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Stock Based Compensation (Details)
 12 Months Ended
Dec. 31, 2021
USD ($)
plan
$ / shares
shares
Dec. 31, 2020
USD ($)
$ / shares
shares
Dec. 31, 2019
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 | $ $ 3,450,000 $ 2,670,000 $ 3,350,000
Shares Available for Grant      
Balance at the beginning of the period (in shares) 577,208 664,502 675,059
Granted (in shares) (79,072) (147,576) (74,336)
Forfeited (in shares) 250 49 1,241
Balance at the end of the period (in shares) 560,755 577,208 664,502
Stock award plans      
Additional disclosures      
Stock-based compensation expense | $ $ 4,210,000 $ 3,290,000 $ 2,760,000
Income tax benefit due to stock-based compensation expenses | $ 990,000 $ 770,000 $ 650,000
Unrecognized stock-based compensation expense | $ $ 7,250,000    
Weighted-average period for unrecognized compensation cost to recognize 3 years 2 months 1 day    
Executive Incentive Plan      
Shares Available for Grant      
Shares authorized 62,369 60,233 62,538
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) 291,494 218,991 253,819
Granted (in shares) 79,072 147,576 74,336
Vested (in shares) (92,622) (74,203) (100,299)
Forfeited (in shares) (3,798) (870) (8,865)
Stock awards outstanding at the end of the period (in shares) 274,146 291,494 218,991
Weighted-Average Grant-Date Fair Value      
Stock awards outstanding at the beginning of the period (in dollars per share) | $ / shares $ 33.71 $ 29.60 $ 28.59
Granted (in dollars per share) | $ / shares 36.22 37.41 31.44
Vested (in dollars per share) | $ / shares 32.53 28.95 28.35
Forfeited (in dollars per share) | $ / shares 32.13 31.82 30.28
Stock awards outstanding at the end of the period (in dollars per share) | $ / shares $ 34.86 33.71 29.60
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 $ 49.98 $ 34.35 $ 43.81
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) (0.42%)    
ESPP offering through June 2021      
Additional disclosures      
Value of shares to be purchased | $ $ 187,740    
Stock value of most recent offering (in dollars per share ) | $ / shares $ 49.98