Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting [Line Items]
Gross premiums written									$ 4,718,235,000		$ 4,633,054,000		$ 4,885,266,000
Net premiums written	920,300,000	1,043,200,000	1,136,000,000	1,473,300,000	879,900,000	1,079,600,000	1,056,500,000	1,470,400,000	4,572,860,000		4,486,329,000		4,705,116,000
(Increase) decrease in unearned premiums									(86,921,000)		161,425,000		71,355,000
Net premiums earned	1,168,100,000	1,237,100,000	1,090,900,000	989,800,000	1,181,400,000	1,294,300,000	1,107,500,000	1,064,600,000	4,485,939,000		4,647,754,000		4,776,471,000
Losses and loss expenses and life policy benefits									(2,805,000,000)		(4,373,000,000)		(3,284,000,000)
Acquisition costs	(245,500,000)	(247,100,000)	(232,700,000)	(211,600,000)	(238,800,000)	(262,500,000)	(229,200,000)	(207,900,000)	(936,909,000)		(938,361,000)		(972,537,000)
Technical result									744,000,000		(663,000,000)		520,000,000
Other income	3,800,000	2,700,000	2,600,000	2,800,000	3,100,000	1,400,000	1,600,000	1,800,000	11,920,000		7,915,000		10,470,000
Other operating expenses	(112,300,000)	(94,700,000)	(106,200,000)	(98,200,000)	(113,000,000)	(103,800,000)	(113,700,000)	(104,300,000)	(411,374,000)		(434,846,000)		(539,751,000)
Underwriting result									345,000,000		(1,090,000,000)		(10,000,000)
Net investment income	135,700,000	135,300,000	153,500,000	146,900,000	155,500,000	163,700,000	158,300,000	151,600,000	571,338,000		629,148,000		672,782,000
Net realized and unrealized investment gains	5,100,000	257,400,000	38,100,000	192,700,000	74,600,000	26,100,000	78,200,000	(112,200,000)	493,409,000		66,692,000		401,482,000
Interest expense	(12,200,000)	(12,200,000)	(12,200,000)	(12,200,000)	(12,300,000)	(12,200,000)	(12,200,000)	(12,300,000)	(48,895,000)		(48,949,000)		(44,413,000)
Amortization of intangible assets	(5,100,000)	(8,900,000)	(8,900,000)	(8,900,000)	(8,900,000)	(9,500,000)	(9,200,000)	(8,800,000)	(31,799,000)		(36,405,000)		(31,461,000)
Net foreign exchange (losses) gains	(3,400,000)	(2,000,000)	7,700,000	(2,600,000)	14,700,000	10,600,000	8,700,000	700,000	(175,000)		34,675,000		(20,686,000)
Income tax expense	(22,800,000)	(64,100,000)	(50,100,000)	(67,200,000)	(3,300,000)	(41,800,000)	(50,100,000)	26,300,000	(204,284,000)		(68,972,000)		(128,784,000)
Interest in earnings (losses) of equity investments	1,000,000	4,300,000	(500,000)	5,100,000	(1,400,000)	(4,500,000)	(1,200,000)	700,000	9,954,000		(6,372,000)		12,597,000
Net income (loss)	111,500,000	486,700,000	176,100,000	360,100,000	(17,600,000)	180,100,000	124,200,000	(807,000,000)	1,134,514,000		(520,291,000)		852,552,000
[LifeMember]
Segment Reporting [Line Items]
Gross premiums written									802,000,000		790,000,000		749,000,000
Net premiums written									799,000,000		786,000,000		742,000,000
(Increase) decrease in unearned premiums									(4,000,000)		6,000,000		2,000,000
Net premiums earned									795,000,000		792,000,000		744,000,000
Losses and loss expenses and life policy benefits									(647,000,000)		(650,000,000)		(624,000,000)
Acquisition costs									(116,000,000)		(117,000,000)		(116,000,000)
Technical result									32,000,000		25,000,000		4,000,000
Other income									4,000,000		1,000,000		2,000,000
Other operating expenses									(52,000,000)		(53,000,000)		(57,000,000)
Underwriting result									(16,000,000)		(27,000,000)		(51,000,000)
Net investment income									64,000,000		66,000,000		71,000,000
Allocated underwriting result									48,000,000	[1]	39,000,000	[1]	20,000,000	[1]
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written									6,000,000		12,000,000		4,000,000
Net premiums written									6,000,000		12,000,000		2,000,000
(Increase) decrease in unearned premiums									1,000,000		0		1,000,000
Net premiums earned									7,000,000		12,000,000		3,000,000
Losses and loss expenses and life policy benefits									(3,000,000)		(6,000,000)		(3,000,000)
Acquisition costs									0		0		0
Technical result									4,000,000		6,000,000		0
Other income									3,000,000		3,000,000		3,000,000
Other operating expenses									(102,000,000)		(99,000,000)		(166,000,000)
Net investment income									507,000,000		563,000,000		602,000,000
Net realized and unrealized investment gains									494,000,000		67,000,000		402,000,000
Interest expense									(49,000,000)		(49,000,000)		(44,000,000)
Amortization of intangible assets									(32,000,000)		(36,000,000)		(31,000,000)
Net foreign exchange (losses) gains									0		34,000,000		(21,000,000)
Income tax expense									(204,000,000)		(69,000,000)		(129,000,000)
Interest in earnings (losses) of equity investments									10,000,000		(6,000,000)		13,000,000
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written									3,910,000,000		3,831,000,000		4,132,000,000
