Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting [Line Items]
Gross premiums written	$ 1,340,582,000		$ 1,163,243,000		$ 3,097,467,000		$ 2,730,726,000
Net premiums written	1,309,318,000		1,136,046,000		2,945,750,000		2,609,331,000
(Increase) decrease in unearned premiums	(100,682,000)		(45,168,000)		(590,434,000)		(528,623,000)
Net premiums earned	1,208,636,000		1,090,878,000		2,355,316,000		2,080,708,000
Losses and loss expenses and life policy benefits	(867,000,000)		(706,000,000)		(1,528,000,000)		(1,283,000,000)
Acquisition costs	(241,743,000)		(232,723,000)		(475,942,000)		(444,330,000)
Technical result	100,000,000		152,000,000		351,000,000		354,000,000
Other income	3,878,000		2,654,000		7,805,000		5,400,000
Other operating expenses	(144,833,000)		(106,184,000)		(260,874,000)		(204,358,000)
Underwriting result	(41,000,000)		49,000,000		98,000,000		155,000,000
Net investment income	124,503,000		153,506,000		248,207,000		300,402,000
Net realized and unrealized investment (losses) gains	(299,215,000)		38,132,000		(276,272,000)		230,867,000
Interest expense	(12,232,000)		(12,223,000)		(24,460,000)		(24,443,000)
Amortization of intangible assets	(7,045,000)		(8,893,000)		(14,091,000)		(17,786,000)
Net foreign exchange (losses) gains	(10,584,000)		7,770,000		(8,543,000)		5,181,000
Income tax (benefit) expense	74,569,000		(50,136,000)		32,894,000		(117,310,000)
Interest in (losses) earnings of equity investments	(3,479,000)		(498,000)		3,736,000		4,579,000
Net (loss) income	(174,388,000)		176,146,000		59,982,000		536,287,000
[LifeAndHealthMember]
Segment Reporting [Line Items]
Gross premiums written	233,000,000		200,000,000		486,000,000		417,000,000
Net premiums written	232,000,000		199,000,000		481,000,000		414,000,000
(Increase) decrease in unearned premiums	0		1,000,000		(25,000,000)		(20,000,000)
Net premiums earned	232,000,000		200,000,000		456,000,000		394,000,000
Losses and loss expenses and life policy benefits	(181,000,000)		(173,000,000)		(363,000,000)		(322,000,000)
Acquisition costs	(33,000,000)		(26,000,000)		(59,000,000)		(55,000,000)
Technical result	18,000,000		1,000,000		34,000,000		17,000,000
Other income	3,000,000		1,000,000		6,000,000		2,000,000
Other operating expenses	(17,000,000)		(13,000,000)		(35,000,000)		(26,000,000)
Underwriting result	4,000,000		(11,000,000)		5,000,000		(7,000,000)
Net investment income	15,000,000		17,000,000		30,000,000		33,000,000
Allocated underwriting result	19,000,000	[1]	6,000,000	[1]	35,000,000	[1]	26,000,000	[1]
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written	2,000,000		3,000,000		4,000,000		6,000,000
Net premiums written	1,000,000		3,000,000		2,000,000		6,000,000
(Increase) decrease in unearned premiums	(1,000,000)		(2,000,000)		(2,000,000)		(4,000,000)
Net premiums earned	0		1,000,000		0		2,000,000
Losses and loss expenses and life policy benefits	0		0		1,000,000		0
Acquisition costs	0		0		0		0
Technical result	0		1,000,000		1,000,000		2,000,000
Other income	1,000,000		2,000,000		2,000,000		2,000,000
Other operating expenses	(68,000,000)		(27,000,000)		(100,000,000)		(49,000,000)
Net investment income	110,000,000		136,000,000		218,000,000		267,000,000
Net realized and unrealized investment (losses) gains	(299,000,000)		38,000,000		(276,000,000)		231,000,000
Interest expense	(12,000,000)		(12,000,000)		(24,000,000)		(24,000,000)
Amortization of intangible assets	(7,000,000)		(9,000,000)		(14,000,000)		(18,000,000)
Net foreign exchange (losses) gains	(11,000,000)		8,000,000		(9,000,000)		5,000,000
Income tax (benefit) expense	75,000,000		(50,000,000)		33,000,000		(117,000,000)
Interest in (losses) earnings of equity investments	(4,000,000)		(1,000,000)		4,000,000		4,000,000
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written	1,106,000,000		960,000,000		2,607,000,000		2,308,000,000
Net premiums written	1,076,000,000		934,000,000		2,463,000,000		2,189,000,000
(Increase) decrease in unearned premiums	(99,000,000)		(44,000,000)		(564,000,000)		(504,000,000)
Net premiums earned	977,000,000		890,000,000		1,899,000,000		1,685,000,000
