Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 129,858	$ 107,300
Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 107,300	$ 119,800