Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation (Details) - USD ($)	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Stock based compensation expense recognized	$ 1,052,000	$ 817,000
Number of unvested stock options (in shares)	1,968,663	1,691,102
Fair value of unvested stock options	$ 5,555,672	$ 4,596,379
Amount remaining to be recognized as expense	$ 4,387,864	$ 3,453,675

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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