GOODWILL AND OTHER INTANGIBLES - Schedule of Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Goodwill and Other Intangible Assets [Line Items]
Cost	$ 135,666	$ 133,372
Accumulated Amortization	(120,622)	(117,624)
Total	15,044	15,748
Client-related
Goodwill and Other Intangible Assets [Line Items]
Cost	63,163	63,168
Accumulated Amortization	(56,988)	(55,877)
Total	6,175	7,291
Technology
Goodwill and Other Intangible Assets [Line Items]
Cost	67,142	64,843
Accumulated Amortization	(58,273)	(56,386)
Total	8,869	8,457
Other
Goodwill and Other Intangible Assets [Line Items]
Cost	5,361	5,361
Accumulated Amortization	(5,361)	(5,361)
Total	$ 0	$ 0
Minimum | Client-related
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	4 years	4 years
Minimum | Technology
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	2 years	2 years
Minimum | Other
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	1 year	1 year
Maximum | Client-related
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	10 years	10 years
Maximum | Technology
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	10 years	10 years
Maximum | Other
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	5 years	5 years