Property, Plant and Equipment - Summary of property, plant and equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	$ 28,703	$ 28,209
Less accumulated depreciation	(17,416)	(17,188)
Total property, plant and equipment	11,287	11,021
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	7	7
Buildings
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	10,293	10,288
Machinery, equipment and tooling
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	17,373	17,249
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	$ 1,030	$ 665