Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
License revenue	$ 152	$ 152	$ 152
Research and development:
Non-cash stock expense associated with in licensing agreements	100	4,000	0
Noncash compensation	5,811	5,598	5,647
Other research and development	148,269	149,793	96,886
Total research and development	154,180	159,391	102,533
General and administrative:
Noncash compensation	5,523	7,288	10,298
Other general and administrative	9,504	7,873	6,033
Total general and administrative	15,027	15,161	16,331
Total costs and expenses	169,207	174,552	118,864
Operating loss	(169,055)	(174,400)	(118,712)
Other (income) expense:
Interest expense	5,287	877	845
Other income	(1,471)	(1,795)	(1,081)
Total other (income) expense, net	3,816	(918)	(236)
Net loss	$ (172,871)	$ (173,482)	$ (118,476)
Basic and diluted net loss per common share (in dollars per share)	$ (1.96)	$ (2.30)	$ (1.91)
Weighted average shares used in computing basic and diluted net loss per common share (in shares)	88,368,844	75,466,813	62,069,570