OTHER NON-OPERATING EXPENSE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income and Expenses [Abstract]
Interest income	$ 523	$ 573	$ 426
Foreign currency exchange gain / (loss), net	17,185	7,149	(11,550)
Change in fair value of derivatives (Note 14)	(1,783)	(10,213)	4,848
Other income / (expense), net	396	417	(17,333)
Total other non-operating income / (expense)	$ 16,321	$ (2,074)	$ (23,609)