Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting [Line Items]
Gross premiums written	$ 1,056,076,000		$ 1,095,326,000		$ 3,786,802,000		$ 3,735,091,000
Net premiums written	1,043,240,000		1,079,557,000		3,652,571,000		3,606,444,000
Decrease (increase) in unearned premiums	193,851,000		214,762,000		(334,772,000)		(140,091,000)
Net premiums earned	1,237,091,000		1,294,319,000		3,317,799,000		3,466,353,000
Losses and loss expenses and life policy benefits	(721,000,000)		(882,000,000)		(2,004,000,000)		(3,303,000,000)
Acquisition costs	(247,058,000)		(262,489,000)		(691,388,000)		(699,589,000)
Technical result	269,000,000		150,000,000		623,000,000		(537,000,000)
Other income	2,744,000		1,434,000		8,143,000		4,843,000
Other operating expenses	(94,697,000)		(103,822,000)		(299,055,000)		(321,813,000)
Underwriting result	177,000,000		47,000,000		332,000,000		(854,000,000)
Net investment income	135,266,000		163,647,000		435,669,000		473,608,000
Net realized and unrealized investment gains (losses)	257,429,000		26,139,000		488,296,000		(7,860,000)
Interest expense	(12,224,000)		(12,216,000)		(36,668,000)		(36,730,000)
Amortization of intangible assets	(8,893,000)		(9,520,000)		(26,679,000)		(27,512,000)
Net foreign exchange (losses) gains	(2,015,000)		10,587,000		3,165,000		20,020,000
Income tax expense	(64,149,000)		(41,803,000)		(181,458,000)		(65,632,000)
Interest in earnings (losses) of equity investments	4,349,000		(4,527,000)		8,929,000		(4,970,000)
Net income (loss)	486,706,000		180,123,000		1,022,994,000		(502,648,000)
[LifeMember]
Segment Reporting [Line Items]
Gross premiums written	187,000,000		194,000,000		604,000,000		597,000,000
Net premiums written	187,000,000		194,000,000		601,000,000		596,000,000
Decrease (increase) in unearned premiums	8,000,000		9,000,000		(12,000,000)		(8,000,000)
Net premiums earned	195,000,000		203,000,000		589,000,000		588,000,000
Losses and loss expenses and life policy benefits	(157,000,000)		(168,000,000)		(479,000,000)		(479,000,000)
Acquisition costs	(27,000,000)		(33,000,000)		(82,000,000)		(89,000,000)
Technical result	11,000,000		2,000,000		28,000,000		20,000,000
Other income	1,000,000		0		3,000,000		0
Other operating expenses	(12,000,000)		(12,000,000)		(38,000,000)		(38,000,000)
Underwriting result	0		(10,000,000)		(7,000,000)		(18,000,000)
Net investment income	15,000,000		19,000,000		49,000,000		50,000,000
Allocated underwriting result	15,000,000	[1]	9,000,000	[1]	42,000,000	[1]	32,000,000	[1]
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written	0		6,000,000		6,000,000		12,000,000
Net premiums written	0		6,000,000		6,000,000		12,000,000
Decrease (increase) in unearned premiums	4,000,000		2,000,000		(1,000,000)		(4,000,000)
Net premiums earned	4,000,000		8,000,000		5,000,000		8,000,000
Losses and loss expenses and life policy benefits	(3,000,000)		(4,000,000)		(3,000,000)		(5,000,000)
Acquisition costs	(1,000,000)		0		0		0
Technical result	0		4,000,000		2,000,000		3,000,000
Other income	1,000,000		0		3,000,000		1,000,000
Other operating expenses	(25,000,000)		(23,000,000)		(74,000,000)		(78,000,000)
Net investment income	120,000,000		145,000,000		387,000,000		424,000,000
Net realized and unrealized investment gains (losses)	257,000,000		26,000,000		488,000,000		(8,000,000)
Interest expense	(12,000,000)		(12,000,000)		(37,000,000)		(37,000,000)
Amortization of intangible assets	(9,000,000)		(9,000,000)		(27,000,000)		(27,000,000)
Net foreign exchange (losses) gains	(2,000,000)		11,000,000		3,000,000		20,000,000
Income tax expense	(64,000,000)		(42,000,000)		(181,000,000)		(66,000,000)
Interest in earnings (losses) of equity investments	5,000,000		(5,000,000)		9,000,000		(5,000,000)
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written	869,000,000		895,000,000		3,177,000,000		3,126,000,000
Net premiums written	856,000,000		880,000,000		3,045,000,000		2,998,000,000
Decrease (increase) in unearned premiums	182,000,000		203,000,000		(321,000,000)		(128,000,000)
Net premiums earned	1,038,000,000		1,083,000,000		2,724,000,000		2,870,000,000
