Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting [Line Items]
Gross premiums written									$ 4,633,054,000	$ 4,885,266,000	$ 4,000,888,000
Net premiums written	879,900,000	1,079,600,000	1,056,500,000	1,470,400,000	820,600,000	987,600,000	1,112,700,000	1,784,200,000	4,486,329,000	4,705,116,000	3,948,704,000
Decrease in unearned premiums									161,425,000	71,355,000	171,121,000
Net premiums earned	1,181,400,000	1,294,300,000	1,107,500,000	1,064,600,000	1,204,600,000	1,313,400,000	1,104,600,000	1,153,800,000	4,647,754,000	4,776,471,000	4,119,825,000
Losses and loss expenses and life policy benefits									(4,373,000,000)	(3,284,000,000)	(2,296,000,000)
Acquisition costs	(238,800,000)	(262,500,000)	(229,200,000)	(207,900,000)	(246,600,000)	(261,600,000)	(244,100,000)	(220,100,000)	(938,361,000)	(972,537,000)	(885,214,000)
Technical result									(663,000,000)	520,000,000	939,000,000
Other income	3,100,000	1,400,000	1,600,000	1,800,000	5,100,000	3,300,000	800,000	1,300,000	7,915,000	10,470,000	22,312,000
Other operating expenses	(113,000,000)	(103,800,000)	(113,700,000)	(104,300,000)	(133,200,000)	(118,200,000)	(160,200,000)	(128,100,000)	(434,846,000)	(539,751,000)	(430,808,000)
Underwriting result									(1,090,000,000)	(10,000,000)	530,000,000
Net investment income	155,500,000	163,700,000	158,300,000	151,600,000	160,800,000	164,400,000	174,500,000	173,100,000	629,148,000	672,782,000	596,071,000
Net realized and unrealized investment gains (losses)	74,600,000	26,100,000	78,200,000	(112,200,000)	(83,200,000)	293,200,000	46,000,000	145,500,000	66,692,000	401,482,000	591,707,000
Net realized gain on purchase of capital efficient notes									0	0	88,427,000
Interest expense	(12,300,000)	(12,200,000)	(12,200,000)	(12,300,000)	(12,200,000)	(12,300,000)	(12,800,000)	(7,100,000)	(48,949,000)	(44,413,000)	(28,301,000)
Amortization of intangible assets	(8,900,000)	(9,500,000)	(9,200,000)	(8,800,000)	(8,800,000)	(10,000,000)	(7,800,000)	(4,800,000)	(36,405,000)	(31,461,000)	6,133,000
Net foreign exchange (losses) gains	14,700,000	10,600,000	8,700,000	700,000	(8,300,000)	(27,100,000)	11,000,000	3,600,000	34,675,000	(20,686,000)	(1,464,000)
Income tax expense	(3,300,000)	(41,800,000)	(50,100,000)	26,300,000	(10,900,000)	(72,600,000)	(17,800,000)	(27,600,000)	(68,972,000)	(128,784,000)	(262,090,000)
Interest in (losses) earnings of equity investments	(1,400,000)	(4,500,000)	(1,200,000)	700,000	7,500,000	1,300,000	1,300,000	2,400,000	(6,372,000)	12,597,000	15,552,000
Net (loss) income	(17,600,000)	180,100,000	124,200,000	(807,000,000)	57,000,000	524,900,000	190,900,000	79,600,000	(520,291,000)	852,552,000	1,536,854,000
[LifeMember]
Segment Reporting [Line Items]
Gross premiums written									790,000,000	749,000,000	595,000,000
Net premiums written									786,000,000	742,000,000	591,000,000
Decrease in unearned premiums									6,000,000	2,000,000	(4,000,000)
Net premiums earned									792,000,000	744,000,000	587,000,000
Losses and loss expenses and life policy benefits									(650,000,000)	(624,000,000)	(440,000,000)
Acquisition costs									(117,000,000)	(116,000,000)	(113,000,000)
Technical result									25,000,000	4,000,000	34,000,000
Other income									1,000,000	2,000,000	2,000,000
Other operating expenses									(53,000,000)	(57,000,000)	(47,000,000)
Underwriting result									(27,000,000)	(51,000,000)	(11,000,000)
Net investment income									66,000,000	71,000,000	62,000,000
Allocated underwriting result									39,000,000	20,000,000	51,000,000
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written									12,000,000	4,000,000	8,000,000
Net premiums written									12,000,000	2,000,000	7,000,000
Decrease in unearned premiums									0	1,000,000	1,000,000
Net premiums earned									12,000,000	3,000,000	8,000,000
Losses and loss expenses and life policy benefits									(6,000,000)	(3,000,000)	2,000,000
Acquisition costs									0	0	0
Technical result									6,000,000	0	10,000,000
Other income									3,000,000	3,000,000	7,000,000
Other operating expenses									(99,000,000)	(166,000,000)	(131,000,000)
Net investment income									563,000,000	602,000,000	534,000,000
Net realized and unrealized investment gains (losses)									67,000,000	402,000,000	591,000,000
