Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 8.7	$ 15.6	$ 20.7	$ 23.2
Cash based awards	0.4	0.7	1.5	1.1
Income tax benefit	(2.0)	(3.5)	(4.8)	(5.3)
Share-based payment expense, net of tax	6.7	12.1	15.9	17.9
Performance Shares Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	5.0	10.9	12.9	15.8
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.9	3.5	5.6	5.4
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.4	0.4	0.7	0.7
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.0	$ 0.1	$ 0.0	$ 0.2