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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Narrative) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development $ (78.8) $ (6.8)
Personal Automobile Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development   16.4
Other Personal Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (10.3)  
Surety Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development 14.1 9.2
Specialty [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (34.8) 43.1
Specialty [Member] | Professional and Executive Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development   (28.6)
Specialty [Member] | Favorable Catastrophe Development [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development 24.8  
Core Commercial [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (39.7) 1.2
Core Commercial [Member] | Commercial Multiple Peril Line [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development   (13.5)
Core Commercial [Member] | Workers' Compensation Line [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development   17.7
Core Commercial [Member] | Certain Liability Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (10.7)  
Core Commercial [Member] | Commercial Multiple Peril, Commercial Automobile, Workers' Compensation And Other Commercial Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (25.6)  
Personal Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development (4.3) 35.1
Personal Lines [Member] | Personal Automobile Lines [Member]    
Liability For Claims And Claims Adjustment Expense [Line Items]    
(Favorable) unfavorable loss and LAE development $ (13.4) $ 11.4