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Note 2 - Stock Option Valuation Assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Minimum [Member]      
Expected volatility 66.00% 70.00%  
Risk-free interest rate 1.07% 0.69% 2.04%
Expected life (in years) 3 years 3 years 6 years 91 days
Maximum [Member]      
Expected volatility 71.00% 73.00%  
Risk-free interest rate 2.42% 2.73% 3.84%
Expected life (in years) 10 years 10 years 10 years
Weighted-average grant date fair value per share (in dollars per share) $ 17.95 $ 8.23 $ 11.37
Expected volatility     73.00%
Dividend yield