Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Net premiums written	$ 8,110	$ 7,902		$ 23,229	$ 22,193
Net premiums earned	7,908	7,807		22,599	21,816
Losses and loss expenses	4,868	6,247		13,457	14,182
Policy benefits	127	169		428	500
Policy acquisition costs	1,504	1,488		4,432	4,334
Administrative expenses	719	714		2,158	2,096
Underwriting income (loss)	690	(811)		2,124	704
Net investment income (loss)	823	813		2,457	2,328
Other (income) expense	(145)	(118)		(307)	(333)
Amortization of purchased intangibles	83	65		253	194
Segment Income (loss)	1,575	55		4,635	3,171
Net realized gains (losses) including OTTI	19	(10)		35	84
Interest expense	164	150		488	451
Chubb integration expenses	16	50		39	233
Income tax expense	183	(85)		536	243
Net income (loss)	1,231	(70)		3,607	2,328
Prior Period Reclassification Adjustment		3			16
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,199	3,086	[1]	9,342	9,019	[2]
Net premiums earned	3,019	3,016		9,325	9,156
Losses and loss expenses	1,881	2,580		5,873	6,376
Policy benefits	0	0		0	0
Policy acquisition costs	458	469		1,378	1,420
Administrative expenses	251	256		735	728
Underwriting income (loss)	429	(289)		1,339	632
Net investment income (loss)	503	497		1,516	1,465
Other (income) expense	(1)	(4)		(20)	(4)
Amortization of purchased intangibles	0	0		0	0
Segment Income (loss)	933	212		2,875	2,101
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,218	1,194		3,601	3,433
Net premiums earned	1,167	1,117		3,463	3,296
Losses and loss expenses	860	1,062		2,474	2,378
Policy benefits	0	0		0	0
Policy acquisition costs	236	226		701	673
Administrative expenses	69	61		202	192
Underwriting income (loss)	2	(232)		86	53
Net investment income (loss)	59	57		177	168
Other (income) expense	0	1		1	3
Amortization of purchased intangibles	4	4		10	12
Segment Income (loss)	57	(180)		252	206
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	884	926		1,380	1,390
Net premiums earned	857	898		1,251	1,256
Losses and loss expenses	719	759		955	976
Policy benefits	0	0		0	0
Policy acquisition costs	49	49		74	75
Administrative expenses	2	(1)		0	(4)
Underwriting income (loss)	87	91		222	209
Net investment income (loss)	7	6		20	18
Other (income) expense	0	0		1	1
Amortization of purchased intangibles	7	8		21	22
Segment Income (loss)	87	89		220	204
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,081	1,966	[1]	6,664	6,185	[2]
Net premiums earned	2,157	2,064		6,425	6,018
Losses and loss expenses	1,114	1,281		3,263	3,316
Policy benefits	0	0		0	0
Policy acquisition costs	582	569		1,754	1,653
Administrative expenses	252	246		757	734
Underwriting income (loss)	209	(32)		651	315
Net investment income (loss)	155	164		461	460
Other (income) expense	(7)	(10)		(12)	(14)
Amortization of purchased intangibles	8	11		29	33
Segment Income (loss)	363	131		1,095	756
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	164	191		554	580
Net premiums earned	157	185		492	542
Losses and loss expenses	86	295		236	435
Policy benefits	0	0		0	0
Policy acquisition costs	40	43		120	137
Administrative expenses	10	11		29	33
Underwriting income (loss)	21	(164)		107	(63)
Net investment income (loss)	63	80		192	207
Other (income) expense	(13)	(3)		(26)	(2)
Amortization of purchased intangibles	0	0		0	0
Segment Income (loss)	97	(81)		325	146
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	564	539		1,688	1,586
Net premiums earned	551	527		1,643	1,548
Losses and loss expenses	195	181		584	556
Policy benefits	127	169		428	500
Policy acquisition costs	139	132		405	376
Administrative expenses	77	77		235	226
Underwriting income (loss)	13	(32)		(9)	(110)
Net investment income (loss)	85	78		253	230
Other (income) expense	20	(19)		24	(60)
Amortization of purchased intangibles	0	1		1	2
Segment Income (loss)	78	64		219	178
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	13	89		72	145
Policy benefits	0	0		0	0
Policy acquisition costs	0	0		0	0
Administrative expenses	58	64		200	187
Underwriting income (loss)	(71)	(153)		(272)	(332)
Net investment income (loss)	(49)	(69)		(162)	(220)
Other (income) expense	(144)	(83)		(275)	(257)
Amortization of purchased intangibles	64	41		192	125
Segment Income (loss)	(40)	(180)		(351)	(420)
Net realized gains (losses) including OTTI	19	(10)		35	84
Interest expense	164	150		488	451
Chubb integration expenses	16	50		39	233
Income tax expense	183	(85)		536	243
Net income (loss)	$ (384)	$ (305)		$ (1,379)	$ (1,263)

[1] The 2017 net premiums written amount was revised to reflect the transfer of certain multinational accounts ($3 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 ($16 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.