Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Data
Collaboration revenue	$ 10	$ 11	$ 796	$ 11	$ 42	$ 844	$ 202	$ 41,035	$ 828	$ 42,123	$ 10,594
Operating expenses:
Research and development	22,858	16,546	15,413	12,219	12,430	13,701	13,766	15,143	67,036	55,040	59,915
General and administrative	5,914	5,881	5,171	4,936	4,212	4,436	4,192	4,443	21,902	17,283	16,334
Total operating expenses	28,772	22,427	20,584	17,155	16,642	18,137	17,958	19,586	88,938	72,323	76,249
Other income:
Interest income	10	19	89	306	441	608	725	452	424	2,226	1,398
Interest expense	(92)	(92)	(87)	(88)	(87)	(107)	(40)	0	(359)	(234)	0
Total other income (expense), net	(82)	(73)	2	218	354	501	685	452	65	1,992	1,398
Net loss	$ (28,844)	$ (22,489)	$ (19,786)	$ (16,926)	$ (16,246)	$ (16,792)	$ (17,071)	$ 21,901	$ (88,045)	$ (28,208)	$ (64,257)
Net income (loss) per share - basic (in dollars per share)	$ (0.42)	$ (0.33)	$ (0.33)	$ (0.35)	$ (0.34)	$ (0.35)	$ (0.36)	$ 0.72
Net income (loss) per share - diluted (in dollars per share)	$ (0.42)	$ (0.33)	$ (0.33)	$ (0.35)	$ (0.34)	$ (0.35)	$ (0.36)	$ 0.70
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders — basic (in shares)					47,886,144	47,833,607	47,708,085	30,299,650
Weighted-average number of shares of common stock used in net income (loss) per share attributable to common stockholders — diluted (in shares)					47,886,144	47,833,607	47,708,085	31,461,696
Weighted-average number of common shares used in net loss per share attributable to common stockholders — basic and diluted (in shares)	68,630,078	68,419,192	60,748,225	47,988,630					61,485,205	43,492,113	23,032,250