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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 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Schedule of Basic and Diluted Earnings Per Share [Line Items]        
Net Income $ 122.3 $ 129.0 $ 312.4 $ 406.4
Dilutive Effect on Income of Equity-based Compensation Equivalent Shares 0.0 0.0 0.0 0.0
Diluted Net Income Attributed to Unrestricted Shares $ 122.2 $ 128.6 $ 312.1 $ 405.0
Weighted-Average Unrestricted Shares Outstanding (in shares) 65,362,500 66,622,400 65,710,800 65,621,800
Equity-based Compensation Equivalent Shares (in shares) 1,271,000.0 585,300 1,086,600 719,400
Weighted-Average Unrestricted Shares and Equivalent Shares Outstanding Assuming Dilution (in shares) 66,633,500 67,207,700 66,797,400 66,341,200
Basic (in dollars per share) $ 1.87 $ 1.93 $ 4.75 $ 6.17
Diluted (in dollars per share) $ 1.83 $ 1.91 $ 4.67 $ 6.10
Common Stock        
Schedule of Basic and Diluted Earnings Per Share [Line Items]        
Net Income $ 122.2 $ 128.6 $ 312.1 $ 405.0
Participating Awards        
Schedule of Basic and Diluted Earnings Per Share [Line Items]        
Net Income $ 0.1 $ 0.4 $ 0.3 $ 1.4