Subsequent Events - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
Oct. 01, 2018
Sep. 30, 2018
Dec. 31, 2017
Redeemable Convertible Preferred Stock      
Subsequent Event [Line Items]      
Redeemable convertible preferred stock, shares outstanding   493,615,703 173,750,421
IPO | Subsequent Event | Redeemable Convertible Preferred Stock      
Subsequent Event [Line Items]      
Redeemable convertible preferred stock, shares outstanding 0    
IPO | Common Stock | Subsequent Event      
Subsequent Event [Line Items]      
Common stock price, per share $ 15.00    
Issuance of common stock, shares 5,667,000    
Gross proceeds from issuance of initial public offering $ 85.0    
Net proceeds from issuance of initial public offering $ 74.4    
IPO | Common Stock | Subsequent Event | Redeemable Convertible Preferred Stock      
Subsequent Event [Line Items]      
Convertible preferred stock shares issued upon conversion 16,028,462    
Warrant to purchase stock 71,812    
IPO | Warrants | Subsequent Event | Redeemable Convertible Preferred Stock      
Subsequent Event [Line Items]      
Convertible preferred stock shares issued upon conversion 2,370,799    
Private Placement | Merck | Common Stock | Subsequent Event      
Subsequent Event [Line Items]      
Common stock price, per share $ 15.00    
Issuance of common stock, shares 666,666    
Gross proceeds from issuance of private placement $ 10.0