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Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2021
USD ($)
$ / shares
shares
Number of Shares (in shares)  
Beginning balance (in shares) | shares 322,700
Granted (in shares) | shares 81,300
Ending balance (in shares) | shares 404,000
Exercisable (in shares) | shares 194,200
Weighted Average Exercise Price (in dollars per share)  
Beginning balance (in dollars per share) | $ / shares $ 79.60
Granted (in dollars per share) | $ / shares 126.00
Ending balance (in dollars per share) | $ / shares 88.94
Exercisable (in dollars per share) | $ / shares $ 69.92
Weighted Average Remaining Contractual Term (Years)  
Outstanding (in years) 4 years 10 months 24 days
Exercisable (in years) 3 years 7 months 6 days
Aggregate Intrinsic Value  
Outstanding | $ $ 22.5
Exercisable | $ $ 14.5