Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Net premiums written	$ 8,343	$ 8,015	$ 15,656	$ 15,119
Net premiums earned	7,891	7,664	15,028	14,691
Losses and loss expenses	4,715	4,487	8,813	8,589
Policy benefits	161	150	357	301
Policy acquisition costs	1,544	1,464	3,008	2,928
Administrative expenses	758	747	1,468	1,439
Underwriting income (loss)	713	816	1,382	1,434
Net investment income (loss)	859	828	1,695	1,634
Other (income) expense	(230)	(115)	(269)	(162)
Amortization of purchased intangibles	77	85	153	170
Segment Income (loss)	1,725	1,674	3,193	3,060
Net Realized Gains Losses	(223)	18	(320)	16
Interest expense	140	167	280	324
Chubb integration expenses	4	13	7	23
Income tax expense	208	218	396	353
Net income	1,150	1,294	2,190	2,376
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,534	3,331	6,485	6,143
Net premiums earned	3,390	3,277	6,475	6,306
Losses and loss expenses	2,214	2,084	4,187	3,992
Policy benefits	0	0	0	0
Policy acquisition costs	459	448	918	920
Administrative expenses	259	253	499	484
Underwriting income (loss)	458	492	871	910
Net investment income (loss)	521	510	1,031	1,013
Other (income) expense	2	(13)	(3)	(19)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	977	1,015	1,905	1,942
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,309	1,335	2,365	2,383
Net premiums earned	1,168	1,156	2,322	2,296
Losses and loss expenses	747	728	1,504	1,614
Policy benefits	0	0	0	0
Policy acquisition costs	237	228	468	465
Administrative expenses	71	68	139	133
Underwriting income (loss)	113	132	211	84
Net investment income (loss)	64	59	128	118
Other (income) expense	1	1	1	1
Amortization of purchased intangibles	3	3	6	6
Segment Income (loss)	173	187	332	195
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	466	388	596	496
Net premiums earned	378	351	433	394
Losses and loss expenses	316	289	289	236
Policy benefits	0	0	0	0
Policy acquisition costs	27	26	34	25
Administrative expenses	4	1	5	(2)
Underwriting income (loss)	31	35	105	135
Net investment income (loss)	4	6	14	13
Other (income) expense	1	1	1	1
Amortization of purchased intangibles	7	7	14	14
Segment Income (loss)	27	33	104	133
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,258	2,199	4,653	4,583
Net premiums earned	2,225	2,161	4,339	4,268
Losses and loss expenses	1,125	1,071	2,231	2,149
Policy benefits	0	0	0	0
Policy acquisition costs	629	584	1,225	1,172
Administrative expenses	265	266	514	505
Underwriting income (loss)	206	240	369	442
Net investment income (loss)	151	155	295	306
Other (income) expense	3	(12)	7	(5)
Amortization of purchased intangibles	12	11	23	21
Segment Income (loss)	342	396	634	732
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	197	197	399	390
Net premiums earned	159	167	327	335
Losses and loss expenses	90	83	166	150
Policy benefits	0	0	0	0
Policy acquisition costs	42	40	85	80
Administrative expenses	7	9	17	19
Underwriting income (loss)	20	35	59	86
Net investment income (loss)	55	65	111	129
Other (income) expense	(15)	(6)	(24)	(13)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	90	106	194	228
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	579	565	1,158	1,124
Net premiums earned	571	552	1,132	1,092
Losses and loss expenses	189	184	391	389
Policy benefits	161	150	357	301
Policy acquisition costs	150	138	278	266
Administrative expenses	78	80	157	158
Underwriting income (loss)	(7)	0	(51)	(22)
Net investment income (loss)	97	85	186	168
Other (income) expense	(7)	8	(47)	4
Amortization of purchased intangibles	1	0	1	1
Segment Income (loss)	96	77	181	141
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	34	48	45	59
Policy benefits	0	0	0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	74	70	137	142
Underwriting income (loss)	(108)	(118)	(182)	(201)
Net investment income (loss)	(33)	(52)	(70)	(113)
Other (income) expense	(215)	(94)	(204)	(131)
Amortization of purchased intangibles	54	64	109	128
Segment Income (loss)	20	(140)	(157)	(311)
Net Realized Gains Losses	(223)	18	(320)	16
Interest expense	140	167	280	324
Chubb integration expenses	4	13	7	23
Income tax expense	208	218	396	353
Net income	$ (555)	$ (520)	$ (1,160)	$ (995)