Earnings Per Share (Computation of Basic and Diluted Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income	$ 590.1	$ 497.1	$ 464.0
Weighted-average shares outstanding - basic (in shares)	35.5	35.7	37.2
Add: Potential effect of diluted securities attributable to stock-based payments	0.2	0.1	0.3
Weighted-average shares outstanding - diluted (in shares)	35.7	35.8	37.5
Earnings per share – Basic:
Basic Earnings (Loss) per Share (in usd per share)	$ 16.61	$ 13.92	$ 12.47
Earnings per share – Diluted:
Diluted Earnings (Loss) per Share (in usd per share)	$ 16.54	$ 13.88	$ 12.39