Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 7.6	$ 8.4
Cash based awards	0.4	0.8
Income tax benefit	(1.8)	(1.9)
Share-based payment expense, net of tax	5.8	6.5
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	4.9	5.1
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.9	1.7
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.1
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.0	0.3
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.1	$ 0.4

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

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