Segment Information (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting [Line Items]
Gross premiums written									$ 5,569,706,000		$ 4,718,235,000		$ 4,633,054,000
Net premiums written	1,186,000,000	1,265,000,000	1,309,000,000	1,636,000,000	920,000,000	1,043,000,000	1,136,000,000	1,473,000,000	5,396,526,000		4,572,860,000		4,486,329,000
(Increase) decrease in unearned premiums									(198,316,000)		(86,921,000)		161,425,000
Net premiums earned	1,421,000,000	1,421,000,000	1,209,000,000	1,147,000,000	1,168,000,000	1,237,000,000	1,091,000,000	990,000,000	5,198,210,000		4,485,939,000		4,647,754,000
Losses and loss expenses and life policy benefits									(3,158,000,000)		(2,805,000,000)		(4,373,000,000)
Acquisition costs	(318,000,000)	(283,000,000)	(242,000,000)	(235,000,000)	(245,000,000)	(247,000,000)	(233,000,000)	(212,000,000)	(1,077,628,000)		(936,909,000)		(938,361,000)
Technical result									962,000,000		744,000,000		(663,000,000)
Other income	3,000,000	5,000,000	4,000,000	4,000,000	3,000,000	3,000,000	3,000,000	2,000,000	16,565,000		11,920,000		7,915,000
Other operating expenses	(131,000,000)	(108,000,000)	(145,000,000)	(116,000,000)	(112,000,000)	(95,000,000)	(106,000,000)	(98,000,000)	(500,466,000)		(411,374,000)		(434,846,000)
Underwriting result									479,000,000		345,000,000		(1,090,000,000)
Net investment income	114,000,000	122,000,000	125,000,000	124,000,000	136,000,000	135,000,000	153,000,000	147,000,000	484,367,000		571,338,000		629,148,000
Net realized and unrealized investment (losses) gains	99,000,000	16,000,000	(299,000,000)	23,000,000	5,000,000	257,000,000	38,000,000	193,000,000	(160,735,000)		493,409,000		66,692,000
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)	(48,929,000)		(48,895,000)		(48,949,000)
Amortization of intangible assets	(6,000,000)	(7,000,000)	(7,000,000)	(7,000,000)	(5,000,000)	(9,000,000)	(9,000,000)	(9,000,000)	(27,180,000)		(31,799,000)		(36,405,000)
Net foreign exchange losses (gains)	(8,000,000)	(1,000,000)	(11,000,000)	2,000,000	(3,000,000)	(2,000,000)	8,000,000	(3,000,000)	(18,203,000)		(175,000)		34,675,000
Income tax expense	(11,000,000)	(70,000,000)	75,000,000	(42,000,000)	(23,000,000)	(64,000,000)	(50,000,000)	(67,000,000)	(48,416,000)		(204,284,000)		(68,972,000)
Interest in earnings (losses) of equity investments	4,000,000	6,000,000	(4,000,000)	7,000,000	1,000,000	5,000,000	(1,000,000)	5,000,000	13,665,000		9,954,000		(6,372,000)
Net income (loss)	276,000,000	338,000,000	(174,000,000)	234,000,000	112,000,000	487,000,000	176,000,000	360,000,000	673,442,000		1,134,514,000		(520,291,000)
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written									4,590,000,000		3,910,000,000		3,831,000,000
Net premiums written									4,427,000,000		3,768,000,000		3,688,000,000
(Increase) decrease in unearned premiums									(192,000,000)		(84,000,000)		156,000,000
Net premiums earned									4,235,000,000		3,684,000,000		3,844,000,000
Losses and loss expenses and life policy benefits									(2,400,000,000)		(2,155,000,000)		(3,717,000,000)
Acquisition costs									(953,000,000)		(821,000,000)		(821,000,000)
Technical result									882,000,000		708,000,000		(694,000,000)
Other income									3,000,000		5,000,000		4,000,000
Other operating expenses									(259,000,000)		(257,000,000)		(283,000,000)
Underwriting result									626,000,000		456,000,000		(973,000,000)
Reinsurance Ratios [Abstract]
Loss ratio									56.70%	[1]	58.50%	[1]	96.70%	[1]
Acquisition ratio									22.50%	[2]	22.30%	[2]	21.30%	[2]
Technical ratio									79.20%	[3]	80.80%	[3]	118.00%	[3]
Other operating expense ratio									6.10%	[4]	7.00%	[4]	7.40%	[4]
Combined ratio									85.30%	[4],[5]	87.80%	[5]	125.40%	[5]
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written									1,601,000,000		1,221,000,000		1,104,000,000
Net premiums written									1,587,000,000		1,219,000,000		1,104,000,000
(Increase) decrease in unearned premiums									(54,000,000)		(43,000,000)		31,000,000
Net premiums earned									1,533,000,000		1,176,000,000		1,135,000,000
Losses and loss expenses and life policy benefits									(975,000,000)		(816,000,000)		(741,000,000)
Acquisition costs									(351,000,000)		(291,000,000)		(276,000,000)
Technical result									207,000,000		69,000,000		118,000,000
Reinsurance Ratios [Abstract]
Loss ratio									63.60%	[1]	69.40%	[1]	65.30%	[1]
Acquisition ratio									22.90%	[2]	24.70%	[2]	24.30%	[2]
