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Note 2 - Significant Accounting Policies - Fair Value Assumption (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Expected stock volatility 33.11% 31.29% 31.68%
Weighted-average stock volatility 33.11% 31.29% 31.68%
Expected dividend yield 1.20% 1.13% 0.00%
Risk-free interest rate 0.80% 2.45% 2.69%
Expected life (Year) 6 years 6 years 6 years
Weighted-average fair value of stock options granted (in dollars per share) $ 6.36 $ 8.37 $ 10.31