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
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2018
shares
Dec. 31, 2017
shares
May 31, 2017
USD ($)
shares
Apr. 30, 2017
USD ($)
shares
Jan. 31, 2017
USD ($)
shares
Weighted Average Grant Date Fair Value                          
The aggregate intrinsic value of restricted stock awards vested | $           $ 34,000 $ 900 $ 100          
Executive officer                          
Weighted Average Grant Date Fair Value                          
Exercised       1,400,000                  
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           5,227,014              
Vested           (3,667,613)              
Unvested restricted common stock end of period           1,559,401 5,227,014            
Weighted Average Grant Date Fair Value                          
Outstanding at the ending (in dollars per share) | $ / shares           $ 0.514              
Weighted Average Grant Date Fair Value- vested (in dollars per share) | $ / shares           0.519              
Ending at the beginning (in dollars per share) | $ / shares           $ 0.502 $ 0.514            
Total unvested restricted common stock including early exercise of stock options           670,834              
Shares issued           5,227,014 5,227,014   1,559,401 5,227,014      
Number of shares exercised             1,400,000            
Unvested shares of restricted common stock, including shares from the early exercise of a stock option                 2,230,235        
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                        
Interest on promissory note forgiven | $ 100                        
Unvested restricted common stock                 670,834 1,050,000      
Shares exchanged           5,227,014 5,227,014   1,559,401 5,227,014      
Common stock Award                 1,559,401        
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                          
Shares issued     460,000 460,000               460,000  
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                        
Shares exchanged     460,000 460,000               460,000  
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                          
Shares issued     1,100,000   3,667,014           1,100,000   3,667,014
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                    
Shares exchanged     1,100,000   3,667,014           1,100,000   3,667,014
Market-Based Stock Options                          
Weighted Average Grant Date Fair Value                          
Weighted average grant-date fair value (per share) | $ / shares           $ 11.88              
Exercised           0              
Forfeited           0              
Market-Based Stock Options | Vesting based on service period | Executive officer                          
Weighted Average Grant Date Fair Value                          
Restricted stock exercise price | $ / shares   $ 16.43