Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Property & All Other	$ 1,612	$ 1,509	$ 2,967	$ 2,628
Casualty	1,542	1,454	3,022	2,807
Life, Accident & Health	1,178	1,104	2,313	2,205
Net premiums earned	4,332	4,067	8,302	7,640
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	423	368	828	715
Casualty	1,018	967	1,999	1,869
Life, Accident & Health	101	93	202	182
Net premiums earned	1,542	1,428	3,029	2,766
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	330	351	433	403
Casualty	0	0	0	0
Life, Accident & Health	0	0	0	0
Net premiums earned	330	351	433	403
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	727	658	1,420	1,243
Casualty	395	374	764	713
Life, Accident & Health	587	531	1,137	1,066
Net premiums earned	1,709	1,563	3,321	3,022
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	132	132	286	267
Casualty	129	113	259	225
Net premiums earned	261	245	545	492
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	490	480	974	957
Net premiums earned	$ 490	$ 480	$ 974	$ 957