Unaudited Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue	$ 0	$ 1,661	$ 0	$ 3,323
Operating expenses:
Research and development	8,516	15,657	16,202	47,636
General and administrative	6,332	8,552	11,977	21,711
Total operating expenses	14,848	24,209	28,179	69,347
Loss from operations	(14,848)	(22,548)	(28,179)	(66,024)
Interest income	602	344	1,075	722
Other expense	0	(1)	(16)	(22)
Loss before income tax provision (benefit)	(14,246)	(22,205)	(27,120)	(65,324)
Income tax provision (benefit)	(1,037)	(1)	(838)	2
Net loss	$ (13,209)	$ (22,204)	$ (26,282)	$ (65,326)
Net loss per common share:
Basic and diluted (in dollars per share)	$ (0.37)	$ (0.62)	$ (0.73)	$ (1.82)
Weighted average common shares outstanding:
Basic and diluted (in shares)	36,188	35,858	36,171	35,831