Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,218	$ 7,807	$ 7,237	$ 6,772	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 29,034	$ 28,749	$ 17,213
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums earned									12,191	12,217	5,634
North America Commercial P&C Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,899	1,963	1,040
North America Commercial P&C Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									9,554	9,552	4,175
North America Commercial P&C Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									738	702	419
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,399	4,319	948
North American Personal P&C [Member] | Automobiles [Member]
Segment Reporting Information [Line Items]
Net premiums earned									742	699	186
North American Personal P&C [Member] | Personal homeowner [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,014	3,007	579
North American Personal P&C [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									643	613	183
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,508	1,316	1,364
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums earned									8,131	8,132	6,471
Overseas General Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,076	2,133	1,833
Overseas General Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,266	2,177	1,361
Overseas General Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,180	2,196	2,066
Overseas General Insurance | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,609	1,626	1,211
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									704	710	849
Global Reinsurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									132	118	155
Global Reinsurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									374	407	475
Global Reinsurance | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned									198	185	219
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									2,101	2,055	1,947
Life Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,121	1,053	1,016
Life Insurance | Life Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 980	$ 1,002	$ 931