ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 616,881	$ 521,033	$ 463,705
Other comprehensive (loss)/gain	(1,960)	5,207	(1,735)
Ending balance	691,618	616,881	521,033
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,642)	(6,849)	(5,114)
Ending balance	(3,602)	(1,642)	$ (6,849)
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(1,303)
Other comprehensive (loss)/gain	(2,982)
Ending balance	(4,285)	(1,303)
Postretirement benefit plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(339)
Other comprehensive (loss)/gain	1,022
Ending balance	$ 683	$ (339)