Segment Information (Net Premiums Earned For Segment By Product) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Net premiums earned	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 4,207	$ 4,719	$ 4,360	$ 3,927	$ 28,749	$ 17,213	$ 17,426
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums earned									12,217	5,634	5,547
North America Commercial P&C Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,963	1,040	1,113
North America Commercial P&C Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									9,552	4,175	4,021
North America Commercial P&C Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									702	419	413
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums earned									4,319	948	560
North American Personal P&C [Member] | Automobiles [Member]
Segment Reporting Information [Line Items]
Net premiums earned									699	186	130
North American Personal P&C [Member] | Personal homeowner [Member]
Segment Reporting Information [Line Items]
Net premiums earned									3,007	579	309
North American Personal P&C [Member] | Other Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									613	183	121
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums earned									1,316	1,364	1,526
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums earned									8,132	6,471	6,805
Overseas General Insurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,133	1,833	1,892
Overseas General Insurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,177	1,361	1,425
Overseas General Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									2,196	2,066	2,284
Overseas General Insurance | Personal lines [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,626	1,211	1,204
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums earned									710	849	1,026
Global Reinsurance | Property and other short-tail [Member]
Segment Reporting Information [Line Items]
Net premiums earned									118	155	246
Global Reinsurance | Casualty and all other [Member]
Segment Reporting Information [Line Items]
Net premiums earned									407	475	527
Global Reinsurance | Property catastrophe [Member]
Segment Reporting Information [Line Items]
Net premiums earned									185	219	253
Life Insurance
Segment Reporting Information [Line Items]
Net premiums earned									2,055	1,947	1,962
Life Insurance | Accident and Health [Member]
Segment Reporting Information [Line Items]
Net premiums earned									1,053	1,016	981
Life Insurance | Life Insurance Product Line [Member]
Segment Reporting Information [Line Items]
Net premiums earned									$ 1,002	$ 931	$ 981