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Stockholders' Equity - Summary of Stock Option Activity Under 2018 Plan and 2006 Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
2018 Employee Stock Purchase Plan      
Weighted- Average Exercise Price      
Vested and exercisable (USD per share) $ 2.16 $ 6.99  
Weighted- Average Remaining Contractual Term      
Vested and expected to vest 10 years    
2018 Employee Stock Purchase Plan | 2018 Plan and 2006 Plan      
Shares of Stock under Stock Options      
Beginning balance (shares) 1,282,847 436,341  
Granted (shares) 716,960 886,023  
Exercised (shares) (50,665) (2,062)  
Canceled (shares) (206,230) (37,455)  
Ending balance (shares) 1,742,912 1,282,847 436,341
Vested and expected to vest (shares) 1,709,588    
Vested and exercisable (shares) 817,026    
Weighted- Average Exercise Price      
Beginning balance (USD per share) $ 6.90 $ 5.14  
Granted (USD per share) 3.78 7.69  
Exercised (USD per share) 1.30 1.70  
Canceled (USD per share) 7.20 5.20  
Ending balance (USD per share) 5.73 $ 6.90 $ 5.14
Vested and expected to vest (USD per share) 5.81    
Vested and exercisable (USD per share) $ 6.33    
Weighted- Average Remaining Contractual Term      
Outstanding 8 years 2 months 12 days 9 years 2 months 12 days 9 years
Vested and expected to vest 8 years 2 months 12 days    
Vested and exercisable 7 years 4 months 24 days    
Aggregate Intrinsic Value      
Beginning balance  
Exercised 151,819 6,046  
Ending balance 4,356
Vested and expected to vest 4,356    
Vested and exercisable $ 0    
Share-based Payment Arrangement, Option      
Aggregate Intrinsic Value      
Vesting period 4 years