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Pension Plans - Reconciliations of Plan Benefit Obligation, Fair Value of Plan Assets and Funded Status of Pension Plans (Details) - USD ($)
$ in Millions
 1 Months Ended 12 Months Ended
Dec. 31, 2021
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]        
Benefit obligation at January 1   $ 454.7 $ 406.3  
Interest cost   6.7 8.4 $ 11.1
Actuarial (gain) / loss   (30.4) 45.4  
Exchange difference   (4.0) 11.7  
Benefits paid   (15.4) (17.2)  
Prior service cost   0.0 0.1  
Benefit obligation at December 31 $ 411.6 411.6 454.7 406.3
Change in plan assets        
Fair value of plan assets at January 1   403.9 371.1  
Actual return on assets   20.4 33.0  
Exchange difference   (3.7) 11.2  
Contributions from employer   18.2 5.8  
Benefits paid   (15.4) (17.2)  
Fair value of plan assets at December 31 423.4 423.4 403.9 371.1
Funded status        
Net benefit surplus / (obligation) 11.8 11.8 (50.8)  
U.K.        
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]        
Benefit obligation at January 1   404.0 359.2  
Interest cost   5.6 7.0 9.2
Actuarial (gain) / loss   (29.6) 40.7  
Exchange difference   (4.0) 11.7  
Benefits paid (12.7) (13.1) (14.7)  
Prior service cost   0.0 0.1  
Benefit obligation at December 31 362.9 362.9 404.0 359.2
Change in plan assets        
Fair value of plan assets at January 1   358.9 328.7  
Actual return on assets   16.3 27.9  
Exchange difference   (3.7) 11.2  
Contributions from employer   18.2 5.8  
Benefits paid   (13.1) (14.7)  
Fair value of plan assets at December 31 376.6 376.6 358.9 328.7
Funded status        
Net benefit surplus / (obligation) 13.7 13.7 (45.1)  
U.S. / other        
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]        
Benefit obligation at January 1   50.7 47.1  
Interest cost   1.1 1.4 1.9
Actuarial (gain) / loss   (0.8) 4.7  
Exchange difference   0.0 0.0  
Benefits paid   (2.3) (2.5)  
Prior service cost   0.0 0.0  
Benefit obligation at December 31 48.7 48.7 50.7 47.1
Change in plan assets        
Fair value of plan assets at January 1   45.0 42.4  
Actual return on assets   4.1 5.1  
Exchange difference   0.0 0.0  
Contributions from employer   0.0 0.0  
Benefits paid   (2.3) (2.5)  
Fair value of plan assets at December 31 46.8 46.8 45.0 $ 42.4
Funded status        
Net benefit surplus / (obligation) $ (1.9) $ (1.9) $ (5.7)