Pension and Post-Employment Benefits - Movements in PBO and Fair Value of Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Change in PBO:
Actuarial losses/(gains)	$ 17.5	$ 69.9	$ 28.2
Fair value of plan assets:
Opening balance	48.1
Closing balance	35.1	48.1
United Kingdom Plan [Member]
Fair value of plan assets:
Opening balance	452.1	838.9
Benefits paid	(35.6)	(45.6)
Actual return on assets	17.9	(258.7)
Exchange effect	24.8	(82.5)
Closing balance	459.2	452.1	838.9
German Plan [Member]
Change in PBO:
Opening balance	8.2	13.2
Interest cost	0.3	0.1	0.1
Service cost	0.1	0.1	0.1
Benefits paid	(0.3)	(0.3)
Actuarial losses/(gains)	0.6	(4.0)
Exchange effect	0.2	(0.9)
Closing balance	9.1	8.2	13.2
Pension Benefit Obligation [Member] | United Kingdom Plan [Member]
Change in PBO:
Opening balance	404.0	679.1
Interest cost	19.6	10.1
Service cost	3.5	2.2
Benefits paid	(35.6)	(45.6)
Plan amendments	0.0	0.4
Actuarial losses/(gains)	10.1	174.2
Exchange effect	22.5	(68.0)
Closing balance	$ 424.1	$ 404.0	$ 679.1