Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Net premiums written	$ 8,015	$ 7,581		$ 15,119	$ 14,291
Net premiums earned	7,664	7,237		14,691	14,009
Losses and loss expenses	4,487	4,146		8,589	7,935
Policy benefits	150	163		301	331
Policy acquisition costs	1,464	1,449		2,928	2,846
Administrative expenses	747	706		1,439	1,382
Underwriting income (loss)	816	773		1,434	1,515
Net investment income (loss)	828	770		1,634	1,515
Other (income) expense	(115)	(145)		(162)	(215)
Amortization of purchased intangibles	85	65		170	129
Segment Income (loss)	1,674	1,623		3,060	3,116
Net realized gains (losses) including OTTI	18	101		16	94
Interest expense	167	147		324	301
Chubb integration expenses	13	72		23	183
Income tax expense	218	200		353	328
Net income	1,294	1,305		2,376	2,398
Prior Period Reclassification Adjustment		1			13
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,331	3,203	[1]	6,143	5,933	[2]
Net premiums earned	3,277	3,099		6,306	6,140
Losses and loss expenses	2,084	1,936		3,992	3,796
Policy benefits	0	0		0	0
Policy acquisition costs	448	464		920	951
Administrative expenses	253	241		484	472
Underwriting income (loss)	492	458		910	921
Net investment income (loss)	510	490		1,013	968
Other (income) expense	(13)	(4)		(19)	0
Amortization of purchased intangibles	0	0		0	0
Segment Income (loss)	1,015	952		1,942	1,889
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,335	1,255		2,383	2,239
Net premiums earned	1,156	1,093		2,296	2,179
Losses and loss expenses	728	683		1,614	1,316
Policy benefits	0	0		0	0
Policy acquisition costs	228	230		465	447
Administrative expenses	68	66		133	131
Underwriting income (loss)	132	114		84	285
Net investment income (loss)	59	56		118	111
Other (income) expense	1	1		1	2
Amortization of purchased intangibles	3	5		6	8
Segment Income (loss)	187	164		195	386
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	388	403		496	464
Net premiums earned	351	344		394	358
Losses and loss expenses	289	290		236	217
Policy benefits	0	0		0	0
Policy acquisition costs	26	27		25	26
Administrative expenses	1	2		(2)	(3)
Underwriting income (loss)	35	25		135	118
Net investment income (loss)	6	6		13	12
Other (income) expense	1	1		1	1
Amortization of purchased intangibles	7	7		14	14
Segment Income (loss)	33	23		133	115
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,199	2,007	[1]	4,583	4,219	[2]
Net premiums earned	2,161	2,018		4,268	3,954
Losses and loss expenses	1,071	964		2,149	2,035
Policy benefits	0	0		0	0
Policy acquisition costs	584	555		1,172	1,084
Administrative expenses	266	243		505	488
Underwriting income (loss)	240	256		442	347
Net investment income (loss)	155	148		306	296
Other (income) expense	(12)	(3)		(5)	(4)
Amortization of purchased intangibles	11	11		21	22
Segment Income (loss)	396	396		732	625
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	197	190		390	389
Net premiums earned	167	168		335	357
Losses and loss expenses	83	46		150	140
Policy benefits	0	0		0	0
Policy acquisition costs	40	43		80	94
Administrative expenses	9	12		19	22
Underwriting income (loss)	35	67		86	101
Net investment income (loss)	65	65		129	127
Other (income) expense	(6)	1		(13)	1
Amortization of purchased intangibles	0	0		0	0
Segment Income (loss)	106	131		228	227
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	565	523		1,124	1,047
Net premiums earned	552	515		1,092	1,021
Losses and loss expenses	184	182		389	375
Policy benefits	150	163		301	331
Policy acquisition costs	138	130		266	244
Administrative expenses	80	77		158	149
Underwriting income (loss)	0	(37)		(22)	(78)
Net investment income (loss)	85	77		168	152
Other (income) expense	8	(12)		4	(41)
Amortization of purchased intangibles	0	0		1	1
Segment Income (loss)	77	52		141	114
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Net premiums written	0	0		0	0
Net premiums earned	0	0		0	0
Losses and loss expenses	48	45		59	56
Policy benefits	0	0		0	0
Policy acquisition costs	0	0		0	0
Administrative expenses	70	65		142	123
Underwriting income (loss)	(118)	(110)		(201)	(179)
Net investment income (loss)	(52)	(72)		(113)	(151)
Other (income) expense	(94)	(129)		(131)	(174)
Amortization of purchased intangibles	64	42		128	84
Segment Income (loss)	(140)	(95)		(311)	(240)
Net realized gains (losses) including OTTI	18	101		16	94
Interest expense	167	147		324	301
Chubb integration expenses	13	72		23	183
Income tax expense	218	200		353	328
Net income	$ (520)	$ (413)		$ (995)	$ (958)

[1] The 2017 net premiums written amount was revised to reflect the transfer of certain multinational accounts ($1 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.

[2] The 2017 net premiums written amount was revised to reflect the transfer of certain multinational accounts ($13 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.