Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Options (Details)

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Summary of Significant Accounting Policies - Options (Details) - £ / shares
 2 Months Ended 12 Months Ended
Feb. 28, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Number of options granted (in shares)   20,771,970 29,924,787 19,404,373
Nominal value of ordinary shares £ 0.001 £ 0.001 £ 0.001  
Exercise price determined by reference to the market value of an ADS at the date of grant        
Number of options granted (in shares) 10,807,200      
Exercise price equal to the nominal value of the ordinary shares        
Number of options granted (in shares) 6,498,126