GOODWILL AND OTHER INTANGIBLE ASSETS - Schedule of Amortizable Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Goodwill and Other Intangible Assets [Line Items]
Cost	$ 133,293	$ 128,218
Accumulated Amortization	(112,573)	(106,983)
Net Book Value	20,720	21,235
Client-related
Goodwill and Other Intangible Assets [Line Items]
Cost	63,089	63,115
Accumulated Amortization	(53,960)	(51,224)
Net Book Value	$ 9,129	$ 11,891
Client-related | Minimum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	4 years	4 years
Client-related | Maximum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	10 years	10 years
Technology
Goodwill and Other Intangible Assets [Line Items]
Cost	$ 64,843	$ 59,742
Accumulated Amortization	(53,252)	(50,398)
Net Book Value	$ 11,591	$ 9,344
Technology | Minimum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	2 years	2 years
Technology | Maximum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	10 years	10 years
Other
Goodwill and Other Intangible Assets [Line Items]
Cost	$ 5,361	$ 5,361
Accumulated Amortization	(5,361)	(5,361)
Net Book Value	$ 0	$ 0
Other | Minimum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	1 year	1 year
Other | Maximum
Goodwill and Other Intangible Assets [Line Items]
Useful Lives	5 years	5 years