Stockholders' Equity - Summary of Option Activity under 2018 Plan, 2013 Plan and 2010 Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Options
Number of Shares, Options outstanding at beginning of period	1,240,354
Number of Shares, Options Granted	687,500
Number of Shares, Cancelled/forfeited	(18,879)
Number of Shares, Options outstanding at end of period	1,908,975	1,240,354
Number of Shares, Options exercisable at end of period	858,426
Weighted Average Exercise Price, Options
Weighted Average Exercise Price, Options outstanding at beginning of period	$ 11.16
Weighted Average Exercise Price, Options Granted	3.48
Weighted Average Exercise Price, Cancelled/forfeited	20.37
Weighted Average Exercise Price, Options outstanding at end of period	8.30	$ 11.16
Weighted Average Exercise Price, Options exercisable at end of period	$ 11.70
Weighted Average Remaining Contract Term, Options
Weighted Average Remaining Contractual Term, Options outstanding at beginning of period	8 years	7 years 9 months 18 days
Weighted Average Remaining Contractual Term, Options exercisable at end of period	6 years 10 months 24 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value, Options outstanding at end of period	$ 12
Aggregate Intrinsic Value, Options exercisable at end of period	$ 1