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Pension and Post-employment Benefits - Movements in PBO and Fair Value of Plan Assets (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in PBO:      
Actuarial losses/(gains) $ 39.5 $ 3.7 $ 3.2
Fair value of plan assets:      
Opening balance 48.0    
Closing balance 116.0 48.0  
United Kingdom Plan [Member]      
Change in PBO:      
Benefits paid (37.8) (39.7)  
Fair value of plan assets:      
Opening balance 758.2 795.2  
Actual benefits paid (37.8) (39.7)  
Actual contributions by employer 1.0 1.1  
Actual return on assets 43.0 142.2  
Exchange effect 73.0 (140.6)  
Closing balance 837.4 758.2 795.2
German Plan [Member]      
Change in PBO:      
Opening balance 10.1 9.2  
Interest cost 0.2 0.2 0.2
Service cost 0.2 0.2 0.2
Benefits paid (0.2) (0.2)  
Actuarial losses/(gains) 0.2 1.0  
Exchange effect 1.3 (0.3)  
Closing balance 11.8 10.1 9.2
Fair value of plan assets:      
Actual benefits paid (0.2) (0.2)  
Pension Benefit Obligation [Member] | United Kingdom Plan [Member]      
Change in PBO:      
Opening balance 710.2 739.7  
Interest cost 15.2 20.7  
Service cost 0.9 1.0  
Benefits paid (37.8) (39.7)  
Actuarial losses/(gains) (32.7) 119.8  
Exchange effect 65.6 (131.3)  
Closing balance 721.4 710.2 $ 739.7
Fair value of plan assets:      
Actual benefits paid $ (37.8) $ (39.7)