INTANGIBLE ASSETS - Schedule of Identifiable Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Identifiable intangible assets [Abstract]
Gross Carrying Amount	$ 327,974	$ 330,063
Accumulated Amortization	226,691	208,403
Customer relationships & lists
Identifiable intangible assets [Abstract]
Gross Carrying Amount	240,992	243,557
Accumulated Amortization	$ 169,690	158,051
Customer relationships & lists | Minimum
Identifiable intangible assets [Abstract]
Amortization Period	10 years
Customer relationships & lists | Maximum
Identifiable intangible assets [Abstract]
Amortization Period	20 years
Trademarks & trade names
Identifiable intangible assets [Abstract]
Gross Carrying Amount	$ 43,140	43,208
Accumulated Amortization	$ 28,047	24,974
Trademarks & trade names | Minimum
Identifiable intangible assets [Abstract]
Amortization Period	2 years
Trademarks & trade names | Maximum
Identifiable intangible assets [Abstract]
Amortization Period	17 years
Developed technology
Identifiable intangible assets [Abstract]
Gross Carrying Amount	$ 20,288	20,437
Accumulated Amortization	$ 14,409	13,693
Developed technology | Minimum
Identifiable intangible assets [Abstract]
Amortization Period	5 years
Developed technology | Maximum
Identifiable intangible assets [Abstract]
Amortization Period	12 years
Other
Identifiable intangible assets [Abstract]
Gross Carrying Amount	$ 23,554	22,861
Accumulated Amortization	$ 14,545	$ 11,685
Other | Minimum
Identifiable intangible assets [Abstract]
Amortization Period	2 years
Other | Maximum
Identifiable intangible assets [Abstract]
Amortization Period	18 years