Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Net premiums written	$ 8,622	$ 8,110	$ 24,278	$ 23,229
Net premiums earned	8,327	7,908	23,355	22,599
Losses and loss expenses	5,052	4,868	13,865	13,457
Policy benefits	158	127	515	428
Policy acquisition costs	1,603	1,504	4,611	4,432
Administrative expenses	752	719	2,220	2,158
Underwriting income (loss)	762	690	2,144	2,124
Net investment income (loss)	873	823	2,568	2,457
Other (income) expense	(57)	(145)	(326)	(307)
Amortization of purchased intangibles	76	83	229	253
Segment Income (loss)	1,616	1,575	4,809	4,635
Net Realized Gains Losses	(155)	19	(475)	35
Interest expense	138	164	418	488
Chubb integration expenses	2	16	9	39
Income tax expense	230	183	626	536
Net income	1,091	1,231	3,281	3,607
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,452	3,199	9,937	9,342
Net premiums earned	3,185	3,019	9,660	9,325
Losses and loss expenses	2,051	1,881	6,238	5,873
Policy benefits	0	0	0	0
Policy acquisition costs	459	458	1,377	1,378
Administrative expenses	256	251	755	735
Underwriting income (loss)	419	429	1,290	1,339
Net investment income (loss)	532	503	1,563	1,516
Other (income) expense	(1)	(1)	(4)	(20)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	952	933	2,857	2,875
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,251	1,218	3,616	3,601
Net premiums earned	1,187	1,167	3,509	3,463
Losses and loss expenses	674	860	2,178	2,474
Policy benefits	0	0	0	0
Policy acquisition costs	240	236	708	701
Administrative expenses	72	69	211	202
Underwriting income (loss)	201	2	412	86
Net investment income (loss)	66	59	194	177
Other (income) expense	1	0	2	1
Amortization of purchased intangibles	3	4	9	10
Segment Income (loss)	263	57	595	252
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	938	884	1,534	1,380
Net premiums earned	941	857	1,374	1,251
Losses and loss expenses	866	719	1,155	955
Policy benefits	0	0	0	0
Policy acquisition costs	56	49	90	74
Administrative expenses	4	2	9	0
Underwriting income (loss)	15	87	120	222
Net investment income (loss)	8	7	22	20
Other (income) expense	0	0	1	1
Amortization of purchased intangibles	7	7	21	21
Segment Income (loss)	16	87	120	220
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,228	2,081	6,881	6,664
Net premiums earned	2,256	2,157	6,595	6,425
Losses and loss expenses	1,154	1,114	3,385	3,263
Policy benefits	0	0	0	0
Policy acquisition costs	630	582	1,855	1,754
Administrative expenses	257	252	771	757
Underwriting income (loss)	215	209	584	651
Net investment income (loss)	148	155	443	461
Other (income) expense	3	(7)	10	(12)
Amortization of purchased intangibles	11	8	34	29
Segment Income (loss)	349	363	983	1,095
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	141	164	540	554
Net premiums earned	160	157	487	492
Losses and loss expenses	79	86	245	236
Policy benefits	0	0	0	0
Policy acquisition costs	42	40	127	120
Administrative expenses	9	10	26	29
Underwriting income (loss)	30	21	89	107
Net investment income (loss)	55	63	166	192
Other (income) expense	(16)	(13)	(40)	(26)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	101	97	295	325
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	612	564	1,770	1,688
Net premiums earned	598	551	1,730	1,643
Losses and loss expenses	190	195	581	584
Policy benefits	158	127	515	428
Policy acquisition costs	176	139	454	405
Administrative expenses	80	77	237	235
Underwriting income (loss)	(6)	13	(57)	(9)
Net investment income (loss)	92	85	278	253
Other (income) expense	(10)	20	(57)	24
Amortization of purchased intangibles	1	0	2	1
Segment Income (loss)	95	78	276	219
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	38	13	83	72
Policy benefits	0	0	0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	74	58	211	200
Underwriting income (loss)	(112)	(71)	(294)	(272)
Net investment income (loss)	(28)	(49)	(98)	(162)
Other (income) expense	(34)	(144)	(238)	(275)
Amortization of purchased intangibles	54	64	163	192
Segment Income (loss)	(160)	(40)	(317)	(351)
Net Realized Gains Losses	(155)	19	(475)	35
Interest expense	138	164	418	488
Chubb integration expenses	2	16	9	39
Income tax expense	230	183	626	536
Net income	$ (685)	$ (384)	$ (1,845)	$ (1,379)