Defined benefit pension plans - Components of Net Periodic Defined Benefit Pension Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Defined benefit pension plans [Line Items]
Service cost	$ 1.6	$ 1.6		$ 4.9	$ 4.7
Interest cost	5.3	5.5		15.9	16.6
Expected return on plan assets	(5.5)	(5.1)		(16.7)	(15.5)
Recognized net actuarial losses	1.0	1.0		2.9	2.9
Settlements			$ 6.2		6.2
Total	2.4	$ 3.0		7.0	$ 14.9
Pension Plans Defined Benefit [Member]
Defined benefit pension plans [Line Items]
Expected contribution	$ 18.0			$ 18.0