MARKETABLE SECURITIES - Schedule of Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	$ 603.4	$ 600.8
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	333.4	337.1
Level 1 | Mutual Funds and Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	312.5	313.8
Level 1 | Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	20.9	23.3
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	270.0	263.7
Level 2 | U.S. Government Issued Debt Securities (Agency and Treasury)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	80.8	70.2
Level 2 | Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	41.8	50.9
Level 2 | Asset-Backed Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	14.3	21.2
Level 2 | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	121.2	110.7
Level 2 | Other Fixed Income Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Marketable securities recorded at fair value on a recurring basis	$ 11.9	$ 10.7