Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 17,967	$ 19,633
Less accumulated depreciation and amortization	(8,259)	(7,803)
Property and equipment, net	9,708	11,830
Depreciation and amortization expense	2,524	2,276	$ 1,591
Laboratory equipment
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	8,430	7,932
Computer equipment and software
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	281	265
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	706	706
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	8,550	10,469
Construction in progress
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 0	$ 261