Insurance Liabilities - Reconciliation of the Net Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	$ 41,795	$ 40,142	$ 40,104	$ 43,065
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	28,871	29,026	30,289	32,102
Liability for unpaid losses and loss adjustment expenses	70,666	69,168	$ 70,393	$ 75,167
Reinsurance recoverables	40,700	$ 40,500
Discount on workers' compensation lines | Reconciling Items
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for unpaid losses and loss adjustment expenses	(2,063)
Other product lines | Reconciling Items
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	7,900
Liability for unpaid losses and loss adjustment expenses	16,351
Reinsurance recoverables	8,400
Unallocated loss adjustment expenses | Reconciling Items
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for unpaid losses and loss adjustment expenses	2,729
Reportable Subsegments
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	33,070
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	20,579
Liability for unpaid losses and loss adjustment expenses	53,649
Reportable Subsegments | U.S. Workers' Compensation (before discount)
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	3,445
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	4,633
Liability for unpaid losses and loss adjustment expenses	8,078
Reportable Subsegments | U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	3,153
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,961
Liability for unpaid losses and loss adjustment expenses	6,114
Reportable Subsegments | U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	4,651
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,170
Liability for unpaid losses and loss adjustment expenses	7,821
Reportable Subsegments | U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	5,270
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,516
Liability for unpaid losses and loss adjustment expenses	6,786
Reportable Subsegments | U.S. Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	4,142
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	990
Liability for unpaid losses and loss adjustment expenses	5,132
Reportable Subsegments | U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	705
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,986
Liability for unpaid losses and loss adjustment expenses	2,691
Reportable Subsegments | UK/Europe Casualty and Financial lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	8,288
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,376
Liability for unpaid losses and loss adjustment expenses	10,664
Reportable Subsegments | UK/Europe Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	2,176
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,214
Liability for unpaid losses and loss adjustment expenses	4,390
Reportable Subsegments | UK/Europe and Japan Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Net liability for unpaid losses and loss adjustment expenses as presented in the disaggregated tables below	1,240
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	733
Liability for unpaid losses and loss adjustment expenses	$ 1,973