Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Net premiums written										$ 32,275	$ 30,579	$ 29,244
Net premiums earned		$ 7,935	$ 8,327	$ 7,891	$ 7,137	$ 7,465	$ 7,908	$ 7,664	$ 7,027	31,290	30,064	29,034
Losses and loss expenses		4,865	5,052	4,715	4,098	4,610	4,868	4,487	4,102	18,730	18,067	18,454
Policy benefits		225	158	161	196	162	127	150	151	740	590	676
Policy Acquisition Costs										6,153	5,912	5,781
Administrative expenses										3,030	2,886	2,833
Underwriting income (loss)										2,637	2,609	1,290
Net Investment Income		858	873	859	836	848	823	828	806	3,426	3,305	3,125
Other (Income) Expense										(596)	(434)	(400)
Amortization of Purchased Intangibles										305	339	260
Segment Underwriting Income Loss and Net Investment Income Loss										6,354	6,009	4,555
Interest expense										552	641	607
Chubb integration expenses										23	59	310
Income tax expense (benefit)										795	695	(139)
Net income		1,173	1,091	1,150	1,040	355	1,231	1,294	1,082	4,454	3,962	3,861
Derivative, Gain (Loss) on Derivative, Net										585	330	(87)
Other (income) expense										(596)	(434)	(400)
(Gains) losses from fair value changes in separate account assets	[1]									44	(38)	97
Net Realized Gains Losses		$ (55)	$ (155)	$ (223)	$ (97)	$ (687)	$ 19	$ 18	$ (2)	(530)	(652)	84
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums written										13,375	12,485	12,019
Net premiums earned										12,922	12,402	12,191
Losses and loss expenses										8,206	8,000	8,287
Policy benefits										0	0	0
Policy Acquisition Costs										1,831	1,829	1,873
Administrative expenses										1,028	966	981
Underwriting income (loss)										1,857	1,607	1,050
Net Investment Income										2,082	2,033	1,961
Other (Income) Expense										(3)	(25)	1
Amortization of Purchased Intangibles										0	0	0
Segment Underwriting Income Loss and Net Investment Income Loss										3,942	3,665	3,010
Segment Insurance North American Personal P&C [Member] [Member]
Segment Reporting Information [Line Items]
Net premiums written										4,787	4,674	4,533
Net premiums earned										4,694	4,593	4,399
Losses and loss expenses										3,043	3,229	3,265
Policy benefits										0	0	0
Policy Acquisition Costs										948	939	899
Administrative expenses										286	269	264
Underwriting income (loss)										417	156	(29)
Net Investment Income										258	236	226
Other (Income) Expense										3	1	4
Amortization of Purchased Intangibles										12	13	16
Segment Underwriting Income Loss and Net Investment Income Loss										660	378	177
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Net premiums written										1,810	1,577	1,516
Net premiums earned										1,795	1,569	1,508
Losses and loss expenses										1,608	1,111	1,036
Policy benefits										0	0	0
Policy Acquisition Costs										84	79	81
Administrative expenses										6	(9)	(8)
Underwriting income (loss)										97	388	399
Net Investment Income										30	28	25
Other (Income) Expense										1	2	2
Amortization of Purchased Intangibles										28	28	29
Segment Underwriting Income Loss and Net Investment Income Loss										98	386	393
Segment Income Loss Including Gains Losses On Crop Derivatives										89
Derivative, Gain (Loss) on Derivative, Net										8
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums written										9,262	8,902	8,350
Net premiums earned										8,882	8,612	8,131
Losses and loss expenses										4,606	4,429	4,281
Policy benefits										0	0	0
Policy Acquisition Costs										2,501	2,346	2,221
Administrative expenses										1,033	1,014	982
Underwriting income (loss)										742	823	647
Net Investment Income										588	619	610
Other (Income) Expense										12	0	(4)
Amortization of Purchased Intangibles										45	41	45
Segment Underwriting Income Loss and Net Investment Income Loss										1,273	1,401	1,216
Segment Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written										649	671	685
Net premiums earned										654	670	704
Losses and loss expenses										352	479	561
Policy benefits										0	0	0
Policy Acquisition Costs										169	162	177
Administrative expenses										35	41	44
Underwriting income (loss)										98	(12)	(78)
Net Investment Income										220	257	273
Other (Income) Expense										(58)	(32)	(1)
Amortization of Purchased Intangibles										0	0	0
Segment Underwriting Income Loss and Net Investment Income Loss										376	277	196
Life Insurance
Segment Reporting Information [Line Items]
Net premiums written										2,392	2,270	2,141
Net premiums earned										2,343	2,218	2,101
Losses and loss expenses										757	766	739
Policy benefits										740	590	676
Policy Acquisition Costs										620	557	530
Administrative expenses										323	310	303
Underwriting income (loss)										(97)	(5)	(147)
Net Investment Income										373	341	313
Other (Income) Expense										(92)	26	(84)
Amortization of Purchased Intangibles										2	2	2
Segment Underwriting Income Loss and Net Investment Income Loss										366	308	248
Management underwriting income loss Insurance										320
(Gains) losses from fair value changes in separate account assets										(44)
Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written										0	0	0
Net premiums earned										0	0	0
Losses and loss expenses										158	53	285
Policy benefits										0	0	0
Policy Acquisition Costs										0	0	0
Administrative expenses										319	295	267
Underwriting income (loss)										(477)	(348)	(552)
Net Investment Income										(125)	(209)	(283)
Other (Income) Expense										(459)	(406)	(318)
Amortization of Purchased Intangibles										218	255	168
Segment Underwriting Income Loss and Net Investment Income Loss										(361)	(406)	(685)
Interest expense										552	641	607
Chubb integration expenses										23	59	310
Income tax expense (benefit)										795	695	(139)
Net income										(2,261)	(2,453)	(1,379)
Net Realized Gains Losses										$ (530)	$ (652)	$ 84

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