MARKETABLE SECURITIES - Fair Value Measurement (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	$ 569.2	$ 545.7
Fair Value, Inputs, Level 1 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	328.8	312.2
Fair Value, Inputs, Level 1 [Member] | Mutual Funds and Equities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	274.0	285.3
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	54.8	26.9
Fair Value, Inputs, Level 2 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	240.4	233.5
Fair Value, Inputs, Level 2 [Member] | U.S. Government Issued Debt Securities (Agency and Treasury) [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	63.0	46.6
Fair Value, Inputs, Level 2 [Member] | Corporate Bond Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	41.1	43.9
Fair Value, Inputs, Level 2 [Member] | Asset-Backed Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	18.5	20.0
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	107.5	111.4
Fair Value, Inputs, Level 2 [Member] | Other Fixed Income Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total marketable securities	$ 10.3	$ 11.6