ACCUMULATED OTHER COMPREHENSIVE INCOME, Components of 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 loss [Roll Forward]
Balance	$ 521,033	$ 463,705	$ 391,898
Other comprehensive (loss)/gain	5,207	(1,735)	(2,463)
Balance	616,881	521,033	463,705
AOCI Attributable to Parent [Member]
Components of accumulated other comprehensive loss [Roll Forward]
Balance	(6,849)	(5,114)	(2,651)
Balance	(1,642)	(6,849)	$ (5,114)
Foreign Currency Translation Adjustment [Member]
Components of accumulated other comprehensive loss [Roll Forward]
Balance	(6,707)
Other comprehensive (loss)/gain	5,404
Balance	(1,303)	(6,707)
Postretirement Benefit Plan [Member]
Components of accumulated other comprehensive loss [Roll Forward]
Balance	(142)
Other comprehensive (loss)/gain	(197)
Balance	$ (339)	$ (142)