Allowance for Loan Losses and Credit Quality Information - Schedule of Consumer Credit Exposure (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,234,959	$ 3,202,498	$ 3,004,672
Total	100.00%	100.00%
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1.00%
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	540,429	545,872
Total	100.00%	100.00%
Total Residential and Consumer Loan [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	20,906	22,042
Total	4.00%	4.00%
Total Residential and Consumer Loan [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	504	521
Total Residential and Consumer Loan [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	519,019	523,309
Total	96.00%	96.00%
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	216,197	218,329	218,153
Residential [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	14,749	15,666
Residential [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	496	512
Residential [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	200,952	202,151
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	324,232	327,543	301,311
Consumer [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	6,157	6,376
Consumer [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	8	9
Consumer [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 318,067	$ 321,158