Allowance for Loan Losses and Credit Quality Information - Schedule of Consumer Credit Exposure Credit Risk Profile Based on Payment Activity (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2012 Mortgages [Member]	Jun. 30, 2012 Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructured mortgages and home equity installment loans	$ 22.2	$ 22.0	$ 10.1	$ 11.0