Insurance Liabilities - Reconciliation of the Net Incurred and Paid Claims Development (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
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	$ 43,065	$ 43,813	$ 43,289	$ 47,423
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	32,102	35,213	34,431	31,069
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	75,167	79,026	$ 77,720	$ 78,328
Reinsurance recoverables	73,700	$ 76,300
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 2022 and $14 in 2021	(2,532)
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	6,600
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	15,072
Reinsurance recoverables	8,500
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 2022 and $14 in 2021	2,472
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	36,295
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	23,860
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	60,155
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	3,962
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	5,573
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	9,535
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	3,638
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,701
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	7,339
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	3,858
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,872
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	7,730
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	5,899
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,773
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	7,672
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	6,815
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	3,295
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	10,110
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	794
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	2,052
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	2,846
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	6,984
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,538
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	8,522
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	2,717
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	1,464
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	4,181
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	1,628
Reinsurance recoverable on unpaid losses and loss adjustment expenses included in the disaggregated tables below	592
Liability for unpaid losses and loss adjustment expenses, including allowance for credit losses of $14 in 2022 and $14 in 2021	$ 2,220