Property, Plant and Equipment - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Apr. 30, 2018
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, at cost	$ 48,677	$ 46,100
Accumulated depreciation	(25,209)	(21,834)
Property, plant and equipment, net	23,468	24,266
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, at cost	6,524	6,405
Machinery and equipment
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, at cost	40,829	38,454	$ 6,900
Construction in progress
Property, Plant and Equipment [Line Items]
Total property, plant and equipment, at cost	$ 1,324	$ 1,241