Allowance for Loan Losses and Credit Quality Information - Schedule of Residential and Consumer Credit Exposure (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-performing loans of troubled debt restructured mortgages and home equity installment loans	$ 11,800	$ 11,400
Acquired nonimpaired loans	121,780	145,800	$ 156,671
Total Residential and Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 21,100	$ 26,000