GOODWILL AND INTANGIBLE ASSETS, NET - Intangible assets, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Intangible Assets, Net
Gross carrying amount	$ 259,682		$ 259,682		$ 262,712
Accumulated amortization	(218,866)		(218,866)		(216,386)
Net book value	40,816		40,816		46,326
Amortization expense for definite lived intangible assets	2,911	$ 2,840	5,510	$ 7,049
2021	9,500		9,500
2022	5,100		5,100
2023	5,100		5,100
2024	5,100		5,100
2025	5,100		5,100
Customer related intangible assets
Intangible Assets, Net
Gross carrying amount	214,973		214,973		214,973
Accumulated amortization	(192,352)		(192,352)		(187,923)
Net book value	22,621		22,621		27,050
Operating agreement
Intangible Assets, Net
Gross carrying amount	35,000		35,000		35,000
Accumulated amortization	(20,001)		(20,001)		(19,126)
Net book value	14,999		14,999		15,874
Trademarks and trade names
Intangible Assets, Net
Gross carrying amount	9,709		9,709		9,709
Accumulated amortization	(6,513)		(6,513)		(6,307)
Net book value	3,196		3,196		3,402
Non-compete agreements
Intangible Assets, Net
Gross carrying amount	0		0		1,230
Accumulated amortization	0		0		(1,230)
Net book value	0		0		0
Other intangible assets
Intangible Assets, Net
Gross carrying amount	0		0		1,800
Accumulated amortization	0		0		(1,800)
Net book value	$ 0		$ 0		$ 0
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)			16 years
Weighted Average | Non-compete agreements
Intangible Assets, Net
Weighted average estimated useful life (in years)			0 years
Weighted Average | Other intangible assets
Intangible Assets, Net
Weighted average estimated useful life (in years)			0 years