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Note 9 - Stockholders' Equity - Valuation Assumptions (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee Stock Option [Member]      
Risk-free interest rate 2.80% 2.10% 1.70%
Dividend yield
Volatility 70.40% 74.00% 89.10%
Expected life (Year) 6 years 6 years 6 years
Employee Stock Purchase Plan [Member]      
Risk-free interest rate 2.40% 1.20% 0.40%
Dividend yield
Volatility 59.70% 50.00% 68.30%
Expected life (Year) 180 days 180 days 180 days