Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Collaboration revenue	$ 26,678	$ 129,160	$ 0
Operating expenses:
Research and development expense	193,382	143,183	74,460
General and administrative	46,480	32,349	15,680
Total operating expenses	239,862	175,532	90,140
Loss from operations	(213,184)	(46,372)	(90,140)
Interest and other income, net	15,219	10,132	1,955
Loss before income taxes	(197,965)	(36,240)	(88,185)
Income tax benefit	351	0	0
Net loss	(197,614)	(36,240)	(88,185)
Other comprehensive income (loss):
Net unrealized gain (loss) on marketable securities, net of tax	999	(281)	5
Comprehensive loss	$ (196,615)	$ (36,521)	$ (88,180)
Net loss per share, basic and diluted (usd per share)	$ (2.07)	$ (0.39)	$ (5.89)
Weighted average number of shares outstanding, basic and diluted (in shares)	95,608,208	92,621,991	14,964,144