Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
License revenue	$ 152	$ 152	$ 152
Research and development:
Non cash stock expense associated with in-licensing agreements	4,000	0	0
Noncash compensation	5,598	5,647	2,742
Other research and development	149,793	96,886	66,490
Total research and development	159,391	102,533	69,232
General and administrative:
Noncash compensation	7,288	10,298	4,767
Other general and administrative	7,873	6,033	5,122
Total general and administrative	15,161	16,331	9,889
Total costs and expenses	174,552	118,864	79,121
Operating loss	(174,400)	(118,712)	(78,969)
Other (income) expense:
Interest income	(857)	(295)	(323)
Other (income) expense	(61)	59	(393)
Total other income, net	(918)	(236)	(716)
Net loss	$ (173,482)	$ (118,476)	$ (78,253)
Basic and diluted net loss per common share (in dollars per share)	$ (2.30)	$ (1.91)	$ (1.60)
Weighted average shares used in computing basic and diluted net loss per common share (in shares)	75,466,813	62,069,570	49,041,354