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Note 12 - Equity Compensation Plans - Stock Options Granted Weighted Average Assumptions (Details) - $ / shares
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Expected volatility 23.66% 18.41%
Risk-free interest rate 0.39% 1.53%
Expected term in years (Year) 4 years 3 months 18 days 4 years 3 months 18 days
Dividend yield 0.63% 0.71%
Weighted average grant date fair value per stock option (in dollars per share) $ 35.14 $ 25.77