Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting [Line Items]
Gross premiums written									$ 5,932,003,000		$ 5,569,706,000		$ 4,718,235,000
Net premiums written	1,220,000,000	1,343,000,000	1,419,000,000	1,738,000,000	1,186,000,000	1,265,000,000	1,309,000,000	1,636,000,000	5,719,884,000		5,396,526,000		4,572,860,000
(Increase) decrease in unearned premiums									(110,689,000)		(198,316,000)		(86,921,000)
Net premiums earned	1,446,000,000	1,557,000,000	1,353,000,000	1,254,000,000	1,421,000,000	1,421,000,000	1,209,000,000	1,147,000,000	5,609,195,000		5,198,210,000		4,485,939,000
Losses and loss expenses and life policy benefits									(3,463,000,000)		(3,158,000,000)		(2,805,000,000)
Acquisition costs	(325,000,000)	(322,000,000)	(303,000,000)	(265,000,000)	(318,000,000)	(283,000,000)	(242,000,000)	(235,000,000)	(1,213,822,000)		(1,077,628,000)		(936,909,000)
Technical result									932,000,000		962,000,000		744,000,000
Other income	4,000,000	2,000,000	9,000,000	0	3,000,000	5,000,000	4,000,000	4,000,000	16,190,000		16,565,000		11,920,000
Other expenses	(123,000,000)	(108,000,000)	(107,000,000)	(111,000,000)	(131,000,000)	(108,000,000)	(145,000,000)	(116,000,000)	(449,688,000)		(500,466,000)		(411,374,000)
Underwriting result									498,000,000		479,000,000		345,000,000
Net investment income	115,000,000	118,000,000	130,000,000	117,000,000	114,000,000	122,000,000	125,000,000	124,000,000	479,696,000		484,367,000		571,338,000
Net realized and unrealized investment gains (losses)	98,000,000	(34,000,000)	166,000,000	142,000,000	99,000,000	16,000,000	(299,000,000)	23,000,000	371,796,000		(160,735,000)		493,409,000
Interest expense	(12,000,000)	(12,000,000)	(12,000,000)	(12,000,000)	(12,000,000)	(12,000,000)	(12,000,000)	(12,000,000)	(48,963,000)		(48,929,000)		(48,895,000)
Amortization of intangible assets	(6,000,000)	(7,000,000)	(7,000,000)	(7,000,000)	(6,000,000)	(7,000,000)	(7,000,000)	(7,000,000)	(27,486,000)		(27,180,000)		(31,799,000)
Net foreign exchange gains (losses)	7,000,000	8,000,000	2,000,000	0	(8,000,000)	(1,000,000)	(11,000,000)	2,000,000	18,201,000		(18,203,000)		(175,000)
Income tax expense	(53,000,000)	(46,000,000)	(78,000,000)	(62,000,000)	(11,000,000)	(70,000,000)	75,000,000	(42,000,000)	(239,506,000)		(48,416,000)		(204,284,000)
Interest in earnings of equity method investments	(1,000,000)	5,000,000	5,000,000	6,000,000	4,000,000	6,000,000	(4,000,000)	7,000,000	15,270,000		13,665,000		9,954,000
Net income	280,000,000	201,000,000	274,000,000	313,000,000	276,000,000	338,000,000	(174,000,000)	234,000,000	1,068,113,000		673,442,000		1,134,514,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written									4,667,000,000		4,590,000,000		3,910,000,000
Net premiums written									4,500,000,000		4,427,000,000		3,768,000,000
(Increase) decrease in unearned premiums									(113,000,000)		(192,000,000)		(84,000,000)
Net premiums earned									4,387,000,000		4,235,000,000		3,684,000,000
Losses and loss expenses and life policy benefits									(2,463,000,000)		(2,400,000,000)		(2,155,000,000)
Acquisition costs									(1,065,000,000)		(953,000,000)		(821,000,000)
Technical result									859,000,000		882,000,000		708,000,000
Other income									3,000,000		3,000,000		5,000,000
Other expenses									(252,000,000)		(259,000,000)		(257,000,000)
Underwriting result									610,000,000		626,000,000		456,000,000
Reinsurance Ratios [Abstract]
Loss ratio									56.10%	[1]	56.70%	[1]	58.50%	[1]
Acquisition ratio									24.30%	[2]	22.50%	[2]	22.30%	[2]
Technical ratio									80.40%	[3]	79.20%	[3]	80.80%	[3]
Other expense ratio									5.80%	[4]	6.10%	[4]	7.00%	[4]
Combined ratio									86.20%	[5]	85.30%	[5]	87.80%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written									1,642,000,000		1,601,000,000		1,221,000,000
Net premiums written									1,630,000,000		1,587,000,000		1,219,000,000
(Increase) decrease in unearned premiums									(33,000,000)		(54,000,000)		(43,000,000)
Net premiums earned									1,597,000,000		1,533,000,000		1,176,000,000
Losses and loss expenses and life policy benefits									(1,000,000,000)		(975,000,000)		(816,000,000)
Acquisition costs									(401,000,000)		(351,000,000)		(291,000,000)
Technical result									196,000,000		207,000,000		69,000,000
Reinsurance Ratios [Abstract]
Loss ratio									62.60%	[1]	63.60%	[1]	69.40%	[1]
Acquisition ratio									25.10%	[2]	22.90%	[2]	24.70%	[2]
