Redeemable Convertible Preferred Stock - Schedule of Warrants Outstanding and Exercisable (Details) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Class Of Warrant Or Right [Line Items]    
Shares 3,603,019 3,331,972
Estimated fair value $ 867 $ 1,708
Series B Redeemable Convertible Preferred Stock    
Class Of Warrant Or Right [Line Items]    
Expiration date 2018-06 2018-06
Exercise price per share $ 0.8822 $ 0.8822
Shares   170,030
Estimated fair value $ 0 $ 116
Series C Redeemable Convertible Preferred Stock    
Class Of Warrant Or Right [Line Items]    
Exercise price per share [1] $ 0.4797 $ 0.4797
Shares [1] 1,920,148 2,479,712
Estimated fair value [1] $ 336 $ 1,263
Series C Redeemable Convertible Preferred Stock | Minimum    
Class Of Warrant Or Right [Line Items]    
Expiration date [1] 2020-07  
Series C Redeemable Convertible Preferred Stock | Maximum    
Class Of Warrant Or Right [Line Items]    
Expiration date [1] 2021-11  
Series D-2 Redeemable Convertible Preferred Stock    
Class Of Warrant Or Right [Line Items]    
Expiration date 2027-08 2027-08
Exercise price per share $ 0.6596 $ 0.6596
Shares   682,230
Estimated fair value $ 0 $ 329
Series E Redeemable Convertible Preferred Stock    
Class Of Warrant Or Right [Line Items]    
Expiration date 2027-08 2027-08
Exercise price per share $ 0.2674 $ 0.2674
Shares 1,682,871 0
Estimated fair value $ 531  
[1] On October 1, 2018, 1,232,220 shares of the Series C redeemable convertible preferred warrants will be canceled, and the remaining 687,928 shares will be converted to warrants to purchase common stock on a 1-for-0.0370 basis. All Series E redeemable convertible preferred warrants will be converted to warrants to purchase common stock at a on a 1-for-0.0275 basis.