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Note 8 - Stockholders' Equity - Option Valuation Assumptions (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Employee Stock Option [Member]      
Risk-free interest rate 1.70% 1.70% 1.90%
Dividend yield
Volatility 89.10% 92.70% 99.00%
Expected life (years) (Year) 6 years 6 years 6 years
Employee Stock Purchase Plan [Member]      
Risk-free interest rate 0.40% 0.10% 0.10%
Dividend yield
Volatility 68.30% 86.70% 53.80%
Expected life (years) (Year) 180 days 180 days 180 days