Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Property & All Other	$ 1,355	$ 1,119
Casualty	1,480	1,353
Life, Accident & Health	1,135	1,101
Net premiums earned	3,970	3,573
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	405	347
Casualty	981	902
Life, Accident & Health	101	89
Net premiums earned	1,487	1,338
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	103	52
Casualty	0	0
Life, Accident & Health	0	0
Net premiums earned	103	52
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	693	585
Casualty	369	339
Life, Accident & Health	550	535
Net premiums earned	1,612	1,459
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	154	135
Casualty	130	112
Net premiums earned	284	247
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	484	477
Net premiums earned	$ 484	$ 477