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Note 10 - Common Stock and Stock-based Compensation Plans - Valuation Assumptions (Details) - Employee Stock Purchase Plan [Member]
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Expected volatility, minimum 37.00%   32.00% 42.00%
Expected volatility, maximum 60.00%   60.00% 43.00%
Expected volatility   42.00%    
Risk-free interest rate, minimum 0.10%   0.10% 2.00%
Risk-free interest rate, maximum 1.80%   1.90% 2.50%
Risk-free interest rate   2.00%    
Contractual term of up to (Month) 24 months 24 months 24 months 24 months