Stock-Based Compensation - Stock-Based Compensation Expense and Stock Option Fair Value Assumptions (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Stock option expense	$ 5	$ 3	$ 7
Restricted stock and unit awards expense	89	96	69
Total stock-based compensation expense	94	99	76
Tax benefit	$ 19	$ 38	$ 29
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	1.10%
Weighted-average grant-date fair value per option (USD per share)	$ 112.98
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free average interest rate	2.60%
Volatility	21.80%
Expected life (years)	5 years 8 months 1 day
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free average interest rate	2.70%
Volatility	22.00%
Expected life (years)	6 years 26 days

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

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The noncash expense that represents the cost of restricted stock or unit distributed to employees as compensation.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The noncash expense that accounts for the value of stock or unit options distributed to employees as compensation.

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