Segment Information (Operations By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Net investment income	$ 531	$ 544
(Gains) losses from fair value changes in separate account assets	(4)	(18)
Net premiums written	3,798	3,572
Net premiums earned	3,573	3,381
Losses and loss expenses	1,926	1,804
Policy benefits	131	147
Policy acquisition costs	614	582
Administrative expenses	514	510
Underwriting income (loss)	388	338
Net realized gains (losses) including OTTI	206	260
Interest (income) expense	60	62
Other	(6)	15
Income tax expense (benefit)	122	110
Net income (loss)	953	973
Other (income) expense	(10)	(3)
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net investment income	251	267
Net premiums written	1,284	1,174
Net premiums earned	1,338	1,228
Losses and loss expenses	878	811
Policy acquisition costs	143	137
Administrative expenses	125	150
Underwriting income (loss)	192	130
Net realized gains (losses) including OTTI	26	(1)
Interest (income) expense	(2)	3
Other	(15)	(9)
Income tax expense (benefit)	94	83
Net income (loss)	392	319
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Net investment income	6	7
Net premiums written	113	119
Net premiums earned	52	59
Losses and loss expenses	32	38
Policy acquisition costs	4	(10)
Administrative expenses	5	(3)
Underwriting income (loss)	11	34
Net realized gains (losses) including OTTI	0	0
Interest (income) expense	0	0
Other	8	8
Income tax expense (benefit)	2	8
Net income (loss)	7	25
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net investment income	132	131
Net premiums written	1,620	1,528
Net premiums earned	1,459	1,391
Losses and loss expenses	747	705
Policy acquisition costs	339	335
Administrative expenses	236	229
Underwriting income (loss)	137	122
Net realized gains (losses) including OTTI	34	20
Interest (income) expense	1	1
Other	(1)	0
Income tax expense (benefit)	46	38
Net income (loss)	257	234
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net investment income	72	71
Net premiums written	279	263
Net premiums earned	247	230
Losses and loss expenses	106	102
Policy acquisition costs	48	43
Administrative expenses	12	12
Underwriting income (loss)	81	73
Net realized gains (losses) including OTTI	20	13
Interest (income) expense	1	1
Other	(8)	(5)
Income tax expense (benefit)	8	6
Net income (loss)	172	155
Life [Member]
Segment Reporting Information [Line Items]
Management underwriting income loss Insurance	91
Net investment income	63	61
(Gains) losses from fair value changes in separate account assets	(4)	(18)
Net premiums written	502	488
Net premiums earned	477	473
Losses and loss expenses	157	148
Policy benefits	131	147
Policy acquisition costs	80	76
Administrative expenses	85	78
Underwriting income (loss)	24	24
Net realized gains (losses) including OTTI	127	231
Interest (income) expense	4	3
Other	0	9
Income tax expense (benefit)	13	11
Net income (loss)	201	311
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Net investment income	7	7
Losses and loss expenses	6	0
Policy acquisition costs		1
Administrative expenses	51	44
Underwriting income (loss)	(57)	(45)
Net realized gains (losses) including OTTI	(1)	(3)
Interest (income) expense	56	54
Other	10	12
Income tax expense (benefit)	(41)	(36)
Net income (loss)	$ (76)	$ (71)