Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net premiums written	$ 9,078	$ 8,622	$ 25,410	$ 24,278
Net premiums earned	8,765	8,327	24,687	23,355
Losses and loss expenses	5,835	5,052	16,897	13,865
Policy benefits	198	158	550	515
Policy acquisition costs	1,645	1,603	4,853	4,611
Administrative expenses	733	752	2,201	2,220
Underwriting income (loss)	354	762	186	2,144
Net investment income (loss)	840	873	2,528	2,568
Other (income) expense	(485)	(57)	(372)	(326)
Amortization of purchased intangibles	72	76	217	229
Segment Income (loss)	1,607	1,616	2,869	4,809
Net gains (losses) recognized during the period	(141)	(155)	(1,069)	(475)
Interest expense	130	138	390	418
Chubb integration expenses	0	2	0	9
Income tax expense	142	230	295	626
Net income	1,194	1,091	1,115	3,281
Segment Reconciling Items
Segment Reporting Information [Line Items]
Net premiums written	0	0	0	0
Net premiums earned	0	0	0	0
Losses and loss expenses	1	(14)	(2)	(8)
Policy benefits	24	(7)	8	20
Policy acquisition costs	0	0	0	0
Administrative expenses	0	0	0	0
Underwriting income (loss)	(25)	21	(6)	(12)
Net investment income (loss)	(32)	(21)	(64)	(62)
Other (income) expense	(56)	(14)	(72)	(82)
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	(1)	14	2	8
Net gains (losses) recognized during the period	1	(14)	(2)	(8)
Interest expense	0	0	0	0
Chubb integration expenses		0		0
Income tax expense	0	0	0	0
Net income	0	0	0	0
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	3,778	3,452	10,750	9,937
Net premiums earned	3,456	3,185	10,427	9,660
Losses and loss expenses	2,444	2,051	8,123	6,238
Policy benefits	0	0		0
Policy acquisition costs	489	459	1,452	1,377
Administrative expenses	243	256	751	755
Underwriting income (loss)	280	419	101	1,290
Net investment income (loss)	510	538	1,544	1,584
Other (income) expense	7	5	19	17
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	783	952	1,626	2,857
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,285	1,251	3,719	3,616
Net premiums earned	1,231	1,187	3,623	3,509
Losses and loss expenses	961	674	2,406	2,178
Policy benefits	0	0	0	0
Policy acquisition costs	248	240	724	708
Administrative expenses	65	72	199	211
Underwriting income (loss)	(43)	201	294	412
Net investment income (loss)	64	66	195	194
Other (income) expense	1	1	4	2
Amortization of purchased intangibles	2	3	8	9
Segment Income (loss)	18	263	477	595
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	986	938	1,604	1,534
Net premiums earned	971	941	1,441	1,374
Losses and loss expenses	845	880	1,223	1,163
Policy benefits	0	0	0	0
Policy acquisition costs	56	56	96	90
Administrative expenses	5	4	12	9
Underwriting income (loss)	65	1	110	112
Net investment income (loss)	7	8	23	22
Other (income) expense	0	0	1	1
Amortization of purchased intangibles	7	7	20	21
Segment Income (loss)	65	2	112	112
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,238	2,228	6,857	6,881
Net premiums earned	2,337	2,256	6,838	6,595
Losses and loss expenses	1,192	1,154	3,935	3,385
Policy benefits	0	0	0	0
Policy acquisition costs	637	630	1,903	1,855
Administrative expenses	260	257	759	771
Underwriting income (loss)	248	215	241	584
Net investment income (loss)	130	147	396	444
Other (income) expense	1	2	10	11
Amortization of purchased intangibles	10	11	33	34
Segment Income (loss)	367	349	594	983
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	181	141	606	540
Net premiums earned	171	160	520	487
Losses and loss expenses	154	79	314	245
Policy benefits	0	0	0	0
Policy acquisition costs	40	42	127	127
Administrative expenses	9	9	28	26
Underwriting income (loss)	(32)	30	51	89
Net investment income (loss)	85	71	214	206
Other (income) expense	0	0	1	0
Amortization of purchased intangibles	0	0	0	0
Segment Income (loss)	53	101	264	295
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	610	612	1,874	1,770
Net premiums earned	599	598	1,838	1,730
Losses and loss expenses	183	190	556	581
Policy benefits	174	165	542	495
Policy acquisition costs	175	176	551	454
Administrative expenses	80	80	238	237
Underwriting income (loss)	(13)	(13)	(49)	(37)
Net investment income (loss)	95	92	285	278
Other (income) expense	(23)	(17)	(52)	(37)
Amortization of purchased intangibles	1	1	3	2
Segment Income (loss)	104	95	285	276
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	55	38	342	83
Policy benefits	0	0	0	0
Policy acquisition costs	0	0	0	0
Administrative expenses	71	74	214	211
Underwriting income (loss)	(126)	(112)	(556)	(294)
Net investment income (loss)	(19)	(28)	(65)	(98)
Other (income) expense	(415)	(34)	(283)	(238)
Amortization of purchased intangibles	52	54	153	163
Segment Income (loss)	218	(160)	(491)	(317)
Net gains (losses) recognized during the period	(142)	(141)	(1,067)	(467)
Interest expense	130	138	390	418
Chubb integration expenses		2		9
Income tax expense	142	230	295	626
Net income	$ (196)	$ (671)	$ (2,243)	$ (1,837)