Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Numerator for basic and diluted net income per common share:
Net income	$ 80.4	$ 155.7
Denominator for net income per common share:
Basic (in shares)	74,100	75,300
Plus dilutive effect of stock awards (in shares)	600	800
Diluted (in shares)	74,700	76,100
Basic
Basic net income (in dollars per share)	$ 1.09	$ 2.07
Diluted
Diluted net income (in dollars per share)	$ 1.08	$ 2.05