INTANGIBLE ASSETS, NET - Summary (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Intangible Assets
Accumulated amortization	$ (81,032)	$ (73,343)
Total	43,216	50,905
Finite lived intangible assets, primarily customer relationships
Intangible Assets
Finite lived intangible assets, primarily customer relationships	115,517	115,517
Indefinite lived intangible assets — Trade name
Intangible Assets
Indefinite lived intangible assets — Trade name	8,731	8,731
FMS
Intangible Assets
Accumulated amortization	(51,545)	(49,031)
Total	6,141	8,655
FMS | Finite lived intangible assets, primarily customer relationships
Intangible Assets
Finite lived intangible assets, primarily customer relationships	57,686	57,686
FMS | Indefinite lived intangible assets — Trade name
Intangible Assets
Indefinite lived intangible assets — Trade name	0	0
SCS
Intangible Assets
Accumulated amortization	(24,748)	(20,047)
Total	25,501	30,202
SCS | Finite lived intangible assets, primarily customer relationships
Intangible Assets
Finite lived intangible assets, primarily customer relationships	50,249	50,249
SCS | Indefinite lived intangible assets — Trade name
Intangible Assets
Indefinite lived intangible assets — Trade name	0	0
DTS
Intangible Assets
Accumulated amortization	(4,739)	(4,265)
Total	2,843	3,317
DTS | Finite lived intangible assets, primarily customer relationships
Intangible Assets
Finite lived intangible assets, primarily customer relationships	7,582	7,582
DTS | Indefinite lived intangible assets — Trade name
Intangible Assets
Indefinite lived intangible assets — Trade name	0	0
CSS
Intangible Assets
Accumulated amortization	0	0
Total	8,731	8,731
CSS | Finite lived intangible assets, primarily customer relationships
Intangible Assets
Finite lived intangible assets, primarily customer relationships	0	0
CSS | Indefinite lived intangible assets — Trade name
Intangible Assets
Indefinite lived intangible assets — Trade name	$ 8,731	$ 8,731