Property, Plant and Equipment - Summary of property, plant and equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	$ 29,174	$ 28,209
Less accumulated depreciation	(17,672)	(17,188)
Total property, plant and equipment	11,502	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	10,384	10,288
Machinery, equipment and tooling
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	18,007	17,249
Construction in progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Gross	$ 776	$ 665