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Incentive Plans (Summary of Additional Stock Option Information) (Details)
 12 Months Ended
Dec. 31, 2020
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Options outstanding (in shares) 322,700
Options outstanding, average remaining life (in years) 4 years 7 months 6 days
Options exercisable (in shares) 110,700
Options exercisable, average remaining life (in years) 3 years 3 months 18 days
$50.00 to $59.99  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 50.00
Upper range limit of exercise price (in dollars per share) | $ / shares $ 59.99
Options outstanding (in shares) 7,300
Options outstanding, average remaining life (in years) 2 years 1 month 6 days
Options exercisable (in shares) 7,300
Options exercisable, average remaining life (in years) 2 years 1 month 6 days
$60.00 to $69.99  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 60.00
Upper range limit of exercise price (in dollars per share) | $ / shares $ 69.99
Options outstanding (in shares) 69,500
Options outstanding, average remaining life (in years) 3 years 1 month 6 days
Options exercisable (in shares) 69,500
Options exercisable, average remaining life (in years) 3 years 1 month 6 days
$70.00 to $79.99  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 70.00
Upper range limit of exercise price (in dollars per share) | $ / shares $ 79.99
Options outstanding (in shares) 166,700
Options outstanding, average remaining life (in years) 4 years 7 months 6 days
Options exercisable (in shares) 33,900
Options exercisable, average remaining life (in years) 3 years 10 months 24 days
$80.00 to $89.99  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 80.00
Upper range limit of exercise price (in dollars per share) | $ / shares $ 89.99
Options outstanding (in shares) 3,600
Options outstanding, average remaining life (in years) 6 years 2 months 12 days
Options exercisable (in shares) 0
Options exercisable, average remaining life (in years)
$100.00 to $109.99  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Lower range limit of exercise price (in dollars per share) | $ / shares $ 100.00
Upper range limit of exercise price (in dollars per share) | $ / shares $ 109.99
Options outstanding (in shares) 75,600
Options outstanding, average remaining life (in years) 6 years 1 month 6 days
Options exercisable (in shares) 0
Options exercisable, average remaining life (in years)