Stock-Based Compensation - Valuation Assumptions on PSU Grants (Details) - Performance Share Units	12 Months Ended
Dec. 31, 2018 Rate	Dec. 31, 2017 Rate	Dec. 31, 2016 Rate
Share-based Compensation Arrangement by Share-based Payment Award
Expected Volatility Rate	24.00%	24.00%	22.00%
Expected Life	3 years	3 years	3 years
Risk Free Interest Rate	2.32%	1.52%	0.88%

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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- Definition

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

+ References

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- Definition

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.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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