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Income from Continuing Operations Per Unrestricted Share - Reconciliation of Numerator and Denominator in Earnings Per Share (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net Income (Loss) $ (120.5) $ 409.9 $ 531.1
Dilutive Effect on Income of Equity-based Compensation Equivalent Shares $ 0.0 $ 0.0 $ 0.0
Weighted-average Unrestricted Shares Outstanding (in shares) 64,264,400 65,636,100 65,880,900
Equity-based Compensation Equivalent Shares (in shares) 0 1,093,700 667,200
Weighted-average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares) 64,264,400 66,729,800 66,548,100
Basic Income from Continuing Operations Per Unrestricted Share (in dollars per share)     $ 8.04
Diluted Income from Continuing Operations Per Unrestricted Share (in dollars per share)     $ 7.96
Common Stock      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net Income (Loss) $ (120.5) $ 409.5 $ 529.4
Diluted Net Income (Loss) Attributed to Unrestricted Shares $ (120.5) 409.5 529.4
Participating Awards      
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]      
Net Income (Loss)   $ 0.4 $ 1.7