Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gross Case Reserves	$ 12,102	$ 12,256
Gross IBNR Reserves	10,561	11,015
Total Gross Carried Claim and Claim Adjustment Expense Reserves	22,663	23,271	$ 24,089	$ 24,763
Net Case Reserves	9,836	10,190
Net IBNR Reserves	8,740	8,737
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,576	18,927	$ 19,117	$ 19,637
Specialty [Member]
Gross Case Reserves	2,011	2,136
Gross IBNR Reserves	4,258	4,093
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,269	6,229
Net Case Reserves	1,810	1,929
Net IBNR Reserves	3,758	3,726
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,568	5,655
Commercial [Member]
Gross Case Reserves	4,975	5,298
Gross IBNR Reserves	4,208	4,216
Total Gross Carried Claim and Claim Adjustment Expense Reserves	9,183	9,514
Net Case Reserves	4,651	4,947
Net IBNR Reserves	3,925	3,906
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,576	8,853
International [Member]
Gross Case Reserves	622	752
Gross IBNR Reserves	725	689
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,347	1,441
Net Case Reserves	531	598
Net IBNR Reserves	688	663
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,219	1,261
Life and Group Non-Core [Member]
Gross Case Reserves	2,973	2,881
Gross IBNR Reserves	247	302
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,220	3,183
Net Case Reserves	2,714	2,572
Net IBNR Reserves	216	271
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,930	2,843
Corporate and Other [Member]
Gross Case Reserves	1,521	1,189
Gross IBNR Reserves	1,123	1,715
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,644	2,904
Net Case Reserves	130	144
Net IBNR Reserves	153	171
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 283	$ 315