FAIR VALUE MEASUREMENTS - Acquisition Related Contingent Consideration (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combination, Acquisition Related Contingent Consideration [Roll Forward]
Change in fair value of contingent consideration	$ 19,389	$ 18,383	$ 13,778
Recurring basis
Business Combination, Acquisition Related Contingent Consideration [Roll Forward]
Balance at January 1,	87,478
Balance at December 31,	90,000	87,478
Recurring basis | Significant unobservable inputs (Level 3)
Business Combination, Acquisition Related Contingent Consideration [Roll Forward]
Balance at January 1,	87,478	59,021
Present value of contingent consideration upon acquisition related to a business combination	0	25,000
Change in fair value of contingent consideration	19,389	18,383
Contractual Payments	(6,006)	(12,826)
Contractual Payments accrued at December 31	(11,012)	(1,988)
Foreign currency impact	151	(112)
Balance at December 31,	$ 90,000	$ 87,478	$ 59,021