Note 11 - Equity Incentive Plan - Assumptions Used to Estimate Fair Value of Stock Options and Stock Appreciation Rights (Details)	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Dividend yield	0.00%	0.00%
Minimum [Member]
Risk-free interest rate	0.30%	0.21%
Expected life of options (in years) (Year)	4 years	4 years
Maximum [Member]
Risk-free interest rate	0.68%	1.59%
Expected life of options (in years) (Year)	6 years 3 months 18 days	6 years 3 months 18 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 risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

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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.

+ 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)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
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-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809

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