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Stock Based Compensation (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) plan $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares
Stock-based compensation |  |  |
Number of stock-based employee compensation plans | plan | 4 |  |
Number of executive stock award plans | plan | 3 |  |
Total fair value of share awards vested | $ | $ 2,670,000 | $ 3,350,000 | $ 3,530,000
Shares Available for Grant |  |  |
Balance at the beginning of the period (in shares) | 664,502 | 675,059 | 676,444
Granted (in shares) | (147,576) | (74,336) | (74,981)
Forfeited (in shares) | 49 | 1,241 | 3,135
Balance at the end of the period (in shares) | 577,208 | 664,502 | 675,059
Stock award plans |  |  |
Additional disclosures |  |  |
Stock-based compensation expense | $ | $ 3,290,000 | $ 2,760,000 | $ 3,550,000
Income tax benefit due to stock-based compensation expenses | $ | 770,000 | $ 650,000 | $ 860,000
Unrecognized stock-based compensation expense | $ | $ 7,630,000 |  |
Weighted-average period for unrecognized compensation cost to recognize | 3 years 5 months 8 days |  |
Executive Incentive Plan |  |  |
Shares Available for Grant |  |  |
Shares authorized | 60,233 | 62,538 | 70,461
Additional disclosures |  |  |
Vesting period | 5 years |  |
1982 Restricted Stock Plan | Maximum |  |  |
Additional disclosures |  |  |
Vesting period | 10 years |  |
Strategic Deployment Incentive Plan |  |  |
Additional disclosures |  |  |
Vesting period | 5 years |  |
Stock options |  |  |
Additional disclosures |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | 0 | 0 | 0
Stock-based compensation expense | $ | $ 0 | $ 0 | $ 0
Stock options | Maximum |  |  |
Additional disclosures |  |  |
Contractual life | 10 years |  |
Non-vested stock awards outstanding |  |  |
Number of Shares |  |  |
Stock awards outstanding at the beginning of the period (in shares) | 218,991 | 253,819 | 295,926
Granted (in shares) | 147,576 | 74,336 | 74,981
Vested (in shares) | (74,203) | (100,299) | (106,513)
Forfeited (in shares) | (870) | (8,865) | (10,575)
Stock awards outstanding at the end of the period (in shares) | 291,494 | 218,991 | 253,819
Weighted-Average Grant-Date Fair Value |  |  |
Stock awards outstanding at the beginning of the period (in dollars per share) | $ / shares | $ 29.60 | $ 28.59 | $ 27.41
Granted (in dollars per share) | $ / shares | 37.41 | 31.44 | 29.11
Vested (in dollars per share) | $ / shares | 28.95 | 28.35 | 25.79
Forfeited (in dollars per share) | $ / shares | 31.82 | 30.28 | 27.51
Stock awards outstanding at the end of the period (in dollars per share) | $ / shares | $ 33.71 | 29.60 | 28.59
Employee Stock Purchase Plan |  |  |
Additional disclosures |  |  |
Required period of service | 2 years |  |
Maximum base rate of employee basic pay to purchase any dollar amount of stock (as a percent) | 25.00% |  |
Maximum stock which can be purchased by employee per year | $ | $ 25,000 |  |
Purchases price (in dollars per share) | $ / shares | $ 34.35 | $ 43.81 | $ 53.84
Percentage of the premium fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent) | 1.78% | 0.14% |
Percentage of the discount fair market value of common stock as on the offering date or the market value as on the purchase date (as a percent) |  |  | (0.09%)
ESPP offering through June 2021 |  |  |
Additional disclosures |  |  |
Value of shares to be purchased | $ | $ 304,635 |  |
Stock value of most recent offering (in dollars per share ) | $ / shares | $ 34.35 |  |

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