Segment Information (Operations By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Net premiums written	$ 4,076	$ 4,185
Net premiums earned	3,927	3,970
Losses and loss expenses	2,122	2,161
Policy benefits	142	114
Policy acquisition costs	707	728
Administrative expenses	554	535
Underwriting income (loss)	402	432
Net investment income	551	553
Net realized gains (losses) including OTTI	(89)	(104)
Interest expense	68	71
(Gains) losses from fair value changes in separate account assets	(11)	6
Other	6	(23)
Other (income) expense	(5)	(17)
Income tax expense (benefit)	120	93
Net income (loss)	681	734
Insurance - North American P&C [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,430	1,418
Net premiums earned	1,526	1,487
Losses and loss expenses	1,035	940
Policy acquisition costs	161	159
Administrative expenses	171	161
Underwriting income (loss)	159	227
Net investment income	263	270
Net realized gains (losses) including OTTI	(6)	(9)
Interest expense	2	3
Other	(7)	(20)
Income tax expense (benefit)	80	83
Net income (loss)	341	422
Insurance - North American Agriculture [Member]
Segment Reporting Information [Line Items]
Underwriting Income Including Losses Related to Crop Derivatives		31
Net premiums written	88	194
Net premiums earned	64	103
Losses and loss expenses	22	126
Policy acquisition costs	(4)	5
Administrative expenses	(1)	1
Underwriting income (loss)	47	(29)
Net investment income	6	7
Net realized gains (losses) including OTTI	0	(2)
Interest expense	0	0
Other	8	8
Income tax expense (benefit)	10	(7)
Net income (loss)	35	(25)
Crop Derivative Gain or Loss		2
Insurance - Overseas General [Member]
Segment Reporting Information [Line Items]
Net premiums written	1,794	1,771
Net premiums earned	1,637	1,612
Losses and loss expenses	814	817
Policy acquisition costs	389	386
Administrative expenses	256	250
Underwriting income (loss)	178	159
Net investment income	138	132
Net realized gains (losses) including OTTI	(10)	(10)
Interest expense	1	1
Other	17	(6)
Income tax expense (benefit)	52	37
Net income (loss)	236	249
Global Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums written	273	308
Net premiums earned	226	284
Losses and loss expenses	99	126
Policy acquisition costs	54	67
Administrative expenses	12	14
Underwriting income (loss)	61	77
Net investment income	75	77
Net realized gains (losses) including OTTI	(11)	(8)
Interest expense	1	1
Other	(5)	(19)
Income tax expense (benefit)	8	10
Net income (loss)	121	154
Life [Member]
Segment Reporting Information [Line Items]
Life underwriting income	77
Net premiums written	491	494
Net premiums earned	474	484
Losses and loss expenses	152	151
Policy benefits	142	114
Policy acquisition costs	107	111
Administrative expenses	73	68
Underwriting income (loss)	0	40
Net investment income	66	64
Net realized gains (losses) including OTTI	(59)	(76)
Interest expense	1	3
(Gains) losses from fair value changes in separate account assets	(11)	6
Other	(12)	7
Income tax expense (benefit)	9	10
Net income (loss)	20	2
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Losses and loss expenses	0	1
Policy acquisition costs		0
Administrative expenses	43	41
Underwriting income (loss)	(43)	(42)
Net investment income	3	3
Net realized gains (losses) including OTTI	(3)	1
Interest expense	63	63
Other	5	7
Income tax expense (benefit)	(39)	(40)
Net income (loss)	$ (72)	$ (68)