Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Collaboration revenue	$ 1,848		$ 4,598
Operating expenses:
Research and development	15,640	$ 13,496	42,897	$ 58,985
General and administrative	5,312	3,510	15,525	8,742
Total operating expenses	20,952	17,006	58,422	67,727
Loss from operations	(19,104)	(17,006)	(53,824)	(67,727)
Other income, net	142	464	725	1,173
Net loss	(18,962)	(16,542)	(53,099)	(66,554)
Cumulative convertible preferred stock dividends		(796)		(7,128)
Net loss attributable to common stockholders	$ (18,962)	$ (17,338)	$ (53,099)	$ (73,682)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.70)	$ (0.97)	$ (2.16)	$ (10.33)
Weighted average number of common shares used in net loss per share attributable to common stockholders, basic and diluted	27,261	17,785	24,621	7,133
Comprehensive loss:
Net loss	$ (18,962)	$ (16,542)	$ (53,099)	$ (66,554)
Other comprehensive (loss) income:
Unrealized (loss) gain on marketable securities	(50)		31
Total other comprehensive (loss) income	(50)		31
Comprehensive loss	$ (19,012)	$ (16,542)	$ (53,068)	$ (66,554)