Claim and Claim Adjustment Expense Reserves (Reconciliation of Liability for Unpaid Claim and Claim Adjustment Expense) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 18,163	$ 16,929		$ 16,300	$ 15,857
Reinsurance receivables	5,141
Total gross liability for unpaid claim and claim adjustment expenses	23,304	22,120	[1]	$ 21,268
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,916	5,563
Reinsurance receivables	1,215
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	9,021	8,430
Reinsurance receivables	1,082
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,276	2,003
Reinsurance receivables	433
Life and Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	582
Reinsurance receivables	93
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Total Net Carried Claim and Claim Adjustment Expense Reserves	368	$ 339
Reinsurance receivables	$ 2,318

[1] As of January 1, 2023, the Company adopted ASU 2018-12, Financial Services-Insurance (Topic 944): Targeted Improvements to the Accounting for Long-Duration Contracts (ASU 2018-12) using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been adjusted to reflect application of the new guidance. See Note A to the Consolidated Financial Statements for additional information.