DEBT - Schedule of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
DEBT
Contractual interest	$ 8.3	$ 10.0	$ 28.0	$ 29.1
Amortization of debt discount	0.2	0.5	0.5	1.3
Amortization of debt issuance cost.	0.3	0.2	1.2	0.5
Interest expense	8.8	10.7	29.7	30.9
Interest income	(0.6)	(1.1)	(1.6)	(2.4)
Interest expense, net	$ 8.2	$ 9.6	$ 28.1	$ 28.5