Equity Plan - Assumptions Used to Estimate Fair Value (Details) - Options	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assumptions Used to Estimate Fair Value
Expected life in months	72 months	72 months	72 months
Volatility (as a percent)	29.00%	24.00%	21.00%
Weighted average risk-free interest rate (as a percent)	0.53%	2.23%	2.88%
Expected dividends (as a percent)	5.71%	3.95%	2.64%

No definition available.

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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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- 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.

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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 award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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