Note 7 - Stock-based Compensation - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2019 USD ($) $ / shares shares
Outstanding (in shares) \| shares	1,576,087
Outstanding - weighted average exercise price (in dollars per share) \| $ / shares	$ 6.58
Granted (in shares) \| shares
Granted - weighted average exercise price (in dollars per share) \| $ / shares
Exercised (in shares) \| shares	(75,427)
Exercised - weighted average exercise price (in dollars per share) \| $ / shares	$ 4.71
Forfeited or Expired (in shares) \| shares
Forfeited or Expired - weighted average exercise price (in dollars per share) \| $ / shares
Outstanding (in shares) \| shares	1,500,660
Outstanding - weighted average exercise price (in dollars per share) \| $ / shares	$ 6.68
Outstanding - weighted average remaining contractual term (Year)	3 years 91 days
Outstanding - aggregate intrinsic value \| $	$ 5,574
Vested and Expected to Vest (in shares) \| shares	1,409,243
Options vested and expected to vest - weighted average exercise price (in dollars per share) \| $ / shares	$ 6.65
Options vested and expected to vest - weighted average remaining contractual term (Year)	3 years 43 days
Options vested and expected to vest - aggregate intrinsic value \| $	$ 5,280
Exercisable (in shares) \| shares	1,031,701
Exercisable - weighted average exercise price (in dollars per share) \| $ / shares	$ 5.98
Exercisable - weighted average remaining contractual term (Year)	2 years 171 days
Exercisable - aggregate intrinsic value \| $	$ 4,377

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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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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Gross number of share options (or share units) granted during the period.

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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 exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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 that were terminated.

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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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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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Weighted average remaining contractual term for fully vested and expected to vest exercisable or convertible options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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