STOCKHOLDERS' EQUITY - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in accumulated other comprehensive income (loss), net of tax
Beginning Balance	$ (43,403)	$ (37,318)	$ (32,682)
Before-Tax Amount	(4,915)	(10,297)	3,147
Tax Benefit (Expense)	4,544	4,212	(7,783)
Total other comprehensive (loss), net of tax	(371)	(6,085)	(4,636)
Ending Balance	(43,774)	(43,403)	(37,318)
Pension and other post-employment liability adjustment
Changes in accumulated other comprehensive income (loss), net of tax
Beginning Balance	(29,405)	(22,832)	(34,777)
Before-Tax Amount	(9,783)	(10,785)	19,728
Tax Benefit (Expense)	4,544	4,212	(7,783)
Total other comprehensive (loss), net of tax	(5,239)	(6,573)	11,945
Ending Balance	(34,644)	(29,405)	(22,832)
Foreign currency translation adjustment
Changes in accumulated other comprehensive income (loss), net of tax
Beginning Balance	(13,998)	(14,486)	2,095
Before-Tax Amount	4,868	488	(16,581)
Tax Benefit (Expense)	0	0	0
Total other comprehensive (loss), net of tax	4,868	488	(16,581)
Ending Balance	$ (9,130)	$ (13,998)	$ (14,486)