Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Net premiums written	$ 37,868	$ 33,820	$ 32,275
Net premiums earned	36,355	33,117	31,290
Losses and loss expenses	21,980	21,710	18,730
Policy benefits	699	784	740
Policy acquisition costs	6,918	6,547	6,153
Administrative expenses	3,136	2,979	3,030
Underwriting income (loss)	3,622	1,097	2,637
Net investment income (loss)	3,456	3,375	3,426
Other (income) expense	(2,365)	(994)	(596)
Amortization of purchased intangibles	287	290	305
Segment income	9,156	5,176	6,354
Net realized gains (losses) (includes $3, $(281), and $(31) reclassified from AOCI)	1,152	(498)	(530)
Interest expense	492	516	552
Income tax expense	1,277	629	795
Chubb integration expenses	0	0	23
Net income	8,539	3,533	4,454
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Net premiums written	0	0	0
Net premiums earned	0	0	0
Losses and loss expenses	(8)	1	(8)
Policy benefits	(8)	58	44
Policy acquisition costs	0	0	0
Administrative expenses	0	0	0
Underwriting income (loss)	16	(59)	(36)
Net investment income (loss)	(179)	(115)	(86)
Other (income) expense	(171)	(173)	(130)
Amortization of purchased intangibles	0	0	0
Segment income	8	(1)	8
Net realized gains (losses) (includes $3, $(281), and $(31) reclassified from AOCI)	(8)	1	(8)
Interest expense	0	0	0
Income tax expense	0	0	0
Chubb integration expenses			0
Net income	0	0	0
North America Commercial P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	16,415	14,474	13,375
Net premiums earned	15,461	13,964	12,922
Losses and loss expenses	10,015	10,129	8,206
Policy benefits	0	0	0
Policy acquisition costs	2,082	1,942	1,831
Administrative expenses	1,052	1,006	1,028
Underwriting income (loss)	2,312	887	1,857
Net investment income (loss)	2,078	2,061	2,109
Other (income) expense	31	23	24
Amortization of purchased intangibles	0	0	0
Segment income	4,359	2,925	3,942
North America Personal P&C Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	5,002	4,920	4,787
Net premiums earned	4,915	4,866	4,694
Losses and loss expenses	2,924	3,187	3,043
Policy benefits	0	0	0
Policy acquisition costs	1,001	974	948
Administrative expenses	276	270	286
Underwriting income (loss)	714	435	417
Net investment income (loss)	249	260	258
Other (income) expense	(2)	5	3
Amortization of purchased intangibles	10	11	12
Segment income	955	679	660
North America Agricultural Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,388	1,846	1,810
Net premiums earned	2,338	1,822	1,795
Losses and loss expenses	1,962	1,544	1,616
Policy benefits	0	0	0
Policy acquisition costs	124	123	84
Administrative expenses	(3)	9	6
Underwriting income (loss)	255	146	89
Net investment income (loss)	28	30	30
Other (income) expense	1	1	1
Amortization of purchased intangibles	26	27	28
Segment income	256	148	90
Overseas General Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	10,713	9,335	9,262
Net premiums earned	10,441	9,285	8,882
Losses and loss expenses	5,143	5,255	4,606
Policy benefits	0	0	0
Policy acquisition costs	2,799	2,568	2,501
Administrative expenses	1,078	1,034	1,033
Underwriting income (loss)	1,421	428	742
Net investment income (loss)	597	534	588
Other (income) expense	0	13	12
Amortization of purchased intangibles	48	45	45
Segment income	1,970	904	1,273
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	873	731	649
Net premiums earned	798	698	654
Losses and loss expenses	632	435	352
Policy benefits	0	0	0
Policy acquisition costs	200	174	169
Administrative expenses	35	37	35
Underwriting income (loss)	(69)	52	98
Net investment income (loss)	331	307	279
Other (income) expense	0	2	1
Amortization of purchased intangibles	0	0	0
Segment income	262	357	376
Life Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	2,477	2,514	2,392
Net premiums earned	2,402	2,482	2,343
Losses and loss expenses	740	724	757
Policy benefits	707	726	696
Policy acquisition costs	712	766	620
Administrative expenses	333	320	323
Underwriting income (loss)	(90)	(54)	(53)
Net investment income (loss)	407	385	373
Other (income) expense	(106)	(74)	(48)
Amortization of purchased intangibles	5	4	2
Segment income	418	401	366
Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written	0	0	0
Net premiums earned	0	0	0
Losses and loss expenses	572	435	158
Policy benefits	0	0	0
Policy acquisition costs	0	0	0
Administrative expenses	365	303	319
Underwriting income (loss)	(937)	(738)	(477)
Net investment income (loss)	(55)	(87)	(125)
Other (income) expense	(2,118)	(791)	(459)
Amortization of purchased intangibles	198	203	218
Segment income	928	(237)	(361)
Net realized gains (losses) (includes $3, $(281), and $(31) reclassified from AOCI)	1,160	(499)	(522)
Interest expense	492	516	552
Income tax expense	1,277	629	795
Chubb integration expenses			23
Net income	$ 319	$ (1,881)	$ (2,253)