GOODWILL (Goodwill Roll Forward) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Goodwill [Roll Forward]
Balance as of January 1, 2021	$ 211,956
2020 acquisitions purchase accounting adjustments	(24)
Effect of net foreign currency exchange rate changes	(1,051)
Balance as of March 31, 2021	210,881
North America
Goodwill [Roll Forward]
Balance as of January 1, 2021	121,132
2020 acquisitions purchase accounting adjustments	0
Effect of net foreign currency exchange rate changes	(353)
Balance as of March 31, 2021	120,779
Europe
Goodwill [Roll Forward]
Balance as of January 1, 2021	90,106
2020 acquisitions purchase accounting adjustments	(24)
Effect of net foreign currency exchange rate changes	(692)
Balance as of March 31, 2021	89,390
Russia
Goodwill [Roll Forward]
Balance as of January 1, 2021	718
2020 acquisitions purchase accounting adjustments	0
Effect of net foreign currency exchange rate changes	(6)
Balance as of March 31, 2021	$ 712