Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - 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	$ 11	$ 844	$ 817	$ 42,081
Operating expenses:
Research and development	16,546	13,701	44,179	42,610
General and administrative	5,881	4,436	15,988	13,072
Total operating expenses	22,427	18,137	60,167	55,682
Other income (expense):
Interest income	19	608	414	1,785
Interest expense	(92)	(107)	(267)	(146)
Total other income (expense), net	(73)	501	147	1,639
Net loss	(22,489)	(16,792)	(59,203)	(11,962)
Other comprehensive loss
Unrealized gain (loss) on marketable securities	(2)	17	(25)	36
Comprehensive loss	(22,491)	(16,775)	(59,228)	(11,926)
Net income (loss) attributable to common stockholders - diluted	(22,489)	(16,792)	(59,203)	(11,962)
Net income (loss) attributable to common stockholders - basic	$ (22,489)	$ (16,792)	$ (59,203)	$ (11,962)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (0.33)	$ (0.35)	$ (1.00)	$ (0.28)
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders - basic (in shares)	68,419,192	47,833,607	59,086,202	42,011,340
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders - diluted (in shares)	68,419,192	47,833,607	59,086,202	42,011,340