Segment Information (Operations By Segment) (Detail) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Net premiums written									$ 17,025,000,000	$ 16,075,000,000	$ 15,372,000,000
Net premiums earned	4,363,000,000	4,610,000,000	4,067,000,000	3,573,000,000	3,848,000,000	4,665,000,000	3,783,000,000	3,381,000,000	16,613,000,000	15,677,000,000	15,387,000,000
Losses and loss expenses	2,517,000,000	2,655,000,000	2,250,000,000	1,926,000,000	2,683,000,000	3,047,000,000	2,119,000,000	1,804,000,000	9,348,000,000	9,653,000,000	9,520,000,000
Policy benefits	136,000,000	138,000,000	110,000,000	131,000,000	142,000,000	130,000,000	102,000,000	147,000,000	515,000,000	521,000,000	401,000,000
Policy acquisition costs									2,659,000,000	2,446,000,000	2,472,000,000
Administrative expenses									2,211,000,000	2,096,000,000	2,068,000,000
Underwriting income (loss)									1,880,000,000	961,000,000	926,000,000
Net investment income	557,000,000	522,000,000	534,000,000	531,000,000	567,000,000	533,000,000	537,000,000	544,000,000	2,144,000,000	2,181,000,000	2,242,000,000
Net realized gains (losses) including OTTI	154,000,000	40,000,000	104,000,000	206,000,000	272,000,000	(60,000,000)	(394,000,000)	260,000,000	504,000,000	78,000,000	(795,000,000)
Interest expense									275,000,000	250,000,000	250,000,000
Other (income) expense									15,000,000	(6,000,000)	81,000,000
(Gains) losses from fair value changes in separate account assets									(16,000,000)	(29,000,000)	36,000,000
Other									31,000,000	23,000,000	45,000,000
Income tax expense (benefit)									480,000,000	270,000,000	502,000,000
Net income (loss)	998,000,000	916,000,000	891,000,000	953,000,000	765,000,000	640,000,000	328,000,000	973,000,000	3,758,000,000	2,706,000,000	1,540,000,000
Insurance - North American Agriculture
Segment Reporting Information [Line Items]
Net premiums written									1,627,000,000	1,859,000,000	1,951,000,000
Net premiums earned									1,678,000,000	1,872,000,000	1,942,000,000
Losses and loss expenses									1,524,000,000	1,911,000,000	1,699,000,000
Policy benefits									0	0
Policy acquisition costs									53,000,000	28,000,000	80,000,000
Administrative expenses									11,000,000	(7,000,000)	(6,000,000)
Underwriting income (loss)									90,000,000	(60,000,000)	169,000,000
Net investment income									26,000,000	25,000,000	22,000,000
Net realized gains (losses) including OTTI									1,000,000	1,000,000	6,000,000
Interest expense									1,000,000	0	2,000,000
(Gains) losses from fair value changes in separate account assets									0	0	0
Other									32,000,000	32,000,000	18,000,000
Income tax expense (benefit)									20,000,000	(29,000,000)	51,000,000
Net income (loss)									64,000,000	(37,000,000)	126,000,000
Insurance - Overseas General
Segment Reporting Information [Line Items]
Net premiums written									6,520,000,000	5,863,000,000	5,629,000,000
Net premiums earned									6,333,000,000	5,740,000,000	5,614,000,000
Losses and loss expenses									3,062,000,000	2,862,000,000	3,029,000,000
Policy benefits
Policy acquisition costs									1,453,000,000	1,353,000,000	1,335,000,000
Administrative expenses									1,008,000,000	935,000,000	939,000,000
Underwriting income (loss)									810,000,000	590,000,000	311,000,000
Net investment income									539,000,000	521,000,000	546,000,000
Net realized gains (losses) including OTTI									18,000,000	103,000,000	(33,000,000)
Interest expense									5,000,000	5,000,000	5,000,000
(Gains) losses from fair value changes in separate account assets
Other									39,000,000	3,000,000	0
Income tax expense (benefit)									222,000,000	133,000,000	164,000,000
Net income (loss)									1,101,000,000	1,073,000,000	721,000,000
Global Reinsurance
Segment Reporting Information [Line Items]
