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Stock Options (Schedule of Qualified Stock Option Activity) (Details) - Qualified Stock Options - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Options outstanding at beginning of year (in shares) 220,820 254,195 301,800
Granted (in shares) 10,750 21,000 29,500
Exercised (in shares) (54,945) (43,775) (75,355)
Canceled (in shares) (3,750) (10,600) (1,750)
Options outstanding at end of year (in shares) 172,875 220,820 254,195
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]      
Outstanding at beginning of year, exercise price (in dollars per share) $ 37.39 $ 34.64 $ 30.73
Granted, exercise price (in dollars per share) 83.99 54.87 53.95
Exercised, exercise price (in dollars per share) 24.83 26.59 26.11
Canceled, exercise price (in dollars per share) 54.57 50.68 53.61
Outstanding at end of year, exercise price (in dollars per share) $ 43.91 $ 37.39 $ 34.64
Options exercisable at end of year (in shares) 111,875 136,220 143,345
Exercisable at end of year, exercise price (in dollars per share) $ 36.44 $ 29.66 $ 25.69
Options available for grant at end of year (in shares) 344,950 353,450 370,750