MARKETABLE SECURITIES - Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	$ 430.4	$ 600.8
Fair Value, Inputs, Level 1
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	269.9	337.1
Fair Value, Inputs, Level 1 | Mutual Funds and Equities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	244.0	313.8
Fair Value, Inputs, Level 1 | Money Market Funds
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	25.9	23.3
Fair Value, Inputs, Level 2
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	160.5	263.7
Fair Value, Inputs, Level 2 | U.S. Government Issued Debt Securities (Agency and Treasury)
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	79.6	70.2
Fair Value, Inputs, Level 2 | Corporate Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	39.5	50.9
Fair Value, Inputs, Level 2 | Asset-Backed Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	14.0	21.2
Fair Value, Inputs, Level 2 | Municipal Bonds
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	19.2	110.7
Fair Value, Inputs, Level 2 | Other Fixed Income Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	$ 8.2	$ 10.7