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Liabilities for Outstanding Claims, Losses and Loss Adjustment Expenses (Schedule of Carried Reserves) (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross $ 7,755.2 $ 7,461.2 $ 7,308.1 $ 7,012.6
Ceded (1,850.3) (1,829.8) (1,795.0) (1,748.6)
Net 5,904.9 5,631.4 $ 5,513.1 $ 5,264.0
Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 3,861.3 3,613.4    
Ceded (716.4) (624.2)    
Net 3,144.9 2,989.2    
Specialty Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 1,732.9 1,675.9    
Ceded (322.9) (334.6)    
Net 1,410.0 1,341.3    
Other Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 29.6 62.9    
Ceded (17.3) (50.3)    
Net 12.3 12.6    
Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 2,131.4 2,109.0    
Ceded (793.7) (820.7)    
Net 1,337.7 1,288.3    
Commercial Multiple Peril Line [Member] | Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 1,712.8 1,626.9    
Ceded (247.0) (202.0)    
Net 1,465.8 1,424.9    
Workers' Compensation Line [Member] | Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 768.7 761.1    
Ceded (129.3) (131.6)    
Net 639.4 629.5    
Commercial Automobile Line [Member] | Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 581.7 513.5    
Ceded (27.1) (22.3)    
Net 554.6 491.2    
General Liability and Umbrella - Occurrence [Member] | Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 697.2 620.7    
Ceded (293.7) (258.2)    
Net 403.5 362.5    
Other Core Commercial [Member] | Core Commercial Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 100.9 91.2    
Ceded (19.3) (10.1)    
Net 81.6 81.1    
Specialty Property & Casualty [Member] | Specialty Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 845.3 837.9    
Ceded (249.7) (257.3)    
Net 595.6 580.6    
General Liability - Claims Made [Member] | Specialty Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 472.9 454.2    
Ceded (30.6) (30.6)    
Net 442.3 423.6    
Other Specialty [Member] | Specialty Lines Segment [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 414.7 383.8    
Ceded (42.6) (46.7)    
Net 372.1 337.1    
Personal Automobile Lines [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 1,655.4 1,635.6    
Ceded (787.6) (817.4)    
Net 867.8 818.2    
Homeowners [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 300.8 323.6    
Ceded (3.9) (1.5)    
Net 296.9 322.1    
Other Personal [Member] | Personal Lines [Member]        
Liability For Claims And Claims Adjustment Expense [Line Items]        
Gross 175.2 149.8    
Ceded (2.2) (1.8)    
Net $ 173.0 $ 148.0