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v3.20.4

Note 1 - Nature of Business and Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2020
Notes Tables |
Fair value of stock option grants | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019 |  | 2018 | ​
Dividend yield | ​ | ​ | 0.60% to 0.81% | ​ | ​ | 0.67% | ​ | ​ | 0.45% to 0.48% | ​
Expected volatility |  | ​ | 25.49% to 25.70% | ​ | ​ | 28.28% | ​ | ​ | 29.51% to 29.59% | ​
Risk-free interest rate |  | ​ | 0.79% to 1.31% | ​ | ​ | 2.90% | ​ | ​ | 2.60% to 2.94% | ​
Expected life of option grants |  | ​ | 6.25 years | ​ | ​ | 6.25 years | ​ | ​ | 6 years | ​
Weighted-average grant date fair value | ​ | $ | 10.07 | ​ | $ | 11.29 | ​ | $ | 14.68 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019 |  | 2018 | ​
Dividend yield | ​ |  | 0.55% to 0.81% | ​ | ​ | 0.69% to 0.75% | ​ | ​ | 0.37% to 0.51% | ​
Expected volatility | ​ |  | 24.59% to 36.38% | ​ | ​ | 20.15% to 21.06% | ​ | ​ | 20.90% to 21.40% | ​
Risk-free interest rate | ​ |  | 0.15% to 1.65% | ​ | ​ | 2.02 % to 2.46% | ​ | ​ | 1.59% to 2.22% | ​
Expected life of purchase grants | ​ |  | 3 to 6 months | ​ | ​ | 3 to 6 months | ​ | ​ | 3 to 6 months | ​
Weighted-average grant date fair value | ​ | $ | 6.03 | ​ | $ | 4.81 | ​ | $ | 6.63 | ​

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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 -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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