Insurance Liabilities - Reconciliation of the Net Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
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	$ 40,104	$ 43,065	$ 43,813	$ 43,289
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	30,289	32,102	35,213	34,431
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	70,393	75,167	$ 79,026	$ 77,720
Reinsurance recoverables	69,800	$ 71,800
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, including allowance for credit losses of $14 in 2023 and $14 in 2022	(2,337)
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	6,000
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	14,739
Reinsurance recoverables	8,700
Unallocated loss adjustment expenses | Reconciling Items
Liability for Future Policy Benefit, by Product Segment [Line Items]
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	2,268
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	34,006
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	21,717
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	55,723
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	3,888
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	5,203
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	9,091
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	3,321
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,272
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	6,593
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	4,112
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,676
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	7,788
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	5,672
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,622
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	7,294
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	4,403
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,494
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	5,897
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	767
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,163
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	2,930
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	7,447
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,951
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	9,398
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	2,913
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,665
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	4,578
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	1,483
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	671
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2023 and $14 in 2022	$ 2,154