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Note 1 - Organization and Significant Accounting Policies - Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Details)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Expected dividend yield 0.00% 0.00% 0.00%
Expected volatility, minimum 39.00% 32.00% 42.00%
Expected volatility, maximum 60.00% 60.00% 43.00%
Risk-free interest rate, minimum 0.10% 0.10% 2.00%
Risk-free interest rate, maximum 1.70% 1.90% 2.50%
Expected forfeiture 0.00% 0.00% 0.00%
Contractual term of up to (months) (Month) 24 months 24 months 24 months