Convertible Preferred Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Jul. 20, 2018
Jul. 31, 2018
Feb. 28, 2018
Jun. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Convertible Preferred Stock              
Proceeds from issuance of convertible preferred stock         $ 100,986 $ 120,067 $ 11,437
Issuance cost         $ 3,548    
Preferred stock authorized           44,070,808  
Preferred stock issued           43,933,006  
Preferred stock outstanding         0 43,933,006  
Preferred stock carrying value           $ 139,790  
Preferred stock, liquidation preference           $ 138,242  
Common stock issuable upon conversion           43,933,006  
Series A redeemable convertible preferred stock              
Convertible Preferred Stock              
Percentage of cumulative dividends per year         8.00%    
Convertible preferred stock redemption period         5 years    
Maximum percentage of change in fair value of preferred stock         10.00%    
Preferred stock authorized           29,703,995  
Preferred stock issued           29,570,662  
Preferred stock outstanding           29,570,662  
Preferred stock carrying value           $ 19,723  
Preferred stock, liquidation preference           $ 17,742  
Common stock issuable upon conversion           29,570,662  
Series B convertible preferred stock              
Convertible Preferred Stock              
Convertible preferred stock issue price (dollars per share)       $ 8.39      
Proceeds from issuance of convertible preferred stock       $ 120,500      
Issuance cost       $ 400      
Preferred stock authorized           14,366,813  
Preferred stock issued       14,362,344   14,362,344  
Preferred stock outstanding           14,362,344  
Preferred stock carrying value           $ 120,067  
Preferred stock, liquidation preference           $ 120,500  
Common stock issuable upon conversion           14,362,344  
Series C convertible preferred stock              
Convertible Preferred Stock              
Convertible preferred stock issue price (dollars per share)     $ 12.79        
Proceeds from issuance of convertible preferred stock     $ 101,200        
Issuance cost     $ 200        
Preferred stock issued     7,912,432        
Common stock              
Convertible Preferred Stock              
Number of common stock issued on conversion 51,845,438 51,845,438     51,845,438