Share-based compensation plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 3.3	$ 7.6	$ 8.5	$ 13.5
Income tax benefit	(1.1)	(2.6)	(2.9)	(4.6)
Share-based payment expense, net of tax	2.2	5.0	5.6	8.9
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.0	1.0	4.5	3.5
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.1	1.9	1.8
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.9	3.4	1.9	5.0
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.1	0.5	0.2	0.5
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.0	$ 2.6	$ 0.0	$ 2.7

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

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section F

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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

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-Name Accounting Standards Codification

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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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