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Stock Options (Schedule of Qualified Stock Option Activity) (Details) - Qualified Stock Options - $ / shares
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Shares      
Options outstanding at beginning of year (in shares) 140,170 172,875 220,820
Granted (in shares) 12,150 9,500 10,750
Exercised (in shares) (28,100) (41,205) (54,945)
Canceled (in shares) (4,500) (1,000) (3,750)
Options outstanding at end of year (in shares) 119,720 140,170 172,875
Exercise Price      
Outstanding at beginning of year, exercise price (in dollars per share) $ 49.78 $ 43.91 $ 37.39
Granted, exercise price (in dollars per share) 105.56 92.50 83.99
Exercised, exercise price (in dollars per share) 38.93 34.89 24.83
Canceled, exercise price (in dollars per share) 58.57 54.49 54.57
Outstanding at end of year, exercise price (in dollars per share) $ 57.65 $ 49.78 $ 43.91
Options exercisable at end of year (in shares) 85,070 96,320 111,875
Exercisable at end of year, exercise price (in dollars per share) $ 47.10 $ 41.56 $ 36.44
Options available for grant at end of year (in shares) 327,250 336,450 344,950