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Retirement benefits - Movement in Defined Benefit Obligations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance $ 66.5 $ 58.9  
Service cost 0.1 0.4 $ 1.5
Interest on obligation 1.8 2.0 3.0
Exchange difference 4.6 (9.6)  
Past service credit 0.0 0.0 (14.9)
Curtailment credit 0.0 (0.6) (3.3)
Net defined benefit liability (asset), ending balance 55.3 66.5 58.9
Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 383.4 395.4  
Service cost 0.1 0.4  
Interest on obligation 10.8 13.5  
Contributions from plan members 0.0 0.1  
Actuarial gains on financial assumptions 10.7 70.1  
Actuarial gains on plan experience 4.0 (3.5)  
Exchange difference 32.0 (59.4)  
Benefits paid (18.2) (18.4)  
Curtailment credit 0.0 (14.8)  
Net defined benefit liability (asset), ending balance 422.8 383.4 395.4
U.K.      
Disclosure of net defined benefit liability (asset) [line items]      
Service cost 0.0 0.3 1.4
Interest on obligation 1.4 1.5 2.5
Actuarial gains on financial assumptions     14.9
Past service credit 0.0 0.0 (14.9)
Curtailment credit 0.0 0.0 (3.3)
U.K. | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 334.8 334.4  
Service cost 0.0 0.3  
Interest on obligation 8.9 10.9  
Contributions from plan members 0.0 0.1  
Actuarial gains on financial assumptions 6.9 67.5  
Actuarial gains on plan experience 3.5 (3.3)  
Exchange difference 31.7 (59.3)  
Benefits paid (16.1) (15.8)  
Curtailment credit 0.0 0.0  
Net defined benefit liability (asset), ending balance 369.7 334.8 334.4
Non-U.K.      
Disclosure of net defined benefit liability (asset) [line items]      
Service cost 0.1 0.1 0.1
Interest on obligation 0.4 0.5 0.5
Past service credit 0.0 0.0 0.0
Curtailment credit 0.0 (0.6) 0.0
Non-U.K. | Present value of defined benefit obligation      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 48.6 61.0  
Service cost 0.1 0.1  
Interest on obligation 1.9 2.6  
Contributions from plan members 0.0 0.0  
Actuarial gains on financial assumptions 3.8 2.6  
Actuarial gains on plan experience 0.5 (0.2)  
Exchange difference 0.3 (0.1)  
Benefits paid (2.1) (2.6)  
Curtailment credit 0.0 (14.8)  
Net defined benefit liability (asset), ending balance $ 53.1 $ 48.6 $ 61.0