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LOAN PORTFOLIO - Corporation's Aging of Loans Held for Investment Portfolio (Parenthetical) (Detail)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
numberofpayments
Dec. 31, 2016
USD ($)
numberofpayments
Accounts Notes And Loans Receivable [Line Items]    
Loans considered to be defaulted if borrower has failed to make payment for a period or more than the period 2 months 29 days 2 months 29 days
Period during which credit card loans continue to accrue finance charges and fees 5 months 27 days 5 months 27 days
Defaulted loans collateralizing Ginnie Mae (GNMA) securities $ 62.1 $ 43.7
Minimum Number of Payments in Arrears to Consider Commercial Mortgage and Construction Loan as Past Due | numberofpayments 2 2
Residential mortgage loans insured by FHA or guaranteed by the VA $ 29.9 $ 29.3
Period of residential mortgage loan that are no longer accruing interest 1 year 3 months 1 year 3 months
Fha Va And Other Government Guaranteed Loans [Member]    
Accounts Notes And Loans Receivable [Line Items]    
30-59 Days past due Mortgages $ 6.0 $ 9.9
Residential Mortgage [Member]    
Accounts Notes And Loans Receivable [Line Items]    
30-59 Days past due Mortgages 224.0 142.8
Commercial Mortgage [Member]    
Accounts Notes And Loans Receivable [Line Items]    
30-59 Days past due Mortgages 9.0 4.6
Residential Construction [Member]    
Accounts Notes And Loans Receivable [Line Items]    
30-59 Days past due Mortgages   0.4
Land Construction [Member]    
Accounts Notes And Loans Receivable [Line Items]    
30-59 Days past due Mortgages $ 2.5 $ 0.7