Annual report pursuant to Section 13 and 15(d)

Stockholders' equity - Underwritten public offering and Registered direct offering (Details)

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Stockholders' equity - Underwritten public offering and Registered direct offering (Details) - USD ($)
12 Months Ended
Sep. 07, 2018
Dec. 31, 2019
Dec. 31, 2018
Stockholders' equity      
Proceeds from issuance of common stock     $ 99,653,000
Common stock      
Stockholders' equity      
Stock offering expenses   $ 0 $ 347,000
Matrix Capital Management Company L P, New Enterprise Associates 16, L.P. New Enterprise Associates 14, L.P. and Syncona Portfolio Limited      
Stockholders' equity      
Issuance of shares represented by American Depositary Shares (in ADSs) 10,000,000    
Share price per ADS (in dollars per share) $ 10.00    
Matrix Capital Management Company L P, New Enterprise Associates 16, L.P. New Enterprise Associates 14, L.P. and Syncona Portfolio Limited | Common stock      
Stockholders' equity      
Issuance of common stock (in shares) 60,000,000    
Proceeds from issuance of common stock $ 99,653,000    
Stock offering expenses $ 347,000