Note 8 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Outstanding, number of options (in shares)	747,500	932,500
Outstanding, option, weighted average exercise price (in dollars per share)	$ 3.33	$ 4.68
Exercised, options (in shares)	(183,000)
Exercised, option, weighted average exercise price (in dollars per share)	$ 1.61
Expired or forfeited, options (in shares)	(80,000)	(185,000)
Expired or forfeitedg, option, weighted average exercise price (in dollars per share)	$ 5.79	$ 10.14
Outstanding, number of options (in shares)	484,500	747,500
Outstanding, option, weighted average exercise price (in dollars per share)	$ 3.57	$ 3.33
Exercisable, options (in shares)	484,500	747,500
Exercisableg, option, weighted average exercise price (in dollars per share)	$ 3.57	$ 3.33
Weighted-Average Remaining Contractual Life (years) (Year)	3 years 255 days	4 years 266 days

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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