Property and Equipment - Net Property and Equipment at Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment
Total property and equipment, at cost	$ 5,829	$ 5,798
Less: accumulated depreciation and amortization	4,417	3,945
Property and equipment, net	1,412	1,853
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	$ 5,158	$ 5,127