Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Gross Case Reserves	$ 11,860	$ 12,102
Gross IBNR Reserves	10,483	10,561
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,343	22,663	$ 23,271	$ 24,089
Net Case Reserves	9,627	9,836
Net IBNR Reserves	8,622	8,740
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,249	18,576	$ 18,927	$ 19,117
Specialty [Member]
Gross Case Reserves	1,871	2,011
Gross IBNR Reserves	4,278	4,258
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,149	6,269
Net Case Reserves	1,681	1,810
Net IBNR Reserves	3,723	3,758
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,404	5,568
Commercial [Member]
Gross Case Reserves	4,661	4,975
Gross IBNR Reserves	4,233	4,208
Total Gross Carried Claim and Claim Adjustment Expense Reserves	8,894	9,183
Net Case Reserves	4,353	4,651
Net IBNR Reserves	3,952	3,925
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,305	8,576
International [Member]
Gross Case Reserves	632	622
Gross IBNR Reserves	696	725
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,328	1,347
Net Case Reserves	548	531
Net IBNR Reserves	653	688
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,201	1,219
Life and Group Non-Core [Member]
Gross Case Reserves	3,172	2,973
Gross IBNR Reserves	186	247
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,358	3,220
Net Case Reserves	2,951	2,714
Net IBNR Reserves	158	216
Total Net Carried Claim and Claim Adjustment Expense Reserves	3,109	2,930
Corporate and Other [Member]
Gross Case Reserves	1,524	1,521
Gross IBNR Reserves	1,090	1,123
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,614	2,644
Net Case Reserves	94	130
Net IBNR Reserves	136	153
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 230	$ 283