NATURE OF OPERATIONS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Variable Interest Entities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Consolidated total debt	$ 3,908,200		$ 3,908,200
Loss from equity investments	(17,483)	$ 1,462	$ (29,960)	$ 141
Consolidated VIEs, aggregated
Variable Interest Entity [Line Items]
Initial term of certain outsourcing agreements			5 years
Consolidated total debt	26,400		$ 26,400		$ 29,300
Consolidated VIEs | Eliminations
Variable Interest Entity [Line Items]
Liabilities associated with the certain outsourcing agreements and purchase options	117,800		117,800		116,500
Consolidated VIEs | Cunningham | Eliminations
Variable Interest Entity [Line Items]
Total payments made under the LMA excluded from liabilities	45,700		45,700		44,000
Total capital leased liabilities excluded from VIE consolidation	4,500		4,500		4,500
VIEs which are not primary beneficiary
Variable Interest Entity [Line Items]
Carrying amount	91,300		91,300		$ 115,700
Loss from equity investments	$ (14,500)	$ 3,800	$ (23,500)	$ 3,500