Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Property & All Other									$ 5,973	$ 5,712	$ 3,898
Casualty									5,292	5,340	5,752
Life, Accident & Health									4,412	4,335	3,854
Net premiums earned	3,848	4,665	3,783	3,381	3,831	4,490	3,757	3,309	15,677	15,387	13,504
Insurance - North American
Segment Reporting Information [Line Items]
Property & All Other									3,242	3,174	1,578
Casualty									3,406	3,380	3,777
Life, Accident & Health									371	357	296
Net premiums earned									7,019	6,911	5,651
Insurance - Overseas General
Segment Reporting Information [Line Items]
Property & All Other									2,236	2,080	1,800
Casualty									1,379	1,415	1,424
Life, Accident & Health									2,125	2,119	1,929
Net premiums earned									5,740	5,614	5,153
Global Reinsurance
Segment Reporting Information [Line Items]
Property & All Other									495	458	520
Casualty									507	545	551
Life, Accident & Health											0
Net premiums earned									1,002	1,003	1,071
Life
Segment Reporting Information [Line Items]
Property & All Other
Casualty											0
Life, Accident & Health									1,916	1,859	1,629
Net premiums earned									$ 1,916	$ 1,859	$ 1,629