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Share capital - Own shares held by ESOP (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
£ / shares
shares
Dec. 31, 2016
£ / shares
shares
Dec. 31, 2015
£ / shares
Disclosure of classes of share capital [line items]            
Beginning balance $ 141.9 $ 169.7 $ 175.4      
Purchase of shares from ESOP   (1.0) 0.1      
Exchange movement on ESOP     0.1      
Ending balance 162.3 141.9 169.7      
Number of treasury shares held | shares       527,616 665,424  
Par value per share (in GBP per share) | £ / shares       £ 0.50 £ 0.5 £ 0.5
Ordinary shares            
Disclosure of classes of share capital [line items]            
Par value per share (in GBP per share) | £ / shares       £ 0.50 £ 0.50  
Own shares held by ESOP            
Disclosure of classes of share capital [line items]            
Beginning balance (0.5) (0.2) (0.4)      
Purchase of shares from ESOP (0.8) (1.0) 0.1      
Exchange movement on ESOP   0.7 0.1      
Utilization of shares from ESOP 0.3 0.7        
Ending balance $ (1.0) $ (0.5) $ (0.2)      
Own shares held by ESOP | Ordinary shares            
Disclosure of classes of share capital [line items]            
Number of treasury shares held | shares       104,709 55,816