Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 8.3	$ 5.9	$ 24.0	$ 20.5
Cash based awards	0.3	1.0	1.0	1.8
Income tax benefit	(1.9)	(1.4)	(5.5)	(4.7)
Share-based payment expense, net of tax	6.4	4.5	18.5	15.8
Performance Shares Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	4.4	2.0	13.0	9.7
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	3.2	2.4	8.8	7.8
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.4	$ 0.5	$ 1.2	$ 1.2

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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+ Details