Segment Information (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Segment Reporting [Line Items]
Gross premiums written	$ 1,281,477,000		$ 1,056,076,000		$ 4,378,944,000		$ 3,786,802,000
Net premiums written	1,264,775,000		1,043,240,000		4,210,525,000		3,652,571,000
Decrease (increase) in unearned premiums	156,694,000		193,851,000		(433,740,000)		(334,772,000)
Net premiums earned	1,421,469,000		1,237,091,000		3,776,785,000		3,317,799,000
Losses and loss expenses and life policy benefits	(751,000,000)		(721,000,000)		(2,279,000,000)		(2,004,000,000)
Acquisition costs	(282,948,000)		(247,058,000)		(758,890,000)		(691,388,000)
Technical result	387,000,000		269,000,000		739,000,000		623,000,000
Other income	5,399,000		2,744,000		13,205,000		8,143,000
Other operating expenses	(108,467,000)		(94,697,000)		(369,340,000)		(299,055,000)
Underwriting result	284,000,000		177,000,000		383,000,000		332,000,000
Net investment income	121,811,000		135,266,000		370,017,000		435,669,000
Net realized and unrealized investment gains (losses)	16,118,000		257,429,000		(260,154,000)		488,296,000
Interest expense	(12,233,000)		(12,224,000)		(36,694,000)		(36,668,000)
Amortization of intangible assets	(7,045,000)		(8,893,000)		(21,136,000)		(26,679,000)
Net foreign exchange losses (gains)	1,279,000		2,015,000		9,822,000		(3,165,000)
Income tax expense	(70,232,000)		(64,149,000)		(37,338,000)		(181,458,000)
Interest in earnings of equity investments	5,941,000		4,349,000		9,677,000		8,929,000
Net income	337,535,000		486,706,000		397,517,000		1,022,994,000
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written	1,041,000,000		869,000,000		3,649,000,000		3,177,000,000
Net premiums written	1,026,000,000		856,000,000		3,489,000,000		3,045,000,000
Decrease (increase) in unearned premiums	147,000,000		182,000,000		(416,000,000)		(321,000,000)
Net premiums earned	1,173,000,000		1,038,000,000		3,073,000,000		2,724,000,000
Losses and loss expenses and life policy benefits	(557,000,000)		(561,000,000)		(1,723,000,000)		(1,522,000,000)
Acquisition costs	(259,000,000)		(219,000,000)		(677,000,000)		(609,000,000)
Technical result	357,000,000		258,000,000		673,000,000		593,000,000
Other income	2,000,000		1,000,000		3,000,000		2,000,000
Other operating expenses	(62,000,000)		(58,000,000)		(189,000,000)		(187,000,000)
Underwriting result	297,000,000		201,000,000		487,000,000		408,000,000
Reinsurance Ratios [Abstract]
Loss ratio	47.50%	[1]	54.10%	[1]	56.10%	[1]	55.90%	[1]
Acquisition ratio	22.10%	[2]	21.10%	[2]	22.00%	[2]	22.30%	[2]
Technical ratio	69.60%	[3]	75.20%	[3]	78.10%	[3]	78.20%	[3]
Other operating expense ratio	5.30%	[4]	5.50%	[4]	6.10%	[4]	6.90%	[4]
Combined ratio	74.90%	[4]	80.70%	[5]	84.20%	[5]	85.10%	[5]
[NonLifeMember] | [NorthAmericaSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	409,000,000		311,000,000		1,228,000,000		924,000,000
Net premiums written	408,000,000		311,000,000		1,215,000,000		922,000,000
Decrease (increase) in unearned premiums	17,000,000		24,000,000		(99,000,000)		(59,000,000)
Net premiums earned	425,000,000		335,000,000		1,116,000,000		863,000,000
Losses and loss expenses and life policy benefits	(197,000,000)		(251,000,000)		(682,000,000)		(568,000,000)
Acquisition costs	(101,000,000)		(83,000,000)		(253,000,000)		(218,000,000)
Technical result	127,000,000		1,000,000		181,000,000		77,000,000
Reinsurance Ratios [Abstract]
Loss ratio	46.30%	[1]	74.90%	[1]	61.10%	[1]	65.90%	[1]
Acquisition ratio	23.90%	[2]	24.80%	[2]	22.70%	[2]	25.20%	[2]
Technical ratio	70.20%	[3]	99.70%	[3]	83.80%	[3]	91.10%	[3]
