Note 9 - Share-based Compensation - Stock Options Valuation Assumptions (Details) - Common Stock [Member]	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expected dividend yield at date of grant	1.84%	2.60%	2.59%
Expected stock price volatility	33.62%	34.01%	32.47%
Risk-free interest rate	1.35%	2.38%	2.51%
Expected life of options (in years) (Year)	7 years 4 months 20 days	7 years 5 months 15 days	7 years 3 months 10 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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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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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
-SubTopic 10
-Section 50
-Paragraph 2
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