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Related-Party Transactions - Additional Information (Details)
$ in Thousands
1 Months Ended 9 Months Ended 12 Months Ended
Apr. 04, 2018
USD ($)
Apr. 30, 2018
USD ($)
Jan. 31, 2018
USD ($)
shares
Oct. 31, 2017
USD ($)
Promissory
shares
Jul. 31, 2016
USD ($)
shares
Apr. 30, 2016
USD ($)
shares
Dec. 31, 2015
USD ($)
shares
Sep. 30, 2019
USD ($)
PromissoryNote
Executive
Sep. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Related Party Transaction [Line Items]                    
Number of promissory notes | PromissoryNote               3    
Number of executive officers | Executive               2    
Proceeds from convertible preferred stock                 $ 59,899  
Executive Officer                    
Related Party Transaction [Line Items]                    
Due from related parties       $ 2,100            
Restricted stock purchased by related party, shares | shares       625,084            
Forgiveness of promissory note accounted as modification of share based payment                   $ 1,500
Stockholders | Financing Activities                    
Related Party Transaction [Line Items]                    
Proceeds from convertible preferred stock               $ 3,000    
Promissory Note | Executive Officer                    
Related Party Transaction [Line Items]                    
Number of promissory notes | Promissory       2            
Due from related parties     $ 400   $ 200 $ 200 $ 200      
Interest rate     2.50%   2.50% 2.50% 2.50%      
Promissory Note | Common Stock | Executive Officer                    
Related Party Transaction [Line Items]                    
Early exercise options in aggregate | shares     114,406   667,253 667,253 667,253      
Full-recourse Notes | Executive Officer                    
Related Party Transaction [Line Items]                    
Repayment of promissory notes $ 200                  
Promissory Note One | Executive Officer                    
Related Party Transaction [Line Items]                    
Due from related parties       $ 1,600            
Interest rate       1.85%            
Promissory Note Two | Executive Officer                    
Related Party Transaction [Line Items]                    
Due from related parties       $ 500            
Interest rate       1.85%            
Non-Recourse Promissory Note | Executive Officer                    
Related Party Transaction [Line Items]                    
Outstanding principal and accrued interest, forgiveness   $ 1,600                
Repayment of promissory notes $ 500