Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator for basic and diluted net income per common share:
Net income	$ 143.0	$ 339.3	$ 298.7	$ 381.4
Denominator for net income per common share:
Basic (in shares)	75,300	76,200	75,300	76,400
Plus dilutive effect of stock awards (in shares)	1,600	1,000	1,200	1,000
Diluted (in shares)	76,900	77,200	76,500	77,400
Basic
Basic net income (in dollars per share)	$ 1.90	$ 4.45	$ 3.97	$ 4.99
Diluted
Diluted net income (in dollars per share)	$ 1.86	$ 4.39	$ 3.90	$ 4.92