Insurance Liabilities - Reconciliation of the Net Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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	$ 40,142	$ 40,104	$ 43,065	$ 43,813
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	29,026	30,289	32,102	35,213
Liability for unpaid losses and loss adjustment expenses	69,168	70,393	$ 75,167	$ 79,026
Reinsurance recoverables	40,500	$ 41,400
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	(2,111)
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	7,000
Liability for unpaid losses and loss adjustment expenses	15,384
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	2,688
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	32,522
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	20,685
Liability for unpaid losses and loss adjustment expenses	53,207
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	3,468
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	4,852
Liability for unpaid losses and loss adjustment expenses	8,320
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	3,208
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,139
Liability for unpaid losses and loss adjustment expenses	6,347
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	4,387
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,416
Liability for unpaid losses and loss adjustment expenses	7,803
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	5,422
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,614
Liability for unpaid losses and loss adjustment expenses	7,036
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	4,297
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,233
Liability for unpaid losses and loss adjustment expenses	5,530
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	836
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,048
Liability for unpaid losses and loss adjustment expenses	2,884
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	7,280
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,952
Liability for unpaid losses and loss adjustment expenses	9,232
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	2,355
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,761
Liability for unpaid losses and loss adjustment expenses	4,116
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	1,269
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	670
Liability for unpaid losses and loss adjustment expenses	$ 1,939