CREDIT CARD AND LOAN RECEIVABLES - Allowance for Loan Loss and Delinquencies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Billed and accrued finance charges	$ 933.5		$ 977.3
Total credit card and loan receivables	17,731.9		19,463.1
Less: Credit card receivables - restricted for securitization investors	12,033.4		13,504.2
Other credit card and loan receivables	5,698.5		5,958.9
Unbilled finance charges	262.9
Allowance for Loan Loss
Balance at beginning of period	1,171.1	$ 1,038.3
Provision for loan loss	655.9	252.1
Recoveries	67.9	59.8
Principal charge-offs	(388.1)	(329.1)
Balance at end of period	$ 2,150.8	1,021.1
Number of days a loan is contractually past due before resulting in charge-off	180 days
Number of days after notification of creditor's bankruptcy or death when an account is charged-off	60 days
Actual charge-offs for unpaid interest and fees	$ 231.9	218.8
Period for which interest and fee income accrue until balance, interest and fees paid or charged off	180 days
Period an account becomes past due before a proprietary collection scoring algorithm automatically scores the risk of an account becoming further delinquent	30 days
Amortized cost of receivables balances contractually delinquent:
Total delinquent	$ 1,288.5		1,391.4
Modified Credit Card Receivables
Maximum period of time temporary programs' concessions remain in place	12 months
Impaired credit card and loan receivables	$ 332.6		$ 308.7
Maximum percentage of credit card receivables to total portfolio	2.00%		2.00%
Average recorded investment in impaired credit card receivables	$ 314.7	297.7
Interest income on modified credit card receivables	5.5	$ 5.7
Adjustment | ASU 2016-13
Allowance for Loan Loss
Balance at beginning of period	$ 644.0
Group A (Current, risk score - high)
Allowance for Loan Loss
Estimated Life (in months)	14 months
Group B (Current, risk score - low)
Allowance for Loan Loss
Estimated Life (in months)	19 months
Group C (Delinquent, risk score - high)
Allowance for Loan Loss
Estimated Life (in months)	17 months
Group D (Delinquent, risk score - low)
Allowance for Loan Loss
Estimated Life (in months)	26 months
Credit Card and Loan Receivables
Principal receivables	$ 16,725.4		$ 18,413.1
Other
Principal receivables	$ 73.0		$ 72.7