Property and Equipment - Net Property and Equipment at Cost (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment
Total property and equipment, at cost	$ 5,936	$ 5,932
Less: accumulated depreciation and amortization	4,616	4,460
Property and equipment, net	1,320	1,472
Leasehold improvements
Property, Plant and Equipment
Total property and equipment, at cost	671	671
Laboratory equipment and other
Property, Plant and Equipment
Total property and equipment, at cost	$ 5,265	$ 5,261