Share-Based Compensation - Assumptions For Stock Options (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-Based Compensation
Share-based compensation expense (benefit) before tax	$ 23.9	$ 51.4	$ 15.9	$ (29.2)
Stock Options
Share-Based Compensation
Granted (in shares)			2,081,047
Total grant date fair value of employee stock options that vested			$ 36.4	33.9
Share-based compensation expense (benefit) before tax	$ 18.4	$ 16.6	$ 52.2	$ 44.8
Method and assumptions on valuation of stock options
Expected volatility (as a percent)			33.80%	36.20%
Risk-free interest rate (as a percent)			2.40%	2.70%
Expected term of awards (in years)			5 years 9 months 18 days	6 years 3 months 18 days
Expected dividend yield (as a percent)			0.00%	0.00%

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Rate of weighted-average expected volatility for award under share-based payment arrangement.

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Net number of share options (or share units) granted during the period.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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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