Condensed Consolidated Statements of Operations and Comprehensive Loss - 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 revenue	$ 28,520		$ 49,385
Operating expenses:
Research and development	43,751	$ 30,877	103,800	$ 51,112
General and administrative	6,905	5,311	12,693	11,122
Total operating expenses	50,656	36,188	116,493	62,234
Operating loss	(22,136)	(36,188)	(67,108)	(62,234)
Other income (expense):
Interest income	608	270	1,044	504
Other income	10	139	4	153
Net loss	$ (21,518)	$ (35,779)	$ (66,060)	$ (61,577)
Net loss per share - basic and diluted	$ (0.53)	$ (0.95)	$ (1.66)	$ (1.65)
Weighted-average number of common shares - basic and diluted	40,819,957	37,811,056	39,795,282	37,342,324
Comprehensive loss:
Net loss	$ (21,518)	$ (35,779)	$ (66,060)	$ (61,577)
Other comprehensive loss - unrealized loss on securities	(75)	63	(254)	37
Comprehensive loss	$ (21,593)	$ (35,716)	$ (66,314)	$ (61,540)