Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	$ 11,670	$ 11,860
Gross IBNR Reserves	10,334	10,483
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,004	22,343	$ 22,663	$ 23,271
Net Case Reserves	9,645	9,627
Net IBNR Reserves	8,425	8,622
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,070	18,249	$ 18,576	$ 18,927
Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,805	1,871
Gross IBNR Reserves	4,043	4,278
Total Gross Carried Claim and Claim Adjustment Expense Reserves	5,848	6,149
Net Case Reserves	1,656	1,681
Net IBNR Reserves	3,523	3,723
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,179	5,404
Commercial
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	4,364	4,661
Gross IBNR Reserves	4,221	4,233
Total Gross Carried Claim and Claim Adjustment Expense Reserves	8,585	8,894
Net Case Reserves	4,047	4,353
Net IBNR Reserves	3,917	3,952
Total Net Carried Claim and Claim Adjustment Expense Reserves	7,964	8,305
International
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	744	632
Gross IBNR Reserves	892	696
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,636	1,328
Net Case Reserves	640	548
Net IBNR Reserves	792	653
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,432	1,201
Life & Group
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	3,386	3,172
Gross IBNR Reserves	113	186
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,499	3,358
Net Case Reserves	3,208	2,951
Net IBNR Reserves	82	158
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,290	3,109
Corporate & Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Gross Case Reserves	1,371	1,524
Gross IBNR Reserves	1,065	1,090
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,436	2,614
Net Case Reserves	94	94
Net IBNR Reserves	111	136
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 205	$ 230