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Note 12 - Equity Compensation Plans - Stock Options Granted Weighted Average Assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Expected volatility 23.60% 18.17%
Risk-free interest rate 0.37% 1.59%
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) $ 36.09 $ 25.49