Claim and Claim Adjustment Expense Reserves - Gross and Net Carried Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	$ 7,738	$ 7,577
Gross IBNR Reserves	15,566	14,543
Total Gross Carried Claim and Claim Adjustment Expense Reserves	23,304	22,120
Net Case Reserves	5,663	5,509
Net IBNR Reserves	12,500	11,420
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,163	16,929	$ 16,300	$ 15,857
Property and casualty
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	5,759	5,502
Gross IBNR Reserves	14,184	13,174
Total Gross Carried Claim and Claim Adjustment Expense Reserves	19,943	18,676
Net Case Reserves	4,978	4,805
Net IBNR Reserves	12,235	11,191
Total Net Carried Claim and Claim Adjustment Expense Reserves	17,213	15,996
Other insurance operations
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	1,979	2,075
Gross IBNR Reserves	1,382	1,369
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,361	3,444
Net Case Reserves	685	704
Net IBNR Reserves	265	229
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 950	$ 933