INTANGIBLE ASSETS - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 214,215	$ 214,600
Accumulated Amortization	(120,987)	(111,318)
Net Carrying Amount	93,228	103,282
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	175,214	175,418
Accumulated Amortization	(85,736)	(76,541)
Net Carrying Amount	89,478	98,877
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,519	5,569
Accumulated Amortization	(5,519)	(5,569)
Net Carrying Amount	0	0
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	24,776	24,870
Accumulated Amortization	(22,134)	(21,794)
Net Carrying Amount	2,642	3,076
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,852	7,879
Accumulated Amortization	(6,837)	(6,691)
Net Carrying Amount	1,015	1,188
Licenses
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	854	864
Accumulated Amortization	(761)	(723)
Net Carrying Amount	$ 93	$ 141