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Equity Incentive Plans - Stock options (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Oct. 31, 2019
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Sep. 30, 2020
Dec. 31, 2019
Options                
Outstanding balance at the beginning of the period (in shares)       4,024,566        
Granted (in shares)       2,547,500        
Exercised (in shares)       (32,778)        
Forfeited (in shares)       (260,435)        
Outstanding balance at the end of the period (in shares)   6,278,853   6,278,853   4,024,566    
Options exercisable at the end of the period (in shares)             3,113,663  
Options vested and expected to vest at end of period (in shares)             6,278,853  
Weighted Average Exercise Price Per Share                
Outstanding balance at the beginning of the period (in dollars per share)       $ 7.48        
Granted (in dollars per share)       2.12        
Exercised (in dollars per share)       1.53        
Forfeited (in dollars per share)       6.95        
Outstanding balance at the end of the period (in dollars per share)   $ 5.35   $ 5.35   $ 7.48    
Exercisable at the end of the period (in dollars per share)             $ 8.45  
Options vested and expected to vest at end of the period (in dollars per share)             $ 5.35  
Weighted Average Remaining Life in Years                
Outstanding balance       8 years 2 months 5 days   7 years 11 months 23 days    
Options exercisable at end of period       7 years 1 month 13 days        
Options vested and expected to vest at end of the period       8 years 2 months 5 days        
Aggregate Intrinsic Value                
Outstanding balance             $ 1,082 $ 2
Options exercisable at the end of the period             359  
Options vested and expected to vest at end of the period             1,082  
Stock-based compensation expense   $ 591 $ 950 $ 1,687 $ 2,796      
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       0.00%   0.00%    
Expected term (in years)       6 years 10 months 6 days   6 years 9 months 7 days    
Expiration period       10 years        
Compensation cost related to the non-vested awards not yet recognized             $ 4,453  
Weighted average period for recognition of compensation cost       2 years 2 months 16 days        
Research and development                
Aggregate Intrinsic Value                
Stock-based compensation expense   282 181 $ 720 536      
General and administrative                
Aggregate Intrinsic Value                
Stock-based compensation expense   $ 309 $ 769 $ 967 $ 2,260      
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        
Restricted Stock Units (RSUs)                
Assumptions used to determine grant-date fair value                
Weighted average period for recognition of compensation cost       2 years 2 months 1 day        
Restricted Stock Units (RSUs) | Executive Officer [Member]                
Aggregate Intrinsic Value                
Stock-based compensation expense       $ 170        
Assumptions used to determine grant-date fair value                
Vesting period       3 years        
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]                
Awarded       660,606        
Settled in cash 90,500              
Restricted Stock Units (RSUs) | 2016 Plan                
Assumptions used to determine grant-date fair value                
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value   $ 1.49   $ 1.49   $ 1.74 $ 1.49 $ 1.74
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]                
Outstanding at the beginning       90,500        
Awarded       753,106        
Settled in cash       (90,500)        
Outstanding at the end   753,106   753,106   90,500    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]                
Outstanding at beginning( in dollars per share)       $ 1.74        
Awarded (in dollars per share)       1.49        
Settled in cash(in dollars per share)       1.74        
Outstanding at end(in dollars per share)   $ 1.49   $ 1.49   $ 1.74