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Net Income Per Unrestricted Share - Reconciliation of Numerator and Denominator Used in Calculation (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Schedule of Basic and Diluted Earnings Per Share [Line Items]    
Net Income $ 123.2 $ 64.0
Dilutive Effect on Income of Equity-based Compensation Equivalent Shares 0.0 0.0
Diluted Net Income Attributed to Unrestricted Shares $ 123.1 $ 63.9
Weighted-Average Unrestricted Shares Outstanding (in shares) 65,424,600 66,515,900
Equity-based Compensation Equivalent Shares (in shares) 1,128,200 458,300
Weighted-Average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares) 66,552,800 66,974,200
Basic (in dollars per share) $ 1.88 $ 0.96
Diluted (in dollars per share) $ 1.85 $ 0.95
Common Stock    
Schedule of Basic and Diluted Earnings Per Share [Line Items]    
Net Income $ 123.1 $ 63.9
Participating Awards    
Schedule of Basic and Diluted Earnings Per Share [Line Items]    
Net Income $ 0.1 $ 0.1