Incentive Plans (Summary of Changes in Stock Options Outstanding) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019
Number of Shares (in shares)
Beginning balance (in shares)	310,362
Granted (in shares)	99,400
Exercised (in shares)	(12,662)
Forfeited (in shares)	(4,800)
Ending balance (in shares)	392,300
Exercisable (in shares)	174,300
Average Exercise Price (in dollars per share)
Beginning balance (in dollars per share)	$ 65.71
Granted (in dollars per share)	76.15
Exercised (in dollars per share)	56.63
Forfeited (in dollars per share)	76.15
Ending balance (in dollars per share)	68.52
Shares exercisable, average exercise price (in dollars per share)	$ 64.00
Options outstanding, average remaining life (in years)	4 years 3 months 18 days
Options exercisable, average remaining life (in years)	3 years
Outstanding, aggregate intrinsic value	$ 19.0
Options exercisable, aggregate intrinsic value	$ 9.2

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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- Definition

The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.