Segment Information (Net Premiums Earned For Segment By Product) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Property & All Other	$ 1,319	$ 1,355
Casualty	1,506	1,480
Life, Accident & Health	1,102	1,135
Net premiums earned	3,927	3,970
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,526	1,487
Insurance - North American P&C [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	410	405
Insurance - North American P&C [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,015	981
Insurance - North American P&C [Member] | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	101	101
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Net premiums earned	64	103
Insurance - North American Agriculture [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	64	103
Insurance - North American Agriculture [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Insurance - North American Agriculture [Member] | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net premiums earned	1,637	1,612
Insurance - Overseas General [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	728	693
Insurance - Overseas General [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	382	369
Insurance - Overseas General [Member] | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	527	550
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	226	284
Global Reinsurance [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	117	154
Global Reinsurance [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	109	130
Global Reinsurance [Member] | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Life [Member]
Segment Reporting Information [Line Items]
Net premiums earned	474	484
Life [Member] | Property Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Life [Member] | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	0	0
Life [Member] | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 474	$ 484