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Reserve for Unpaid Losses and Loss Adjustment Expenses (Details)
$ in Millions
 3 Months Ended
Dec. 31, 2024
USD ($)
Asbestos and Environmental  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Increase (Decrease) in Deferred Revenue $ 62
Liability for Asbestos and Environmental Claims, Gross, Period Increase (Decrease) 203
Asbestos and Environmental | Maximum | Reinsurance Contract [Axis]: Asbestos and Environmental  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Reinsurance, Excess Retention, Amount Reinsured, Per Policy 1,500
Navigator's ADC  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Increase (Decrease) in Deferred Revenue 58
Asbestos Issue  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 167
Environmental Issue  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 36
Property, Liability and Casualty Insurance Product Line  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 101
Supplemental Information for Property Casualty Insurance Underwriters Prior Year Claims and Claims Adjustment Expense, net of retroactive reinsurance benefit 97
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Increase (Decrease) in Deferred Revenue 4
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Catastrophe  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (49)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Asbestos and Environmental  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 141
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Change in Workers Compensation Discount Including Accretion  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 10
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | General Liability  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 130
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Marine  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 0
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Package Business  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 0
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Property Insurance | P&C Commercial Lines  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 0
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Property Insurance | P&C Personal Lines  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (13)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Professional Liability Insurance  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (20)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Surety Product Line  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (34)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Assumed Reinsurance  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 0
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Automobiles | P&C Commercial Lines  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 21
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Automobiles | P&C Personal Lines  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (17)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Uncollectible Reinsurance  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense (19)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Insurance, Other  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense 17
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment | Workers' Compensation  
Liability for Claims and Claims Adjustment Expense [Line Items]  
Prior Year Claims and Claims Adjustment Expense $ (70)