Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 10.9	$ 5.8	$ 19.3	$ 13.0
Cash based awards	(0.2)	0.4	0.6	0.4
Income tax benefit	(2.5)	(1.2)	(4.4)	(2.8)
Share-based payment expense, net of tax	8.4	4.6	14.9	10.2
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	7.4	2.4	12.5	7.0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.9	1.7	4.6	3.0
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.5	0.3	0.6	0.6
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.0	0.5	0.3	1.2
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.3	$ 0.5	$ 0.7	$ 0.8

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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