Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Collaboration revenue - related party	$ 3,594		$ 6,929
Operating expenses:
Research and development	15,513	$ 13,448	29,719	$ 33,113
General and administrative	6,766	5,526	12,781	10,776
Restructuring charges				1,944
Total operating expenses	22,279	18,974	42,500	45,833
Loss from operations	(18,685)	(18,974)	(35,571)	(45,833)
Other income (expense):
Interest (expense) income	(13)	(13)	(27)	151
Other income	3		3	1
Total other income (expense), net	(10)	(13)	(24)	152
Net loss	$ (18,695)	$ (18,987)	$ (35,595)	$ (45,681)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.17)	$ (0.39)	$ (0.35)	$ (0.95)
Weighted average shares of common stock outstanding, basic and diluted	112,298,982	48,163,094	100,850,878	48,111,186