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Stock-Based Compensation (Stock Options) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Exercised, shares	(1.0)	(0.3)	(1.3)
Exercised, weighted-average exercise price (in dollars per share)	$ 37.85	$ 31.21	$ 23.33
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected dividend rate (percentage)		0.00%	0.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Options outstanding, beginning balance (in shares)	5.2
Options outstanding, weighted-average exercise price, beginning balance (in dollars per share)	$ 46.99
Granted, shares	0.0
Granted, weighted-average exercise price (in dollars per share)	$ 0.00
Exercised, shares	(1.0)
Exercised, weighted-average exercise price (in dollars per share)	$ 37.85
Forfeited, shares	(0.2)
Forfeited, weighted-average exercise price (in dollars per share)	$ 56.35
Expired, shares	(0.3)
Expired, weighted-average exercise price (in dollars per share)	$ 54.91
Options outstanding, ending balance (in shares)	3.7	5.2
Options outstanding, weighted-average exercise price, ending balance (in dollars per share)	$ 48.49	$ 46.99
Options outstanding, weighted-average remaining contractual term (years)	5 years 9 months 25 days
Options outstanding, aggregate intrinsic value	$ 23.4
Options exercisable (in shares)	2.7
Options exercisable, weighted-average exercise price (in dollars per share)	$ 45.65
Options exercisable, weighted-average remaining contractual term (years)	5 years 2 months 8 days
Options exercisable, aggregate intrinsic value	$ 23.5
Options exercisable and expected to vest thereafter (in shares)	3.7
Options exercisable and expected to vest thereafter, weighted-average exercise price (in dollars per share)	$ 48.52
Options exercisable and expected to vest thereafter, weighted-average remaining contractual term (years)	5 years 9 months 3 days
Options exercisable and expected to vest thereafter, aggregate intrinsic value	$ 23.5
Options available for future grants (in shares)	5.4
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average grant-date fair value of options granted (in dollars per share)	$ 0.00	$ 17.96	$ 19.38
Total intrinsic value of stock options exercised	$ 11.9	$ 5.3	$ 51.9
Minimum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate (percentage)		1.16%	0.76%
Expected term (in years)		4 years	2 years
Expected volatility rate (percentage)		29.00%	24.00%
Maximum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Risk-free interest rate (percentage)		1.55%	1.86%
Expected term (in years)		7 years	7 years
Expected volatility rate (percentage)		31.00%	34.00%
AwardsGranted2016 [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period (in years)	10 years
Vesting period (in years)	4 years
Awards Granted 2015 and Prior Years [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period (in years)	10 years
Vesting period (in years)	4 years

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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 estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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

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