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Note 10 - Common Stock and Stock-based Compensation Plans - Valuation Assumptions (Details) - Employee Stock Purchase Plan [Member]
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Expected dividend yield 0.00% 0.00%
Expected volatility, minimum 39.00% 32.00%
Expected volatility, maximum 56.00% 42.00%
Risk-free interest rate, minimum 0.10% 1.50%
Risk-free interest rate, maximum 1.70% 1.90%
Contractual term of up to (Month) 24 months 24 months