Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting [Line Items]
Gross premiums written	$ 1,748,933,000		$ 1,871,740,000
Net premiums written	1,653,215,000		1,738,494,000
(Increase) decrease in unearned premiums	(418,493,000)		(484,712,000)
Net premiums earned	1,234,722,000		1,253,782,000
Losses and loss expenses and life policy benefits	(721,000,000)		(749,000,000)
Acquisition costs	(275,791,000)		(264,608,000)
Technical result	238,000,000		240,000,000
Other income (loss)	4,292,000		404,000
Other expenses	(124,750,000)		(111,462,000)
Underwriting result	117,000,000		129,000,000
Net investment income	104,631,000		116,867,000
Net realized and unrealized investment gains	115,645,000		142,172,000
Interest expense	(12,245,000)		(12,238,000)
Amortization of intangible assets	(6,768,000)		(7,002,000)
Net foreign exchange gains	13,147,000		670,000
Income tax expense	(79,665,000)		(62,305,000)
Interest in (losses) earnings of equity method investments	(3,838,000)		6,064,000
Net income	248,099,000		312,887,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written	1,425,000,000		1,583,000,000
Net premiums written	1,340,000,000		1,456,000,000
(Increase) decrease in unearned premiums	(404,000,000)		(464,000,000)
Net premiums earned	936,000,000		992,000,000
Losses and loss expenses and life policy benefits	(481,000,000)		(534,000,000)
Acquisition costs	(242,000,000)		(233,000,000)
Technical result	213,000,000		225,000,000
Other income (loss)	0		1,000,000
Other expenses	(52,000,000)		(65,000,000)
Underwriting result	161,000,000		161,000,000
Reinsurance Ratios [Abstract]
Technical ratio	77.30%	[1]	77.40%	[1]
Loss Ratio	51.40%	[2]	53.80%	[2]
Acquisition ratio	25.90%	[3]	23.60%	[3]
Other expense Ratio	5.50%	[4]	6.50%	[4]
Combined Ratio	82.80%	[5]	83.90%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	473,000,000		530,000,000
Net premiums written	471,000,000		527,000,000
(Increase) decrease in unearned premiums	(132,000,000)		(148,000,000)
Net premiums earned	339,000,000		379,000,000
Losses and loss expenses and life policy benefits	(172,000,000)		(260,000,000)
Acquisition costs	(93,000,000)		(92,000,000)
Technical result	74,000,000		27,000,000
Reinsurance Ratios [Abstract]
Technical ratio	78.20%	[1]	92.90%	[1]
Loss Ratio	50.70%	[2]	68.60%	[2]
Acquisition ratio	27.50%	[3]	24.30%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written	334,000,000		364,000,000
Net premiums written	331,000,000		361,000,000
(Increase) decrease in unearned premiums	(157,000,000)		(182,000,000)
Net premiums earned	174,000,000		179,000,000
Losses and loss expenses and life policy benefits	(119,000,000)		(94,000,000)
Acquisition costs	(52,000,000)		(54,000,000)
Technical result	3,000,000		31,000,000
Reinsurance Ratios [Abstract]
Technical ratio	98.50%	[1]	82.50%	[1]
Loss Ratio	68.40%	[2]	52.40%	[2]
Acquisition ratio	30.10%	[3]	30.10%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written	427,000,000		479,000,000
Net premiums written	362,000,000		389,000,000
(Increase) decrease in unearned premiums	3,000,000		(34,000,000)
Net premiums earned	365,000,000		355,000,000
Losses and loss expenses and life policy benefits	(170,000,000)		(201,000,000)
Acquisition costs	(93,000,000)		(79,000,000)
Technical result	102,000,000		75,000,000
Reinsurance Ratios [Abstract]
Technical ratio	72.10%	[1]	79.00%	[1]
Loss Ratio	46.70%	[2]	56.60%	[2]
Acquisition ratio	25.40%	[3]	22.40%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	191,000,000		210,000,000
Net premiums written	176,000,000		179,000,000
(Increase) decrease in unearned premiums	(118,000,000)		(100,000,000)
Net premiums earned	58,000,000		79,000,000
Losses and loss expenses and life policy benefits	(20,000,000)		21,000,000
Acquisition costs	(4,000,000)		(8,000,000)
Technical result	34,000,000		92,000,000
Reinsurance Ratios [Abstract]
Technical ratio	40.70%	[1]	(16.40%)	[1]
Loss Ratio	33.70%	[2]	(26.50%)	[2]
Acquisition ratio	7.00%	[3]	10.10%	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written	324,000,000		289,000,000
Net premiums written	313,000,000		282,000,000
(Increase) decrease in unearned premiums	(14,000,000)		(20,000,000)
Net premiums earned	299,000,000		262,000,000
Losses and loss expenses and life policy benefits	(240,000,000)		(215,000,000)
Acquisition costs	(34,000,000)		(32,000,000)
Technical result	25,000,000		15,000,000
Other income (loss)	1,000,000		1,000,000
Other expenses	(15,000,000)		(17,000,000)
Underwriting result	11,000,000		(1,000,000)
Net investment income	14,000,000		15,000,000
Allocated underwriting result	25,000,000	[6]	14,000,000	[6]
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written	0		0
Net premiums written	0		0
(Increase) decrease in unearned premiums	0		0
Net premiums earned	0		0
Losses and loss expenses and life policy benefits	0		0
Acquisition costs	0		0
Technical result	0		0
Other income (loss)	3,000,000		(2,000,000)
Other expenses	(58,000,000)		(29,000,000)
Net investment income	91,000,000		102,000,000
Net realized and unrealized investment gains	116,000,000		142,000,000
Interest expense	(12,000,000)		(12,000,000)
Amortization of intangible assets	(7,000,000)		(7,000,000)
Net foreign exchange gains	13,000,000		0
Income tax expense	(80,000,000)		(62,000,000)
Interest in (losses) earnings of equity method investments	$ (4,000,000)		$ 6,000,000

[1] Technical ratio is defined as the sum of the loss ratio and the acquisition ratio.

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

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

[4] Other expense ratio is obtained by dividing other expenses by net premiums earned.

[5] Combined ratio is defined as the sum of the technical ratio and the other expense ratio.

[6] 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 expenses.