GOODWILL AND INTANGIBLE ASSETS, NET - Intangible assets, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Intangible Assets, Net
Gross carrying amount	$ 266,388		$ 266,388		$ 324,796
Accumulated amortization	(197,772)		(197,772)		(233,143)
Net book value	68,616		68,616		91,653
Amortization expense for definite lived intangible assets	3,544	$ 7,544	12,191	$ 14,691
2019	19,400		19,400
2020	13,300		13,300
2021	10,800		10,800
2022	5,300		5,300
2023	5,300		5,300
Customer related intangible assets
Intangible Assets, Net
Gross carrying amount	214,973		214,973		273,172
Accumulated amortization	(170,756)		(170,756)		(207,639)
Net book value	44,217		44,217		65,533
Operating agreement
Intangible Assets, Net
Gross carrying amount	35,000		35,000		35,000
Accumulated amortization	(16,501)		(16,501)		(15,632)
Net book value	18,499		18,499		19,368
Trademarks and trade names
Intangible Assets, Net
Gross carrying amount	11,140		11,140		11,349
Accumulated amortization	(6,342)		(6,342)		(6,244)
Net book value	4,798		4,798		5,105
Non-compete agreements
Intangible Assets, Net
Gross carrying amount	1,230		1,230		1,230
Accumulated amortization	(1,180)		(1,180)		(1,026)
Net book value	50		50		204
Intellectual property
Intangible Assets, Net
Gross carrying amount	300		300		300
Accumulated amortization	(160)		(160)		(145)
Net book value	140		140		155
Other intangible assets
Intangible Assets, Net
Gross carrying amount	3,745		3,745		3,745
Accumulated amortization	(2,833)		(2,833)		(2,457)
Net book value	$ 912		$ 912		$ 1,288
Weighted Average | Customer related intangible assets
Intangible Assets, Net
Weighted average estimated useful life (in years)			9 years
Weighted Average | Operating agreement
Intangible Assets, Net
Weighted average estimated useful life (in years)			20 years
Weighted Average | Trademarks and trade names
Intangible Assets, Net
Weighted average estimated useful life (in years)			15 years
Weighted Average | Non-compete agreements
Intangible Assets, Net
Weighted average estimated useful life (in years)			4 years
Weighted Average | Intellectual property
Intangible Assets, Net
Weighted average estimated useful life (in years)			10 years
Weighted Average | Other intangible assets
Intangible Assets, Net
Weighted average estimated useful life (in years)			5 years