|**Net Income Per Common Share Computations (Tables)** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
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|**Earnings Per Share [Abstract]**                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Schedule of Earnings Per Share Calculation            | The following is a reconciliation of the numerator and denominator of the net income per common share computations:Years Ended December 31,(in millions, except per share data)202320222021Numerator for basic net income per common share:Net income$417.3$439.4$249.1Denominator for net income per common share:Basic75.275.977.2Plus dilutive effect of stock awards0.91.11.2Diluted76.177.078.4Net income per common share data:Basic net income$5.55$5.79$3.22Diluted net income$5.49$5.71$3.18
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|                                                      | Years Ended December 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|(in millions, except per share data)                  | 2023                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Numerator for basic net income per common share:      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
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|Net income                                            | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
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|Denominator for net income per common share:          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Basic                                                 | 75.2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Plus dilutive effect of stock awards                  | 0.9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Diluted                                               | 76.1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
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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.
+ ReferencesReference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482662/260-10-50-1
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