Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 9.4	$ 8.7	$ 28.7	$ 21.7
Cash based awards	0.2	0.4	0.8	0.8
Income tax benefit	(2.1)	(1.9)	(6.5)	(4.7)
Share-based payment expense, net of tax	7.3	6.8	22.2	17.0
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	6.2	5.5	18.7	12.5
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.4	1.5	7.0	4.5
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.4	0.3	1.0	0.9
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.0	0.5	0.3	1.7
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.2	$ 0.5	$ 0.9	$ 1.3

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of tax benefit for recognition of expense of award under share-based payment arrangement.

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