Note 10 - Share-based Compensation - Valuation Assumptions for Employee Stock Options (Details) - Share-based Payment Arrangement, Option [Member]	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Dividend yield	0.00%	0.00%	0.00%
Expected term (Year)		6 years 91 days
Minimum [Member]
Risk-free interest rate	1.40%	2.66%	1.76%
Volatility	74.00%	81.00%	81.00%
Expected term (Year)	6 years 91 days		5 years
Maximum [Member]
Risk-free interest rate	2.14%	2.89%	2.25%
Volatility	94.00%	83.00%	89.00%
Expected term (Year)	10 years		6 years 91 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 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.

+ 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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