|**Net Income Per Common Share Computations (Tables)**        | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
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|**Earnings Per Share [Abstract]**                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Schedule of Earnings Per Share, Basic and Diluted            | The following is a reconciliation of the numerator and denominator of the net income per common share computations:Three Months Ended June 30,Six Months Ended June 30,(in millions, except per share data)2022202120222021Numerator for basic and diluted net income per common share:Net income$339.3$108.3$381.4$144.4Denominator for net income per common share:Basic38.138.738.238.8Plus dilutive effect of stock awards0.50.60.50.6Diluted38.639.338.739.4Net income per common share data:Basic net income$8.91$2.80$9.98$3.72Diluted net income$8.79$2.76$9.85$3.66
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|                                                             | Three Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|(in millions, except per share data)                         | 2022                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Numerator for basic and diluted net income per common share: |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Net income                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Denominator for net income per common share:                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Basic                                                        | 38.1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Plus dilutive effect of stock awards                         | 0.5                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Diluted                                                      | 38.6                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|Net income per common share data:                            |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|Basic net income                                             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Diluted net income                                           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

X- ReferencesNo definition available.
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duration X- DefinitionTabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations.
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