Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 6.5	$ 14.3	$ 27.2	$ 37.5
Cash based awards	0.1	0.1	1.6	1.2
Income tax benefit	(1.5)	(3.4)	(6.3)	(8.7)
Share-based payment expense, net of tax	5.0	10.9	20.9	28.8
Performance Shares Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	3.7	10.5	16.6	26.3
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.4	3.3	8.0	8.7
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
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.0	$ 0.1	$ 0.0	$ 0.3