Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 7.2	$ 9.4
Income tax benefit	(1.6)	(2.2)
Share-based payment expense, net of tax	5.6	7.2
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	4.6	5.8
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.3	2.0
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.3
Performance-Based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.7	1.1
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.3	$ 0.2

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

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