Unaudited Statements of Operations - USD ($) shares in Thousands, $ 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	$ 1,661	$ 28,198	$ 3,323	$ 43,919
Operating expenses:
Research and development	15,657	48,262	47,636	86,032
General and administrative	8,552	10,489	21,711	25,185
Total operating expenses	24,209	58,751	69,347	111,217
Loss from operations	(22,548)	(30,553)	(66,024)	(67,298)
Interest income	344	446	722	892
Other loss	(1)	(98)	(22)	(68)
Loss before income tax provision (benefit)	(22,205)	(30,205)	(65,324)	(66,474)
Income tax provision (benefit)	(1)	(260)	2	(228)
Net loss	$ (22,204)	$ (29,945)	$ (65,326)	$ (66,246)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (0.62)	$ (0.85)	$ (1.82)	$ (1.88)
Weighted average common shares outstanding:
Basic and diluted (in shares)	35,858	35,392	35,831	35,324