Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net premiums written		$ 33,820	$ 32,275	$ 30,579
Net premiums earned		33,117	31,290	30,064
Losses and loss expenses		21,710	18,730	18,067
Policy benefits		784	740	590
Policy Acquisition Costs		6,547	6,153	5,912
Administrative expenses		2,979	3,030	2,886
Underwriting income (loss)		1,097	2,637	2,609
Net Investment Income		3,375	3,426	3,305
Other (Income) Expense		(994)	(596)	(434)
Amortization of Purchased Intangibles		290	305	339
Segment Underwriting Income Loss and Net Investment Income Loss		5,176	6,354	6,009
Interest expense		516	552	641
Chubb integration expenses		0	23	59
Income tax expense (benefit)		629	795	695
Net income		3,533	4,454	3,962
Derivative, Gain (Loss) on Derivative, Net		228	585	330
Other (income) expense		(994)	(596)	(434)
(Gains) losses from fair value changes in separate account assets	[1]	58	44	(38)
Realized Investment Gains (Losses)		(498)	(530)	(652)
Interest Income (Expense), Net		516	552	641
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		1	(8)	(3)
Policy benefits		58	44	(38)
Policy Acquisition Costs		0	0	0
Administrative expenses		0	0	0
Net Investment Income		(115)	(86)	(63)
Other (Income) Expense		(173)	(130)	(25)
Amortization of Purchased Intangibles		0	0	0
Segment Underwriting Income Loss and Net Investment Income Loss		(1)	8	3
Chubb integration expenses			0	0
Income tax expense (benefit)		0	0	0
Net income		0	0	0
Realized Investment Gains (Losses)		1	(8)	(3)
Underwriting Income (Loss)		(59)	(36)	41
Interest Income (Expense), Net		0	0	0
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums written		14,474	13,375	12,485
Net premiums earned		13,964	12,922	12,402
Losses and loss expenses		10,129	8,206	8,000
Policy benefits		0	0	0
Policy Acquisition Costs		1,942	1,831	1,829
Administrative expenses		1,006	1,028	966
Underwriting income (loss)		887	1,857	1,607
Net Investment Income		2,061	2,109	2,061
Other (Income) Expense		23	24	3
Amortization of Purchased Intangibles		0	0	0
Segment Underwriting Income Loss and Net Investment Income Loss		2,925	3,942	3,665
Segment Insurance North American Personal P&C [Member] [Member]
Segment Reporting Information [Line Items]
Net premiums written		4,920	4,787	4,674
Net premiums earned		4,866	4,694	4,593
Losses and loss expenses		3,187	3,043	3,229
Policy benefits		0	0	0
Policy Acquisition Costs		974	948	939
Administrative expenses		270	286	269
Underwriting income (loss)		435	417	156
Net Investment Income		260	258	236
Other (Income) Expense		5	3	1
Amortization of Purchased Intangibles		11	12	13
Segment Underwriting Income Loss and Net Investment Income Loss		679	660	378
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums written		1,846	1,810	1,577
Net premiums earned		1,822	1,795	1,569
Losses and loss expenses		1,544	1,616	1,114
Policy benefits		0	0	0
Policy Acquisition Costs		123	84	79
Administrative expenses		9	6	(9)
Underwriting income (loss)		146	89	385
Net Investment Income		30	30	28
Other (Income) Expense		1	1	2
Amortization of Purchased Intangibles		27	28	28
Segment Underwriting Income Loss and Net Investment Income Loss		148	90	383
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums written		9,335	9,262	8,902
Net premiums earned		9,285	8,882	8,612
Losses and loss expenses		5,255	4,606	4,429
Policy benefits		0	0	0
Policy Acquisition Costs		2,568	2,501	2,346
Administrative expenses		1,034	1,033	1,014
Underwriting income (loss)		428	742	823
Net Investment Income		534	588	622
Other (Income) Expense		13	12	3
Amortization of Purchased Intangibles		45	45	41
Segment Underwriting Income Loss and Net Investment Income Loss		904	1,273	1,401
Segment Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written		731	649	671
Net premiums earned		698	654	670
Losses and loss expenses		435	352	479
Policy benefits		0	0	0
Policy Acquisition Costs		174	169	162
Administrative expenses		37	35	41
Underwriting income (loss)		52	98	(12)
Net Investment Income		307	279	289
Other (Income) Expense		2	1	0
Amortization of Purchased Intangibles		0	0	0
Segment Underwriting Income Loss and Net Investment Income Loss		357	376	277
Life Insurance
Segment Reporting Information [Line Items]
Net premiums written		2,514	2,392	2,270
Net premiums earned		2,482	2,343	2,218
Losses and loss expenses		724	757	766
Policy benefits		726	696	628
Policy Acquisition Costs		766	620	557
Administrative expenses		320	323	310
Underwriting income (loss)		(54)	(53)	(43)
Net Investment Income		385	373	341
Other (Income) Expense		(74)	(48)	(12)
Amortization of Purchased Intangibles		4	2	2
Segment Underwriting Income Loss and Net Investment Income Loss		401	366	308
Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written		0	0	0
Net premiums earned		0	0	0
Losses and loss expenses		435	158	53
Policy benefits		0	0	0
Policy Acquisition Costs		0	0	0
Administrative expenses		303	319	295
Underwriting income (loss)		(738)	(477)	(348)
Net Investment Income		(87)	(125)	(209)
Other (Income) Expense		(791)	(459)	(406)
Amortization of Purchased Intangibles		203	218	255
Segment Underwriting Income Loss and Net Investment Income Loss		(237)	(361)	(406)
Interest expense		516	552	641
Chubb integration expenses			23	59
Income tax expense (benefit)		629	795	695
Net income		(1,881)	(2,253)	(2,450)
Realized Investment Gains (Losses)		$ (499)	$ (522)	$ (649)

[1] Related to gains (losses) from fair value changes in separate account assets that do not qualify for separate account reporting under GAAP.