Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Property & All Other	$ 2,197	$ 2,066	$ 4,623	$ 4,275
Casualty	1,366	1,303	3,925	4,035
Life, Accident & Health	1,102	1,121	3,281	3,246
Net premiums earned	4,665	4,490	11,829	11,556
Insurance - North American [Member]
Segment Reporting Information [Line Items]
Property & All Other	1,519	1,395	2,631	2,413
Casualty	860	815	2,505	2,570
Life, Accident & Health	93	89	275	266
Net premiums earned	2,472	2,299	5,411	5,249
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	547	553	1,637	1,517
Casualty	356	366	1,027	1,056
Life, Accident & Health	529	552	1,579	1,591
Net premiums earned	1,432	1,471	4,243	4,164
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	131	118	355	345
Casualty	150	122	393	409
Net premiums earned	281	240	748	754
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	480	480	1,427	1,389
Net premiums earned	$ 480	$ 480	$ 1,427	$ 1,389