GOODWILL AND INTANGIBLE ASSETS, NET - Intangible Assets, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Intangible Assets, Net
Gross carrying amount	$ 262,712	$ 264,957
Accumulated amortization	(216,386)	(203,911)
Net book value	46,326	61,046
Amortization expense for definite lived intangible assets	14,720	19,021	$ 28,412
Amortization expense, 2021	9,500
Amortization expense, 2022	5,100
Amortization expense, 2023	5,100
Amortization expense, 2024	5,100
Amortization expense, 2025	5,100
Customer related intangible assets
Intangible Assets, Net
Gross carrying amount	214,973	214,973
Accumulated amortization	(187,923)	(176,043)
Net book value	$ 27,050	38,930
Customer related intangible assets | Weighted average
Intangible Assets, Net
Estimated useful life	9 years
Operating agreement
Intangible Assets, Net
Gross carrying amount	$ 35,000	35,000
Accumulated amortization	(19,126)	(17,376)
Net book value	$ 15,874	17,624
Operating agreement | Weighted average
Intangible Assets, Net
Estimated useful life	20 years
Trademarks and trade names
Intangible Assets, Net
Gross carrying amount	$ 9,709	9,709
Accumulated amortization	(6,307)	(5,893)
Net book value	$ 3,402	3,816
Trademarks and trade names | Weighted average
Intangible Assets, Net
Estimated useful life	16 years
Non-compete agreements
Intangible Assets, Net
Gross carrying amount	$ 1,230	1,230
Accumulated amortization	(1,230)	(1,215)
Net book value	$ 0	15
Non-compete agreements | Weighted average
Intangible Assets, Net
Estimated useful life	4 years
Intellectual property
Intangible Assets, Net
Gross carrying amount	$ 0	300
Accumulated amortization	0	(175)
Net book value	$ 0	125
Intellectual property | Weighted average
Intangible Assets, Net
Estimated useful life	0 years
Other intangible assets
Intangible Assets, Net
Gross carrying amount	$ 1,800	3,745
Accumulated amortization	(1,800)	(3,209)
Net book value	$ 0	$ 536
Other intangible assets | Weighted average
Intangible Assets, Net
Estimated useful life	5 years