SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Comprehensive Income (Details) $ in Thousands	6 Months Ended
	Jun. 30, 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	(311,166)
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)	(43,091)
Divestiture of grinding media business	9,203
Balance at the end of the period	(205,287)
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)	529
Divestiture of grinding media business	1,215
Balance at the end of the period	8,101
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
Divestiture of grinding media business	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)	(42,562)
Divestiture of grinding media business	10,418
Balance at the end of the period	$ (311,166)