Earnings Per Share (Computation Of Basic And Diluted Earnings Per Share) (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share [Abstract]
Net (loss) income attributable to common shareholders	$ (59.3)	$ 116.6	$ 303.9	$ 363.3
Average basic shares outstanding (in shares)	56.1	58.7	56.5	58.5
Effect of dilutive stock options (in shares)	0.0	0.8	0.8	0.7
Average diluted shares outstanding (in shares)	56.1	59.5	57.3	59.2
Earnings per basic share (in dollars per share)	$ (1.06)	$ 1.99	$ 5.38	$ 6.21
Earnings per diluted share (in dollars per share)	$ (1.06)	$ 1.96	$ 5.30	$ 6.14