Losses and loss expenses and life policy benefits	(686,000,000)		(533,000,000)		(1,166,000,000)		(961,000,000)
Acquisition costs	(209,000,000)		(207,000,000)		(417,000,000)		(389,000,000)
Technical result	82,000,000		150,000,000		316,000,000		335,000,000
Other income	0		0		0		1,000,000
Other operating expenses	(60,000,000)		(66,000,000)		(126,000,000)		(129,000,000)
Underwriting result	22,000,000		84,000,000		190,000,000		207,000,000
Reinsurance Ratios [Abstract]
Loss ratio	70.30%	[2]	59.90%	[2]	61.40%	[2]	57.00%	[2]
Acquisition ratio	21.40%	[3]	23.20%	[3]	22.00%	[3]	23.10%	[3]
Technical ratio	91.70%	[4]	83.10%	[4]	83.40%	[4]	80.10%	[4]
Other operating expense ratio	6.10%	[5]	7.50%	[5]	6.60%	[5]	7.70%	[5]
Combined ratio	97.80%	[6]	90.60%	[6]	90.00%	[6]	87.80%	[6]
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written	372,000,000		271,000,000		819,000,000		613,000,000
Net premiums written	360,000,000		270,000,000		807,000,000		611,000,000
(Increase) decrease in unearned premiums	(3,000,000)		20,000,000		(117,000,000)		(84,000,000)
Net premiums earned	357,000,000		290,000,000		690,000,000		527,000,000
Losses and loss expenses and life policy benefits	(245,000,000)		(185,000,000)		(485,000,000)		(317,000,000)
Acquisition costs	(79,000,000)		(69,000,000)		(151,000,000)		(134,000,000)
Technical result	33,000,000		36,000,000		54,000,000		76,000,000
Reinsurance Ratios [Abstract]
Loss ratio	68.60%	[2]	63.70%	[2]	70.20%	[2]	60.10%	[2]
Acquisition ratio	22.10%	[3]	23.80%	[3]	21.90%	[3]	25.50%	[3]
Technical ratio	90.70%	[4]	87.50%	[4]	92.10%	[4]	85.60%	[4]
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written	160,000,000		130,000,000		532,000,000		477,000,000
Net premiums written	158,000,000		128,000,000		525,000,000		474,000,000
(Increase) decrease in unearned premiums	11,000,000		36,000,000		(190,000,000)		(150,000,000)
Net premiums earned	169,000,000		164,000,000		335,000,000		324,000,000
Losses and loss expenses and life policy benefits	(106,000,000)		(119,000,000)		(173,000,000)		(217,000,000)
Acquisition costs	(34,000,000)		(39,000,000)		(84,000,000)		(78,000,000)
Technical result	29,000,000		6,000,000		78,000,000		29,000,000
Reinsurance Ratios [Abstract]
Loss ratio	62.90%	[2]	72.30%	[2]	51.80%	[2]	67.00%	[2]
Acquisition ratio	19.90%	[3]	23.90%	[3]	24.90%	[3]	23.90%	[3]
Technical ratio	82.80%	[4]	96.20%	[4]	76.70%	[4]	90.90%	[4]
[NonLifeMember] | [GlobalSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written	413,000,000		400,000,000		857,000,000		817,000,000
Net premiums written	409,000,000		391,000,000		771,000,000		744,000,000
(Increase) decrease in unearned premiums	(37,000,000)		(28,000,000)		(62,000,000)		(73,000,000)
Net premiums earned	372,000,000		363,000,000		709,000,000		671,000,000
Losses and loss expenses and life policy benefits	(284,000,000)		(213,000,000)		(469,000,000)		(408,000,000)
Acquisition costs	(90,000,000)		(93,000,000)		(165,000,000)		(162,000,000)
Technical result	(2,000,000)		57,000,000		75,000,000		101,000,000
Reinsurance Ratios [Abstract]
Loss ratio	76.60%	[2]	58.80%	[2]	66.10%	[2]	60.80%	[2]
Acquisition ratio	24.10%	[3]	25.50%	[3]	23.30%	[3]	24.20%	[3]
Technical ratio	100.70%	[4]	84.30%	[4]	89.40%	[4]	85.00%	[4]
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	161,000,000		159,000,000		399,000,000		401,000,000
Net premiums written	149,000,000		145,000,000		360,000,000		360,000,000
(Increase) decrease in unearned premiums	(70,000,000)		(72,000,000)		(195,000,000)		(197,000,000)
Net premiums earned	79,000,000		73,000,000		165,000,000		163,000,000
Losses and loss expenses and life policy benefits	(51,000,000)		(16,000,000)		(39,000,000)		(19,000,000)
Acquisition costs	(6,000,000)		(6,000,000)		(17,000,000)		(15,000,000)
Technical result	$ 22,000,000		$ 51,000,000		$ 109,000,000		$ 129,000,000
Reinsurance Ratios [Abstract]
Loss ratio	64.10%	[2]	22.50%	[2]	23.80%	[2]	11.30%	[2]
Acquisition ratio	8.50%	[3]	8.30%	[3]	10.50%	[3]	9.50%	[3]
Technical ratio	72.60%	[4]	30.80%	[4]	34.30%	[4]	20.80%	[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.