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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Expected dividend yield
Expected volatility, minimum 42.00% 35.00% 28.00%
Expected volatility, maximum 43.00% 42.00% 46.00%
Risk-free interest rate, minimum 2.00% 0.70% 0.50%
Risk-free interest rate, maximum 2.50% 2.20% 1.10%
Expected forfeiture 0.00% 0.00% 0.00%
Contractual term of up to (months) (Month) 2 years 2 years 2 years