Stock-Based Compensation - Assumptions Used in Options Pricing Model (Details)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Average option life	6 years 6 months	6 years	6 years
Future dividend yield (as a percentage)	2.28%	1.79%	2.86%
Historical share price volatility (as a percentage)	0.282%	0.287%	0.297%
Weighted-average risk-free interest rate (as a percentage)	2.80%	2.10%	1.30%

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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

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the expected term of a nonvested share or option award issued to other than an employee.

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- Definition

The estimated measure of the percentage amount by which a share price is expected to fluctuate during the expected term of a nonvested share or option award issued to other than an employee.

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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.

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