Share-Based Compensation - Summary of Assumptions (Details) - 2007 Long-Term Incentive Plan	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Director and Officers
Expected dividend yield	26.00%	26.00%	22.00%
Expected volatility	29.73%	30.81%	41.19%
Risk-free interest rate	2.20%	1.90%	2.00%
Expected life (in years)	5 years	5 years	7 years 8 months 5 days
Employees
Expected dividend yield	26.00%	26.00%	25.00%
Expected volatility	29.82%	30.67%	34.50%
Risk-free interest rate	2.51%	1.89%	1.73%
Expected life (in years)	5 years	5 years	5 years 8 months 9 days

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.

+ References

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- Definition

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

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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
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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