Credit Risk Exposure Indicators by Portfolio Segment (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 1,540,600	$ 1,514,028	$ 1,512,312
Residential real estate loans
Financing Receivable, Recorded Investment [Line Items]
Loans	571,293	578,262	580,248
Residential real estate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	555,036	560,926
Residential real estate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	1,143	876
Residential real estate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	15,114	16,460
Commercial real estate loans
Financing Receivable, Recorded Investment [Line Items]
Loans	501,283	470,061	458,348
Commercial real estate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	435,924	413,489
Commercial real estate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	15,524	8,134
Commercial real estate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	49,835	48,438
Commercial loans
Financing Receivable, Recorded Investment [Line Items]
Loans	178,283	185,045	191,060
Commercial loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans	153,528	157,141
Commercial loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Loans	7,204	8,998
Commercial loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans	17,551	18,335
Commercial loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans		571
Home equity loans
Financing Receivable, Recorded Investment [Line Items]
Loans	274,176	268,782	270,468
Home equity loans | Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	272,471	266,112
Home equity loans | Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	1,705	2,670
Consumer loans
Financing Receivable, Recorded Investment [Line Items]
Loans	15,565	11,878	12,188
Consumer loans | Performing Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	15,305	11,726
Consumer loans | Nonperforming Financing Receivable
Financing Receivable, Recorded Investment [Line Items]
Loans	$ 260	$ 152