Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares
Number of Shares (in shares)
Beginning balance (in shares) | shares	392,300
Granted (in shares) | shares	79,200
Ending balance (in shares) | shares	471,500
Exercisable (in shares) | shares	257,500
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
Ending balance (in dollars per share) | $ / shares	74.75
Exercisable (in dollars per share) | $ / shares	$ 65.30
Options outstanding, average remaining life (in years)	4 years 6 months
Options exercisable, average remaining life (in years)	3 years 3 months 18 days
Options outstanding, aggregate intrinsic value | $	$ 6.2
Options exercisable, aggregate intrinsic value | $	$ 4.9