Allowance for Loan and Lease Losses ("ALLL") and Credit Quality Indicators - Schedule of Aging Analysis Past Due Loans and Leases, Loans Purchased and Non-Purchased by FDIC Loss Share Agreements (Parenthetical) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
30-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and leases on nonaccrual status	$ 0.7	$ 0.9	$ 1.8
Non Accrual Loans Current [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Loans and leases on nonaccrual status	$ 2.5	$ 0.4	$ 2.8