Technical ratio									86.50%	[3]	94.10%	[3]	89.60%	[3]
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written									818,000,000		684,000,000		682,000,000
Net premiums written									811,000,000		681,000,000		678,000,000
(Increase) decrease in unearned premiums									(68,000,000)		(3,000,000)		81,000,000
Net premiums earned									743,000,000		678,000,000		759,000,000
Losses and loss expenses and life policy benefits									(373,000,000)		(415,000,000)		(567,000,000)
Acquisition costs									(196,000,000)		(167,000,000)		(191,000,000)
Technical result									174,000,000		96,000,000		1,000,000
Reinsurance Ratios [Abstract]
Loss ratio									50.20%	[1]	61.30%	[1]	74.70%	[1]
Acquisition ratio									26.40%	[2]	24.60%	[2]	25.10%	[2]
Technical ratio									76.60%	[3]	85.90%	[3]	99.80%	[3]
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written									1,676,000,000		1,505,000,000		1,446,000,000
Net premiums written									1,579,000,000		1,415,000,000		1,344,000,000
(Increase) decrease in unearned premiums									(73,000,000)		(42,000,000)		32,000,000
Net premiums earned									1,506,000,000		1,373,000,000		1,376,000,000
Losses and loss expenses and life policy benefits									(920,000,000)		(821,000,000)		(950,000,000)
Acquisition costs									(362,000,000)		(321,000,000)		(328,000,000)
Technical result									224,000,000		231,000,000		98,000,000
Reinsurance Ratios [Abstract]
Loss ratio									61.10%	[1]	59.80%	[1]	69.10%	[1]
Acquisition ratio									24.00%	[2]	23.40%	[2]	23.80%	[2]
Technical ratio									85.10%	[3]	83.20%	[3]	92.90%	[3]
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written									495,000,000		500,000,000		599,000,000
Net premiums written									450,000,000		453,000,000		562,000,000
(Increase) decrease in unearned premiums									3,000,000		4,000,000		12,000,000
Net premiums earned									453,000,000		457,000,000		574,000,000
Losses and loss expenses and life policy benefits									(132,000,000)		(103,000,000)		(1,459,000,000)
Acquisition costs									(44,000,000)		(42,000,000)		(26,000,000)
Technical result									277,000,000		312,000,000		(911,000,000)
Reinsurance Ratios [Abstract]
Loss ratio									29.00%	[1]	22.40%	[1]	254.20%	[1]
Acquisition ratio									9.70%	[2]	9.30%	[2]	4.50%	[2]
Technical ratio									38.70%	[3]	31.70%	[3]	258.70%	[3]
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written									972,000,000		802,000,000		790,000,000
Net premiums written									964,000,000		799,000,000		786,000,000
(Increase) decrease in unearned premiums									(7,000,000)		(4,000,000)		6,000,000
Net premiums earned									957,000,000		795,000,000		792,000,000
Losses and loss expenses and life policy benefits									(760,000,000)		(647,000,000)		(650,000,000)
Acquisition costs									(125,000,000)		(116,000,000)		(117,000,000)
Technical result									72,000,000		32,000,000		25,000,000
Other income									11,000,000		4,000,000		1,000,000
Other operating expenses									(71,000,000)		(52,000,000)		(53,000,000)
Underwriting result									12,000,000		(16,000,000)		(27,000,000)
Net investment income									61,000,000		64,000,000		66,000,000
Allocated underwriting result									73,000,000	[6]	48,000,000	[6]	39,000,000	[6]
Corporate And Other [Member]
Segment Reporting [Line Items]
Gross premiums written									8,000,000		6,000,000		12,000,000
Net premiums written									6,000,000		6,000,000		12,000,000
(Increase) decrease in unearned premiums									0		1,000,000		0
Net premiums earned									6,000,000		7,000,000		12,000,000
Losses and loss expenses and life policy benefits									2,000,000		(3,000,000)		(6,000,000)
Acquisition costs									0		0		0
Technical result									8,000,000		4,000,000		6,000,000
Other income									3,000,000		3,000,000		3,000,000
Other operating expenses									(170,000,000)		(102,000,000)		(99,000,000)
Net investment income									423,000,000		507,000,000		563,000,000
Net realized and unrealized investment (losses) gains									(161,000,000)		494,000,000		67,000,000
Interest expense									(49,000,000)		(49,000,000)		(49,000,000)
Amortization of intangible assets									(27,000,000)		(32,000,000)		(36,000,000)
Net foreign exchange losses (gains)									(18,000,000)		0		34,000,000
Income tax expense									(49,000,000)		(204,000,000)		(69,000,000)
Interest in earnings (losses) of equity investments									$ 14,000,000		$ 10,000,000		$ (6,000,000)

[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.