SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Comprehensive Income (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	$ (279,022)
Balance at the end of the period	(276,629)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	(171,399)
Current-period comprehensive income (loss)	3,256
Balance at the end of the period	(168,143)
Gain/(Loss) on Hedging Activities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	6,357
Current-period comprehensive income (loss)	(863)
Balance at the end of the period	5,494
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	(113,980)
Current-period comprehensive income (loss)	0
Balance at the end of the period	(113,980)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at the beginning of the period	(279,022)
Current-period comprehensive income (loss)	2,393
Balance at the end of the period	$ (276,629)