Allowance for Loan Losses and Credit Quality Information - Schedule of Commercial Credit Exposure (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired non-impaired loans	$ 354,694	$ 371,100	$ 141,716
Total Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Acquired non-impaired loans	$ 265,700	$ 277,000