INCOME TAXES - Components of Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Allowance for doubtful accounts	$ 5.3	$ 6.9
Accrued payroll & employee benefits	9.0	11.4
Workers’ compensation	7.9	10.9
Amortization of intangible assets	26.0	56.3
Share-based compensation	6.1	7.8
Net operating loss carryforwards	20.1	44.2
Tax credit carryforwards	4.6	4.8
Other	2.4	1.1
Gross deferred tax assets	81.4	143.4
Less: valuation allowance	(0.7)	(0.4)
Net deferred tax assets	80.7	143.0
Deferred tax (liabilities):
Property and equipment	(4.0)	(7.8)
Deferred revenue	(18.0)	(23.2)
Investment in partnerships	(2.1)	(3.2)
Other liabilities	(0.5)	(0.9)
Gross deferred tax liabilities	(24.6)	(35.1)
Net deferred tax assets (liabilities)	$ 56.1	$ 107.9