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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, 2021
Dec. 31, 2020
Employees    
Number of Options    
Beginning balance (in shares) 7,302,176  
Granted (in shares) 2,628,000  
Exercised (in shares) (153,278)  
Forfeited (in shares) (62,577)  
Ending balance (in shares) 9,714,321 7,302,176
Vested (in shares) 3,064,130  
Vested and expected to vest (in shares) 8,920,548  
Weighted-average exercise price    
Beginning balance (in dollars per share) $ 3.98  
Granted (in dollars per share) 3.13  
Exercised (in dollars per share) 1.47  
Forfeited (in dollars per share) 5.19  
Ending balance (in dollars per share) 3.78 $ 3.98
Vested (in dollars per share) 5.46  
Vested and expected to vest (in dollars per share) $ 3.88  
Weighted-average remaining contractual term    
Outstanding, term 8 years 7 months 24 days 8 years 5 months 4 days
Vested, term 7 years 7 months 17 days  
Vested and expected to vest, term 8 years 7 months 2 days  
Aggregate intrinsic value    
Outstanding $ 15,665 $ 4,456
Vested 4,313  
Vested and expected to vest $ 14,231  
Non-employee consultants    
Number of Options    
Beginning balance (in shares) 473,073  
Granted (in shares) 0  
Exercised (in shares) 0  
Forfeited (in shares) 0  
Ending balance (in shares) 473,073 473,073
Vested (in shares) 462,186  
Vested and expected to vest (in shares) 473,073  
Weighted-average exercise price    
Beginning balance (in dollars per share) $ 5.89  
Granted (in dollars per share) 0  
Exercised (in dollars per share) 0  
Forfeited (in dollars per share) 0  
Ending balance (in dollars per share) 5.89 $ 5.89
Vested (in dollars per share) 5.82  
Vested and expected to vest (in dollars per share) $ 5.89  
Weighted-average remaining contractual term    
Outstanding, term 4 years 11 months 23 days 5 years 2 months 23 days
Vested, term 4 years 11 months 1 day  
Vested and expected to vest, term 4 years 11 months 23 days  
Aggregate intrinsic value    
Outstanding $ 366 $ 86
Vested 366  
Vested and expected to vest $ 366