Note 7 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based Payment Arrangement, Option [Member]
Outstanding (in shares)	844,455
Outstanding, weighted average exercise price (in dollars per share)	$ 2.00
Exercised (in shares)	(85,555)
Exercised, weighted average exercise price (in dollars per share)	$ 1.37
Forfeited (in shares)	(1,146)
Forfeited, weighted average exercise price (in dollars per share)	$ 5.69
Expired (in shares)	(35,714)
Expired, weighted average exercise price (in dollars per share)	$ 1.73
Outstanding (in shares)	722,040
Outstanding at March 31, 2021 (in dollars per share)	$ 2.09
Options exercisable (in shares)	716,936
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.07
Performance Shares [Member]
Outstanding (in shares)		686,001
Outstanding, weighted average exercise price (in dollars per share)		$ 1.64
Exercised (in shares)		(77,310)
Exercised, weighted average exercise price (in dollars per share)		$ 1.64
Outstanding (in shares)		608,691
Outstanding at March 31, 2021 (in dollars per share)		$ 1.64
Options exercisable (in shares)		608,691
Options exercisable, weighted average exercise price (in dollars per share)		$ 1.64

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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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 option holders acquired shares when converting their stock options into shares.

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

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