Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting [Line Items]
Gross premiums written	$ 1,361,280,000		$ 1,281,477,000		$ 4,695,327,000		$ 4,378,944,000
Net premiums written	1,342,690,000		1,264,775,000		4,499,849,000		4,210,525,000
Decrease (increase) in unearned premiums	213,924,000		156,694,000		(336,384,000)		(433,740,000)
Net premiums earned	1,556,614,000		1,421,469,000		4,163,465,000		3,776,785,000
Losses and loss expenses and life policy benefits	(960,000,000)		(751,000,000)		(2,593,000,000)		(2,279,000,000)
Acquisition costs	(321,756,000)		(282,948,000)		(888,937,000)		(758,890,000)
Technical result	275,000,000		387,000,000		681,000,000		739,000,000
Other (loss) income	2,223,000		5,399,000		11,892,000		13,205,000
Other operating expenses	(108,615,000)		(108,467,000)		(327,149,000)		(369,340,000)
Underwriting result	169,000,000		284,000,000		366,000,000		383,000,000
Net investment income	118,176,000		121,811,000		365,010,000		370,017,000
Net realized and unrealized investment (losses) gains	(34,420,000)		16,118,000		273,468,000		(260,154,000)
Interest expense	(12,241,000)		(12,233,000)		(36,719,000)		(36,694,000)
Amortization of intangible assets	(7,003,000)		(7,045,000)		(21,007,000)		(21,136,000)
Net foreign exchange gains (losses)	8,206,000		(1,279,000)		10,900,000		(9,822,000)
Income tax expense	(45,617,000)		(70,232,000)		(186,363,000)		(37,338,000)
Interest in earnings of equity method investments	5,294,000		5,941,000		16,283,000		9,677,000
Net income	201,318,000		337,535,000		787,996,000		397,517,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written	1,025,000,000		1,041,000,000		3,744,000,000		3,649,000,000
Net premiums written	1,019,000,000		1,026,000,000		3,583,000,000		3,489,000,000
Decrease (increase) in unearned premiums	208,000,000		147,000,000		(323,000,000)		(416,000,000)
Net premiums earned	1,227,000,000		1,173,000,000		3,260,000,000		3,073,000,000
Losses and loss expenses and life policy benefits	(688,000,000)		(557,000,000)		(1,853,000,000)		(1,723,000,000)
Acquisition costs	(284,000,000)		(259,000,000)		(778,000,000)		(677,000,000)
Technical result	255,000,000		357,000,000		629,000,000		673,000,000
Other (loss) income	(1,000,000)		2,000,000		1,000,000		3,000,000
Other operating expenses	(62,000,000)		(62,000,000)		(187,000,000)		(189,000,000)
Underwriting result	192,000,000		297,000,000		443,000,000		487,000,000
Reinsurance Ratios [Abstract]
Loss ratio	56.10%	[1]	47.50%	[1]	56.80%	[1]	56.10%	[1]
Acquisition ratio	23.10%	[2]	22.10%	[2]	23.90%	[2]	22.00%	[2]
Technical ratio	79.20%		69.60%	[3]	80.70%		78.10%	[3]
Other operating expense ratio	5.00%	[4]	5.30%	[4]	5.70%	[4]	6.10%	[4]
Combined ratio	84.20%		74.90%	[5]	86.40%		84.20%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	372,000,000		409,000,000		1,302,000,000		1,228,000,000
Net premiums written	372,000,000		408,000,000		1,291,000,000		1,215,000,000
Decrease (increase) in unearned premiums	52,000,000		17,000,000		(99,000,000)		(99,000,000)
Net premiums earned	424,000,000		425,000,000		1,192,000,000		1,116,000,000
Losses and loss expenses and life policy benefits	(247,000,000)		(197,000,000)		(747,000,000)		(682,000,000)
Acquisition costs	(106,000,000)		(101,000,000)		(299,000,000)		(253,000,000)
Technical result	71,000,000		127,000,000		146,000,000		181,000,000
Reinsurance Ratios [Abstract]
Loss ratio	58.20%	[1]	46.30%	[1]	62.60%	[1]	61.10%	[1]
Acquisition ratio	24.90%	[2]	23.90%	[2]	25.10%	[2]	22.70%	[2]
Technical ratio	83.10%		70.20%	[3]	87.70%		83.80%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written	162,000,000		157,000,000		682,000,000		690,000,000
Net premiums written	164,000,000		157,000,000		672,000,000		682,000,000
Decrease (increase) in unearned premiums	38,000,000		38,000,000		(104,000,000)		(152,000,000)
Net premiums earned	202,000,000		195,000,000		568,000,000		530,000,000
Losses and loss expenses and life policy benefits	(123,000,000)		(90,000,000)		(319,000,000)		(263,000,000)
Acquisition costs	(56,000,000)		(50,000,000)		(162,000,000)		(134,000,000)
