Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 3,569	$ 3,143
Less: accumulated depreciation and amortization	(3,078)	(2,729)
Total property and equipment, net	491	414
Depreciation and amortization	351	304	$ 243
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,624	2,330
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	183	156
Less: accumulated depreciation and amortization	(147)	(123)
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property and equipment, gross	584	483
Office equipment, furniture, and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 178	$ 174