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Note 10 - Common Stock and Stock-based Compensation Plans - Valuation Assumptions (Details) - Employee Stock Purchase Plan [Member]
6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Expected dividend yield 0.00% 0.00%
Expected volatility, minimum 32.00%  
Expected volatility, maximum 42.00%  
Expected volatility   43.00%
Risk-free interest rate, minimum 1.50%  
Risk-free interest rate, maximum 1.90%  
Risk-free interest rate   2.50%
Contractual term of up to (Month) 24 months 24 months