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Reserve for Unpaid Losses and Loss Adjustment Expenses - Reconciliation of Loss Development to Liability for Unpaid Losses and Loss Adjustment Expenses, P&C (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid losses and loss adjustment expenses $ 44,610 $ 42,318    
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 300      
Deferred Revenue 64 209    
Asbestos and Environmental        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid for Accident Years not Displayed in Triangles 224      
Asbestos and Environmental | Reinsurance Policy, Type [Axis]: Asbestos and Environmental        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 1,500      
Deferred Revenue 850      
General liability | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 105      
Marine [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 38      
Property Insurance [Member] | P&C Business Insurance [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 3      
Automobile liability | P&C Business Insurance [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 14      
Professional liability | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 95      
Surety Product Line [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 1      
Assumed Reinsurance [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 27      
Short-Duration Insurance, Other [Member] | Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Policyholder Benefits and Claims Incurred, Ceded 17      
Property and Casualty Insurance Products        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 71,145      
Cumulative Paid for Accident Years Displayed in Triangles (47,764)      
Unpaid for Accident Years not Displayed in Triangles 5,381      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 1,222      
Amount of discount 333 339 $ 347  
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 29,651 27,348 26,618 $ 25,368
Reinsurance and Other Recoverables 6,753 6,696 6,465 6,081
Unpaid losses and loss adjustment expenses 36,404 34,044 33,083 $ 31,449
Policyholder Benefits and Claims Incurred, Ceded 1,241 $ 1,043 $ 1,338  
Segment Reconciling Items | Asbestos and Environmental        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid for Accident Years not Displayed in Triangles 315      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 0      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 315      
Reinsurance and Other Recoverables 2,038      
Unpaid losses and loss adjustment expenses 2,353      
Segment Reconciling Items | Workers' Compensation        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 18,389      
Cumulative Paid for Accident Years Displayed in Triangles (9,665)      
Unpaid for Accident Years not Displayed in Triangles 3,836      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 419      
Amount of discount 319      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 12,660      
Reinsurance and Other Recoverables 1,721      
Unpaid losses and loss adjustment expenses 14,381      
Segment Reconciling Items | General liability        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 9,416      
Cumulative Paid for Accident Years Displayed in Triangles (4,177)      
Unpaid for Accident Years not Displayed in Triangles 494      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 202      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 5,935      
Reinsurance and Other Recoverables 1,230      
Unpaid losses and loss adjustment expenses 7,165      
Segment Reconciling Items | Marine [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 1,427      
Cumulative Paid for Accident Years Displayed in Triangles (1,114)      
Unpaid for Accident Years not Displayed in Triangles 15      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 15      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 343      
Reinsurance and Other Recoverables 244      
Unpaid losses and loss adjustment expenses 587      
Segment Reconciling Items | Package Business [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 8,630      
Cumulative Paid for Accident Years Displayed in Triangles (6,279)      
Unpaid for Accident Years not Displayed in Triangles 109      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 130      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 2,590      
Reinsurance and Other Recoverables 31      
Unpaid losses and loss adjustment expenses 2,621      
Segment Reconciling Items | Property Insurance [Member] | P&C Business Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 4,244      
Cumulative Paid for Accident Years Displayed in Triangles (3,676)      
Unpaid for Accident Years not Displayed in Triangles 13      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 31      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 612      
Reinsurance and Other Recoverables 299      
Unpaid losses and loss adjustment expenses 911      
Segment Reconciling Items | Property Insurance [Member] | P&C Personal Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 5,876      
Cumulative Paid for Accident Years Displayed in Triangles (5,521)      
Unpaid for Accident Years not Displayed in Triangles 6      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 42      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 403      
Reinsurance and Other Recoverables 5      
Unpaid losses and loss adjustment expenses 408      
Segment Reconciling Items | Automobile liability | P&C Business Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 4,606      
Cumulative Paid for Accident Years Displayed in Triangles (3,106)      
Unpaid for Accident Years not Displayed in Triangles 16      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 38      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 1,554      
Reinsurance and Other Recoverables 113      
Unpaid losses and loss adjustment expenses 1,667      
Segment Reconciling Items | Automobile liability | P&C Personal Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 10,862      
Cumulative Paid for Accident Years Displayed in Triangles (9,240)      
Unpaid for Accident Years not Displayed in Triangles 31      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 74      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 1,727      
Reinsurance and Other Recoverables 18      
Unpaid losses and loss adjustment expenses 1,745      
Segment Reconciling Items | Commercial automobile physical damage | P&C Business Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 239      
Cumulative Paid for Accident Years Displayed in Triangles (213)      
Unpaid for Accident Years not Displayed in Triangles 4      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 1      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 31      
Reinsurance and Other Recoverables 0      
Unpaid losses and loss adjustment expenses 31      
Segment Reconciling Items | Professional liability        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 3,042      
Cumulative Paid for Accident Years Displayed in Triangles (1,567)      
Unpaid for Accident Years not Displayed in Triangles 55      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 47      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 1,577      
Reinsurance and Other Recoverables 611      
Unpaid losses and loss adjustment expenses 2,188      
Segment Reconciling Items | Surety Product Line [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 667      
Cumulative Paid for Accident Years Displayed in Triangles (276)      
Unpaid for Accident Years not Displayed in Triangles 19      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 33      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 443      
Reinsurance and Other Recoverables 14      
Unpaid losses and loss adjustment expenses 457      
Segment Reconciling Items | Assumed Reinsurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 2,107      
Cumulative Paid for Accident Years Displayed in Triangles (1,356)      
Unpaid for Accident Years not Displayed in Triangles 0      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 7      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 758      
Reinsurance and Other Recoverables 30      
Unpaid losses and loss adjustment expenses 788      
Segment Reconciling Items | Personal automobile physical damage | P&C Personal Insurance [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Cumulative Incurred for Accident Years Displayed in Triangles 1,640      
Cumulative Paid for Accident Years Displayed in Triangles (1,574)      
Unpaid for Accident Years not Displayed in Triangles 7      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 5      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 78      
Reinsurance and Other Recoverables 0      
Unpaid losses and loss adjustment expenses 78      
Segment Reconciling Items | Short-Duration Insurance, Other [Member]        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid for Accident Years not Displayed in Triangles 168      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 7      
Amount of discount 14      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 161      
Reinsurance and Other Recoverables 356      
Unpaid losses and loss adjustment expenses 517      
Segment Reconciling Items | Other operations        
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]        
Unpaid for Accident Years not Displayed in Triangles 293      
Unpaid Unallocated Loss Adjustment Expenses, Net of Reinsurance 171      
Amount of discount 0      
Unpaid Losses and Loss Adjustment Expenses, Net of Reinsurance 464      
Reinsurance and Other Recoverables 43      
Unpaid losses and loss adjustment expenses $ 507