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Note 8 - Stock-based Compensation - Assumptions Used in the Valuation of Options (Details) - Share-based Payment Arrangement, Option [Member]
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Expected life (years) (Year) 6 years 3 months 6 years 3 months
Expected dividend yield 0.00% 0.00%
Minimum [Member]    
Risk-free interest rate 0.50% 2.35%
Expected volatility 42.40% 42.52%
Maximum [Member]    
Risk-free interest rate 1.47% 2.58%
Expected volatility 43.83% 42.74%