Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 141,716	$ 145,800
Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired nonimpaired loans	$ 117,500	$ 119,800