INTANGIBLE ASSETS - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 214,456	$ 214,600
Accumulated Amortization	(117,930)	(111,318)
Net Carrying Amount	96,526	103,282
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	175,349	175,418
Accumulated Amortization	(82,773)	(76,541)
Net Carrying Amount	92,576	98,877
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,567	5,569
Accumulated Amortization	(5,567)	(5,569)
Net Carrying Amount	0	0
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	24,819	24,870
Accumulated Amortization	(22,050)	(21,794)
Net Carrying Amount	2,769	3,076
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,863	7,879
Accumulated Amortization	(6,791)	(6,691)
Net Carrying Amount	1,072	1,188
Licenses
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	858	864
Accumulated Amortization	(749)	(723)
Net Carrying Amount	$ 109	$ 141