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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Liability For Claims And Claims Adjustment Expense [Line Items]        
Catastrophe losses $ 276.3 $ 375.9 $ 690.1  
(Favorable) unfavorable loss and LAE development (96.4) (109.4) (15.9)  
Loss and loss adjustment expense reserves $ 7,755.2 7,461.2 7,308.1 $ 7,012.6
U.S. Companies [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross incurred claims, percentage 4.00%      
Asbestos Issue [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Loss and loss adjustment expense reserves $ 27.5 55.8    
Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development (18.2) 0.1 40.9  
Loss and loss adjustment expense reserves 2,131.4 2,109.0    
Core Commercial [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development (18.1) (50.0) (2.0)  
Loss and loss adjustment expense reserves 3,861.3 3,613.4    
Specialty [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development (60.8) (60.9) (57.1)  
Loss and loss adjustment expense reserves 1,732.9 1,675.9    
Workers' Compensation Line [Member] | Core Commercial [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development     (2.0)  
Loss and loss adjustment expense reserves 768.7 761.1    
Personal Automobile Lines [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development     $ 40.9  
Loss and loss adjustment expense reserves 1,655.4 1,635.6    
Specialty Property and Casualty Lines Division, Professional and Executive Lines Division and Marine Division [Member] | Specialty [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development (60.8) (60.9)    
Commercial Multiple Peril Line [Member] | Core Commercial [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development (18.1)      
Loss and loss adjustment expense reserves $ 1,712.8 1,626.9    
Homeowners Line Segment [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development   (18.2)    
Favorable Catastrophe Development [Member] | Core Commercial [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development   (32.3)    
Other Personal and Homeowners Lines [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
(Favorable) unfavorable loss and LAE development   $ 0.1