GOODWILL - Roll Forward (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Goodwill [Roll Forward]
Balance as of January 1, 2017	$ 109,289
Effect of net foreign currency exchange rate changes	3,131
Balance as of September 30, 2017	119,452
North America
Goodwill [Roll Forward]
Balance as of January 1, 2017	76,812
Effect of net foreign currency exchange rate changes	470
Balance as of September 30, 2017	77,196
Europe
Goodwill [Roll Forward]
Balance as of January 1, 2017	32,477
Effect of net foreign currency exchange rate changes	2,661
Balance as of September 30, 2017	42,256
Other Acquisitions
Goodwill [Roll Forward]
Goodwill acquired	4,732
Purchase accounting adjustments	2,300
Other Acquisitions | North America
Goodwill [Roll Forward]
Goodwill acquired	199
Purchase accounting adjustments	(285)
Other Acquisitions | Europe
Goodwill [Roll Forward]
Goodwill acquired	4,533
Purchase accounting adjustments	$ 2,585