Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Net premiums written										$ 17,713	$ 17,799	$ 17,025
Net premiums earned	$ 4,207	$ 4,719	$ 4,360	$ 3,927		$ 4,370	$ 4,754	$ 4,332	$ 3,970	17,213	17,426	16,613
Losses and loss expenses	2,302	2,643	2,417	2,122		2,416	2,684	2,388	2,161	9,484	9,649	9,348
Policy benefits	159	89	153	142		134	125	144	114	543	517	515
Policy acquisition costs										2,941	3,075	2,659
Administrative expenses										2,270	2,245	2,211
Underwriting income (loss)										1,975	1,940	1,880
Net investment income	532	549	562	551		577	566	556	553	2,194	2,252	2,144
Net realized gains (losses) including OTTI	(60)	(397)	126	(89)		(210)	(120)	(73)	(104)	(420)	(507)	504
Interest expense										300	280	275
Other (income) expense										(51)	(190)	(80)
Amortization of Intangible Assets										171	108	95
Chubb integration expenses										33	0	0
(Gains) losses from fair value changes in separate account assets										19	(2)	(16)
Other										(70)	(188)	(64)
Income tax expense (benefit)										462	634	480
Net income (loss)	$ 683	$ 528	$ 942	$ 681	[1]	$ 555	$ 785	$ 779	$ 734	2,834	2,853	3,758
Insurance - North American Agriculture
Segment Reporting Information [Line Items]
Underwriting Income Including Gains Related to Crop Derivatives										197
Crop Derivative Gain or Loss										9
Net premiums written										1,346	1,590	1,627
Net premiums earned										1,364	1,526	1,678
Losses and loss expenses										1,088	1,351	1,524
Policy benefits										0	0
Policy acquisition costs										69	81	53
Administrative expenses										1	9	11
Underwriting income (loss)										206	85	90
Net investment income										23	26	26
Net realized gains (losses) including OTTI										(8)	54	1
Interest expense										0	0	1
Amortization of Intangible Assets										30	31	32
Chubb integration expenses										0
(Gains) losses from fair value changes in separate account assets										0	0	0
Other										1	2	0
Income tax expense (benefit)										40	33	20
Net income (loss)										150	99	64
Insurance - Overseas General
Segment Reporting Information [Line Items]
Net premiums written										6,634	6,999	6,520
Net premiums earned										6,471	6,805	6,333
Losses and loss expenses										3,052	3,189	3,062
Policy acquisition costs										1,581	1,625	1,453
Administrative expenses										997	1,026	1,008
Underwriting income (loss)										841	965	810
Net investment income										534	545	539
Net realized gains (losses) including OTTI										(38)	(78)	18
Interest expense										8	6	5
Amortization of Intangible Assets										61	74	60
Chubb integration expenses										0
Other										(16)	(63)	(21)
Income tax expense (benefit)										232	378	222
Net income (loss)										1,052	1,037	1,101
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums written										828	935	991
Net premiums earned										849	1,026	976
Losses and loss expenses										290	431	396
Policy acquisition costs										214	257	197
Administrative expenses										49	54	50
Underwriting income (loss)										296	284	333
Net investment income										300	316	280
Net realized gains (losses) including OTTI										(32)	(29)	53
Interest expense										5	4	5
Amortization of Intangible Assets										0	0	0
Chubb integration expenses										0
Other										(6)	(54)	(19)
Income tax expense (benefit)										26	38	36
Net income (loss)										539	583	644
Life
Segment Reporting Information [Line Items]
Management underwriting income loss Insurance										282
Net premiums written										1,998	2,012	1,972
Net premiums earned										1,947	1,962	1,905
Losses and loss expenses										601	589	582
Policy benefits										543	517	515
Policy acquisition costs										476	478	358
Administrative expenses										291	285	343
Underwriting income (loss)										36	93	107
Net investment income										265	268	251
Net realized gains (losses) including OTTI										(229)	(383)	360
Interest expense										5	11	15
Amortization of Intangible Assets										2	3	3
Chubb integration expenses										0
(Gains) losses from fair value changes in separate account assets										19	(2)	(16)
Other										(39)	(1)	10
Income tax expense (benefit)										30	46	34
Net income (loss)										55	(79)	672
Corporate and Other
Segment Reporting Information [Line Items]
Losses and loss expenses										3	3	8
Policy acquisition costs										1	0	1
Administrative expenses										184	193	198
Underwriting income (loss)										(188)	(196)	(207)
Net investment income										16	12	27
Net realized gains (losses) including OTTI										(5)	(4)
Interest expense										280	250	244
Amortization of Intangible Assets										0	0	0
Chubb integration expenses										33
Other										19	29	24
Income tax expense (benefit)										(182)	(167)	(179)
Net income (loss)										(327)	(300)	(269)
Insurance - North American P&C
Segment Reporting Information [Line Items]
Net premiums written										6,907	6,263	5,915
Net premiums earned										6,582	6,107	5,721
Losses and loss expenses										4,450	4,086	3,776
Policy benefits										0	0
Policy acquisition costs										600	634	597
Administrative expenses										748	678	601
Underwriting income (loss)										784	709	747
Net investment income										1,056	1,085	1,021
Net realized gains (losses) including OTTI										(108)	(67)	72
Interest expense										2	9	5
Amortization of Intangible Assets										78	0	0
Chubb integration expenses										0
(Gains) losses from fair value changes in separate account assets										0	0	0
Other										(29)	(101)	(58)
Income tax expense (benefit)										316	306	347
Net income (loss)										$ 1,365	$ 1,513	$ 1,546

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