Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator for basic and diluted net income per common share:
Net income	$ 57.0	$ 61.4	$ 438.4	$ 205.8
Denominator for net income per common share:
Basic (in shares)	37.8	38.6	38.1	38.7
Plus dilutive effect of stock awards (in shares)	0.6	0.6	0.5	0.6
Diluted (in shares)	38.4	39.2	38.6	39.3
Net income (loss) per common share data: Basic
Basic net income (in dollars per share)	$ 1.51	$ 1.59	$ 11.52	$ 5.31
Net income (loss) per common share data: Diluted
Diluted net income (in dollars per share)	$ 1.49	$ 1.57	$ 11.36	$ 5.23