Technical ratio									87.70%	[3]	86.50%	[3]	94.10%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written									803,000,000		818,000,000		684,000,000
Net premiums written									794,000,000		811,000,000		681,000,000
(Increase) decrease in unearned premiums									(26,000,000)		(68,000,000)		(3,000,000)
Net premiums earned									768,000,000		743,000,000		678,000,000
Losses and loss expenses and life policy benefits									(438,000,000)		(373,000,000)		(415,000,000)
Acquisition costs									(222,000,000)		(196,000,000)		(167,000,000)
Technical result									108,000,000		174,000,000		96,000,000
Reinsurance Ratios [Abstract]
Loss ratio									57.00%	[1]	50.20%	[1]	61.30%	[1]
Acquisition ratio									28.90%	[2]	26.40%	[2]	24.60%	[2]
Technical ratio									85.90%	[3]	76.60%	[3]	85.90%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written									1,797,000,000		1,676,000,000		1,505,000,000
Net premiums written									1,696,000,000		1,579,000,000		1,415,000,000
(Increase) decrease in unearned premiums									(58,000,000)		(73,000,000)		(42,000,000)
Net premiums earned									1,638,000,000		1,506,000,000		1,373,000,000
Losses and loss expenses and life policy benefits									(963,000,000)		(920,000,000)		(821,000,000)
Acquisition costs									(400,000,000)		(362,000,000)		(321,000,000)
Technical result									275,000,000		224,000,000		231,000,000
Reinsurance Ratios [Abstract]
Loss ratio									58.80%	[1]	61.10%	[1]	59.80%	[1]
Acquisition ratio									24.40%	[2]	24.00%	[2]	23.40%	[2]
Technical ratio									83.20%	[3]	85.10%	[3]	83.20%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written									425,000,000		495,000,000		500,000,000
Net premiums written									380,000,000		450,000,000		453,000,000
(Increase) decrease in unearned premiums									4,000,000		3,000,000		4,000,000
Net premiums earned									384,000,000		453,000,000		457,000,000
Losses and loss expenses and life policy benefits									(62,000,000)		(132,000,000)		(103,000,000)
Acquisition costs									(42,000,000)		(44,000,000)		(42,000,000)
Technical result									280,000,000		277,000,000		312,000,000
Reinsurance Ratios [Abstract]
Loss ratio									16.10%	[1]	29.00%	[1]	22.40%	[1]
Acquisition ratio									11.00%	[2]	9.70%	[2]	93.00%	[2]
Technical ratio									27.10%	[3]	38.70%	[3]	115.40%	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written									1,265,000,000		972,000,000		802,000,000
Net premiums written									1,220,000,000		964,000,000		799,000,000
(Increase) decrease in unearned premiums									2,000,000		(7,000,000)		(4,000,000)
Net premiums earned									1,222,000,000		957,000,000		795,000,000
Losses and loss expenses and life policy benefits									(1,000,000,000)		(760,000,000)		(647,000,000)
Acquisition costs									(149,000,000)		(125,000,000)		(116,000,000)
Technical result									73,000,000		72,000,000		32,000,000
Other income									8,000,000		11,000,000		4,000,000
Other expenses									(68,000,000)		(71,000,000)		(52,000,000)
Underwriting result									13,000,000		12,000,000		(16,000,000)
Net investment income									60,000,000		61,000,000		64,000,000
Allocated underwriting result									73,000,000		73,000,000	[6]	48,000,000
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written									0		8,000,000		6,000,000
Net premiums written									0		6,000,000		6,000,000
(Increase) decrease in unearned premiums									0		0		1,000,000
Net premiums earned									0		6,000,000		7,000,000
Losses and loss expenses and life policy benefits									0		2,000,000		(3,000,000)
Acquisition costs									0		0		0
Technical result									0		8,000,000		4,000,000
Other income									5,000,000		3,000,000		3,000,000
Other expenses									(130,000,000)		(170,000,000)		(102,000,000)
Net investment income									420,000,000		423,000,000		507,000,000
Net realized and unrealized investment gains (losses)									372,000,000		(161,000,000)		494,000,000
Interest expense									(49,000,000)		(49,000,000)		(49,000,000)
Amortization of intangible assets									(27,000,000)		(27,000,000)		(32,000,000)
Net foreign exchange gains (losses)									18,000,000		(18,000,000)		0
Income tax expense									(239,000,000)		(49,000,000)		(204,000,000)
Interest in earnings of equity method investments									$ 15,000,000		$ 14,000,000		$ 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 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.