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Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
$ / shares
shares
Number of Shares (in shares)  
Beginning balance (in shares) | shares 392,300
Granted (in shares) | shares 79,200
Exercised (in shares) | shares (148,800)
Ending balance (in shares) | shares 322,700
Exercisable (in shares) | shares 110,700
Average Exercise Price (in dollars per share)  
Beginning balance (in dollars per share) | $ / shares $ 68.52
Granted (in dollars per share) | $ / shares 105.57
Exercised (in dollars per share) | $ / shares 64.21
Ending balance (in dollars per share) | $ / shares 79.60
Shares exercisable, average exercise price (in dollars per share) | $ / shares $ 66.91
Options outstanding, average remaining life (in years) 4 years 7 months 6 days
Options exercisable, average remaining life (in years) 3 years 3 months 18 days
Outstanding, aggregate intrinsic value | $ $ 16.5
Options exercisable, aggregate intrinsic value | $ $ 7.1