Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 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
Weighted 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
Exercisable (in dollars per share) | $ / shares	$ 66.91
Options outstanding, average remaining life (in years)	4 years 10 months 24 days
Options exercisable, average remaining life (in years)	3 years 6 months
Options outstanding, aggregate intrinsic value | $	$ 15.7
Options exercisable, aggregate intrinsic value | $	$ 6.8