Stock-Based Compensation (Stock Options Activity) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average remaining contractual life of options outstanding	6 years 4 months 24 days
Weighted average remaining contractual life of options exercisable	5 years 2 months
Weighted average grant date fair value of options granted	$ 20.16	$ 18.08	$ 12.12
Aggregate intrinsic value of total options outstanding	$ 117.5
Aggregate intrinsic value of total options vested	89.3
Intrinsic value of options exercised	61.3	$ 91.7	$ 49.4
Grant date fair value of options vested	$ 8.3	$ 6.7	$ 5.8
Stock Options Activity [Roll Forward]
Options outstanding, Beginning	3.5	4.5	5.0
Granted	0.5	0.5	0.7
Exercised	(1.0)	(1.5)	(1.1)
Forfeited	0.0	0.0	(0.1)
Options outstanding, Ending	3.0	3.5	4.5
Options exercisable	1.7	1.9	2.7
Stock Options, Weighted Average Exercise Price [Roll Forward]
Options outstanding, Beginning (in usd per share)	$ 48.76	$ 38.11	$ 31.77
Granted (in usd per share)	90.36	84.09	61.98
Exercised (in usd per share)	35.95	26.15	22.50
Forfeited (in usd per share)	75.32	60.92	45.91
Options outstanding, Ending (in usd per share)	58.93	48.76	38.11
Options exercisable (in usd per share)	$ 45.32	$ 35.44	$ 27.17
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Awards, expiration period	10 years
Risk-free interest rate	2.70%	2.00%	1.40%
Stock volatility	19.80%	19.90%	20.40%
Dividend yield	0.70%	0.70%	0.90%
Expected life	6 years	6 years	6 years

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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- Definition

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.