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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Change in PBO:      
Actuarial gains $ 9.5 $ (15.7) $ 39.5
Fair value of plan assets:      
Opening balance 95.9    
Closing balance 115.9 95.9  
United Kingdom Plan [Member]      
Change in PBO:      
Benefits paid (42.1) (43.7)  
Fair value of plan assets:      
Opening balance 739.1 837.4  
Benefits paid (42.1) (43.7)  
Actual contributions by employer 0.4 1.1  
Actual return on assets 89.1 (10.3)  
Exchange effect 30.4 (45.4)  
Closing balance 816.9 739.1 837.4
German Plan [Member]      
Change in PBO:      
Opening balance 11.3 11.8  
Interest cost 0.2 0.2 0.2
Service cost 0.1 0.2  
Benefits paid (0.3) (0.3)  
Actuarial gains 2.2 0.0  
Exchange effect (0.2) (0.6)  
Closing balance 13.3 11.3 11.8
Fair value of plan assets:      
Benefits paid (0.3) (0.3)  
Pension Benefit Obligation [Member] | United Kingdom Plan [Member]      
Change in PBO:      
Opening balance 643.2 721.4  
Interest cost 15.2 15.0  
Service cost 0.9 1.2  
Benefits paid (42.1) (43.7)  
Actuarial gains 57.7 (14.6)  
Plan amendments 0.0 3.3  
Exchange effect 26.1 (39.4)  
Closing balance 701.0 643.2 $ 721.4
Fair value of plan assets:      
Benefits paid $ (42.1) $ (43.7)