Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Less: accumulated depreciation and amortization	$ (15,638)	$ (15,083)
Net book value	1,070	1,404
Research Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	6,984	6,789
Office Furniture and Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	1,043	1,045
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	2,311	2,283
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Property, plant and equipment	$ 6,370	$ 6,370