Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 4.0	$ 2.1	$ 8.5	$ 4.9
Income tax benefit	(1.5)	(0.7)	(3.1)	(1.7)
Share-based payment expense, net of tax	2.5	1.4	5.4	3.2
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.9	1.0	4.5	2.3
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.0	0.0	0.1	0.2
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.2	0.9	2.4	2.0
Deferred Stock Units and fees paid in stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.1	0.5	0.3
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.6	$ 0.1	$ 1.0	$ 0.1

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

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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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

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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