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Stock Option Plan (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2019
Share-based Payment Arrangement [Abstract] |
Stock Options Issued |
| Directors |  | Officers
Grant date | May 5, 2017 | May 11, 2018 | May 3, 2019 |  | May 5, 2017 | May 11, 2018 | May 3, 2019
Exercise price | $ | 59.41 |  | $ | 49.46 |  | $ | 55.71 |  |  | $ | 59.41 |  | $ | 49.46 |  | $ | 55.71 |
Volatility | 0.173 |  | 0.192 |  | 0.236 |  |  | 0.170 |  | 0.177 |  | 0.206 |
Expected life (years) | 5.0 |  | 5.0 |  | 5.0 |  |  | 7.0 |  | 7.0 |  | 7.0 |
Assumed yield | 3.45 | % | 3.70 | % | 3.75 | % |  | 3.50 | % | 3.75 | % | 3.80 | %
Risk-free rate | 1.89 | % | 2.84 | % | 2.33 | % |  | 2.17 | % | 2.94 | % | 2.43 | %
Summary of Option Activity | The table below summarizes the option activity for the years 2019, 2018, and 2017:
| 2019 |  | 2018 |  | 2017
| Shares |  | Weighted Average Exercise Price |  | Shares |  | Weighted Average Exercise Price |  | Shares |  | Weighted Average Exercise Price
Outstanding at January 1 | 1,114,169 |  |  | $ | 52.40 |  |  | 913,320 |  |  | $ | 52.80 |  |  | 833,630 |  |  | $ | 49.92 |
Granted | 260,000 |  |  | 55.71 |  |  | 245,000 |  |  | 49.46 |  |  | 232,500 |  |  | 59.41 |
Exercised | (57,055 | ) |  | 44.53 |  |  | (39,151 | ) |  | 42.98 |  |  | (149,060 | ) |  | 46.97 |
Expired/Forfeited | (7,500 | ) |  | 56.07 |  |  | (5,000 | ) |  | 54.78 |  |  | (3,750 | ) |  | 53.73 |
Outstanding December 31 | 1,309,614 |  |  | 53.38 |  |  | 1,114,169 |  |  | 52.40 |  |  | 913,320 |  |  | 52.80 |
Exercisable at December 31 | 763,614 |  |  | 52.43 |  |  | 600,919 |  |  | 50.93 |  |  | 430,945 |  |  | 48.94 |

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