INTANGIBLE ASSETS - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 214,340	$ 214,600
Accumulated Amortization	(114,476)	(111,318)
Net Carrying Amount	99,864	103,282
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	175,289	175,418
Accumulated Amortization	(79,584)	(76,541)
Net Carrying Amount	95,705	98,877
Non-compete agreements
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	5,541	5,569
Accumulated Amortization	(5,541)	(5,569)
Net Carrying Amount	0	0
Trade names
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	24,797	24,870
Accumulated Amortization	(21,893)	(21,794)
Net Carrying Amount	2,904	3,076
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	7,858	7,879
Accumulated Amortization	(6,727)	(6,691)
Net Carrying Amount	1,131	1,188
Licenses
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	855	864
Accumulated Amortization	(731)	(723)
Net Carrying Amount	$ 124	$ 141