CREDIT CARD AND LOAN RECEIVABLES - Credit Quality (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 18,413.1	$ 16,869.9
Percentage of Principal Receivables Outstanding	100.00%	100.00%
No Score
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 290.0	$ 249.0
Percentage of Principal Receivables Outstanding	1.60%	1.50%
27.1% and higher
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 1,591.4	$ 1,394.0
Percentage of Principal Receivables Outstanding	8.60%	8.20%
17.1% - 27.0%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 1,065.4	$ 770.1
Percentage of Principal Receivables Outstanding	5.80%	4.60%
12.6% - 17.0%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 1,127.7	$ 1,047.6
Percentage of Principal Receivables Outstanding	6.10%	6.20%
3.7% - 12.5%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 7,985.8	$ 6,877.6
Percentage of Principal Receivables Outstanding	43.40%	40.80%
1.9% - 3.6%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 3,285.3	$ 3,060.7
Percentage of Principal Receivables Outstanding	17.80%	18.10%
Lower than 1.9%
Probability of an Account Becoming 91 or More Days Past Due or Becoming Charged-off (within the next 12 months)
Principal Receivables Outstanding	$ 3,067.5	$ 3,470.9
Percentage of Principal Receivables Outstanding	16.70%	20.60%