MARKETABLE SECURITIES - Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	$ 462.6	$ 430.4
Fair Value, Inputs, Level 1
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	274.5	269.9
Fair Value, Inputs, Level 1 | Mutual Funds and Equities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	228.5	244.0
Fair Value, Inputs, Level 1 | Money Market Funds
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	46.0	25.9
Level 2
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	188.1	160.5
Level 2 | U.S. Government Issued Debt Securities (Agency and Treasury)
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	96.8	79.6
Level 2 | Corporate Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	44.0	39.5
Level 2 | Asset-Backed Debt Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	12.9	14.0
Level 2 | Municipal Bonds
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	26.7	19.2
Level 2 | Other Fixed Income Securities
Debt and Equity Securities, FV-NI [Line Items]
Total Marketable Securities	$ 7.7	$ 8.2