Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
License revenue	$ 38	$ 38	$ 114	$ 114
Research and development:
Non-cash stock expense associated with in-licensing agreements	0	0	4,000	0
Noncash compensation	644	1,814	4,391	5,387
Other research and development	32,754	25,335	98,724	71,150
Total research and development	33,398	27,149	107,115	76,537
General and administrative:
Noncash compensation	(817)	3,076	7,037	6,988
Other general and administrative	1,785	1,398	6,212	4,266
Total general and administrative	968	4,474	13,249	11,254
Total costs and expenses	34,366	31,623	120,364	87,791
Operating loss	(34,328)	(31,585)	(120,250)	(87,677)
Other (income) expense:
Interest income	(258)	(79)	(591)	(174)
Other (income) expense	(119)	30	(37)	113
Total other income, net	(377)	(49)	(628)	(61)
Net loss	$ (33,951)	$ (31,536)	$ (119,622)	$ (87,616)
Basic and diluted net loss per common share	$ (0.43)	$ (0.48)	$ (1.61)	$ (1.45)
Weighted average shares used in computing basic and diluted net loss per common share	78,221,069	65,079,128	74,399,243	60,552,084