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Share capital - Ordinary share capital (Details)
$ in Millions
 1 Months Ended 12 Months Ended
Jun. 30, 2015
$ / £
shares
Mar. 31, 2015
$ / £
shares
Feb. 28, 2015
$ / £
shares
Aug. 31, 2014
$ / £
shares
May 30, 2014
shares
Jan. 31, 2014
$ / £
shares
Mar. 31, 2013
$ / £
shares
Oct. 31, 2012
$ / £
shares
Feb. 28, 2007
$ / £
shares
Dec. 31, 2017
USD ($)
£ / $
shares
Dec. 31, 2017
£ / shares
Dec. 31, 2016
USD ($)
shares
Dec. 31, 2016
£ / shares
Dec. 31, 2015
£ / shares
May 31, 2014
$ / £
Disclosure of classes of share capital [line items]                              
Number of shares authorized                   769,463,688,000   769,463,688,000      
Number of shares alloted, called up and fully paid                   769,440,844,799   769,440,844,799      
Shares authorized, value | $                   $ 186.6   $ 186.6      
Shares alloted, called up and fully paid, value | $                   $ 176.2   $ 176.2      
Closing foreign exchange rate 1.5715 1.4847 1.5436 1.6580   1.6487 1.5173 1.6129 1.9613 0.7401         1.6760
Number of shares issued 27,679 3,866 8,563 58,399 24,292 12,076 1,924 7,000,000 20,000,000            
Par value per share (in GBP per share) | £ / shares                     £ 0.50   £ 0.5 £ 0.5  
Number of shares outstanding                   25,929,312          
Ordinary shares                              
Disclosure of classes of share capital [line items]                              
Number of shares authorized                   40,000,000   40,000,000      
Number of shares alloted, called up and fully paid                   27,136,799   27,136,799      
Shares authorized, value | $                   $ 35.7   $ 35.7      
Shares alloted, called up and fully paid, value | $                   $ 25.3   $ 25.3      
Par value per share (in GBP per share) | £ / shares                     0.50   £ 0.50    
Deferred shares                              
Disclosure of classes of share capital [line items]                              
Number of shares authorized                   769,423,688,000   769,423,688,000      
Number of shares alloted, called up and fully paid                   769,413,708,000   769,413,708,000      
Shares authorized, value | $                   $ 150.9   $ 150.9      
Shares alloted, called up and fully paid, value | $                   $ 150.9   $ 150.9      
Par value per share (in GBP per share) | £ / shares                     £ 0.0001        
Multiple of amount paid up on ordinary shares                   100