Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Derivative, Gain (Loss) on Derivative, Net									$ (126)	$ (193)	$ (442)
Net premiums written									28,145	17,713	17,799
Net premiums earned	$ 7,059	$ 7,688	$ 7,405	$ 6,597	$ 4,207	$ 4,719	$ 4,360	$ 3,927	28,749	17,213	17,426
Losses and loss expenses	3,855	4,269	4,254	3,674	2,302	2,643	2,417	2,122	16,052	9,484	9,649
Policy benefits	161	155	146	126	159	89	153	142	588	543	517
Policy Acquisition Costs									5,904	2,941	3,075
Administrative expenses									3,081	2,270	2,245
Underwriting income (loss)									3,124	1,975	1,940
Net Investment Income	744	739	708	674	532	549	562	551	2,865	2,194	2,252
Other (Income) Expense									(222)	(51)	(190)
Segment Underwriting Income Loss and Net Investment Income Loss									6,192	4,049	4,274
Net realized gains (losses) including OTTI	365	100	(216)	(394)	(60)	(397)	126	(89)	(145)	(420)	(507)
Interest expense									605	300	280
Other (income) expense									(222)	(51)	(190)
Amortization of Purchased Intangibles									19	171	108
Chubb integration expenses									492	33	0
(Gains) losses from fair value changes in separate account assets									11	(19)	2
Income tax expense (benefit)									815	462	634
Net income	$ 1,610	$ 1,360	$ 726	$ 439	$ 683	$ 528	$ 942	$ 681	4,135	2,834	2,853
North America Commercial P&C Insurance
Segment Reporting Information [Line Items]
Net premiums written									11,740	5,715	5,685
Net premiums earned									12,217	5,634	5,547
Losses and loss expenses									7,439	3,661	3,476
Policy benefits									0		0
Policy Acquisition Costs									2,023	531	518
Administrative expenses									1,125	621	599
Underwriting income (loss)									1,630	821	954
Net Investment Income									1,860	1,032	1,060
Other (Income) Expense									(2)	(7)	(12)
Segment Underwriting Income Loss and Net Investment Income Loss									3,492	1,860	2,026
Amortization of Purchased Intangibles									0	0	0
North American Personal P&C [Member]
Segment Reporting Information [Line Items]
Net premiums written									4,153	1,192	578
Net premiums earned									4,319	948	560
Losses and loss expenses									2,558	590	368
Policy benefits									0		0
Policy Acquisition Costs									966	69	116
Administrative expenses									363	123	74
Underwriting income (loss)									432	166	2
Net Investment Income									207	25	22
Other (Income) Expense									6	2	1
Segment Underwriting Income Loss and Net Investment Income Loss									614	111	23
Amortization of Purchased Intangibles									19	78	0
North America Agricultural Insurance
Segment Reporting Information [Line Items]
Segment Income Loss Including Gains Losses On Crop Derivatives									341
Derivative, Gain (Loss) on Derivative, Net									5
Net premiums written									1,328	1,346	1,590
Net premiums earned									1,316	1,364	1,526
Losses and loss expenses									893	1,088	1,351
Policy benefits									0	0	0
Policy Acquisition Costs									83	69	81
Administrative expenses									(6)	1	9
Underwriting income (loss)									346	206	85
Net Investment Income									20	23	26
Other (Income) Expense									1	1	2
Segment Underwriting Income Loss and Net Investment Income Loss									336	198	78
Amortization of Purchased Intangibles									29	30	31
Overseas General Insurance
Segment Reporting Information [Line Items]
Net premiums written									8,124	6,634	6,999
Net premiums earned									8,132	6,471	6,805
Losses and loss expenses									4,005	3,052	3,189
Policy benefits									0	0	0
Policy Acquisition Costs									2,136	1,581	1,625
Administrative expenses									1,057	997	1,026
Underwriting income (loss)									934	841	965
Net Investment Income									600	534	545
Other (Income) Expense									(11)	(17)	(18)
Segment Underwriting Income Loss and Net Investment Income Loss									1,497	1,331	1,454
Amortization of Purchased Intangibles									48	61	74
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums written									676	828	935
Net premiums earned									710	849	1,026
Losses and loss expenses									325	290	431
Policy benefits									0	0	0
Policy Acquisition Costs									187	214	257
Administrative expenses									52	49	54
Underwriting income (loss)									146	296	284
Net Investment Income									263	300	316
Other (Income) Expense									(4)	(6)	(4)
Segment Underwriting Income Loss and Net Investment Income Loss									413	602	604
Amortization of Purchased Intangibles									0	0	0
Life Insurance
Segment Reporting Information [Line Items]
Management underwriting income loss Insurance									282
Net premiums written									2,124	1,998	2,012
Net premiums earned									2,055	1,947	1,962
Losses and loss expenses									663	601	589
Policy benefits									588	543	517
Policy Acquisition Costs									509	476	478
Administrative expenses									307	291	285
Underwriting income (loss)									(12)	36	93
Net Investment Income									283	265	268
Other (Income) Expense									5	23	3
Segment Underwriting Income Loss and Net Investment Income Loss									263	276	355
Amortization of Purchased Intangibles									3	2	3
(Gains) losses from fair value changes in separate account assets									11
Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written									0	0	0
Net premiums earned									0	0	0
Losses and loss expenses									169	202	245
Policy benefits									0	0	0
Policy Acquisition Costs									0	1	0
Administrative expenses									183	188	198
Underwriting income (loss)									(352)	(391)	(443)
Net Investment Income									(368)	15	15
Other (Income) Expense									(217)	(47)	(162)
Segment Underwriting Income Loss and Net Investment Income Loss									(423)	(329)	(266)
Net realized gains (losses) including OTTI									(145)	(420)	(507)
Interest expense									605	300	280
Amortization of Purchased Intangibles									(80)	0	0
Chubb integration expenses									492	33
Income tax expense (benefit)									815	462	634
Net income									$ (2,480)	$ (1,544)	$ (1,687)