Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Less: accumulated depreciation and amortization	$ (16,199)	$ (15,638)
Net book value	991	1,070
Research Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	7,313	6,984
Office Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	1,046	1,043
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	2,461	2,311
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	$ 6,370	$ 6,370