MARKETABLE SECURITIES - Schedule of Contractual Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Less than one year	$ 28.5
One to five years	48.7
Six to ten years	61.8
Greater than ten years	153.8
Total Debt Securities	292.8	$ 284.9
Fair Value
Less than one year	28.5
One to five years	48.3
Six to ten years	61.2
Greater than ten years	152.9
Total Debt Securities	$ 290.9	$ 287.0