SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently Adopted Accounting Standards (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan loss	$ 2,150.8	$ 1,171.1	$ 1,021.1	$ 1,038.3
ASU 2016-13 | Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for loan loss		$ 644.0