ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of accumulated other comprehensive income (loss)
Balance	$ 568,545	$ 524,029	$ 462,676
Change during the period
Other comprehensive earnings/(loss)	24,119	1,103	(5,690)
Balance	653,924	568,545	524,029
Pension Liability Adjustment
Components of accumulated other comprehensive income (loss)
Balance	(49,200)	(50,905)
Change during the period
Before-tax amount	18,980	3,045
Tax benefit	(4,821)	(1,340)
Other comprehensive earnings/(loss)	14,159	1,705
Balance	(35,041)	(49,200)	(50,905)
Foreign Currency Translation
Components of accumulated other comprehensive income (loss)
Balance	(20,875)	(20,273)
Change during the period
Before-tax amount	9,960	(602)
Tax benefit	0	0
Other comprehensive earnings/(loss)	9,960	(602)
Balance	(10,915)	(20,875)	(20,273)
Total
Components of accumulated other comprehensive income (loss)
Balance	(70,075)	(71,178)	(65,488)
Change during the period
Before-tax amount	28,940	2,443
Tax benefit	(4,821)	(1,340)
Other comprehensive earnings/(loss)	24,119	1,103
Balance	$ (45,956)	$ (70,075)	$ (71,178)