Allowance for Loan Losses and Credit Quality Information - Schedule of Consumer Credit Exposure Credit Risk Profile Based on Payment Activity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,787,437		$ 2,772,440
Total	100.00%		100.00%
Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	521,160		532,628
Total	100.00%		100.00%
Residential and Consumer Loan [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	25,788		24,812
Total	5.00%		5.00%
Residential and Consumer Loan [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	495,372		507,816
Total	95.00%		95.00%
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	237,854		243,627
Residential [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	19,578	[1]	18,483
Residential [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	218,276		225,144
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	283,306		289,001
Consumer [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	6,210	[1]	6,329
Consumer [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 277,096		$ 282,672

[1] Includes $10.8 million of troubled debt restructured mortgages and home equity installment loans performing in accordance with modified terms and are accruing interest