Note 2 - Summary of Significant Accounting Policies - Stock-based Compensation (Details) - USD ($)	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017
Stock based compensation expense recognized	$ 1,584,000	$ 1,329,000
Number of unvested stock options (in shares)	1,936,163	1,658,602
Fair value of unvested stock options	$ 5,499,104	$ 4,553,854
Amount remaining to be recognized as expense	$ 3,886,278	$ 2,966,049

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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