Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 8.4	$ 6.5	$ 26.4	$ 27.2
Cash based awards	0.9	0.1	2.3	1.6
Income tax benefit	(1.9)	(1.5)	(6.0)	(6.3)
Share-based payment expense, net of tax	6.5	5.0	20.4	20.9
Performance Shares Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	4.7	3.7	15.5	16.6
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.5	2.4	7.6	8.0
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.3	$ 0.3	$ 1.0	$ 1.0