Note 8 - Stock Based Compensation - Summary of Stock Option Activity (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Outstanding, shares (in shares)	42,283	63,197
Outstanding, weighted-average exercise price (in dollars per share)	$ 6.20	$ 7.01
Outstanding, weighted-average remaining contractual term (Year)		292 days	1 year 292 days
Cancelled/Forfeited, shares (in shares)	(36,071)	(20,914)
Cancelled/Forfeited, weighted-average exercise price (in dollars per share)	$ 5.54	$ 8.64
Expired, shares (in shares)	(6,212)
Expired, weighted-average exercise price (in dollars per share)	$ 10.01
Outstanding, shares (in shares)		42,283	63,197
Outstanding, weighted-average exercise price (in dollars per share)		$ 6.20	$ 7.01
Outstanding, aggregate intrinsic value
Exercisable, shares (in shares)
Exercisable, weighted-average exercise price (in dollars per share)
Exercisable, weighted-average remaining contractual term (Year)
Exercisable, aggregate intrinsic value

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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Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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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.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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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 grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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