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Note 6 - Employee Benefit Plans - Stock Options, Valuation Assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Weighted average fair value per share of options granted during the period (in dollars per share) $ 5.21  
Twenty Eleven Stock Incentive Plan [Member]    
Expected life (in years) (Year) 4 years 164 days 4 years 167 days
Volatility 38.77% 45.26%
Risk-free interest rate 0.88% 2.55%
Expected dividend
Weighted average fair value per share of options granted during the period (in dollars per share) $ 5.21 $ 4.51
Employee Stock Purchase Plan [Member]    
Expected life (Year) 1 year 91 days 1 year 91 days
Volatility 34.25% 45.19%
Risk-free interest rate 1.43% 2.52%
Expected dividend 0.00% 0.00%
Weighted average fair value of purchase rights granted during the period (in dollars per share) $ 4.83 $ 3.67