DEBT - Schedule of Interest Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
DEBT
Contractual interest	$ 9.5	$ 10.9	$ 18.9	$ 18.5
Amortization of debt discount	0.2	0.2	0.4	0.2
Amortization of debt issuance cost.	0.5	1.4	0.9	2.3
Interest expense	10.2	12.5	20.2	21.0
Interest income	(0.9)		(1.3)	(0.6)
Interest expense, net	$ 9.3	$ 12.5	$ 18.9	$ 20.4