Note 1 - Nature of Business and Significant Accounting Policies - Option Pricing Model Valuation Assumptions (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expected life of option grants (Year)	10 years
Weighted-average grant date fair value (in dollars per share)	$ 10.07	$ 11.29	$ 14.68
Employee Stock Option
Dividend yield		0.67%
Expected volatility		28.28%
Risk-free interest rate		2.90%
Expected life of option grants (Year)	6 years 3 months	6 years 3 months	6 years
Weighted-average grant date fair value (in dollars per share)	$ 10.07	$ 11.29	$ 14.68
Employee Stock Option \| Minimum
Dividend yield	0.60%		0.45%
Expected volatility	25.49%		29.51%
Risk-free interest rate	0.79%		2.60%
Employee Stock Option \| Maximum
Dividend yield	0.81%		0.48%
Expected volatility	25.70%		29.59%
Risk-free interest rate	1.31%		2.94%
Stock Purchase Grants [Member]
Weighted-average grant date fair value (in dollars per share)	$ 6.03	$ 4.81	$ 6.63
Stock Purchase Grants [Member] \| Minimum
Dividend yield	0.55%	0.69%	0.37%
Expected volatility	24.59%	20.15%	20.90%
Risk-free interest rate	0.15%	2.02%	1.59%
Expected life of option grants (Year)	3 months	3 months	3 months
Stock Purchase Grants [Member] \| Maximum
Dividend yield	0.81%	0.75%	0.51%
Expected volatility	36.38%	21.06%	21.40%
Risk-free interest rate	1.65%	2.46%	2.22%
Expected life of option grants (Year)	6 months	6 months	6 months

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

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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
-SubTopic 10
-Section 50
-Paragraph 2
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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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