Activity in Allowance for Loan Losses by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	$ 23,262	$ 22,989	$ 23,011	$ 22,293	$ 22,293
Loans charged off	(1,416)	(1,400)	(3,366)	(3,417)	(5,025)
Recoveries	146	235	530	865	1,002
Provision (reduction)	859	1,187	2,676	3,270	4,741
Ending balance	22,851	23,011	22,851	23,011	23,011
Ending Balance: Individually evaluated for impairment	3,094	5,372	3,094	5,372	3,671
Ending Balance: Collectively evaluated for impairment	19,757	17,639	19,757	17,639	19,340
Ending Balance: Individually evaluated for impairment	25,435	27,746	25,435	27,746	27,386
Ending Balance: Collectively evaluated for impairment	1,515,165	1,484,566	1,515,165	1,484,566	1,486,642
Loans ending balance	1,540,600	1,512,312	1,540,600	1,512,312	1,514,028
Residential real estate loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,352	6,109	6,398	3,273	3,273
Loans charged off	(578)	(239)	(1,024)	(1,036)	(1,216)
Recoveries	5	1	73	114	120
Provision (reduction)	860	75	1,192	3,595	4,221
Ending balance	6,639	5,946	6,639	5,946	6,398
Ending Balance: Individually evaluated for impairment	2,071	2,669	2,071	2,669	1,364
Ending Balance: Collectively evaluated for impairment	4,568	3,277	4,568	3,277	5,034
Ending Balance: Individually evaluated for impairment	12,554	12,305	12,554	12,305	12,715
Ending Balance: Collectively evaluated for impairment	558,739	567,943	558,739	567,943	565,547
Loans ending balance	571,293	580,248	571,293	580,248	578,262
Commercial real estate loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	4,837	6,324	5,702	8,198	8,198
Loans charged off		(621)	(209)	(946)	(1,633)
Recoveries	53	124	219	307	374
Provision (reduction)	(215)	179	(1,037)	(1,553)	(1,237)
Ending balance	4,675	6,006	4,675	6,006	5,702
Ending Balance: Individually evaluated for impairment	343	1,411	343	1,411	961
Ending Balance: Collectively evaluated for impairment	4,332	4,595	4,332	4,595	4,741
Ending Balance: Individually evaluated for impairment	7,121	9,596	7,121	9,596	7,830
Ending Balance: Collectively evaluated for impairment	494,162	448,752	494,162	448,752	462,231
Loans ending balance	501,283	458,348	501,283	458,348	470,061
Commercial loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	6,368	4,473	4,846	5,633	5,633
Loans charged off	(730)	(325)	(1,146)	(1,080)	(1,256)
Recoveries	85	83	205	239	296
Provision (reduction)	73	633	1,891	72	173
Ending balance	5,796	4,864	5,796	4,864	4,846
Ending Balance: Individually evaluated for impairment	376	831	376	831	815
Ending Balance: Collectively evaluated for impairment	5,420	4,033	5,420	4,033	4,031
Ending Balance: Individually evaluated for impairment	3,829	4,343	3,829	4,343	4,019
Ending Balance: Collectively evaluated for impairment	174,454	186,717	174,454	186,717	181,026
Loans ending balance	178,283	191,060	178,283	191,060	185,045
Home equity loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	2,319	2,478	2,704	2,051	2,051
Loans charged off	(70)	(205)	(921)	(325)	(861)
Recoveries	1	25	21	195	196
Provision (reduction)	108	188	554	565	1,318
Ending balance	2,358	2,486	2,358	2,486	2,704
Ending Balance: Individually evaluated for impairment	265	370	265	370	440
Ending Balance: Collectively evaluated for impairment	2,093	2,116	2,093	2,116	2,264
Ending Balance: Individually evaluated for impairment	1,668	1,343	1,668	1,343	2,670
Ending Balance: Collectively evaluated for impairment	272,508	269,125	272,508	269,125	266,112
Loans ending balance	274,176	270,468	274,176	270,468	268,782
Consumer loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	164	453	420	202	202
Loans charged off	(38)	(10)	(66)	(30)	(59)
Recoveries	2	2	12	10	16
Provision (reduction)	34	(12)	(204)	251	261
Ending balance	162	433	162	433	420
Ending Balance: Individually evaluated for impairment	39	91	39	91	91
Ending Balance: Collectively evaluated for impairment	123	342	123	342	329
Ending Balance: Individually evaluated for impairment	263	159	263	159	152
Ending Balance: Collectively evaluated for impairment	15,302	12,029	15,302	12,029	11,726
Loans ending balance	15,565	12,188	15,565	12,188	11,878
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Beginning balance	3,222	3,152	2,941	2,936	2,936
Provision (reduction)	(1)	124	280	340	5
Ending balance	3,221	3,276	3,221	3,276	2,941
Ending Balance: Collectively evaluated for impairment	$ 3,221	$ 3,276	$ 3,221	$ 3,276	$ 2,941