Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 10,694	$ 10,989
Gross IBNR Reserves	12,012	10,731
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,706	21,720	$ 21,984	$ 22,004
Net Case Reserves	8,930	9,178
Net IBNR Reserves	9,771	8,707
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,701	17,885	$ 17,965	$ 18,070
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,567	1,481
Gross IBNR Reserves	4,181	3,757
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,748	5,238
Net Case Reserves	1,410	1,343
Net IBNR Reserves	3,488	3,333
Total Net Carried Claim and Claim Adjustment Expense Reserves	4,898	4,676
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,724	3,937
Gross IBNR Reserves	5,317	4,719
Total Gross Carried Claim and Claim Adjustment Expense Reserves	9,041	8,656
Net Case Reserves	3,357	3,543
Net IBNR Reserves	4,847	4,306
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,204	7,849
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	892	858
Gross IBNR Reserves	1,199	1,018
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,091	1,876
Net Case Reserves	777	759
Net IBNR Reserves	1,045	869
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,822	1,628
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,406	3,576
Gross IBNR Reserves	337	140
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,743	3,716
Net Case Reserves	3,298	3,441
Net IBNR Reserves	317	116
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,615	3,557
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,105	1,137
Gross IBNR Reserves	978	1,097
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,083	2,234
Net Case Reserves	88	92
Net IBNR Reserves	74	83
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 162	$ 175