CREDIT CARD AND LOAN RECEIVABLES - Age of Credit Card and Loan Receivable Accounts (Details) item in Millions, $ in Millions	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($) item
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	25.5	24.2
Percentage of Active Accounts with Balances	100.00%	100.00%
Principal Receivables Outstanding | $	$ 18,413.1	$ 16,869.9
Percentage of Principal Receivables Outstanding	100.00%	100.00%
0-12 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	7.1	6.5
Percentage of Active Accounts with Balances	27.80%	26.70%
Principal Receivables Outstanding | $	$ 4,585.1	$ 4,099.9
Percentage of Principal Receivables Outstanding	24.90%	24.30%
13-24 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	3.9	4.2
Percentage of Active Accounts with Balances	15.30%	17.10%
Principal Receivables Outstanding | $	$ 2,826.6	$ 2,887.8
Percentage of Principal Receivables Outstanding	15.40%	17.10%
25-36 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	3.2	3.1
Percentage of Active Accounts with Balances	12.60%	13.00%
Principal Receivables Outstanding | $	$ 2,423.1	$ 2,428.9
Percentage of Principal Receivables Outstanding	13.20%	14.40%
37-48 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	2.5	2.2
Percentage of Active Accounts with Balances	9.70%	9.10%
Principal Receivables Outstanding | $	$ 2,051.7	$ 1,795.0
Percentage of Principal Receivables Outstanding	11.10%	10.70%
49-60 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	1.8	1.7
Percentage of Active Accounts with Balances	7.00%	7.10%
Principal Receivables Outstanding | $	$ 1,509.7	$ 1,367.2
Percentage of Principal Receivables Outstanding	8.20%	8.10%
Over 60 Months
Age of Accounts Since Origination
Number of Active Accounts with Balances | item	7.0	6.5
Percentage of Active Accounts with Balances	27.60%	27.00%
Principal Receivables Outstanding | $	$ 5,016.9	$ 4,291.1
Percentage of Principal Receivables Outstanding	27.20%	25.40%