Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Property & All Other	$ 2,079	$ 2,067	$ 5,046	$ 4,695
Casualty	1,493	1,430	4,515	4,237
Life, Accident & Health	1,182	1,113	3,495	3,318
Net premiums earned	4,754	4,610	13,056	12,250
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Property & All Other	420	388	1,248	1,103
Casualty	993	957	2,992	2,826
Life, Accident & Health	105	99	307	281
Net premiums earned	1,518	1,444	4,547	4,210
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Property & All Other	766	849	1,199	1,252
Casualty	0	0	0	0
Life, Accident & Health	0	0	0	0
Net premiums earned	766	849	1,199	1,252
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Property & All Other	744	689	2,164	1,932
Casualty	394	375	1,158	1,088
Life, Accident & Health	588	547	1,725	1,613
Net premiums earned	1,726	1,611	5,047	4,633
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Property & All Other	149	141	435	408
Casualty	106	98	365	323
Net premiums earned	255	239	800	731
Life [Member]
Segment Reporting Information [Line Items]
Life, Accident & Health	489	467	1,463	1,424
Net premiums earned	$ 489	$ 467	$ 1,463	$ 1,424