UNSECURED BORROWINGS - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest paid on unsecured borrowings	$ 213.8	$ 136.1	$ 110.8
Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Junior subordinated notes maturity term	30 years
Face amount of junior subordinated notes	$ 100.0
Interest paid on unsecured borrowings	$ 7.5	$ 7.5	$ 7.5