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Note 10 - Summary of Stock Options - Valuation Assumption for Employee Stock Purchase Plan (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Expected term (Year)   5 years 5 years 5 years
Average volatility   70.00% 73.00% 73.00%
Risk-free interest rate   2.86% 2.49% 2.67%
Dividend yield   0.00% 0.00% 0.00%
2017 Employee Stock Purchase Plan [Member]        
Expected term (Year) 91 days 91 days 91 days 91 days
Average volatility 98.00% 67.00% 84.00% 67.00%
Risk-free interest rate 2.21% 1.93% 2.40% 1.71%
Dividend yield 0.00% 0.00% 0.00% 0.00%