Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration revenue	$ 1,020	$ 2,043	$ 7,211	$ 5,929
Operating expenses:
Research and development	10,335	10,252	33,355	27,520
General and administrative	2,721	2,367	8,274	5,410
Total operating expenses	13,056	12,619	41,629	32,930
Loss from operations	(12,036)	(10,576)	(34,418)	(27,001)
Other income (expense):
Interest income	31	405	206	745
Other income, net	82	3	82	330
Total other income, net	113	408	288	1,075
Net loss	(11,923)	(10,168)	(34,130)	(25,926)
Accretion of redeemable convertible preferred stock to redemption value				(16)
Net loss attributable to common stockholders	$ (11,923)	$ (10,168)	$ (34,130)	$ (25,942)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.39)	$ (0.34)	$ (1.12)	$ (1.12)
Weighted average common shares outstanding, basic and diluted	30,661,125	29,879,476	30,418,752	23,169,348
Comprehensive loss:
Net loss	$ (11,923)	$ (10,168)	$ (34,130)	$ (25,926)
Other comprehensive income (loss):
Unrealized gains (losses) on marketable securities, net of tax of $0		(11)	12	(8)
Total other comprehensive income (loss)		(11)	12	(8)
Comprehensive loss	$ (11,923)	$ (10,179)	$ (34,118)	$ (25,934)