Segment Information (Operations By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Net premiums written	$ 4,391	$ 4,130	$ 8,189	$ 7,702
Net premiums earned	4,067	3,783	7,640	7,164
Losses and loss expenses	2,250	2,119	4,176	3,923
Policy benefits	110	102	241	249
Policy acquisition costs	665	619	1,279	1,201
Administrative expenses	564	514	1,078	1,024
Underwriting income (loss)	478	429	866	767
Net investment income	534	537	1,065	1,081
Net realized gains (losses) including OTTI	104	(394)	310	(134)
Interest (income) expense	73	62	133	124
(Gains) losses from fair value changes in separate account assets	11	14	7	(4)
Other	26	20	20	35
Other (income) expense	37	34	27	31
Income tax expense (benefit)	115	148	237	258
Net income (loss)	891	328	1,844	1,301
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,529	1,368	2,813	2,542
Net premiums earned	1,428	1,268	2,766	2,496
Losses and loss expenses	950	844	1,828	1,655
Policy acquisition costs	142	135	285	272
Administrative expenses	159	153	284	303
Underwriting income (loss)	177	136	369	266
Net investment income	250	265	501	532
Net realized gains (losses) including OTTI	28	18	54	17
Interest (income) expense	3	3	1	6
Other	(10)	2	(25)	(7)
Income tax expense (benefit)	91	96	185	179
Net income (loss)	371	318	763	637
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Net premiums written	453	492	566	611
Net premiums earned	351	384	403	443
Losses and loss expenses	293	319	325	357
Policy acquisition costs	20	22	24	12
Administrative expenses	3	0	8	(3)
Underwriting income (loss)	35	43	46	77
Net investment income	7	6	13	13
Net realized gains (losses) including OTTI	1	0	1	0
Interest (income) expense	0	0	0	0
Other	8	8	16	16
Income tax expense (benefit)	8	11	10	19
Net income (loss)	27	30	34	55
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,630	1,475	3,250	3,003
Net premiums earned	1,563	1,420	3,022	2,811
Losses and loss expenses	768	703	1,515	1,408
Policy acquisition costs	360	332	699	667
Administrative expenses	251	233	487	462
Underwriting income (loss)	184	152	321	274
Net investment income	136	128	268	259
Net realized gains (losses) including OTTI	8	26	42	46
Interest (income) expense	2	1	3	2
Other	17	6	16	6
Income tax expense (benefit)	50	51	96	89
Net income (loss)	259	248	516	482
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	292	309	571	572
Net premiums earned	245	237	492	467
Losses and loss expenses	93	102	199	204
Policy acquisition costs	48	42	96	85
Administrative expenses	12	13	24	25
Underwriting income (loss)	92	80	173	153
Net investment income	71	70	143	141
Net realized gains (losses) including OTTI	31	(17)	51	(4)
Interest (income) expense	1	1	2	2
Other	2	3	(6)	(2)
Income tax expense (benefit)	7	0	15	6
Net income (loss)	184	129	356	284
Life [Member]
Segment Reporting Information [Line Items]
Life underwriting income	96
Net premiums written	487	486	989	974
Net premiums earned	480	474	957	947
Losses and loss expenses	145	151	302	299
Policy benefits	110	102	241	249
Policy acquisition costs	95	88	175	164
Administrative expenses	86	78	171	156
Underwriting income (loss)	44	55	68	79
Net investment income	63	62	126	123
Net realized gains (losses) including OTTI	36	(421)	163	(190)
Interest (income) expense	4	3	8	6
(Gains) losses from fair value changes in separate account assets	11	14	7	(4)
Other	3	5	3	14
Income tax expense (benefit)	10	19	23	30
Net income (loss)	115	(345)	316	(34)
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	1	0	7	0
Policy acquisition costs		0		1
Administrative expenses	53	37	104	81
Underwriting income (loss)	(54)	(37)	(111)	(82)
Net investment income	7	6	14	13
Net realized gains (losses) including OTTI	0	0	(1)	(3)
Interest (income) expense	63	54	119	108
Other	6	(4)	16	8
Income tax expense (benefit)	(51)	(29)	(92)	(65)
Net income (loss)	$ (65)	$ (52)	$ (141)	$ (123)