Note 8 - Stockholders' Equity - Stock Option Activity (Details) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Number of shares outstanding (in shares)	13,463
Number of shares outstanding, weighted average exercise price (in dollars per share)	$ 15.03
Number of shares outstanding, weighted average remaining contractual term (Year)	7 years 313 days	8 years 3 days
Number of shares granted (in shares)	122
Number of shares granted, weighted average exercise price (in dollars per share)	$ 23.06
Number of shares exercised (in shares)	(435)
Number of shares exercised, weighted average exercise price (in dollars per share)	$ 9.66
Expired and forfeited (in shares)	(244)
Number of shares expired and forfeited, weighted average exercise price (in dollars per share)	$ 19.02
Number of shares outstanding (in shares)	12,906	13,463
Number of shares outstanding, weighted average exercise price (in dollars per share)	$ 15.22	$ 15.03

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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Weighted average price of options that were either forfeited or 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 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 per share amount at which grantees can acquire shares of common stock by exercise of options.

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

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