Annual report pursuant to Section 13 and 15(d)

Stockholders' equity - Initial Public Offering (Details)

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Stockholders' equity - Initial Public Offering (Details)
$ in Thousands
 12 Months Ended
May 11, 2015
GBP (£)
shares
May 11, 2015
USD ($)
shares
Dec. 31, 2017
USD ($)
shares
Jun. 30, 2015
USD ($)
shares
Stockholders' equity        
Issuance of common stock     $ 103,167  
Proceeds from issuance of common stock       $ 175,989
IPO        
Stockholders' equity        
Issuance of shares represented by American Depositary Shares (in ADSs) | shares 11,250,000 11,250,000    
Issuance of common stock       175,989
Proceeds from issuance of common stock   $ 175,989    
Common stock        
Stockholders' equity        
Issuance of common stock (in shares) | shares     136,201,338  
Issuance of common stock     $ 170  
Stock issuance costs   $ 13,387 $ 4,774 $ 13,387
Common stock | IPO        
Stockholders' equity        
Issuance of common stock (in shares) | shares 67,500,000 67,500,000   67,500,000
Issuance of common stock £ 67,500 $ 104   $ 104