Credit Risk Exposure Indicators by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 1,536,464	$ 1,514,028	$ 1,551,456
Residential real estate loans
Financing Receivable, Recorded Investment [Line Items]
Loans	569,823	578,262	590,110
Residential real estate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	553,473	560,926
Residential real estate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	1,153	876
Residential real estate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	15,197	16,460
Commercial real estate loans
Financing Receivable, Recorded Investment [Line Items]
Loans	496,411	470,061	465,248
Commercial real estate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	437,714	413,489
Commercial real estate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	12,585	8,134
Commercial real estate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	46,112	48,438
Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	182,428	185,045	214,415
Commercial loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	155,341	157,141
Commercial loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	9,121	8,998
Commercial loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	17,404	18,335
Commercial loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans	562	571
Home equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans	272,309	268,782	269,263
Home equity loans | Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	270,658	266,112
Home equity loans | Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	1,651	2,670
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans	15,493	11,878	12,420
Consumer loans | Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	15,233	11,726
Consumer loans | Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 260	$ 152