Share-Based Compensation Plans - Compenssation Expense (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 17.7	$ 9.5	$ 14.1
Income tax benefit	(3.4)	(3.0)	(4.6)
Share-based payment expense, net of tax	14.3	6.5	9.5
Fair value of shares vested	29.6	22.2	6.6
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	9.5	4.1	8.1
Unrecognized Expense for Nonvested Awards at	$ 14.2
Weighted-average No. of Years Unrecognized Expense to be Recognized	1 year 10 months 24 days
Fair value of shares vested	$ 13.3	8.2	0.0
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	0.3	0.1	2.3
Unrecognized Expense for Nonvested Awards at	$ 0.0
Weighted-average No. of Years Unrecognized Expense to be Recognized	2 months 12 days
Fair value of shares vested	$ 4.3	2.7	0.0
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	4.7	3.8	3.2
Unrecognized Expense for Nonvested Awards at	$ 4.5
Weighted-average No. of Years Unrecognized Expense to be Recognized	1 year 6 months
Fair value of shares vested	$ 7.3	4.1	5.3
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	1.0	0.9	0.5
Unrecognized Expense for Nonvested Awards at	$ 0.2
Weighted-average No. of Years Unrecognized Expense to be Recognized	3 months 18 days
Fair value of shares vested	$ 2.7	1.9	0.2
Performance-based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	2.2	0.6	0.0
Unrecognized Expense for Nonvested Awards at	$ 4.3
Weighted-average No. of Years Unrecognized Expense to be Recognized	1 year 10 months 24 days
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 0.0	0.0	0.0
Unrecognized Expense for Nonvested Awards at	$ 0.1
Weighted-average No. of Years Unrecognized Expense to be Recognized	2 years 9 months 18 days
Fair value of shares vested	$ 2.0	$ 5.3	$ 1.1

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=96867065&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=96867065&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

Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB
-Name Accounting Standards Codification
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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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.

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Fair value of options vested. Excludes equity instruments other than options, for example, but not limited to, share units, stock appreciation rights, restricted stock.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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