PROPERTY, PLANT & EQUIPMENT, & FINANCE LEASE (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Property, Plant and Equipment [Line Items]
Property, plant and equipment - gross	$ 191,222	$ 192,526
Less accumulated depreciation	137,025	135,925
Net property, plant and equipment	54,197	56,601
Finance lease	34,254	30,551
Less finance lease accumulated amortization	2,603	2,074
Net property, plant and equipment, and finance lease	85,848	85,078
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment - gross	1,533	1,584
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment - gross	33,902	34,207
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment - gross	150,605	151,399
Property under construction
Property, Plant and Equipment [Line Items]
Property, plant and equipment - gross	$ 5,182	$ 5,336