Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
License revenue	$ 38	$ 38		$ 76	$ 76
Research and development:
Non-cash stock expense associated with in–licensing agreements	100	3,000		100	4,000
Noncash compensation	1,352	888		2,841	3,747
Other research and development	31,414		$ 34,812	62,310	65,971
Total research and development	32,866	38,700		65,251	73,718
General and administrative:
Noncash compensation	405	3,375		797	7,854
Other general and administrative	2,311	2,308		4,260	4,426
Total general and administrative	2,716	5,683		5,057	12,280
Total costs and expenses	35,582	44,383		70,308	85,998
Other (income) expense:
Other income	(408)	(426)		(715)	(687)
Total other (income) expense, net	(669)	(203)		(1,136)	(251)
Net loss	36,213	44,142	$ (41,529)	71,368	85,671
Interest expense	1,077	223		1,851	436
Other income	408	426		715	687
Total other (income) expense, net	669	203		1,136	251
Operating loss	$ 35,544	$ 44,345		$ 70,232	$ 85,922
Basic and diluted net loss per common share	$ 0.42	$ 0.59		$ 0.85	$ 1.18
Weighted average shares used in computing basic and diluted net loss per common share	86,800,017	74,256,348		84,002,700	72,456,657