Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Numerator for basic and diluted net income per common share:
Net income	$ 61.0	$ 57.0	$ 359.7	$ 438.4
Denominator for net income per common share:
Basic (in shares)	75,200	75,600	75,300	76,100
Plus dilutive effect of stock awards (in shares)	1,900	1,100	1,400	1,100
Diluted (in shares)	77,100	76,700	76,700	77,200
Basic
Basic net income (in dollars per share)	$ 0.81	$ 0.75	$ 4.78	$ 5.76
Diluted
Diluted net income (in dollars per share)	$ 0.79	$ 0.74	$ 4.69	$ 5.68