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LOAN PORTFOLIO - Corporation's Aging of Loans Held for Investment Portfolio (Parenthetical) (Detail)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
numberofpayments
Sep. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
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 $ 75.9 $ 75.9 $ 62.1
Minimum Number of Payments in Arrears to Consider Commercial Mortgage and Construction Loan as Past Due | numberofpayments 2    
Residential mortgage loans insured by FHA or guaranteed by the VA $ 33.9 $ 33.9 $ 29.9
Period of residential mortgage loan that are no longer accruing interest 1 year 3 months 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 $ 7.0 $ 7.0 $ 6.0
Other Residential Mortgage Loans [Member]      
Accounts Notes And Loans Receivable [Line Items]      
30-59 Days past due Mortgages 108.1 108.1 224.0
Commercial Mortgage Loans [Member]      
Accounts Notes And Loans Receivable [Line Items]      
30-59 Days past due Mortgages 4.2 4.2 9.0
Land [Member]      
Accounts Notes And Loans Receivable [Line Items]      
30-59 Days past due Mortgages $ 0.1 $ 0.1 $ 2.5