INCOME TAXES - Components of Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Allowance for doubtful accounts	$ 5.6	$ 5.3
Accrued payroll & employee benefits	11.2	9.0
Workers’ compensation	8.3	7.9
Amortization of intangible assets	14.7	26.0
Share-based compensation	6.9	6.1
Net operating loss carryforwards	5.9	20.1
Tax credit carryforwards	2.8	4.6
Other	2.9	2.4
Gross deferred tax assets	58.3	81.4
Less: valuation allowance	(0.7)	(0.7)
Net deferred tax assets	57.6	80.7
Deferred tax (liabilities):
Property and equipment	(4.4)	(4.0)
Deferred revenue	(13.5)	(18.0)
Investment in partnerships	(3.1)	(2.1)
Other liabilities	(0.8)	(0.5)
Gross deferred tax liabilities	(21.8)	(24.6)
Net deferred tax assets (liabilities)	$ 35.8	$ 56.1