Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,367,991	$ 3,202,498
Total	100.00%	100.00%
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	0.00%
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 540,906	$ 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	$ 22,362	$ 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	$ 387	$ 521
Total Residential and Consumer Loan [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 518,157	$ 523,309
Total	96.00%	96.00%
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 199,696	$ 218,329	$ 229,221
Residential [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	14,613	15,666
Residential [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	380	512
Residential [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	184,703	202,151
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	341,210	327,543	$ 325,786
Consumer [Member] | Nonperforming Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7,749	6,376
Consumer [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7	9
Consumer [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 333,454	$ 321,158