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Pension Plans - Schedule of Net Benefit Costs (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
In respect of defined benefit plans:      
Interest cost $ 11.2 $ 7.5 $ 6.7
Expected return on assets (13.6) (11.1) (12.0)
Settlement loss 8.9 2.0 0.0
Amortization of net actuarial loss 1.8 2.1 3.8
Amortization of prior service credit (0.4) (0.4) (0.4)
Total (credit) / expense for defined benefit plans 7.9 0.1 (1.9)
In respect of defined contribution plans:      
Total charge for defined contribution plans 3.7 3.7 3.6
Total charge / (credit) for benefit plans 11.6 3.8 1.7
U.K.      
In respect of defined benefit plans:      
Interest cost 11.0 6.2 5.6
Expected return on assets (13.8) (10.4) (10.2)
Settlement loss 0.0 0.0 0.0
Amortization of net actuarial loss 1.7 1.8 3.4
Amortization of prior service credit (0.4) (0.4) (0.4)
Total (credit) / expense for defined benefit plans (1.5) (2.8) (1.6)
In respect of defined contribution plans:      
Total charge for defined contribution plans 2.1 2.0 2.1
Total charge / (credit) for benefit plans 0.6 (0.8) 0.5
U.S. / other      
In respect of defined benefit plans:      
Interest cost 0.2 1.3 1.1
Expected return on assets 0.2 (0.7) (1.8)
Settlement loss 8.9 2.0 0.0
Amortization of net actuarial loss 0.1 0.3 0.4
Amortization of prior service credit 0.0 0.0 0.0
Total (credit) / expense for defined benefit plans 9.4 2.9 (0.3)
In respect of defined contribution plans:      
Total charge for defined contribution plans 1.6 1.7 1.5
Total charge / (credit) for benefit plans $ 11.0 $ 4.6 $ 1.2