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Note 11 - Stockholders' Equity - Valuation Assumptions (Details)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Employee Stock Option [Member]    
Risk-free interest rate 2.70% 2.00%
Dividend yield 0.00% 0.00%
Volatility 71.10% 81.40%
Expected life (Year) 6 years 6 years
Employee Stock Purchase Plan [Member]    
Risk-free interest rate 2.10% 1.00%
Dividend yield 0.00% 0.00%
Volatility 59.80% 56.00%
Expected life (Year) 6 years 6 years