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Note 1 - Nature of Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2023
Notes Tables |
Fair value assumptions for stock option grants and stock purchase grants | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2023 |  | 2022 |  | 2021 | ​
Dividend yield | ​ | ​ | 0.45% | ​ | ​ | 0.43% to 0.45% | ​ | ​ | 0.55% | ​
Expected volatility |  | ​ | 22.31% | ​ | ​ | 22.65% to 23.52% | ​ | ​ | 23.57% | ​
Risk-free interest rate |  | ​ | 4.22% | ​ | ​ | 1.87% to 3.12% | ​ | ​ | 1.62% | ​
Expected life of option grants |  | ​ | 6.25 years | ​ | ​ | 6.25 years | ​ | ​ | 6.25 years | ​
Weighted-average grant date fair value | ​ | $ | 16.36 | ​ | $ | 13.94 to 15.09 | ​ | $ | 10.85 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2023 |  | 2022 |  | 2021 | ​
Dividend yield | ​ |  | 0.48% to 0.58% | ​ | ​ | 0.43% to 0.44% | ​ | ​ | 0.49% to 0.61% | ​
Expected volatility | ​ |  | 15.86% to 29.69% | ​ | ​ | 17.97% to 21.16% | ​ | ​ | 37.51% to 38.65% | ​
Risk-free interest rate | ​ |  | 4.67% to 5.47% | ​ | ​ | 0.14% to 2.24% | ​ | ​ | 0% to 0.06% | ​
Expected life of purchase grants | ​ |  | 3 to 6 months | ​ | ​ | 3 to 6 months | ​ | ​ | 3 to 6 months | ​
Weighted-average grant date fair value | ​ | $ | 8.35 | ​ | $ | 7.88 | ​ | $ | 7.67 | ​

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480429/718-10-50-2 + Details
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