DEBT AND OTHER OBLIGATIONS - Schedule of Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Debt, subordinated debt, and subordinated funding obligations	$ 66.5	$ 61.3	$ 62.1
Non-recourse funding obligations, other obligations, and repurchase agreements	172.6	171.9	163.7
Total interest expense	$ 239.1	$ 233.2	$ 225.8