Segment Information (Operations By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Net premiums written	$ 4,729	$ 4,620	$ 13,473	$ 12,809
Net premiums earned	4,754	4,610	13,056	12,250
Losses and loss expenses	2,684	2,655	7,233	6,831
Policy benefits	125	138	383	379
Policy acquisition costs	825	678	2,311	1,957
Administrative expenses	554	563	1,655	1,641
Underwriting income (loss)	566	576	1,474	1,442
Net investment income	566	522	1,675	1,587
Net realized gains (losses) including OTTI	(120)	40	(297)	350
Interest expense	70	72	213	205
(Gains) losses from fair value changes in separate account assets	6	(14)	(5)	(7)
Other	(25)	9	(56)	29
Other (income) expense	(19)	(5)	(61)	22
Income tax expense (benefit)	176	155	402	392
Net income (loss)	785	916	2,298	2,760
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,541	1,500	4,594	4,313
Net premiums earned	1,518	1,444	4,547	4,210
Losses and loss expenses	1,053	963	3,009	2,791
Policy acquisition costs	169	159	480	444
Administrative expenses	165	153	501	437
Underwriting income (loss)	131	169	557	538
Net investment income	277	254	812	755
Net realized gains (losses) including OTTI	(5)	9	(25)	63
Interest expense	2	3	7	4
Other	(31)	(13)	(75)	(38)
Income tax expense (benefit)	73	79	247	264
Net income (loss)	359	363	1,165	1,126
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Underwriting Income Including Losses Related to Crop Derivatives	81
Net premiums written	764	805	1,346	1,371
Net premiums earned	766	849	1,199	1,252
Losses and loss expenses	686	746	1,099	1,071
Policy acquisition costs	41	32	69	56
Administrative expenses	3	5	5	13
Underwriting income (loss)	36	66	26	112
Net investment income	6	6	19	19
Net realized gains (losses) including OTTI	45	0	51	1
Interest expense	0	0	0	0
Other	7	8	24	24
Income tax expense (benefit)	23	14	21	24
Net income (loss)	57	50	51	84
Crop Derivative Gain or Loss	45
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,719	1,571	5,250	4,821
Net premiums earned	1,726	1,611	5,047	4,633
Losses and loss expenses	707	712	2,354	2,227
Policy acquisition costs	418	349	1,206	1,048
Administrative expenses	258	263	764	750
Underwriting income (loss)	343	287	723	608
Net investment income	130	128	398	396
Net realized gains (losses) including OTTI	(75)	(8)	(71)	34
Interest expense	2	1	4	4
Other	6	14	8	30
Income tax expense (benefit)	97	78	189	174
Net income (loss)	293	314	849	830
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	208	265	794	836
Net premiums earned	255	239	800	731
Losses and loss expenses	92	93	327	292
Policy acquisition costs	74	52	201	148
Administrative expenses	13	12	41	36
Underwriting income (loss)	76	82	231	255
Net investment income	81	66	238	209
Net realized gains (losses) including OTTI	6	(5)	(17)	46
Interest expense	1	2	4	4
Other	(10)	(7)	(39)	(13)
Income tax expense (benefit)	11	16	31	31
Net income (loss)	161	132	456	488
Life [Member]
Segment Reporting Information [Line Items]
Life underwriting income	88
Net premiums written	497	479	1,489	1,468
Net premiums earned	489	467	1,463	1,424
Losses and loss expenses	145	141	442	443
Policy benefits	125	138	383	379
Policy acquisition costs	123	86	355	261
Administrative expenses	71	85	212	256
Underwriting income (loss)	25	17	71	85
Net investment income	69	61	199	187
Net realized gains (losses) including OTTI	(89)	43	(237)	206
Interest expense	4	4	10	12
(Gains) losses from fair value changes in separate account assets	6	(14)	(5)	(7)
Other	(3)	4	8	7
Income tax expense (benefit)	12	10	34	33
Net income (loss)	(14)	117	(14)	433
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	1	0	2	7
Policy acquisition costs		0		0
Administrative expenses	44	45	132	149
Underwriting income (loss)	(45)	(45)	(134)	(156)
Net investment income	3	7	9	21
Net realized gains (losses) including OTTI	(2)	1	2	0
Interest expense	61	62	188	181
Other	6	3	18	19
Income tax expense (benefit)	(40)	(42)	(120)	(134)
Net income (loss)	$ (71)	$ (60)	$ (209)	$ (201)