Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Property & All Other									$ 6,382	$ 5,973	$ 5,712
Casualty									5,759	5,292	5,340
Life, Accident & Health									4,472	4,412	4,335
Net premiums earned	4,363	4,610	4,067	3,573	3,848	4,665	3,783	3,381	16,613	15,677	15,387
Insurance - North American P&C
Segment Reporting Information [Line Items]
Property & All Other									1,489	1,370	1,232
Casualty									3,847	3,406	3,380
Life, Accident & Health									385	371	357
Net premiums earned									5,721	5,147	4,969
Insurance - North American Agriculture
Segment Reporting Information [Line Items]
Property & All Other									1,678	1,872	1,942
Casualty											0
Life, Accident & Health											0
Net premiums earned									1,678	1,872	1,942
Insurance - Overseas General
Segment Reporting Information [Line Items]
Property & All Other									2,672	2,236	2,080
Casualty									1,479	1,379	1,415
Life, Accident & Health									2,182	2,125	2,119
Net premiums earned									6,333	5,740	5,614
Global Reinsurance
Segment Reporting Information [Line Items]
Property & All Other									543	495	458
Casualty									433	507	545
Life, Accident & Health											0
Net premiums earned									976	1,002	1,003
Life
Segment Reporting Information [Line Items]
Property & All Other											0
Casualty											0
Life, Accident & Health									1,905	1,916	1,859
Net premiums earned									$ 1,905	$ 1,916	$ 1,859