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Earnings Per Share (Reconciliation of Basic and Diluted Earnings Per Share Computations) (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Net income per share - basic    
Net income attributable to common stockholders $ 55.3 $ 89.3
Weighted average common shares outstanding (in shares) 27,131 30,235
Earnings per common share (in dollars per share) $ 2.04 $ 2.95
Net income per share - diluted    
Net income attributable to common stockholders $ 55.3 $ 89.3
Weighted average common shares outstanding (in shares) 27,131 30,235
Common equivalent shares:    
Dilutive share-based awards (in shares) 357 306
Weighted average common shares outstanding - assuming dilution (in shares) 27,488 30,541
Earnings per common share assuming dilution (in dollars per share) $ 2.01 $ 2.92