Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Property & All Other	$ 1,119	$ 1,029
Casualty	1,353	1,270
Life, Accident & Health	1,101	1,082
Net premiums earned	3,573	3,381
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	347	322
Casualty	902	817
Life, Accident & Health	89	89
Net premiums earned	1,338	1,228
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	52	59
Casualty	0	0
Life, Accident & Health	0	0
Net premiums earned	52	59
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	585	538
Casualty	339	333
Life, Accident & Health	535	520
Net premiums earned	1,459	1,391
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	135	110
Casualty	112	120
Net premiums earned	247	230
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	477	473
Net premiums earned	$ 477	$ 473