Net Income Per Common Share Computations (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	$ 417.3	$ 439.4	$ 249.1
Denominator for net income per common share:
Basic (in shares)	75.2	75.9	77.2
Plus dilutive effect of stock awards (in shares)	0.9	1.1	1.2
Diluted (in shares)	76.1	77.0	78.4
Net income per common share data:
Net income per common share - basic (in dollars per share)	$ 5.55	$ 5.79	$ 3.22
Net income per common share - diluted (in dollars per share)	$ 5.49	$ 5.71	$ 3.18