Net premiums written									991,000,000	1,025,000,000	979,000,000
Net premiums earned									976,000,000	1,002,000,000	1,003,000,000
Losses and loss expenses									396,000,000	553,000,000	621,000,000
Policy benefits
Policy acquisition costs											185,000,000
Administrative expenses									50,000,000	51,000,000	52,000,000
Underwriting income (loss)									333,000,000	226,000,000	145,000,000
Net investment income									280,000,000	290,000,000	287,000,000
Net realized gains (losses) including OTTI									53,000,000	6,000,000	(50,000,000)
Interest expense									5,000,000	4,000,000	2,000,000
(Gains) losses from fair value changes in separate account assets
Other									(19,000,000)	(15,000,000)	(1,000,000)
Income tax expense (benefit)									36,000,000	15,000,000	30,000,000
Net income (loss)									644,000,000	518,000,000	351,000,000
Life
Segment Reporting Information [Line Items]
Net premiums written									1,972,000,000	1,979,000,000	1,913,000,000
Net premiums earned									1,905,000,000	1,916,000,000	1,859,000,000
Losses and loss expenses									582,000,000	611,000,000	593,000,000
Policy benefits									515,000,000	521,000,000	401,000,000
Policy acquisition costs									358,000,000	334,000,000	339,000,000
Administrative expenses									343,000,000	328,000,000	317,000,000
Underwriting income (loss)									107,000,000	122,000,000	209,000,000
Net investment income									251,000,000	251,000,000	226,000,000
Net realized gains (losses) including OTTI									360,000,000	(72,000,000)	806,000,000
Interest expense									15,000,000	12,000,000	11,000,000
(Gains) losses from fair value changes in separate account assets									16,000,000	29,000,000	(36,000,000)
Other									13,000,000	25,000,000	26,000,000
Income tax expense (benefit)									34,000,000	58,000,000	50,000,000
Net income (loss)									672,000,000	235,000,000	(494,000,000)
Corporate and Other
Segment Reporting Information [Line Items]
Net premiums written
Net premiums earned
Losses and loss expenses									8,000,000	1,000,000	1,000,000
Policy benefits
Policy acquisition costs									1,000,000	1,000,000	1,000,000
Administrative expenses									198,000,000	181,000,000	168,000,000
Underwriting income (loss)									(207,000,000)	(183,000,000)	(170,000,000)
Net investment income									27,000,000	28,000,000	13,000,000
Net realized gains (losses) including OTTI									0	(1,000,000)	6,000,000
Interest expense									244,000,000	217,000,000	217,000,000
(Gains) losses from fair value changes in separate account assets
Other									24,000,000	19,000,000	15,000,000
Income tax expense (benefit)									(179,000,000)	(136,000,000)	(137,000,000)
Net income (loss)									(269,000,000)	(256,000,000)	(258,000,000)
Insurance - North American P&C
Segment Reporting Information [Line Items]
Net premiums written									5,915,000,000	5,349,000,000	4,900,000,000
Net premiums earned									5,721,000,000	5,147,000,000	4,969,000,000
Losses and loss expenses									3,776,000,000	3,715,000,000	3,577,000,000
Policy benefits									0	0
Policy acquisition costs									597,000,000	558,000,000	532,000,000
Administrative expenses									601,000,000	608,000,000	598,000,000
Underwriting income (loss)									747,000,000	266,000,000	262,000,000
Net investment income									1,021,000,000	1,066,000,000	1,148,000,000
Net realized gains (losses) including OTTI									72,000,000	41,000,000	28,000,000
Interest expense									5,000,000	12,000,000	13,000,000
(Gains) losses from fair value changes in separate account assets									0	0	0
Other									(58,000,000)	(41,000,000)	(13,000,000)
Income tax expense (benefit)									347,000,000	229,000,000	344,000,000
Net income (loss)									$ 1,546,000,000	$ 1,173,000,000	$ 1,094,000,000