Claim and Claim Adjustment Expense Reserves (Gross and Net Carried Claim and Claim Adjustment Expense Reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Claims and Claims Adjustment Expense [Line Items]
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	$ 22,120	$ 21,269
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
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	2,023	1,604
Gross IBNR Reserves	5,403	5,527
Total Gross Carried Claim and Claim Adjustment Expense Reserves	7,426	7,131
Net Case Reserves	1,697	1,392
Net IBNR Reserves	4,282	4,524
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,979	5,916
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,690	3,291
Gross IBNR Reserves	7,646	6,812
Total Gross Carried Claim and Claim Adjustment Expense Reserves	11,336	10,103
Net Case Reserves	3,135	2,878
Net IBNR Reserves	6,804	6,143
Total Net Carried Claim and Claim Adjustment Expense Reserves	9,939	9,021
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	876	864
Gross IBNR Reserves	2,044	1,845
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,920	2,709
Net Case Reserves	741	708
Net IBNR Reserves	1,675	1,568
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,416	2,276
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	572	626
Gross IBNR Reserves	50	49
Total Gross Carried Claim and Claim Adjustment Expense Reserves	622	675
Net Case Reserves	514	556
Net IBNR Reserves	27	26
Total Net Carried Claim and Claim Adjustment Expense Reserves	541	582
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,241	1,353
Gross IBNR Reserves	1,431	1,333
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,672	2,686
Net Case Reserves	120	129
Net IBNR Reserves	268	239
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 388	$ 368