Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Net premiums written		$ 7,104	$ 6,710
Net premiums earned		7,027	6,772
Losses and loss expenses		4,102	3,789
Policy benefits		151	168
Policy acquisition costs		1,464	1,397
Administrative expenses		692	676
Underwriting income (loss)		618	742
Net investment income		806	745
Other (income) expense		(47)	(70)
Amortization of purchased intangibles		85	64
Segment Income (loss)		1,386	1,493
Net realized gains (losses) including OTTI		(2)	(7)
Interest expense		157	154
Chubb integration expenses		10	111
Income tax expense		135	128
Net income		1,082	1,093
Prior Period Reclassification Adjustment	[1]		12
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		2,812	2,730	[1]
Net premiums earned		3,029	3,041
Losses and loss expenses		1,908	1,860
Policy benefits		0	0
Policy acquisition costs		472	487
Administrative expenses		231	231
Underwriting income (loss)		418	463
Net investment income		503	478
Other (income) expense		(6)	4
Amortization of purchased intangibles		0	0
Segment Income (loss)		927	937
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		1,048	984
Net premiums earned		1,140	1,086
Losses and loss expenses		886	633
Policy benefits		0	0
Policy acquisition costs		237	217
Administrative expenses		65	65
Underwriting income (loss)		(48)	171
Net investment income		59	55
Other (income) expense		0	1
Amortization of purchased intangibles		3	3
Segment Income (loss)		8	222
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		108	61
Net premiums earned		43	14
Losses and loss expenses		(53)	(73)
Policy benefits		0	0
Policy acquisition costs		(1)	(1)
Administrative expenses		(3)	(5)
Underwriting income (loss)		100	93
Net investment income		7	6
Other (income) expense		0	0
Amortization of purchased intangibles		7	7
Segment Income (loss)		100	92
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		2,384	2,212	[1]
Net premiums earned		2,107	1,936
Losses and loss expenses		1,078	1,071
Policy benefits		0	0
Policy acquisition costs		588	529
Administrative expenses		239	245
Underwriting income (loss)		202	91
Net investment income		151	148
Other (income) expense		7	(1)
Amortization of purchased intangibles		10	11
Segment Income (loss)		336	229
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		193	199
Net premiums earned		168	189
Losses and loss expenses		67	94
Policy benefits		0	0
Policy acquisition costs		40	51
Administrative expenses		10	10
Underwriting income (loss)		51	34
Net investment income		64	62
Other (income) expense		(7)	0
Amortization of purchased intangibles		0	0
Segment Income (loss)		122	96
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		559	524
Net premiums earned		540	506
Losses and loss expenses		205	193
Policy benefits		151	168
Policy acquisition costs		128	114
Administrative expenses		78	72
Underwriting income (loss)		(22)	(41)
Net investment income		83	75
Other (income) expense		(4)	(29)
Amortization of purchased intangibles		1	1
Segment Income (loss)		64	62
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Net premiums written		0	0
Net premiums earned		0	0
Losses and loss expenses		11	11
Policy benefits		0	0
Policy acquisition costs		0	0
Administrative expenses		72	58
Underwriting income (loss)		(83)	(69)
Net investment income		(61)	(79)
Other (income) expense		(37)	(45)
Amortization of purchased intangibles		64	42
Segment Income (loss)		(171)	(145)
Net realized gains (losses) including OTTI		(2)	(7)
Interest expense		157	154
Chubb integration expenses		10	111
Income tax expense		135	128
Net income		$ (475)	$ (545)

[1] (1) The 2017 net premiums written amount was revised to reflect the transfer of certain multinational accounts ($12 million) from the North America Commercial P&C Insurance segment to the Overseas General Insurance segment to better align the reporting with the management of these businesses in 2018. There is no impact on a consolidated basis.