v3.26.3
Reinsurance - Credit Quality Information (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Jun. 30, 2026
Dec. 31, 2025
Sep. 30, 2025
Jun. 30, 2025
Dec. 31, 2024
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross $ 5,673   $ 7,260      
Reinsurance Recoverable, Allowance for Credit Loss (72)   (69) $ (76)    
Net Reinsurance Recoverables 5,601   7,191      
Mandatory and Voluntary Risk Pools [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 191   204      
Captives [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 484   465      
Other not rated companies [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 228   243      
AMBestRated [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 4,770   6,348      
AM Best, A++ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 683   2,161      
AM Best, A+ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 2,781   2,791      
AM Best, A Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 692   765      
AM Best, A- Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 588   608      
AM Best, B++ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 4   2      
AM Best, Below B plus plus Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 22   21      
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 5,144   6,750      
Reinsurance Recoverable, Allowance for Credit Loss (69) $ (67) (66) (73) $ (72) $ (72)
Net Reinsurance Recoverables 5,075   6,684      
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Mandatory and Voluntary Risk Pools [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 191   204      
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Captives [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 484   465      
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Other not rated companies [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 222   238      
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Before Disputed Amounts [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverable, Allowance for Credit Loss (25) (23) (24) (22) (21) (24)
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Disputes [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverable, Allowance for Credit Loss (44) $ (44) (42) (51) $ (51) $ (48)
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | AMBestRated [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 4,247   5,843      
Operating Segments [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 322   295      
Reinsurance Recoverable, Allowance for Credit Loss (1)   (1) (1)    
Net Reinsurance Recoverables 321   294      
Operating Segments [Member] | Group Insurance Policy [Member] | Mandatory and Voluntary Risk Pools [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Operating Segments [Member] | Group Insurance Policy [Member] | Captives [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Operating Segments [Member] | Group Insurance Policy [Member] | Other not rated companies [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 6   5      
Operating Segments [Member] | Group Insurance Policy [Member] | AMBestRated [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 316   290      
Operating Segments [Member] | AM Best, A++ Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 683   2,161      
Operating Segments [Member] | AM Best, A++ Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Operating Segments [Member] | AM Best, A+ Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 2,264   2,292      
Operating Segments [Member] | AM Best, A+ Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 312   286      
Operating Segments [Member] | AM Best, A Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 691   764      
Operating Segments [Member] | AM Best, A Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 1   1      
Operating Segments [Member] | AM Best, A- Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 585   603      
Operating Segments [Member] | AM Best, A- Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 3   3      
Operating Segments [Member] | AM Best, B++ Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 2   2      
Operating Segments [Member] | AM Best, B++ Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Operating Segments [Member] | AM Best, Below B plus plus Rating [Member] | Property, Liability and Casualty Insurance Product Line            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 22   21      
Operating Segments [Member] | AM Best, Below B plus plus Rating [Member] | Group Insurance Policy [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 207   215      
Reinsurance Recoverable, Allowance for Credit Loss (2)   (2) $ (2)    
Net Reinsurance Recoverables 205   213      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | Mandatory and Voluntary Risk Pools [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | Captives [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | Other not rated companies [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AMBestRated [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 207   215      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, A++ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, A+ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 205   213      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, A Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, A- Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 0   2      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, B++ Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross 2   0      
Segment Reporting, Reconciling Item, Corporate Nonsegment [Member] | AM Best, Below B plus plus Rating [Member]            
Reinsurance Recoverable, Credit Quality Indicator [Line Items]            
Reinsurance Recoverables, Gross $ 0   $ 0