Note 13 - Equity Incentive Plans - Assumptions of Options (Details) - Share-based Payment Arrangement, Option [Member]	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Minimum [Member]
Risk-free interest rate	0.40%	1.68%	2.79%
Expected term (in years) (Year)	5 years 80 days	5 years 142 days	5 years 306 days
Expected volatility	94.00%	83.00%	78.00%
Maximum [Member]
Risk-free interest rate	0.88%	2.31%	2.80%
Expected term (in years) (Year)	10 years	10 years	6 years 18 days
Expected volatility	111.00%	85.00%	80.00%

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.

+ 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)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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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