Net premiums written									3,768,000,000		3,688,000,000		3,961,000,000
(Increase) decrease in unearned premiums									(84,000,000)		156,000,000		68,000,000
Net premiums earned									3,684,000,000		3,844,000,000		4,029,000,000
Losses and loss expenses and life policy benefits									(2,155,000,000)		(3,717,000,000)		(2,657,000,000)
Acquisition costs									(821,000,000)		(821,000,000)		(856,000,000)
Technical result									708,000,000		(694,000,000)		516,000,000
Other income									5,000,000		4,000,000		5,000,000
Other operating expenses									(257,000,000)		(283,000,000)		(317,000,000)
Underwriting result									456,000,000		(973,000,000)		204,000,000
Reinsurance Ratios [Abstract]
Loss ratio									58.50%	[2]	96.70%	[2]	65.90%	[2]
Acquisition ratio									22.30%	[3]	21.30%	[3]	21.30%	[3]
Technical ratio									80.80%	[4]	118.00%	[4]	87.20%	[4]
Other operating expense ratio									7.00%	[5]	7.40%	[5]	7.80%	[5]
Combined ratio									87.80%	[6]	125.40%	[6]	95.00%	[6]
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written									1,221,000,000		1,104,000,000		1,028,000,000
Net premiums written									1,219,000,000		1,104,000,000		1,026,000,000
(Increase) decrease in unearned premiums									(43,000,000)		31,000,000		12,000,000
Net premiums earned									1,176,000,000		1,135,000,000		1,038,000,000
Losses and loss expenses and life policy benefits									(816,000,000)		(741,000,000)		(577,000,000)
Acquisition costs									(291,000,000)		(276,000,000)		(288,000,000)
Technical result									69,000,000		118,000,000		173,000,000
Reinsurance Ratios [Abstract]
Loss ratio									69.40%	[2]	65.30%	[2]	55.60%	[2]
Acquisition ratio									24.70%	[3]	24.30%	[3]	27.80%	[3]
Technical ratio									94.10%	[4]	89.60%	[4]	83.40%	[4]
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written									684,000,000		682,000,000		909,000,000
Net premiums written									681,000,000		678,000,000		898,000,000
(Increase) decrease in unearned premiums									(3,000,000)		81,000,000		16,000,000
Net premiums earned									678,000,000		759,000,000		914,000,000
Losses and loss expenses and life policy benefits									(415,000,000)		(567,000,000)		(702,000,000)
Acquisition costs									(167,000,000)		(191,000,000)		(227,000,000)
Technical result									96,000,000		1,000,000		(15,000,000)
Reinsurance Ratios [Abstract]
Loss ratio									61.30%	[2]	74.70%	[2]	76.80%	[2]
Acquisition ratio									24.60%	[3]	25.10%	[3]	24.90%	[3]
Technical ratio									85.90%	[4]	99.80%	[4]	101.70%	[4]
[NonLifeMember] | [GlobalNonUsSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written									1,505,000,000		1,446,000,000		1,479,000,000
Net premiums written									1,415,000,000		1,344,000,000		1,391,000,000
(Increase) decrease in unearned premiums									(42,000,000)		32,000,000		14,000,000
Net premiums earned									1,373,000,000		1,376,000,000		1,405,000,000
Losses and loss expenses and life policy benefits									(821,000,000)		(950,000,000)		(985,000,000)
Acquisition costs									(321,000,000)		(328,000,000)		(292,000,000)
Technical result									231,000,000		98,000,000		128,000,000
Reinsurance Ratios [Abstract]
Loss ratio									59.80%	[2]	69.10%	[2]	70.00%	[2]
Acquisition ratio									23.40%	[3]	23.80%	[3]	20.80%	[3]
Technical ratio									83.20%	[4]	92.90%	[4]	90.80%	[4]
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written									500,000,000		599,000,000		716,000,000
Net premiums written									453,000,000		562,000,000		646,000,000
(Increase) decrease in unearned premiums									4,000,000		12,000,000		26,000,000
Net premiums earned									457,000,000		574,000,000		672,000,000
Losses and loss expenses and life policy benefits									(103,000,000)		(1,459,000,000)		(393,000,000)
Acquisition costs									(42,000,000)		(26,000,000)		(49,000,000)
Technical result									$ 312,000,000		$ (911,000,000)		$ 230,000,000
Reinsurance Ratios [Abstract]
Loss ratio									22.40%	[2]	254.20%	[2]	58.50%	[2]
Acquisition ratio									9.30%	[3]	4.50%	[3]	7.20%	[3]
Technical ratio									31.70%	[4]	258.70%	[4]	65.70%	[4]

[1] (1) Allocated underwriting result is defined as net premiums earned, other income or loss and allocated net investment income less life policy benefits, acquisition costs and other operating expenses.

[2] (2) Loss ratio is obtained by dividing losses and loss expenses by net premiums earned.

[3] (3) Acquisition ratio is obtained by dividing acquisition costs by net premiums earned.

[4] (4) Technical ratio is defined as the sum of the loss ratio and the acquisition ratio.

[5] (5) Other operating expense ratio is obtained by dividing other operating expenses by net premiums earned.

[6] (6) Combined ratio is defined as the sum of the technical ratio and the other operating expense ratio.