Net realized gain on purchase of capital efficient notes											89,000,000
Interest expense									(49,000,000)	(44,000,000)	(28,000,000)
Amortization of intangible assets									(36,000,000)	(31,000,000)	6,000,000
Net foreign exchange (losses) gains									34,000,000	(21,000,000)	(1,000,000)
Income tax expense									(69,000,000)	(129,000,000)	(262,000,000)
Interest in (losses) earnings of equity investments									(6,000,000)	13,000,000	16,000,000
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written									3,831,000,000	4,132,000,000	3,398,000,000
Net premiums written									3,688,000,000	3,961,000,000	3,351,000,000
Decrease in unearned premiums									156,000,000	68,000,000	174,000,000
Net premiums earned									3,844,000,000	4,029,000,000	3,525,000,000
Losses and loss expenses and life policy benefits									(3,717,000,000)	(2,657,000,000)	(1,858,000,000)
Acquisition costs									(821,000,000)	(856,000,000)	(772,000,000)
Technical result									(694,000,000)	516,000,000	895,000,000
Other income									4,000,000	5,000,000	13,000,000
Other operating expenses									(283,000,000)	(317,000,000)	(253,000,000)
Underwriting result									(973,000,000)	204,000,000	655,000,000
Reinsurance Ratios [Abstract]
Loss ratio									96.70%	65.90%	52.70%
Acquisition ratio									21.30%	21.30%	21.90%
Technical ratio									118.00%	87.20%	74.60%
Other operating expense ratio									7.40%	7.80%	7.20%
Combined ratio									125.40%	95.00%	81.80%
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written									1,104,000,000	1,028,000,000	1,162,000,000
Net premiums written									1,104,000,000	1,026,000,000	1,162,000,000
Decrease in unearned premiums									31,000,000	12,000,000	48,000,000
Net premiums earned									1,135,000,000	1,038,000,000	1,210,000,000
Losses and loss expenses and life policy benefits									(741,000,000)	(577,000,000)	(728,000,000)
Acquisition costs									(276,000,000)	(288,000,000)	(311,000,000)
Technical result									118,000,000	173,000,000	171,000,000
Reinsurance Ratios [Abstract]
Loss ratio									65.30%	55.60%	60.20%
Acquisition ratio									24.30%	27.80%	25.70%
Technical ratio									89.60%	83.40%	85.90%
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written									682,000,000	909,000,000	677,000,000
Net premiums written									678,000,000	898,000,000	679,000,000
Decrease in unearned premiums									81,000,000	16,000,000	50,000,000
Net premiums earned									759,000,000	914,000,000	729,000,000
Losses and loss expenses and life policy benefits									(567,000,000)	(702,000,000)	(392,000,000)
Acquisition costs									(191,000,000)	(227,000,000)	(174,000,000)
Technical result									1,000,000	(15,000,000)	163,000,000
Reinsurance Ratios [Abstract]
Loss ratio									74.70%	76.80%	53.70%
Acquisition ratio									25.10%	24.90%	23.80%
Technical ratio									99.80%	101.70%	77.50%
[NonLifeMember] | [GlobalNonUsSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written									1,446,000,000	1,479,000,000	1,159,000,000
Net premiums written									1,344,000,000	1,391,000,000	1,113,000,000
Decrease in unearned premiums									32,000,000	14,000,000	3,000,000
Net premiums earned									1,376,000,000	1,405,000,000	1,116,000,000
Losses and loss expenses and life policy benefits									(950,000,000)	(985,000,000)	(732,000,000)
Acquisition costs									(328,000,000)	(292,000,000)	(254,000,000)
Technical result									98,000,000	128,000,000	130,000,000
Reinsurance Ratios [Abstract]
Loss ratio									69.10%	70.00%	65.60%
Acquisition ratio									23.80%	20.80%	22.70%
Technical ratio									92.90%	90.80%	88.30%
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written									599,000,000	716,000,000	400,000,000
Net premiums written									562,000,000	646,000,000	397,000,000
Decrease in unearned premiums									12,000,000	26,000,000	73,000,000
Net premiums earned									574,000,000	672,000,000	470,000,000
Losses and loss expenses and life policy benefits									(1,459,000,000)	(393,000,000)	(6,000,000)
Acquisition costs									(26,000,000)	(49,000,000)	(33,000,000)
Technical result									$ (911,000,000)	$ 230,000,000	$ 431,000,000
Reinsurance Ratios [Abstract]
Loss ratio									254.20%	58.50%	1.30%
Acquisition ratio									4.50%	7.20%	7.00%
Technical ratio									258.70%	65.70%	8.30%