Claim and Claim Adjustment Expense Reserves - Gross and Net Carried Reserves (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	$ 9,063	$ 8,402
Gross IBNR Reserves	17,536	16,574
Total Gross Carried Claim and Claim Adjustment Expense Reserves	26,599	24,976
Net Case Reserves	6,814	6,207
Net IBNR Reserves	13,803	13,056
Total Net Carried Claim and Claim Adjustment Expense Reserves	20,617	19,263	$ 18,163	$ 16,929
Property and casualty
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	7,311	6,589
Gross IBNR Reserves	16,098	15,093
Total Gross Carried Claim and Claim Adjustment Expense Reserves	23,409	21,682
Net Case Reserves	6,189	5,573
Net IBNR Reserves	13,536	12,761
Total Net Carried Claim and Claim Adjustment Expense Reserves	19,725	18,334
Other insurance operations
Gross and Net Carried Reserves [Abstract]
Gross Case Reserves	1,752	1,813
Gross IBNR Reserves	1,438	1,481
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,190	3,294
Net Case Reserves	625	634
Net IBNR Reserves	267	295
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 892	$ 929