GOODWILL AND INTANGIBLE ASSETS, NET - Intangible assets, net (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
Intangible Assets, Net
Gross carrying amount	$ 326,001		$ 326,001		$ 333,787
Accumulated amortization	(220,627)		(220,627)		(213,722)
Net book value	105,374		105,374		120,065
Amortization expense for definite lived intangible assets	7,544	$ 9,393	14,691	$ 18,539
2018	25,700		25,700
2019	20,600		20,600
2020	17,900		17,900
2021	13,400		13,400
2022	7,300		7,300
Customer related intangible assets
Intangible Assets, Net
Gross carrying amount	273,172		273,172		277,828
Accumulated amortization	(196,315)		(196,315)		(188,258)
Net book value	76,857		76,857		89,570
Operating agreement
Intangible Assets, Net
Gross carrying amount	35,000		35,000		35,000
Accumulated amortization	(14,748)		(14,748)		(13,865)
Net book value	20,252		20,252		21,135
Trademarks and trade names
Intangible Assets, Net
Gross carrying amount	12,554		12,554		15,354
Accumulated amortization	(6,480)		(6,480)		(8,881)
Net book value	6,074		6,074		6,473
Non-compete agreements
Intangible Assets, Net
Gross carrying amount	1,230		1,230		1,560
Accumulated amortization	(872)		(872)		(897)
Net book value	358		358		663
Intellectual property
Intangible Assets, Net
Gross carrying amount	300		300		300
Accumulated amortization	(130)		(130)		(115)
Net book value	170		170		185
Other intangible assets
Intangible Assets, Net
Gross carrying amount	3,745		3,745		3,745
Accumulated amortization	(2,082)		(2,082)		(1,706)
Net book value	$ 1,663		$ 1,663		$ 2,039
Weighted Average | Customer related intangible assets
Intangible Assets, Net
Weighted average estimated useful life (in years)			10 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)			14 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