GOODWILL AND INTANGIBLE ASSETS, NET (Goodwill Roll Forward) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Balance beginning of period	$ 166,832	$ 119,531
Effect of currency translation	1,317	(1,798)
Balance end of period	195,043	166,832
North America Segment
Goodwill [Roll Forward]
Balance beginning of period	103,542	77,290
Effect of currency translation	80	(365)
Balance end of period	113,426	103,542
Europe Segment
Goodwill [Roll Forward]
Balance beginning of period	63,290	42,241
Effect of currency translation	1,231	(1,433)
Balance end of period	80,873	63,290
Russia Segment
Goodwill [Roll Forward]
Balance beginning of period	0	0
Effect of currency translation	6	0
Balance end of period	744	0
Continuum
Goodwill [Roll Forward]
Acquisition		26,617
Continuum | North America Segment
Goodwill [Roll Forward]
Acquisition		26,617
Continuum | Europe Segment
Goodwill [Roll Forward]
Acquisition		0
Continuum | Russia Segment
Goodwill [Roll Forward]
Acquisition		0
Think
Goodwill [Roll Forward]
Acquisition		22,482
Purchase accounting adjustments	(2,043)
Think | North America Segment
Goodwill [Roll Forward]
Acquisition		0
Purchase accounting adjustments	0
Think | Europe Segment
Goodwill [Roll Forward]
Acquisition		22,482
Purchase accounting adjustments	(2,043)
Think | Russia Segment
Goodwill [Roll Forward]
Acquisition		$ 0
Purchase accounting adjustments	0
test IO
Goodwill [Roll Forward]
Acquisition	12,150
Purchase accounting adjustments	145
test IO | North America Segment
Goodwill [Roll Forward]
Acquisition	3,301
test IO | Europe Segment
Goodwill [Roll Forward]
Acquisition	8,849
test IO | Russia Segment
Goodwill [Roll Forward]
Acquisition	0
Other 2019 Acquisitions
Goodwill [Roll Forward]
Acquisition	16,787
Other 2019 Acquisitions | North America Segment
Goodwill [Roll Forward]
Acquisition	6,503
Other 2019 Acquisitions | Europe Segment
Goodwill [Roll Forward]
Acquisition	9,546
Other 2019 Acquisitions | Russia Segment
Goodwill [Roll Forward]
Acquisition	$ 738