Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Property & All Other	$ 2,067	$ 2,197	$ 4,695	$ 4,623
Casualty	1,430	1,366	4,237	3,925
Life, Accident & Health	1,113	1,102	3,318	3,281
Net premiums earned	4,610	4,665	12,250	11,829
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	388	353	1,103	1,022
Casualty	957	860	2,826	2,505
Life, Accident & Health	99	93	281	275
Net premiums earned	1,444	1,306	4,210	3,802
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	849	1,166	1,252	1,609
Casualty	0	0	0	0
Life, Accident & Health	0	0	0	0
Net premiums earned	849	1,166	1,252	1,609
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	689	547	1,932	1,637
Casualty	375	356	1,088	1,027
Life, Accident & Health	547	529	1,613	1,579
Net premiums earned	1,611	1,432	4,633	4,243
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	141	131	408	355
Casualty	98	150	323	393
Net premiums earned	239	281	731	748
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	467	480	1,424	1,427
Net premiums earned	$ 467	$ 480	$ 1,424	$ 1,427