Note 3 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Options outstanding (in shares)	884,998
Options outstanding (in dollars per share)	$ 2.26
Granted to employees and non-employee directors (in shares)	289,750	0
Granted to employees and non-employee directors (in dollars per share)	$ 3.20
Exercised (in shares)	(71,667)
Exercised (in dollars per share)	$ 1.67
Canceled/expired/forfeited (in shares)
Canceled/expired/forfeited (in dollars per share)	$ 0
Options outstanding (in shares)	1,103,081
Options outstanding (in dollars per share)	$ 2.54
Options exercisable (in shares)	465,003
Options exercisable (in dollars per share)	$ 2.09

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

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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 per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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