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Retirement benefits - Movement in Fair Value of Plan Assets (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)  
Interest on plan assets (1.8) (2.0) $ (3.0)
Exchange difference (4.6) 9.6  
Contributions from employer 8.9    
Administrative expenses (0.7) (0.9) (1.3)
Net defined benefit liability (asset), ending balance (55.3) (66.5) (58.9)
Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 316.9 336.5  
Interest on plan assets 9.0 11.5  
Actuarial gains 24.2 44.9  
Exchange difference 27.4 (49.8)  
Contributions from employer 8.9 7.2  
Contributions from plan members 0.0 0.1  
Administrative expenses (0.7) (0.9)  
Benefits paid (18.2) (18.4)  
Settlement credit 0.0 (14.2)  
Net defined benefit liability (asset), ending balance 367.5 316.9 336.5
U.K.      
Disclosure of net defined benefit liability (asset) [line items]      
Interest on plan assets (1.4) (1.5) (2.5)
Administrative expenses (0.2) (0.4) (1.0)
U.K. | Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 280.3 287.1  
Interest on plan assets 7.5 9.4  
Actuarial gains 20.4 43.7  
Exchange difference 27.4 (49.8)  
Contributions from employer 7.0 6.0  
Contributions from plan members 0.0 0.1  
Administrative expenses (0.2) (0.4)  
Benefits paid (16.1) (15.8)  
Settlement credit 0.0 0.0  
Net defined benefit liability (asset), ending balance 326.3 280.3 287.1
Non-U.K.      
Disclosure of net defined benefit liability (asset) [line items]      
Interest on plan assets (0.4) (0.5) (0.5)
Administrative expenses (0.5) (0.5) (0.3)
Non-U.K. | Fair value of plan assets      
Disclosure of net defined benefit liability (asset) [line items]      
Net defined benefit liability (asset), beginning balance 36.6 49.4  
Interest on plan assets 1.5 2.1  
Actuarial gains 3.8 1.2  
Exchange difference 0.0 0.0  
Contributions from employer 1.9 1.2  
Contributions from plan members 0.0 0.0  
Administrative expenses (0.5) (0.5)  
Benefits paid (2.1) (2.6)  
Settlement credit 0.0 (14.2)  
Net defined benefit liability (asset), ending balance $ 41.2 $ 36.6 $ 49.4