Segment Information (Net Premiums Earned For Segment By Product) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting [Line Items]
Property & All Other	$ 1,029	$ 913
Casualty	1,270	1,380
Life, Accident & Health	1,082	1,016
Net premiums earned	3,381	3,309
Insurance - North American [Member]
Segment Reporting [Line Items]
Property & All Other	381	370
Casualty	817	890
Life, Accident & Health	89	86
Net premiums earned	1,287	1,346
Insurance - Overseas General [Member]
Segment Reporting [Line Items]
Property & All Other	538	431
Casualty	333	342
Life, Accident & Health	520	505
Net premiums earned	1,391	1,278
Global Reinsurance [Member]
Segment Reporting [Line Items]
Property & All Other	110	112
Casualty	120	148
Net premiums earned	230	260
Life [Member]
Segment Reporting [Line Items]
Life, Accident & Health	473	425
Net premiums earned	$ 473	$ 425