Note 6 - Share-based Compensation - Stock Option Transactions (Details) - $ / shares	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Outstanding options (in shares)	633,877
Outstanding options (in dollars per share)	$ 13.33
Granted options (in shares)	87,385	90,012
Granted options (in dollars per share)	$ 23.59
Exercised options (in shares)	(38,908)
Exercised options (in dollars per share)	$ 10.35
Lapsed options (in shares)
Lapsed options (in dollars per share)
Cancelled options (in shares)	(700)
Cancelled options (in dollars per share)	$ 18.68
Outstanding options (in shares)	681,654
Outstanding options (in dollars per share)	$ 14.81

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

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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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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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