MARKETABLE SECURITIES - Contractual Maturity (Details) $ in Millions	Dec. 31, 2017 USD ($)
Amortized Cost
Less than one year	$ 40.2
One to five years	46.7
Six to ten years	64.7
Greater than ten years	133.3
Total Debt Securities	284.9
Fair Value
Less than one year	40.1
One to five years	47.5
Six to ten years	65.6
Greater than ten years	133.8
Total Debt Securities	$ 287.0