Share-Based Compensation Plans - Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense	$ 6.9	$ 42.7	$ 28.2	$ 17.7
Income tax benefit		(9.2)	(6.5)	(3.4)
Share-based payment expense, net of tax		33.5	21.7	14.3
Fair value of shares vested		46.8	44.4	29.6
Income tax benefit realized		10.2	9.9	9.2
Performance Shares Units PSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		25.8	15.8	9.5
Unrecognized Expense for Nonvested Awards at		$ 16.8
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 6 months
Fair value of shares vested		$ 28.7	25.3	13.3
Market Share Units MSU
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		0.0	0.1	0.3
Unrecognized Expense for Nonvested Awards at		0.0
Fair value of shares vested		0.0	8.2	4.3
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		6.6	6.6	4.7
Unrecognized Expense for Nonvested Awards at		$ 3.8
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 4 months 24 days
Fair value of shares vested		$ 11.8	8.0	7.3
Deferred Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		1.2	1.2	1.0
Unrecognized Expense for Nonvested Awards at		$ 0.3
Weighted-average No. of Years Unrecognized Expense to be Recognized		4 months 24 days
Fair value of shares vested		$ 0.9	0.7	2.7
Performance-based Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		8.1	4.5	2.2
Unrecognized Expense for Nonvested Awards at		$ 2.6
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 1 month 6 days
Fair value of shares vested		$ 5.4	0.0	0.0
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment expense		1.0	0.0	0.0
Unrecognized Expense for Nonvested Awards at		$ 1.7
Weighted-average No. of Years Unrecognized Expense to be Recognized		1 year 9 months 18 days
Fair value of shares vested		$ 0.0	$ 2.2	$ 2.0

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.

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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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Unrecognized cost of unvested share-based compensation awards.

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

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Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement.

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

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