Segment Information (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting [Line Items]
Gross premiums written	$ 1,462,307,000		$ 1,340,582,000		$ 3,334,047,000		$ 3,097,467,000
Net premiums written	1,418,665,000		1,309,318,000		3,157,159,000		2,945,750,000
(Increase) decrease in unearned premiums	(65,596,000)		(100,682,000)		(550,308,000)		(590,434,000)
Net premiums earned	1,353,069,000		1,208,636,000		2,606,851,000		2,355,316,000
Losses and loss expenses and life policy benefits	(884,000,000)		(867,000,000)		(1,633,000,000)		(1,528,000,000)
Acquisition costs	(302,573,000)		(241,743,000)		(567,181,000)		(475,942,000)
Technical result	166,000,000		100,000,000		407,000,000		351,000,000
Other income	9,265,000		3,878,000		9,669,000		7,805,000
Other operating expenses	(107,072,000)		(144,833,000)		(218,534,000)		(260,874,000)
Underwriting result	68,000,000		(41,000,000)		198,000,000		98,000,000
Net investment income	129,967,000		124,503,000		246,834,000		248,207,000
Net realized and unrealized investment gains (losses)	165,717,000		(299,215,000)		307,888,000		(276,272,000)
Interest expense	(12,240,000)		(12,232,000)		(24,477,000)		(24,460,000)
Amortization of intangible assets	(7,003,000)		(7,045,000)		(14,005,000)		(14,091,000)
Net foreign exchange gains (losses)	2,023,000		(10,584,000)		2,693,000		(8,543,000)
Income tax (expense) benefit	(78,440,000)		74,569,000		(140,746,000)		32,894,000
Interest in earnings (losses) of equity method investments	4,925,000		(3,479,000)		10,989,000		3,736,000
Net income (loss)	273,792,000		(174,388,000)		586,678,000		59,982,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written	1,136,000,000		1,106,000,000		2,719,000,000		2,607,000,000
Net premiums written	1,108,000,000		1,076,000,000		2,564,000,000		2,463,000,000
(Increase) decrease in unearned premiums	(66,000,000)		(99,000,000)		(530,000,000)		(564,000,000)
Net premiums earned	1,042,000,000		977,000,000		2,034,000,000		1,899,000,000
Losses and loss expenses and life policy benefits	(632,000,000)		(686,000,000)		(1,165,000,000)		(1,166,000,000)
Acquisition costs	(260,000,000)		(209,000,000)		(494,000,000)		(417,000,000)
Technical result	150,000,000		82,000,000		375,000,000		316,000,000
Other income	1,000,000		0		2,000,000		0
Other operating expenses	(61,000,000)		(60,000,000)		(126,000,000)		(126,000,000)
Underwriting result	90,000,000		22,000,000		251,000,000		190,000,000
Reinsurance Ratios [Abstract]
Loss ratio	60.60%	[1]	70.30%	[1]	57.30%	[1]	61.40%	[1]
Acquisition ratio	25.00%	[2]	21.40%	[2]	24.30%	[2]	22.00%	[2]
Technical ratio	85.60%	[3]	91.70%	[3]	81.60%	[3]	83.40%	[3]
Other operating expense ratio	5.90%	[4]	6.10%	[4]	6.20%	[4]	6.60%	[4]
Combined ratio	91.50%	[5]	97.80%	[5]	87.80%	[5]	90.00%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	400,000,000		372,000,000		930,000,000		819,000,000
Net premiums written	392,000,000		360,000,000		919,000,000		807,000,000
(Increase) decrease in unearned premiums	(2,000,000)		(3,000,000)		(151,000,000)		(117,000,000)
Net premiums earned	390,000,000		357,000,000		768,000,000		690,000,000
Losses and loss expenses and life policy benefits	(240,000,000)		(245,000,000)		(499,000,000)		(485,000,000)
Acquisition costs	(102,000,000)		(79,000,000)		(194,000,000)		(151,000,000)
Technical result	48,000,000		33,000,000		75,000,000		54,000,000
Reinsurance Ratios [Abstract]
Loss ratio	61.50%	[1]	68.60%	[1]	65.00%	[1]	70.20%	[1]
Acquisition ratio	26.10%	[2]	22.10%	[2]	25.20%	[2]	21.90%	[2]
Technical ratio	87.60%	[3]	90.70%	[3]	90.20%	[3]	92.10%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written	155,000,000		160,000,000		519,000,000		532,000,000
Net premiums written	148,000,000		158,000,000		508,000,000		525,000,000
(Increase) decrease in unearned premiums	39,000,000		11,000,000		(141,000,000)		(190,000,000)
Net premiums earned	187,000,000		169,000,000		367,000,000		335,000,000
Losses and loss expenses and life policy benefits	(103,000,000)		(106,000,000)		(196,000,000)		(173,000,000)
