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Credit Quality and Nonperforming Assets - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Loan, classified as troubled debt restructuring $ 51.6  
Loan, classified as troubled debt restructuring, non-accrual status $ 31.8  
Loans Accruals Due 90 days 90 days