Segment Information (Operations By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended									12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014		Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Net premiums written										$ 17,799	$ 17,025	$ 16,075
Net premiums earned	4,370	4,754	4,332	3,970		4,363	4,610	4,067	3,573	17,426	16,613	15,677
Losses and loss expenses	2,416	2,684	2,388	2,161		2,517	2,655	2,250	1,926	9,649	9,348	9,653
Policy benefits	134	125	144	114		136	138	110	131	517	515	521
Policy acquisition costs										3,075	2,659	2,446
Administrative expenses										2,245	2,211	2,096
Underwriting income (loss)										1,940	1,880	961
Net investment income	577	566	556	553		557	522	534	531	2,252	2,144	2,181
Net realized gains (losses) including OTTI	(210)	(120)	(73)	(104)		154	40	104	206	(507)	504	78
Interest expense										280	275	250
Other (income) expense										(82)	15	(6)
(Gains) losses from fair value changes in separate account assets										(2)	(16)	(29)
Other										(80)	31	23
Income tax expense (benefit)										634	480	270
Net income (loss)	555	785	779	734	[1]	998	916	891	953	2,853	3,758	2,706
Insurance - North American Agriculture
Segment Reporting Information [Line Items]
Underwriting Income Including Gains Related to Crop Derivatives										136
Crop Derivative Gain or Loss										51
Net premiums written										1,590	1,627	1,859
Net premiums earned										1,526	1,678	1,872
Losses and loss expenses										1,351	1,524	1,911
Policy benefits										0	0
Policy acquisition costs										81	53	28
Administrative expenses										9	11	(7)
Underwriting income (loss)										85	90	(60)
Net investment income										26	26	25
Net realized gains (losses) including OTTI										54	1	1
Interest expense										0	1	0
(Gains) losses from fair value changes in separate account assets										0	0	0
Other										33	32	32
Income tax expense (benefit)										33	20	(29)
Net income (loss)										99	64	(37)
Insurance - Overseas General
Segment Reporting Information [Line Items]
Net premiums written										6,999	6,520	5,863
Net premiums earned										6,805	6,333	5,740
Losses and loss expenses										3,189	3,062	2,862
Policy acquisition costs										1,625	1,453	1,353
Administrative expenses										1,026	1,008	935
Underwriting income (loss)										965	810	590
Net investment income										545	539	521
Net realized gains (losses) including OTTI										(78)	18	103
Interest expense										6	5	5
Other										11	39	3
Income tax expense (benefit)										378	222	133
Net income (loss)										1,037	1,101	1,073
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums written										935	991	1,025
Net premiums earned										1,026	976	1,002
Losses and loss expenses										431	396	553
Policy acquisition costs										257	197	172
Administrative expenses										54	50	51
Underwriting income (loss)										284	333	226
Net investment income										316	280	290
Net realized gains (losses) including OTTI										(29)	53	6
Interest expense										4	5	4
Other										(54)	(19)	(15)
Income tax expense (benefit)										38	36	15
Net income (loss)										583	644	518
Life
Segment Reporting Information [Line Items]
Management underwriting income loss Insurance										363
Net premiums written										2,012	1,972	1,979
Net premiums earned										1,962	1,905	1,916
Losses and loss expenses										589	582	611
Policy benefits										517	515	521
Policy acquisition costs										478	358	334
Administrative expenses										285	343	328
Underwriting income (loss)										93	107	122
Net investment income										268	251	251
Net realized gains (losses) including OTTI										(383)	360	(72)
Interest expense										11	15	12
(Gains) losses from fair value changes in separate account assets										(2)	(16)	(29)
Other										2	13	25
Income tax expense (benefit)										46	34	58
Net income (loss)										(79)	672	235
Corporate and Other
Segment Reporting Information [Line Items]
Losses and loss expenses										3	8	1
Policy acquisition costs										0	1	1
Administrative expenses										193	198	181
Underwriting income (loss)										(196)	(207)	(183)
Net investment income										12	27	28
Net realized gains (losses) including OTTI										(4)		(1)
Interest expense										250	244	217
Other										29	24	19
Income tax expense (benefit)										(167)	(179)	(136)
Net income (loss)										(300)	(269)	(256)
Insurance - North American P&C
Segment Reporting Information [Line Items]
Net premiums written										6,263	5,915	5,349
Net premiums earned										6,107	5,721	5,147
Losses and loss expenses										4,086	3,776	3,715
Policy benefits										0	0
Policy acquisition costs										634	597	558
Administrative expenses										678	601	608
Underwriting income (loss)										709	747	266
Net investment income										1,085	1,021	1,066
Net realized gains (losses) including OTTI										(67)	72	41
Interest expense										9	5	12
(Gains) losses from fair value changes in separate account assets										0	0	0
Other										(101)	(58)	(41)
Income tax expense (benefit)										306	347	229
Net income (loss)										$ 1,513	$ 1,546	$ 1,173

[1] (1) Net income for the three months ended December 31, 2014 includes $89 million of net charges related to income taxes to correct prior periods. Such amounts are not material to any period presented.