Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Net income	$ 130.4	$ 141.9	$ 445.6	$ 402.7
Weighted average number of shares outstanding, basic (in shares)	35.5	35.4	35.5	35.8
Add: Potential effect of dilutive securities attributable to stock-based payments	0.2	0.1	0.1	0.1
Weighted-average shares outstanding – diluted	35.7	35.5	35.6	35.9
Earnings per share - Basic (in dollars per share)	$ 3.67	$ 4.00	$ 12.55	$ 11.25
Earnings per share - Diluted (in dollars per share)	$ 3.65	$ 3.99	$ 12.51	$ 11.22