Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 6.3	$ 4.0	$ 18.8	$ 12.5
Income tax benefit	(1.5)	(1.5)	(4.4)	(4.6)
Share-based payment expense, net of tax	4.8	2.5	14.4	7.9
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	3.1	1.9	9.7	6.4
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.0	0.1	0.1	0.2
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.6	1.1	4.9	3.5
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.4	0.3	0.9	0.8
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 1.2	$ 0.6	$ 3.2	$ 1.6

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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