Losses and loss expenses and life policy benefits	(561,000,000)		(710,000,000)		(1,522,000,000)		(2,819,000,000)
Acquisition costs	(219,000,000)		(229,000,000)		(609,000,000)		(611,000,000)
Technical result	258,000,000		144,000,000		593,000,000		(560,000,000)
Other income	1,000,000		1,000,000		2,000,000		4,000,000
Other operating expenses	(58,000,000)		(69,000,000)		(187,000,000)		(206,000,000)
Underwriting result	201,000,000		76,000,000		408,000,000		(762,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	54.10%	[2]	65.60%	[2]	55.90%	[2]	98.20%	[2]
Acquisition ratio	21.10%	[3]	21.10%	[3]	22.30%	[3]	21.30%	[3]
Technical ratio	75.20%	[4]	86.70%	[4]	78.20%	[4]	119.50%	[4]
Other operating expense ratio	5.50%	[5]	6.40%	[5]	6.90%	[5]	7.20%	[5]
Combined ratio	80.70%	[6]	93.10%	[6]	85.10%	[6]	126.70%	[6]
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written	311,000,000		288,000,000		924,000,000		868,000,000
Net premiums written	311,000,000		287,000,000		922,000,000		868,000,000
Decrease (increase) in unearned premiums	24,000,000		42,000,000		(59,000,000)		(18,000,000)
Net premiums earned	335,000,000		329,000,000		863,000,000		850,000,000
Losses and loss expenses and life policy benefits	(251,000,000)		(192,000,000)		(568,000,000)		(556,000,000)
Acquisition costs	(83,000,000)		(80,000,000)		(218,000,000)		(210,000,000)
Technical result	1,000,000		57,000,000		77,000,000		84,000,000
Reinsurance Ratios [Abstract]
Loss ratio	74.90%	[2]	58.40%	[2]	65.90%	[2]	65.50%	[2]
Acquisition ratio	24.80%	[3]	24.30%	[3]	25.20%	[3]	24.60%	[3]
Technical ratio	99.70%	[4]	82.70%	[4]	91.10%	[4]	90.10%	[4]
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written	123,000,000		144,000,000		600,000,000		585,000,000
Net premiums written	122,000,000		144,000,000		596,000,000		581,000,000
Decrease (increase) in unearned premiums	50,000,000		49,000,000		(100,000,000)		(14,000,000)
Net premiums earned	172,000,000		193,000,000		496,000,000		567,000,000
Losses and loss expenses and life policy benefits	(110,000,000)		(102,000,000)		(327,000,000)		(379,000,000)
Acquisition costs	(42,000,000)		(52,000,000)		(120,000,000)		(145,000,000)
Technical result	20,000,000		39,000,000		49,000,000		43,000,000
Reinsurance Ratios [Abstract]
Loss ratio	63.90%	[2]	52.90%	[2]	65.90%	[2]	66.80%	[2]
Acquisition ratio	24.90%	[3]	26.90%	[3]	24.30%	[3]	25.70%	[3]
Technical ratio	88.80%	[4]	79.80%	[4]	90.20%	[4]	92.50%	[4]
[NonLifeMember] | [GlobalNonUsSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written	360,000,000		368,000,000		1,178,000,000		1,092,000,000
Net premiums written	354,000,000		360,000,000		1,098,000,000		1,007,000,000
Decrease (increase) in unearned premiums	9,000,000		2,000,000		(64,000,000)		14,000,000
Net premiums earned	363,000,000		362,000,000		1,034,000,000		1,021,000,000
Losses and loss expenses and life policy benefits	(161,000,000)		(247,000,000)		(569,000,000)		(675,000,000)
Acquisition costs	(79,000,000)		(82,000,000)		(241,000,000)		(240,000,000)
Technical result	123,000,000		33,000,000		224,000,000		106,000,000
Reinsurance Ratios [Abstract]
Loss ratio	44.40%	[2]	68.30%	[2]	55.10%	[2]	66.10%	[2]
Acquisition ratio	21.70%	[3]	22.70%	[3]	23.30%	[3]	23.50%	[3]
Technical ratio	66.10%	[4]	91.00%	[4]	78.40%	[4]	89.60%	[4]
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	75,000,000		95,000,000		475,000,000		581,000,000
Net premiums written	69,000,000		89,000,000		429,000,000		542,000,000
Decrease (increase) in unearned premiums	99,000,000		110,000,000		(98,000,000)		(110,000,000)
Net premiums earned	168,000,000		199,000,000		331,000,000		432,000,000
Losses and loss expenses and life policy benefits	(39,000,000)		(169,000,000)		(58,000,000)		(1,209,000,000)
Acquisition costs	(15,000,000)		(15,000,000)		(30,000,000)		(16,000,000)
Technical result	$ 114,000,000		$ 15,000,000		$ 243,000,000		$ (793,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	23.30%	[2]	85.00%	[2]	17.40%	[2]	279.90%	[2]
Acquisition ratio	8.80%	[3]	7.40%	[3]	9.10%	[3]	3.50%	[3]
Technical ratio	32.10%	[4]	92.40%	[4]	26.50%	[4]	283.40%	[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.