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Stock-Based Compensation - Stock options (Details) - 2015 Plan - USD ($)
1 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2018
May 31, 2016
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 24, 2017
Dec. 29, 2015
Stock Based Compensation                
Vesting period       4 years        
Number of shares reserved for issuance under the plan             373,422 311,185
Number of Shares                
Outstanding at the beginning     2,056,891 506,586 189,189      
Granted     247,275 1,570,136 317,397      
Exercised     (53,963) (5,599)        
Forfeitures     (1,250) (14,232)        
Outstanding at the end 2,056,891   2,248,953 2,056,891 506,586 189,189    
Vested and expected to vest at the end 2,056,891   2,248,953 2,056,891        
Exercisable at the end 242,079   450,866 242,079        
Weighted Average Exercise Price                
Outstanding at the beginning (in dollars per share)     $ 4.74 $ 0.94 $ 0.48      
Granted (in dollars per share)     18.18 5.92 1.21      
Exercised (in dollars per share)     1.20 0.84        
Forfeitures (in dollars per share)     7.63 1.11        
Outstanding at the end (in dollars per share) $ 4.74   6.30 4.74 $ 0.94 $ 0.48    
Vested and expected to vest at the end (in dollars per share) 4.74   6.30 4.74        
Exercisable at the end (in dollars per share) $ 0.88   $ 1.15 $ 0.88        
Weighted Average Remaining Contractual Life (Years) And Aggregate Intrinsic Value                
Outstanding (in years) 9 years 4 months 24 days   9 years 1 month 6 days 9 years 4 months 24 days 9 years 8 years 7 months 6 days    
Granted (in years)     10 years 9 years 10 months 24 days        
Vested and expected to vest at the end (in years)       9 years 4 months 24 days        
Exercisable at the end (in years)       7 years 10 months 24 days        
Outstanding at the beginning (in dollars)     $ 5,946,000 $ 137,000 $ 137,000      
Granted (in dollars)       2,684,000        
Outstanding at the end (in dollars) $ 5,946,000   55,305,000 5,946,000 $ 137,000 $ 137,000    
Vested and expected to vest at the end (in dollars) 5,946,000   55,305,000 5,946,000        
Exercisable at the end (in dollars) $ 1,634,000   $ 12,335,000 $ 1,634,000        
Weighted average grant date fair value per share of stock options granted     $ 11.48 $ 3.80 $ 0.80      
Aggregate intrinsic value of stock options exercised     $ 1,600,000 $ 38,000 $ 0      
Option to purchase common stock                
Weighted Average Remaining Contractual Life (Years) And Aggregate Intrinsic Value                
Aggregate grant date fair value       $ 157,000        
Restricted Common Stock                
Stock Based Compensation                
Vesting period   4 years            
Weighted Average Remaining Contractual Life (Years) And Aggregate Intrinsic Value                
Aggregate grant date fair value         $ 29,000