Claim and Claim Adjustment Expense Reserves - Gross and Net Carried Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	$ 8,402	$ 7,738
Gross IBNR Reserves	16,574	15,566
Total Gross Carried Claim and Claim Adjustment Expense Reserves	24,976	23,304
Net Case Reserves	6,207	5,663
Net IBNR Reserves	13,056	12,500
Total Net Carried Claim and Claim Adjustment Expense Reserves	19,263	18,163	$ 16,929	$ 16,300
Property and casualty
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	6,589	5,759
Gross IBNR Reserves	15,093	14,184
Total Gross Carried Claim and Claim Adjustment Expense Reserves	21,682	19,943
Net Case Reserves	5,573	4,978
Net IBNR Reserves	12,761	12,235
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,334	17,213
Other insurance operations
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	1,813	1,979
Gross IBNR Reserves	1,481	1,382
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,294	3,361
Net Case Reserves	634	685
Net IBNR Reserves	295	265
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 929	$ 950