Segment Information (Operations By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Net premiums written	$ 4,709	$ 4,729	$ 13,569	$ 13,473
Net premiums earned	4,719	4,754	13,006	13,056
Losses and loss expenses	2,643	2,684	7,182	7,233
Policy benefits	89	125	384	383
Policy acquisition costs	771	825	2,205	2,311
Administrative expenses	568	554	1,700	1,655
Underwriting income (loss)	648	566	1,535	1,474
Net investment income	549	566	1,662	1,675
Net realized gains (losses) including OTTI	(397)	(120)	(360)	(297)
Interest expense	68	70	207	213
(Gains) losses from fair value changes in separate account assets	49	6	32	(5)
Other	(37)	(52)	(93)	(134)
Other (income) expense	12	(46)	(61)	(139)
Amortization of intangible assets	51	27	136	78
Chubb integration expenses	9	0	9	0
Income tax expense (benefit)	132	176	395	402
Net income (loss)	528	785	2,151	2,298
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,711	1,541	5,116	4,594
Net premiums earned	1,682	1,518	4,896	4,547
Losses and loss expenses	1,175	1,053	3,330	3,009
Policy acquisition costs	155	169	446	480
Administrative expenses	192	165	552	501
Underwriting income (loss)	160	131	568	557
Net investment income	266	277	798	812
Net realized gains (losses) including OTTI	(33)	(5)	(39)	(25)
Interest expense	3	2	7	7
Other	(16)	(31)	(32)	(75)
Amortization of intangible assets	31	0	63	0
Chubb integration expenses	0		0
Income tax expense (benefit)	70	73	239	247
Net income (loss)	305	359	1,050	1,165
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Underwriting Income Including Losses Related to Crop Derivatives	73
Net premiums written	737	764	1,204	1,346
Net premiums earned	739	766	1,124	1,199
Losses and loss expenses	620	686	913	1,099
Policy acquisition costs	42	41	61	69
Administrative expenses	0	3	3	5
Underwriting income (loss)	77	36	147	26
Net investment income	5	6	17	19
Net realized gains (losses) including OTTI	(4)	45	(6)	51
Interest expense	0	0	0	0
Other	(1)	(1)	1	0
Amortization of intangible assets	8	8	22	24
Chubb integration expenses	0		0
Income tax expense (benefit)	15	23	29	21
Net income (loss)	56	57	106	51
Crop Derivative Gain or Loss	4
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,584	1,719	5,047	5,250
Net premiums earned	1,615	1,726	4,896	5,047
Losses and loss expenses	674	707	2,304	2,354
Policy acquisition costs	405	418	1,190	1,206
Administrative expenses	246	258	756	764
Underwriting income (loss)	290	343	646	723
Net investment income	132	130	409	398
Net realized gains (losses) including OTTI	(13)	(75)	(10)	(71)
Interest expense	1	2	3	4
Other	(6)	(13)	(19)	(44)
Amortization of intangible assets	12	19	50	52
Chubb integration expenses	0		0
Income tax expense (benefit)	76	97	197	189
Net income (loss)	326	293	814	849
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	185	208	719	794
Net premiums earned	203	255	649	800
Losses and loss expenses	20	92	191	327
Policy acquisition costs	52	74	166	201
Administrative expenses	12	13	37	41
Underwriting income (loss)	119	76	255	231
Net investment income	76	81	230	238
Net realized gains (losses) including OTTI	(14)	6	(20)	(17)
Interest expense	1	1	3	4
Other	(2)	(10)	(10)	(39)
Amortization of intangible assets	0	0	0	0
Chubb integration expenses	0		0
Income tax expense (benefit)	8	11	23	31
Net income (loss)	174	161	449	456
Life [Member]
Segment Reporting Information [Line Items]
Life underwriting income	64
Net premiums written	492	497	1,483	1,489
Net premiums earned	480	489	1,441	1,463
Losses and loss expenses	153	145	442	442
Policy benefits	89	125	384	383
Policy acquisition costs	117	123	342	355
Administrative expenses	74	71	221	212
Underwriting income (loss)	47	25	52	71
Net investment income	66	69	198	199
Net realized gains (losses) including OTTI	(326)	(89)	(282)	(237)
Interest expense	1	4	4	10
(Gains) losses from fair value changes in separate account assets	49	6	32	(5)
Other	(13)	(3)	(43)	6
Amortization of intangible assets	0	0	1	2
Chubb integration expenses	0		0
Income tax expense (benefit)	8	12	27	34
Net income (loss)	(258)	(14)	(53)	(14)
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	1	1	2	2
Policy acquisition costs		0		0
Administrative expenses	44	44	131	132
Underwriting income (loss)	(45)	(45)	(133)	(134)
Net investment income	4	3	10	9
Net realized gains (losses) including OTTI	(7)	(2)	(3)	2
Interest expense	62	61	190	188
Other	1	6	10	18
Amortization of intangible assets	0	0	0	0
Chubb integration expenses	9		9
Income tax expense (benefit)	(45)	(40)	(120)	(120)
Net income (loss)	$ (75)	$ (71)	$ (215)	$ (209)