Net Income Per Unrestricted Share - Reconciliation of Numerator and Denominator Used in Calculation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income (Loss)	$ (75.3)	$ 122.3	$ (14.7)	$ 312.4
Dilutive Effect on Income of Equity-based Compensation Equivalent Shares	0.0	0.0	0.0	0.0
Diluted Net Income (Loss) Attributed to Unrestricted Shares	$ (75.3)	$ 122.2	$ (14.7)	$ 312.1
Weighted-Average Unrestricted Shares Outstanding (in shares)	63,628,100	65,362,500	64,469,900	65,710,800
Equity-based Compensation Equivalent Shares (in shares)	0	1,271,000	0	1,086,600
Weighted-Average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares)	63,628,100	66,633,500	64,469,900	66,797,400
Basic (in dollars per share)	$ (1.18)	$ 1.87	$ (0.23)	$ 4.75
Diluted (in dollars per share)	$ (1.18)	$ 1.83	$ (0.23)	$ 4.67
Common Stock
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income (Loss)	$ (75.3)	$ 122.2	$ (14.7)	$ 312.1
Participating Awards
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income (Loss)	$ 0.0	$ 0.1	$ 0.0	$ 0.3