Property and Equipment - Net Property and Equipment at Cost (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment
Total property and equipment, at cost	$ 872	$ 871
Less: accumulated depreciation and amortization	721	664
Property and equipment, net	151	207
Leasehold improvements
Property, Plant and Equipment
Total property and equipment, at cost	107	104
Laboratory equipment and other
Property, Plant and Equipment
Total property and equipment, at cost	$ 765	$ 767