Net Income Per Common Share Computations (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator for basic and diluted net income per common share:
Net income	$ 339.3	$ 108.3	$ 381.4	$ 144.4
Denominator for net income per common share:
Basic (in shares)	38.1	38.7	38.2	38.8
Plus dilutive effect of stock awards (in shares)	0.5	0.6	0.5	0.6
Diluted (in shares)	38.6	39.3	38.7	39.4
Net income (loss) per common share data: Basic
Net income (loss) per common share - basic (in dollars per share)	$ 8.91	$ 2.80	$ 9.98	$ 3.72
Net income (loss) per common share data: Diluted
Net income (loss) per common share - diluted (in dollars per share)	$ 8.79	$ 2.76	$ 9.85	$ 3.66