GOODWILL (Goodwill Roll Forward) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Goodwill [Roll Forward]
Balance as of December 31, 2017	$ 119,531
Effect of net foreign currency exchange rate changes	(1,157)
Balance as of June 30, 2018	144,667
North America
Goodwill [Roll Forward]
Balance as of December 31, 2017	77,290
Effect of net foreign currency exchange rate changes	(285)
Balance as of June 30, 2018	103,298
Europe
Goodwill [Roll Forward]
Balance as of December 31, 2017	42,241
Effect of net foreign currency exchange rate changes	(872)
Balance as of June 30, 2018	41,369
Continuum
Goodwill [Roll Forward]
Goodwill acquired	26,293
Balance as of June 30, 2018	26,293
Continuum | North America
Goodwill [Roll Forward]
Goodwill acquired	26,293
Continuum | Europe
Goodwill [Roll Forward]
Goodwill acquired	$ 0