Summary of Option Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Beginning Balance, Options	16.1	16.0	17.2
Granted, Options	2.0	2.1	2.1
Exercised, Options	(3.3)	(1.8)	(3.0)
Cancelled, Options	(0.1)	(0.2)	(0.3)
Ending Balance, Options	14.7	16.1	16.0
Exercisable as of December 31, 2017, Options	8.8
Beginning Balance, Weighted-Average Exercise Price	$ 33.11	$ 30.06	$ 25.89
Granted, Weighted-Average Exercise Price	50.77	48.39	46.75
Exercised, Weighted-Average Exercise Price	23.07	23.06	16.96
Cancelled, Weighted-Average Exercise Price	48.54	43.64	39.60
Ending Balance, Weighted-Average Exercise Price	37.63	$ 33.11	$ 30.06
Exercisable as of December 31, 2017, Weighted-Average Exercise Price	$ 29.82
Outstanding as of December 31, 2017, Weighted-Average Remaining Contractual Term, years	5 years 7 months 6 days
Exercisable as of December 31, 2017, Weighted-Average Remaining Contractual Term, years	4 years
Outstanding as of December 31, 2017, Aggregate Intrinsic Value	$ 412.5
Exercisable as of December 31, 2017, Aggregate Intrinsic Value	$ 314.7

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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