Stock Incentive Plans - Summary of Options Activity (Details) - 2008 Plan, 2016 Plan, and 2018 Inducement Incentive Award Plan - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021
Employees
Number of Options
Beginning balance (in shares)	10,616,800
Granted (in shares)	4,476,500
Exercised (in shares)	(10,000)
Ending balance (in shares)	15,083,300	10,616,800
Vested (in shares)	5,602,046
Vested and expected to vest (in shares)	13,738,523
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 3.99
Granted (in dollars per share)	3.26
Exercised (in dollars per share)	2.10
Ending balance (in dollars per share)	3.78	$ 3.99
Vested (in dollars per share)	4.46
Vested and expected to vest (in dollars per share)	$ 3.82
Weighted-average remaining contractual term
Outstanding, term	8 years 5 months 26 days	8 years 2 months 8 days
Vested, term	7 years 3 months 18 days
Vested and expected to vest, term	8 years 4 months 24 days
Aggregate intrinsic value
Outstanding	$ 0	$ 4,982
Vested	0
Vested and expected to vest	$ 0
Non-employee consultants
Number of Options
Beginning balance (in shares)	423,073
Forfeited (in shares)	(156,834)
Ending balance (in shares)	266,239	423,073
Vested (in shares)	266,239
Vested and expected to vest (in shares)	266,239
Weighted-average exercise price
Beginning balance (in dollars per share)	$ 6.34
Forfeited (in dollars per share)	3.44
Ending balance (in dollars per share)	8.05	$ 6.34
Vested (in dollars per share)	8.05
Vested and expected to vest (in dollars per share)	$ 8.05
Weighted-average remaining contractual term
Outstanding, term	5 years 9 months 29 days	3 years 10 months 6 days
Vested, term	5 years 9 months 29 days
Vested and expected to vest, term	5 years 9 months 29 days
Aggregate intrinsic value
Outstanding	$ 0	$ 42
Vested	0
Vested and expected to vest	$ 0