4,741
Ending Balance: Individually evaluated for impairment	7,362	7,765	7,362	7,765	7,830
Ending Balance: Collectively evaluated for impairment	489,049	457,483	489,049	457,483	462,231
Loans ending balance	496,411	465,248	496,411	465,248	470,061
Commercial loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	5,193	5,437	4,846	5,633	5,633
Loans charged off	(225)	(333)	(416)	(755)	(1,256)
Recoveries	56	61	120	156	296
Provision (reduction)	1,344	(692)	1,818	(561)	173
Ending balance	6,368	4,473	6,368	4,473	4,846
Ending Balance: Individually evaluated for impairment	933	397	933	397	815
Ending Balance: Collectively evaluated for impairment	5,435	4,076	5,435	4,076	4,031
Ending Balance: Individually evaluated for impairment	4,751	3,878	4,751	3,878	4,019
Ending Balance: Collectively evaluated for impairment	177,677	210,537	177,677	210,537	181,026
Loans ending balance	182,428	214,415	182,428	214,415	185,045
Home equity loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,474	1,957	2,704	2,051	2,051
Loans charged off	(464)	(111)	(851)	(120)	(861)
Recoveries	20	6	20	170	196
Provision (reduction)	289	626	446	377	1,318
Ending balance	2,319	2,478	2,319	2,478	2,704
Ending Balance: Individually evaluated for impairment	203	373	203	373	440
Ending Balance: Collectively evaluated for impairment	2,116	2,105	2,116	2,105	2,264
Ending Balance: Individually evaluated for impairment	1,651	1,355	1,651	1,355	2,670
Ending Balance: Collectively evaluated for impairment	270,658	267,908	270,658	267,908	266,112
Loans ending balance	272,309	269,263	272,309	269,263	268,782
Consumer loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	523	238	420	202	202
Loans charged off	(4)	(7)	(28)	(20)	(59)
Recoveries	3	2	10	8	16
Provision (reduction)	(358)	220	(238)	263	261
Ending balance	164	453	164	453	420
Ending Balance: Individually evaluated for impairment	39	108	39	108	91
Ending Balance: Collectively evaluated for impairment	125	345	125	345	329
Ending Balance: Individually evaluated for impairment	263	125	263	125	152
Ending Balance: Collectively evaluated for impairment	15,230	12,295	15,230	12,295	11,726
Loans ending balance	15,493	12,420	15,493	12,420	11,878
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	3,004	3,643	2,941	2,936	2,936
Provision (reduction)	218	(491)	281	216	5
Ending balance	3,222	3,152	3,222	3,152	2,941
Ending Balance: Collectively evaluated for impairment	$ 3,222	$ 3,152	$ 3,222	$ 3,152	$ 2,941