Net Income Per Unrestricted Share - Reconciliation of Numerator and Denominator Used in Calculation (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income	$ (62.6)	$ 126.1	$ 60.6	$ 190.1
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	$ (62.6)	$ 126.0	$ 60.5	$ 189.9
Weighted-Average Unrestricted Shares Outstanding (in shares)	64,376,800	65,257,600	64,897,800	65,886,800
Equity-based Compensation Equivalent Shares (in shares)	0	763,200	1,153,700	781,100
Weighted-Average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares)	64,376,800	66,020,800	66,051,500	66,667,900
Basic (in dollars per share)	$ (0.97)	$ 1.93	$ 0.93	$ 2.88
Diluted (in dollars per share)	$ (0.97)	$ 1.91	$ 0.92	$ 2.85
Common Stock
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income	$ (62.6)	$ 126.0	$ 60.5	$ 189.9
Participating Awards
Schedule of Basic and Diluted Earnings Per Share [Line Items]
Net Income	$ 0.0	$ 0.1	$ 0.1	$ 0.2