MARKETABLE SECURITIES - Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	$ 600.8	$ 569.2
Fair Value, Inputs, Level 1
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	337.1	328.8
Fair Value, Inputs, Level 1 | Mutual Funds and Equities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	313.8	274.0
Fair Value, Inputs, Level 1 | Money Market Funds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	23.3	54.8
Fair Value, Inputs, Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	263.7	240.4
Fair Value, Inputs, Level 2 | U.S. Government Issued Debt Securities (Agency and Treasury)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	70.2	63.0
Fair Value, Inputs, Level 2 | Corporate Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	50.9	41.1
Fair Value, Inputs, Level 2 | Asset-Backed Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	21.2	18.5
Fair Value, Inputs, Level 2 | Municipal Bonds
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	110.7	107.5
Fair Value, Inputs, Level 2 | Other Fixed Income Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Marketable Securities	$ 10.7	$ 10.3