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, 2017 USD ($) numberofpayments	Sep. 30, 2017 USD ($)	Dec. 31, 2016 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	$ 45.1	$ 45.1	$ 43.7
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	$ 28.9	$ 28.9	$ 29.3
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	$ 9.4	$ 9.4	$ 9.9
Commercial Mortgage Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	33.8	33.8	4.6
Commercial Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	0.1	0.1
Residential Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	6.4	6.4	0.4
Land Construction [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	0.9	0.9	0.7
Other Residential Mortgage Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
30-59 Days past due Mortgages	$ 216.1	$ 216.1	$ 142.8