Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net premiums earned	$ 4,207	$ 4,719	$ 4,360	$ 3,927	$ 4,370	$ 4,754	$ 4,332	$ 3,970	$ 17,213	$ 17,426	$ 16,613
Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									6,586	6,687	6,382
Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									6,195	6,080	5,759
Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,432	4,659	4,472
Insurance - North American P&C
Segment Reporting Information [Line Items]
Net premiums earned									6,582	6,107	5,721
Insurance - North American P&C | Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									1,931	1,662	1,489
Insurance - North American P&C | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,232	4,032	3,847
Insurance - North American P&C | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									419	413	385
Insurance - North American Agriculture
Segment Reporting Information [Line Items]
Net premiums earned									1,364	1,526	1,678
Insurance - North American Agriculture | Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									1,364	1,526	1,678
Insurance - Overseas General
Segment Reporting Information [Line Items]
Net premiums earned									6,471	6,805	6,333
Insurance - Overseas General | Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									2,868	2,948	2,672
Insurance - Overseas General | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,537	1,573	1,479
Insurance - Overseas General | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,066	2,284	2,182
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									849	1,026	976
Global Reinsurance | Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									423	551	543
Global Reinsurance | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									426	475	433
Life
Segment Reporting Information [Line Items]
Net premiums earned									1,947	1,962	1,905
Life | Property and all other
Segment Reporting Information [Line Items]
Net premiums earned									0	0	0
Life | Casualty Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									0	0	0
Life | Life Accident And Health Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 1,947	$ 1,962	$ 1,905