Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 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	259,500
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	$ 65.36
Options outstanding, average remaining life (in years)	4 years 3 months 18 days
Options exercisable, average remaining life (in years)	3 years
Options outstanding, aggregate intrinsic value | $	$ 17.8
Options exercisable, aggregate intrinsic value | $	$ 12.3