Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Collaboration and license revenue	$ 5,625	$ 3,549	$ 11,250	$ 7,099
Operating expenses:
Research and development	13,689	9,279	25,606	17,409
General and administrative	5,082	4,056	10,558	7,916
Total operating expenses	18,771	13,335	36,164	25,325
Loss from operations	(13,146)	(9,786)	(24,914)	(18,226)
Interest and other income, net	309	26	530	46
Net loss	(12,837)	(9,760)	(24,384)	(18,180)
Other comprehensive loss	(3)		(58)
Comprehensive loss	(12,840)	(9,760)	(24,442)	(18,180)
Net loss attributable to common stockholders	$ (12,837)	$ (9,760)	$ (24,384)	$ (18,180)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.41)	$ (0.37)	$ (0.78)	$ (0.69)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted	31,200,773	26,337,184	31,151,216	26,284,630