Claim and Claim Adjustment Expense Reserves (Gross and net carried claim and claim adjustment expense reserves) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Gross Case Reserves	$ 12,256	$ 12,262
Gross IBNR Reserves	11,015	11,827
Total Gross Carried Claim and Claim Adjustment Expense Reserves	23,271	24,089	24,763	24,303
Net Case Reserves	10,190	10,176
Net IBNR Reserves	8,737	8,941
Total Net Carried Claim and Claim Adjustment Expense Reserves	18,927	19,117	19,637	19,283
Specialty [Member]
Gross Case Reserves	2,136	2,001
Gross IBNR Reserves	4,093	4,057
Total Gross Carried Claim and Claim Adjustment Expense Reserves	6,229	6,058
Net Case Reserves	1,929	1,793
Net IBNR Reserves	3,726	3,789
Total Net Carried Claim and Claim Adjustment Expense Reserves	5,655	5,582
Commercial [Member]
Gross Case Reserves	5,298	5,570
Gross IBNR Reserves	4,216	4,521
Total Gross Carried Claim and Claim Adjustment Expense Reserves	9,514	10,091
Net Case Reserves	4,947	5,119
Net IBNR Reserves	3,906	3,992
Total Net Carried Claim and Claim Adjustment Expense Reserves	8,853	9,111
International [Member]
Gross Case Reserves	752	803
Gross IBNR Reserves	689	772
Total Gross Carried Claim and Claim Adjustment Expense Reserves	1,441	1,575
Net Case Reserves	598	629
Net IBNR Reserves	663	705
Total Net Carried Claim and Claim Adjustment Expense Reserves	1,261	1,334
Life and Group Non-Core [Member]
Gross Case Reserves	2,881	2,748
Gross IBNR Reserves	302	310
Total Gross Carried Claim and Claim Adjustment Expense Reserves	3,183	3,058
Net Case Reserves	2,572	2,352
Net IBNR Reserves	271	271
Total Net Carried Claim and Claim Adjustment Expense Reserves	2,843	2,623
Corporate and Other [Member]
Gross Case Reserves	1,189	1,140
Gross IBNR Reserves	1,715	2,167
Total Gross Carried Claim and Claim Adjustment Expense Reserves	2,904	3,307
Net Case Reserves	144	283
Net IBNR Reserves	171	184
Total Net Carried Claim and Claim Adjustment Expense Reserves	$ 315	$ 467