Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 8.7	$ 9.8	$ 21.7	$ 35.9
Cash based awards	0.4	0.0	0.8	0.0
Income tax benefit	(1.9)	(2.3)	(4.7)	(8.3)
Share-based payment expense, net of tax	6.8	7.5	17.0	27.6
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	5.5	7.0	12.5	21.6
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.5	1.4	4.5	5.3
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.3	0.9	0.9
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.5	0.8	1.7	7.3
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.5	$ 0.3	$ 1.3	$ 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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