Technical result	23,000,000		55,000,000		87,000,000		133,000,000
Reinsurance Ratios [Abstract]
Loss ratio	61.10%	[1]	46.00%	[1]	56.20%	[1]	49.70%	[1]
Acquisition ratio	27.60%	[2]	25.70%	[2]	28.50%	[2]	25.20%	[2]
Technical ratio	88.70%		71.70%	[3]	84.70%		74.90%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written	432,000,000		396,000,000		1,348,000,000		1,253,000,000
Net premiums written	428,000,000		389,000,000		1,250,000,000		1,159,000,000
Decrease (increase) in unearned premiums	20,000,000		(7,000,000)		(42,000,000)		(68,000,000)
Net premiums earned	448,000,000		382,000,000		1,208,000,000		1,091,000,000
Losses and loss expenses and life policy benefits	(279,000,000)		(228,000,000)		(749,000,000)		(697,000,000)
Acquisition costs	(105,000,000)		(92,000,000)		(283,000,000)		(257,000,000)
Technical result	64,000,000		62,000,000		176,000,000		137,000,000
Reinsurance Ratios [Abstract]
Loss ratio	62.30%	[1]	59.80%	[1]	62.10%	[1]	63.90%	[1]
Acquisition ratio	23.50%	[2]	24.00%	[2]	23.40%	[2]	23.60%	[2]
Technical ratio	85.80%		83.80%	[3]	85.50%		87.50%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	59,000,000		79,000,000		412,000,000		478,000,000
Net premiums written	55,000,000		72,000,000		370,000,000		433,000,000
Decrease (increase) in unearned premiums	98,000,000		99,000,000		(78,000,000)		(97,000,000)
Net premiums earned	153,000,000		171,000,000		292,000,000		336,000,000
Losses and loss expenses and life policy benefits	(39,000,000)		(42,000,000)		(38,000,000)		(81,000,000)
Acquisition costs	(17,000,000)		(16,000,000)		(34,000,000)		(33,000,000)
Technical result	97,000,000		113,000,000		220,000,000		222,000,000
Reinsurance Ratios [Abstract]
Loss ratio	25.20%	[1]	24.50%	[1]	12.90%	[1]	24.20%	[1]
Acquisition ratio	11.70%	[2]	9.00%	[2]	11.50%	[2]	9.70%	[2]
Technical ratio	36.90%		33.50%	[3]	24.40%		33.90%	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written	336,000,000		235,000,000		951,000,000		722,000,000
Net premiums written	325,000,000		234,000,000		918,000,000		715,000,000
Decrease (increase) in unearned premiums	6,000,000		9,000,000		(14,000,000)		(17,000,000)
Net premiums earned	331,000,000		243,000,000		904,000,000		698,000,000
Losses and loss expenses and life policy benefits	(272,000,000)		(195,000,000)		(740,000,000)		(558,000,000)
Acquisition costs	(38,000,000)		(24,000,000)		(111,000,000)		(82,000,000)
Technical result	21,000,000		24,000,000		53,000,000		58,000,000
Other (loss) income	2,000,000		3,000,000		6,000,000		9,000,000
Other operating expenses	(17,000,000)		(17,000,000)		(52,000,000)		(52,000,000)
Underwriting result	6,000,000		10,000,000		7,000,000		15,000,000
Net investment income	14,000,000		15,000,000		45,000,000		45,000,000
Allocated underwriting result	20,000,000	[6]	25,000,000	[6]	52,000,000		60,000,000	[6]
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written	0		5,000,000		0		8,000,000
Net premiums written	(1,000,000)		5,000,000		(1,000,000)		7,000,000
Decrease (increase) in unearned premiums	0		0		0		(1,000,000)
Net premiums earned	(1,000,000)		5,000,000		(1,000,000)		6,000,000
Losses and loss expenses and life policy benefits	0		1,000,000		0		2,000,000
Acquisition costs	0		0		0		0
Technical result	(1,000,000)		6,000,000		(1,000,000)		8,000,000
Other (loss) income	1,000,000		0		5,000,000		1,000,000
Other operating expenses	(29,000,000)		(29,000,000)		(88,000,000)		(128,000,000)
Net investment income	104,000,000		107,000,000		320,000,000		325,000,000
Net realized and unrealized investment (losses) gains	(34,000,000)		16,000,000		273,000,000		(260,000,000)
Interest expense	(12,000,000)		(12,000,000)		(36,000,000)		(37,000,000)
Amortization of intangible assets	(7,000,000)		(7,000,000)		(21,000,000)		(21,000,000)
Net foreign exchange gains (losses)	8,000,000		(1,000,000)		11,000,000		(10,000,000)
Income tax expense	(46,000,000)		(70,000,000)		(186,000,000)		(37,000,000)
Interest in earnings of equity method investments	$ 5,000,000		$ 6,000,000		$ 16,000,000		$ 10,000,000

[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.