Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Numerator for basic and diluted net income per common share:
Net income	$ 155.7	$ 42.1
Denominator for net income per common share:
Basic (in shares)	37,600	38,300
Plus dilutive effect of stock awards (in shares)	500	500
Diluted (in shares)	38,100	38,800
Basic
Basic net income (in dollars per share)	$ 4.14	$ 1.10
Diluted
Diluted net income (in dollars per share)	$ 4.09	$ 1.08