LOAN PORTFOLIO - Corporation's Aging of Loans Held for Investment Portfolio (Parenthetical) (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) numberofpayments	Dec. 31, 2015 USD ($) 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	$ 40.0	$ 38.5
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	$ 34.9	$ 37.3
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	$ 10.0	$ 11.0
Residential Mortgage [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	155.9	162.9
Commercial Mortgage Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	74.8	38.6
Commercial Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	5.2
Residential Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	0.7	0.8
Land Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	$ 0.5	$ 5.7