Acquisition costs	(52,000,000)		(34,000,000)		(107,000,000)		(84,000,000)
Technical result	32,000,000		29,000,000		64,000,000		78,000,000
Reinsurance Ratios [Abstract]
Loss ratio	54.60%	[1]	62.90%	[1]	53.50%	[1]	51.80%	[1]
Acquisition ratio	27.90%	[2]	19.90%	[2]	29.00%	[2]	24.90%	[2]
Technical ratio	82.50%	[3]	82.80%	[3]	82.50%	[3]	76.70%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written	438,000,000		413,000,000		917,000,000		857,000,000
Net premiums written	432,000,000		409,000,000		822,000,000		771,000,000
(Increase) decrease in unearned premiums	(26,000,000)		(37,000,000)		(61,000,000)		(62,000,000)
Net premiums earned	406,000,000		372,000,000		761,000,000		709,000,000
Losses and loss expenses and life policy benefits	(270,000,000)		(284,000,000)		(471,000,000)		(469,000,000)
Acquisition costs	(98,000,000)		(90,000,000)		(178,000,000)		(165,000,000)
Technical result	38,000,000		(2,000,000)		112,000,000		75,000,000
Reinsurance Ratios [Abstract]
Loss ratio	66.50%	[1]	76.60%	[1]	61.90%	[1]	66.10%	[1]
Acquisition ratio	24.20%	[2]	24.10%	[2]	23.40%	[2]	23.30%	[2]
Technical ratio	90.70%	[3]	100.70%	[3]	85.30%	[3]	89.40%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	143,000,000		161,000,000		353,000,000		399,000,000
Net premiums written	136,000,000		149,000,000		315,000,000		360,000,000
(Increase) decrease in unearned premiums	(77,000,000)		(70,000,000)		(177,000,000)		(195,000,000)
Net premiums earned	59,000,000		79,000,000		138,000,000		165,000,000
Losses and loss expenses and life policy benefits	(19,000,000)		(51,000,000)		1,000,000		(39,000,000)
Acquisition costs	(8,000,000)		(6,000,000)		(15,000,000)		(17,000,000)
Technical result	32,000,000		22,000,000		124,000,000		109,000,000
Reinsurance Ratios [Abstract]
Loss ratio	33.40%	[1]	64.10%	[1]	(0.90%)	[1]	23.80%	[1]
Acquisition ratio	13.00%	[2]	8.50%	[2]	11.40%	[2]	10.50%	[2]
Technical ratio	46.40%	[3]	72.60%	[3]	10.50%	[3]	34.30%	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written	326,000,000		233,000,000		615,000,000		486,000,000
Net premiums written	311,000,000		232,000,000		593,000,000		481,000,000
(Increase) decrease in unearned premiums	0		0		(20,000,000)		(25,000,000)
Net premiums earned	311,000,000		232,000,000		573,000,000		456,000,000
Losses and loss expenses and life policy benefits	(252,000,000)		(181,000,000)		(468,000,000)		(363,000,000)
Acquisition costs	(43,000,000)		(33,000,000)		(73,000,000)		(59,000,000)
Technical result	16,000,000		18,000,000		32,000,000		34,000,000
Other income	3,000,000		3,000,000		4,000,000		6,000,000
Other operating expenses	(16,000,000)		(17,000,000)		(34,000,000)		(35,000,000)
Underwriting result	3,000,000		4,000,000		2,000,000		5,000,000
Net investment income	15,000,000		15,000,000		30,000,000		30,000,000
Allocated underwriting result	18,000,000	[6]	19,000,000	[6]	32,000,000	[6]	35,000,000	[6]
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written	0		2,000,000		0		4,000,000
Net premiums written	0		1,000,000		0		2,000,000
(Increase) decrease in unearned premiums	0		(1,000,000)		0		(2,000,000)
Net premiums earned	0		0		0		0
Losses and loss expenses and life policy benefits	0		0		0		1,000,000
Acquisition costs	0		0		0		0
Technical result	0		0		0		1,000,000
Other income	5,000,000		1,000,000		4,000,000		2,000,000
Other operating expenses	(30,000,000)		(68,000,000)		(59,000,000)		(100,000,000)
Net investment income	115,000,000		110,000,000		217,000,000		218,000,000
Net realized and unrealized investment gains (losses)	166,000,000		(299,000,000)		308,000,000		(276,000,000)
Interest expense	(12,000,000)		(12,000,000)		(25,000,000)		(24,000,000)
Amortization of intangible assets	(7,000,000)		(7,000,000)		(14,000,000)		(14,000,000)
Net foreign exchange gains (losses)	2,000,000		(11,000,000)		3,000,000		(9,000,000)
Income tax (expense) benefit	(78,000,000)		75,000,000		(141,000,000)		33,000,000
Interest in earnings (losses) of equity method investments	$ 5,000,000		$ (4,000,000)		$ 11,000,000		$ 4,000,000

[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.