Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 10.0	$ 12.0
Cash based awards	0.7	1.1
Income tax benefit	(2.3)	(2.8)
Share-based payment expense, net of tax	7.7	9.2
Performance Shares Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	6.4	7.9
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.5	2.7
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.4	$ 0.3