Stock-Based Compensation - Restricted Common Stock (Details)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Jun. 21, 2018
USD ($)
Oct. 31, 2018
$ / shares
May 31, 2017
USD ($)
item
$ / shares
shares
Apr. 30, 2017
USD ($)
$ / shares
shares
Jan. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Executive officer                
Weighted Average Grant Date Fair Value                
Restricted stock exercise price | $ / shares       $ 0.18        
Principal balance of promissory note repaid | $ $ 200              
Interest on promissory note repaid | $ 100              
Restricted Common Stock                
Number of shares                
Unvested restricted common stock beginning of period           1,559,401    
Vested           (1,258,837)    
Forfeited           (201,250)    
Unvested restricted common stock end of period           99,314 1,559,401  
Weighted Average Grant Date Fair Value                
Outstanding at the ending (in dollars per share) | $ / shares           $ 0.502    
Weighted Average Grant Date Fair Value- vested (in dollars per share) | $ / shares           0.519    
Weighted Average Grant Date Fair Value- Forfeited (in dollars per share) | $ / shares           0.387    
Ending at the beginning (in dollars per share) | $ / shares           $ 0.526 $ 0.502  
Number of shares exercised               1,400,000
Repurchase of unvested restricted common stock (in shares)           466,667    
Amount recorded in balance sheet due to promissory notes received from sale of common stock | $     $ 0 $ 0 $ 0      
Principal amount of promissory note forgiven | $ 2,500              
Unvested restricted common stock             670,834  
The aggregate intrinsic value of restricted stock awards vested | $           $ 16,300 $ 34,000 $ 900
Incremental share based compensation expense | $             $ 1,500  
Restricted Common Stock | Executive officer                
Number of shares                
Unvested restricted common stock beginning of period     460,000          
Unvested restricted common stock end of period       460,000        
Weighted Average Grant Date Fair Value                
Restricted common stock exercise price | $ / shares       $ 0.19        
Principal amount of promissory note received in exchange for restricted common stock | $       $ 100        
Principal balance of promissory note repaid | $ 100              
Interest on promissory note repaid | $ 100              
Interest on promissory note forgiven | $ $ 100              
Restricted Common Stock | Board of Directors Chairman                
Number of shares                
Unvested restricted common stock end of period     1,100,000   3,667,014      
Weighted Average Grant Date Fair Value                
Restricted common stock exercise price | $ / shares     $ 1.65   $ 0.19      
Principal amount of promissory note received in exchange for restricted common stock | $     $ 2,500          
Number of promissory notes | item     2          
Market-Based Stock Options                
Weighted Average Grant Date Fair Value                
Weighted average grant-date fair value (per share) | $ / shares             $ 11.88  
Market-Based Stock Options | Vesting based on service period | Executive officer                
Weighted Average Grant Date Fair Value                
Restricted stock exercise price | $ / shares   $ 16.43            
Restricted Stock Units                
Number of shares                
Issued           277,200    
Vested           0    
Forfeited           0    
Unvested restricted common stock end of period           277,200    
Weighted Average Grant Date Fair Value                
Weighted Average Grant Date Fair Value- issued (in dollars per share) | $ / shares           $ 10.660    
Ending at the beginning (in dollars per share) | $ / shares           $ 10.660    
Vesting period           3 years