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Nature of the Business (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Jan. 27, 2020
Apr. 15, 2019
Jun. 22, 2018
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Class of Stock                
Net loss       $ (37,315) $ (27,248) $ (99,233) $ (82,905)  
Accumulated deficit       336,383   336,383   $ 237,150
Cash       $ 177,695 $ 229,366 $ 177,695 $ 229,366 $ 210,643
Ordinary Shares | IPO                
Class of Stock                
Number of shares issued in transaction (in shares)     10,147,059          
Per share price of issuance (usd per share)     $ 17.00          
Proceeds of issuance of ordinary shares, net of issuance costs     $ 156,500          
Ordinary Shares | Options                
Class of Stock                
Number of shares issued in transaction (in shares)   630,000 1,323,529          
Ordinary Shares | Underwriters public offering                
Class of Stock                
Number of shares issued in transaction (in shares) 7,250,000 4,830,000            
Per share price of issuance (usd per share) $ 11.00 $ 24.00            
Proceeds of issuance of ordinary shares, net of issuance costs $ 74,000 $ 108,800