Note 11 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Outstanding (in shares)	211,213	154,581
Outstanding (in dollars per share)	$ 61.59	$ 80.44
Granted (in shares)	535,964	60,449
Granted (in dollars per share)	$ 2.69	$ 18.08
Canceled or expired (in shares)	(43,735)	(3,817)
Canceled or expired (in dollars per share)	$ 164.12	$ 135.95
Outstanding (in shares)	703,442	211,213
Outstanding (in dollars per share)	$ 10.34	$ 61.59
Outstanding (Year)	8 years 328 days
Exercisable (in shares)	405,075
Exercisable (in dollars per share)	$ 14.97
Exercisable (Year)	8 years 219 days

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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Weighted average price of options that were either forfeited or expired.

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Net 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 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 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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