Stock-Based Compensation - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Options
Outstanding balance at the beginning of the period (in shares)	4,024,566	2,761,912
Granted (in shares)	2,662,500	1,481,975
Exercised (in shares)	(32,778)
Forfeited (in shares)	(260,435)	(219,321)
Outstanding balance at the end of the period (in shares)	6,393,853	4,024,566	2,761,912
Options exercisable at the end of the period (in shares)	3,482,169
Options vested and expected to vest at end of period (in shares)	6,393,853
Weighted Average Exercise Price Per Share
Outstanding balance at the beginning of the period (in dollars per share)	$ 7.48	$ 11.30
Granted (in dollars per share)	2.10	1.48
Exercised (in dollars per share)	1.53
Forfeited (in dollars per share)	6.95	15.14
Outstanding balance at the end of the period (in dollars per share)	5.29	$ 7.48	$ 11.30
Exercisable at the end of the period (in dollars per share)	7.83
Options vested and expected to vest at end of the period (in dollars per share)	$ 5.29
Weighted Average Remaining Life in Years
Outstanding balance	7 years 11 months 16 days	7 years 11 months 23 days	8 years 4 days
Options exercisable at end of period	7 years 26 days
Options vested and expected to vest at end of the period	7 years 11 months 16 days
Aggregate Intrinsic Value
Outstanding balance	$ 1,961	$ 2
Options exercisable at the end of the period	722
Options vested and expected to vest at end of the period	1,961
Stock-based compensation expense	2,572	3,732
Stock-based compensation	$ 2,324	$ 3,732
Assumptions used to determine grant-date fair value
Expected Volatility	66.94%	66.94%
Weighted average risk-free interest rate	0.66%	2.07%
Expected dividend yield (as a percentage)	0.00%	0.00%
Expected term (in years)	6 years 10 months 6 days	6 years 9 months 7 days
Expiration period	10 years
Unrecognized costs	$ 3,947
Research and development
Aggregate Intrinsic Value
Stock-based compensation expense	1,028	$ 714
General and administrative
Aggregate Intrinsic Value
Stock-based compensation expense	$ 1,296	$ 3,018
Weighted average
Assumptions used to determine grant-date fair value
Weighted average period for recognition of compensation cost	2 years 29 days
Minimum
Assumptions used to determine grant-date fair value
Vesting period	3 years
Maximum
Assumptions used to determine grant-date fair value
Vesting period	4 years