Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 7,469	$ 6,215
Operating expenses:
Research and development	22,493	13,431
General and administrative	7,406	5,732
Total operating expenses	29,899	19,163
Operating loss	(22,430)	(12,948)
Interest income	1,074	317
Net loss	$ (21,356)	$ (12,631)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.51)	$ (0.36)
Weighted average shares outstanding, basic and diluted	42,043	34,723