Defined contribution and defined benefit retirement - Components of Net Periodic Defined Benefit Pension Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
U.K.
Defined Benefit Plan Disclosure [Line Items]
Settlements	$ 6.2
Pension Plans Defined Benefit [Member] | United States
Defined Benefit Plan Disclosure [Line Items]
Interest cost	2.2	$ 1.4	$ 1.3
Expected return on plan assets	(1.9)	(2.0)	(2.1)
Recognized net actuarial losses	2.0	1.9	2.1
Settlements			(0.5)
Total	2.3	1.3	0.8
Pension Plans Defined Benefit [Member] | Non U.S.
Defined Benefit Plan Disclosure [Line Items]
Service cost	6.3	11.3	14.7
Interest cost	19.8	10.6	8.3
Expected return on plan assets	(18.3)	(11.1)	(11.4)
Amortization of prior service cost		0.1	0.2
Recognized net actuarial losses	1.8	12.8	19.5
Settlements	6.5	0.4
Total	$ 16.1	$ 24.1	$ 31.3