Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Property & All Other	$ 1,509	$ 1,397	$ 2,628	$ 2,426
Casualty	1,454	1,289	2,807	2,559
Life, Accident & Health	1,104	1,097	2,205	2,179
Net premiums earned	4,067	3,783	7,640	7,164
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	368	347	715	669
Casualty	967	828	1,869	1,645
Life, Accident & Health	93	93	182	182
Net premiums earned	1,428	1,268	2,766	2,496
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	351	384	403	443
Casualty	0	0	0	0
Life, Accident & Health	0	0	0	0
Net premiums earned	351	384	403	443
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	658	552	1,243	1,090
Casualty	374	338	713	671
Life, Accident & Health	531	530	1,066	1,050
Net premiums earned	1,563	1,420	3,022	2,811
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	132	114	267	224
Casualty	113	123	225	243
Net premiums earned	245	237	492	467
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	480	474	957	947
Net premiums earned	$ 480	$ 474	$ 957	$ 947