Employee Benefit Plans, Assumptions and Resulting Estimates of Fair Value (Details) - $ / shares shares in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Stock Purchase Plan
Estimates of Fair Value
Volatility	25.00%	32.00%	29.00%
Risk-free interest rate	0.90%	0.40%	0.10%
Expected life (years)	6 months	6 months	6 months
Dividend yield	1.50%	1.80%	1.70%
Weighted-average fair value per share	$ 6.04	$ 5.56	$ 5.63
Market-based RSUs
Estimates of Fair Value
Volatility	30.00%	36.00%	34.00%
Risk-free interest rate	1.90%	1.20%	1.40%
Dividend yield	1.40%	1.70%	1.80%
Weighted-average fair value per share	$ 19.30	$ 14.71	$ 14.97
Stock Options
Estimates of Fair Value
Volatility		31.00%
Risk-free interest rate		0.70%
Expected life (years)		1 year 3 months 18 days
Dividend yield		1.70%
Weighted-average fair value per share		$ 16.17
Employee Stock Purchase Plan 2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Offering period (in months)	6 months
Common shares purchased through ESPP (in shares)	2.7	2.7	2.7
Average exercise price of shares purchased through ESPP (in dollar per share)	$ 20.83	$ 19.66	$ 19.25

Employee Stock Purchase Plans offering period duration.

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Stock Issued During Period, Average Price Per Share, Employee Stock Purchase Plans

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- Definition

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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- Definition

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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