PROPERTY, PLANT AND EQUIPMENT (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, plant and equipment
Property, plant and equipment, gross	$ 176,960	$ 174,015
Accumulated depreciation	(125,287)	(123,225)
Property, plant and equipment, net	51,673	50,790
Land, buildings and improvements
Property, plant and equipment
Property, plant and equipment, gross	73,018	71,914
Machinery, equipment and fixtures
Property, plant and equipment
Property, plant and equipment, gross	79,204	78,168
Office furniture, equipment and vehicles
Property, plant and equipment
Property, plant and equipment, gross	23,949	23,633
Construction in progress
Property, plant and equipment
Property, plant and equipment, gross	$ 789	$ 300