Nature of Operations and Significant Accounting Policies (Computation Of Basic And Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Net income attributable to common shareholders	$ 401.1	$ 435.4	$ 181.0
Average basic shares outstanding (in shares)	56.4	58.4	58.3
Effect of dilutive share-based awards (in shares)	0.8	0.8	0.5
Average diluted shares outstanding (in shares)	57.2	59.2	58.8
Basic earnings per share (in dollars per share)	$ 7.11	$ 7.46	$ 3.10
Diluted earnings per share (in dollars per share)	$ 7.01	$ 7.36	$ 3.08