Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 10,989	$ 11,395
Gross IBNR Reserves	10,731	10,589
Total Gross Carried Claim and Claim Adjustment Expense Reserves	21,720	21,984	$ 22,004	$ 22,343
Net Case Reserves	9,178	9,523
Net IBNR Reserves	8,707	8,442
Total Net Carried Claim and Claim Adjustment Expense Reserves	17,885	17,965	$ 18,070	$ 18,249
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,481	1,623
Gross IBNR Reserves	3,757	3,842
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,238	5,465
Net Case Reserves	1,343	1,483
Net IBNR Reserves	3,333	3,348
Total Net Carried Claim and Claim Adjustment Expense Reserves	4,676	4,831
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,937	4,181
Gross IBNR Reserves	4,719	4,562
Total Gross Carried Claim and Claim Adjustment Expense Reserves	8,656	8,743
Net Case Reserves	3,543	3,831
Net IBNR Reserves	4,306	4,167
Total Net Carried Claim and Claim Adjustment Expense Reserves	7,849	7,998
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	858	867
Gross IBNR Reserves	1,018	883
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,876	1,750
Net Case Reserves	759	749
Net IBNR Reserves	869	775
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,628	1,524
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,576	3,516
Gross IBNR Reserves	140	85
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,716	3,601
Net Case Reserves	3,441	3,364
Net IBNR Reserves	116	56
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,557	3,420
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,137	1,208
Gross IBNR Reserves	1,097	1,217
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,234	2,425
Net Case Reserves	92	96
Net IBNR Reserves	83	96
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 175	$ 192