GOODWILL AND INTANGIBLE ASSETS, NET (Goodwill Roll Forward) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Roll Forward]
Balance beginning of period	$ 119,531	$ 109,289
Effect of currency translation	(1,798)	3,695
Balance end of period	166,832	119,531
North America
Goodwill [Roll Forward]
Balance beginning of period	77,290	76,812
Effect of currency translation	(365)	564
Balance end of period	103,542	77,290
Europe
Goodwill [Roll Forward]
Balance beginning of period	42,241	32,477
Effect of currency translation	(1,433)	3,131
Balance end of period	63,290	42,241
Russia
Goodwill [Roll Forward]
Balance beginning of period	0	0
Balance end of period	0	0
Other
Goodwill [Roll Forward]
Balance beginning of period	0	0
Balance end of period	0	0
Other Acquisitions
Goodwill [Roll Forward]
Acquisition		4,732
Purchase accounting adjustments		1,815
Other Acquisitions | North America
Goodwill [Roll Forward]
Acquisition		199
Purchase accounting adjustments		(285)
Other Acquisitions | Europe
Goodwill [Roll Forward]
Acquisition		4,533
Purchase accounting adjustments		$ 2,100
Continuum
Goodwill [Roll Forward]
Acquisition	26,617
Continuum | North America
Goodwill [Roll Forward]
Acquisition	26,617
Continuum | Europe
Goodwill [Roll Forward]
Acquisition	0
Think
Goodwill [Roll Forward]
Acquisition	22,482
Think | North America
Goodwill [Roll Forward]
Acquisition	0
Think | Europe
Goodwill [Roll Forward]
Acquisition	$ 22,482