Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting [Line Items]
Gross premiums written	$ 1,871,740,000		$ 1,756,886,000
Net premiums written	1,738,494,000		1,636,431,000
Increase in unearned premiums	(484,712,000)		(489,751,000)
Net premiums earned	1,253,782,000		1,146,680,000
Losses and loss expenses and life policy benefits	(749,000,000)		(661,000,000)
Acquisition costs	(264,608,000)		(234,200,000)
Technical result	240,000,000		251,000,000
Other income	404,000		3,927,000
Other operating expenses	(111,462,000)		(116,040,000)
Underwriting result	129,000,000		139,000,000
Net investment income	116,867,000		123,704,000
Net realized and unrealized investment gains	142,172,000		22,943,000
Interest expense	(12,238,000)		(12,229,000)
Amortization of intangible assets	(7,002,000)		(7,046,000)
Net foreign exchange gains	670,000		2,043,000
Income tax expense	(62,305,000)		(41,675,000)
Interest in earnings of equity method investments	6,064,000		7,215,000
Net income	312,887,000		234,370,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written	1,583,000,000		1,502,000,000
Net premiums written	1,456,000,000		1,386,000,000
Increase in unearned premiums	(464,000,000)		(463,000,000)
Net premiums earned	992,000,000		923,000,000
Losses and loss expenses and life policy benefits	(534,000,000)		(480,000,000)
Acquisition costs	(233,000,000)		(208,000,000)
Technical result	225,000,000		235,000,000
Other income	1,000,000		0
Other operating expenses	(65,000,000)		(66,000,000)
Underwriting result	161,000,000		169,000,000
Reinsurance Ratios [Abstract]
Loss ratio	53.80%	[1]	52.00%	[1]
Acquisition ratio	23.60%	[2]	22.60%	[2]
Technical ratio	77.40%	[3]	74.60%	[3]
Other operating expense ratio	6.50%	[4]	7.10%	[4]
Combined ratio	83.90%	[4],[5]	81.70%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	530,000,000		447,000,000
Net premiums written	527,000,000		446,000,000
Increase in unearned premiums	(148,000,000)		(113,000,000)
Net premiums earned	379,000,000		333,000,000
Losses and loss expenses and life policy benefits	(260,000,000)		(240,000,000)
Acquisition costs	(92,000,000)		(72,000,000)
Technical result	27,000,000		21,000,000
Reinsurance Ratios [Abstract]
Loss ratio	68.60%	[1]	72.00%	[1]
Acquisition ratio	24.30%	[2]	21.60%	[2]
Technical ratio	92.90%	[3]	93.60%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written	364,000,000		372,000,000
Net premiums written	361,000,000		368,000,000
Increase in unearned premiums	(182,000,000)		(202,000,000)
Net premiums earned	179,000,000		166,000,000
Losses and loss expenses and life policy benefits	(94,000,000)		(67,000,000)
Acquisition costs	(54,000,000)		(50,000,000)
Technical result	31,000,000		49,000,000
Reinsurance Ratios [Abstract]
Loss ratio	52.40%	[1]	40.40%	[1]
Acquisition ratio	30.10%	[2]	30.10%	[2]
Technical ratio	82.50%	[3]	70.50%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written	479,000,000		445,000,000
Net premiums written	389,000,000		361,000,000
Increase in unearned premiums	(34,000,000)		(24,000,000)
Net premiums earned	355,000,000		337,000,000
Losses and loss expenses and life policy benefits	(201,000,000)		(184,000,000)
Acquisition costs	(79,000,000)		(75,000,000)
Technical result	75,000,000		78,000,000
Reinsurance Ratios [Abstract]
Loss ratio	56.60%	[1]	54.60%	[1]
Acquisition ratio	22.40%	[2]	22.40%	[2]
Technical ratio	79.00%	[3]	77.00%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	210,000,000		238,000,000
Net premiums written	179,000,000		211,000,000
Increase in unearned premiums	(100,000,000)		(124,000,000)
Net premiums earned	79,000,000		87,000,000
Losses and loss expenses and life policy benefits	21,000,000		11,000,000
Acquisition costs	(8,000,000)		(11,000,000)
Technical result	92,000,000		87,000,000
Reinsurance Ratios [Abstract]
Loss ratio	(26.50%)	[1]	(12.80%)	[1]
Acquisition ratio	10.10%	[2]	12.30%	[2]
Technical ratio	(16.40%)	[3]	(0.50%)	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written	289,000,000		254,000,000
Net premiums written	282,000,000		249,000,000
Increase in unearned premiums	(20,000,000)		(25,000,000)
Net premiums earned	262,000,000		224,000,000
Losses and loss expenses and life policy benefits	(215,000,000)		(182,000,000)
Acquisition costs	(32,000,000)		(27,000,000)
Technical result	15,000,000		15,000,000
Other income	1,000,000		3,000,000
Other operating expenses	(17,000,000)		(18,000,000)
Underwriting result	(1,000,000)		0
Net investment income	15,000,000		16,000,000
Allocated underwriting result	14,000,000	[6]	16,000,000	[6]
Corporate And Other [Member]
Segment Reporting [Line Items]
Gross premiums written	0		1,000,000
Net premiums written	0		1,000,000
Increase in unearned premiums	0		(1,000,000)
Net premiums earned	0		0
Losses and loss expenses and life policy benefits	0		1,000,000
Acquisition costs	0		0
Technical result	0		1,000,000
Other income	(2,000,000)		1,000,000
Other operating expenses	(29,000,000)		(32,000,000)
Net investment income	102,000,000		108,000,000
Net realized and unrealized investment gains	142,000,000		23,000,000
Interest expense	(12,000,000)		(12,000,000)
Amortization of intangible assets	(7,000,000)		(7,000,000)
Net foreign exchange gains	0		2,000,000
Income tax expense	(62,000,000)		(42,000,000)
Interest in earnings of equity method investments	$ 6,000,000		$ 7,000,000

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

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

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

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

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