Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 3,847,515	$ 3,774,639	$ 3,343,505
Total	100.00%	100.00%
Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 236,657	$ 259,679	212,797
Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	374,634	360,249	$ 328,884
Nonperforming Financing Receivable [Member] | Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	14,416	15,548
Nonperforming Financing Receivable [Member] | Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	7,965	7,664
Performing Financing Receivable [Member] | Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	221,295	243,181
Performing Financing Receivable [Member] | Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 366,665	$ 352,578
Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1.00%	1.00%
Acquired Impaired Loans [Member] | Residential [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 946	$ 950
Acquired Impaired Loans [Member] | Consumer Loan Commitments [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	4	7
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 611,291	$ 619,928
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	$ 22,381	$ 23,212
Total	4.00%	4.00%
Total Residential and Consumer Loan [Member] | Performing Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 587,960	$ 595,759
Total	96.00%	96.00%
Total Residential and Consumer Loan [Member] | Acquired Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 950	$ 957