COMMITMENTS, CONTINGENCIES, AND GUARANTEES (Bluestem) (Details) - SC - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Other Commitments [Line Items]
Purchase obligation	$ 14.9		$ 10.6
Purchase commitment, repurchase rate	9.99%
Purchase commitment, exercise of repurchase rights, retainer rate	20.00%
Bluestem | Purchase New Advances on Personal Revolving Financing Receivable
Other Commitments [Line Items]
Other commitments	$ 3,000.0		3,000.0	$ 3,100.0
Purchases from other commitments	500.0		$ 1,200.0
Bluestem | Purchase of Receivables Related to New Opened Customer Accounts
Other Commitments [Line Items]
Purchases from other commitments	$ 78.8	$ 75.5