Segment Information (Net Premiums Earned For Segment By Product) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting [Line Items]
Property & All Other									$ 5,712	$ 3,898	$ 4,023
Casualty									5,340	5,752	5,587
Life, Accident & Health									4,335	3,854	3,630
Net premiums earned	3,831	4,490	3,757	3,309	3,572	3,422	3,233	3,277	15,387	13,504	13,240
Insurance - North American [Member]
Segment Reporting [Line Items]
Property & All Other									3,174	1,578	1,690
Casualty									3,380	3,777	3,734
Life, Accident & Health									357	296	260
Net premiums earned									6,911	5,651	5,684
Insurance - Overseas General [Member]
Segment Reporting [Line Items]
Property & All Other									2,080	1,800	1,787
Casualty									1,415	1,424	1,420
Life, Accident & Health									2,242	2,016	1,940
Net premiums earned									5,737	5,240	5,147
Global Reinsurance [Member]
Segment Reporting [Line Items]
Property & All Other									458	520	546
Casualty									545	551	433
Net premiums earned									1,003	1,071	979
Life [Member]
Segment Reporting [Line Items]
Life, Accident & Health									1,736	1,542	1,430
Net premiums earned									$ 1,736	$ 1,542	$ 1,430