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written	157,000,000		123,000,000		690,000,000		600,000,000
Net premiums written	157,000,000		122,000,000		682,000,000		596,000,000
Decrease (increase) in unearned premiums	38,000,000		50,000,000		(152,000,000)		(100,000,000)
Net premiums earned	195,000,000		172,000,000		530,000,000		496,000,000
Losses and loss expenses and life policy benefits	(90,000,000)		(110,000,000)		(263,000,000)		(327,000,000)
Acquisition costs	(50,000,000)		(42,000,000)		(134,000,000)		(120,000,000)
Technical result	55,000,000		20,000,000		133,000,000		49,000,000
Reinsurance Ratios [Abstract]
Loss ratio	46.00%	[1]	63.90%	[1]	49.70%	[1]	65.90%	[1]
Acquisition ratio	25.70%	[2]	24.90%	[2]	25.20%	[2]	24.30%	[2]
Technical ratio	71.70%	[3]	88.80%	[3]	74.90%	[3]	90.20%	[3]
[NonLifeMember] | [GlobalSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written	396,000,000		360,000,000		1,253,000,000		1,178,000,000
Net premiums written	389,000,000		354,000,000		1,159,000,000		1,098,000,000
Decrease (increase) in unearned premiums	(7,000,000)		9,000,000		(68,000,000)		(64,000,000)
Net premiums earned	382,000,000		363,000,000		1,091,000,000		1,034,000,000
Losses and loss expenses and life policy benefits	(228,000,000)		(161,000,000)		(697,000,000)		(569,000,000)
Acquisition costs	(92,000,000)		(79,000,000)		(257,000,000)		(241,000,000)
Technical result	62,000,000		123,000,000		137,000,000		224,000,000
Reinsurance Ratios [Abstract]
Loss ratio	59.80%	[1]	44.40%	[1]	63.90%	[1]	55.10%	[1]
Acquisition ratio	24.00%	[2]	21.70%	[2]	23.60%	[2]	23.30%	[2]
Technical ratio	83.80%	[3]	66.10%	[3]	87.50%	[3]	78.40%	[3]
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	79,000,000		75,000,000		478,000,000		475,000,000
Net premiums written	72,000,000		69,000,000		433,000,000		429,000,000
Decrease (increase) in unearned premiums	99,000,000		99,000,000		(97,000,000)		(98,000,000)
Net premiums earned	171,000,000		168,000,000		336,000,000		331,000,000
Losses and loss expenses and life policy benefits	(42,000,000)		(39,000,000)		(81,000,000)		(58,000,000)
Acquisition costs	(16,000,000)		(15,000,000)		(33,000,000)		(30,000,000)
Technical result	113,000,000		114,000,000		222,000,000		243,000,000
Reinsurance Ratios [Abstract]
Loss ratio	24.50%	[1]	23.30%	[1]	24.20%	[1]	17.40%	[1]
Acquisition ratio	9.00%	[2]	8.80%	[2]	9.70%	[2]	9.10%	[2]
Technical ratio	33.50%	[3]	32.10%	[3]	33.90%	[3]	26.50%	[3]
[LifeAndHealthMember]
Segment Reporting [Line Items]
Gross premiums written	235,000,000		187,000,000		722,000,000		604,000,000
Net premiums written	234,000,000		187,000,000		715,000,000		601,000,000
Decrease (increase) in unearned premiums	9,000,000		8,000,000		(17,000,000)		(12,000,000)
Net premiums earned	243,000,000		195,000,000		698,000,000		589,000,000
Losses and loss expenses and life policy benefits	(195,000,000)		(157,000,000)		(558,000,000)		(479,000,000)
Acquisition costs	(24,000,000)		(27,000,000)		(82,000,000)		(82,000,000)
Technical result	24,000,000		11,000,000		58,000,000		28,000,000
Other income	3,000,000		1,000,000		9,000,000		3,000,000
Other operating expenses	(17,000,000)		(12,000,000)		(52,000,000)		(38,000,000)
Underwriting result	10,000,000		0		15,000,000		(7,000,000)
Net investment income	15,000,000		15,000,000		45,000,000		49,000,000
Allocated underwriting result	25,000,000	[6]	15,000,000	[6]	60,000,000	[6]	42,000,000	[6]
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written	5,000,000		0		8,000,000		6,000,000
Net premiums written	5,000,000		0		7,000,000		6,000,000
Decrease (increase) in unearned premiums	0		4,000,000		(1,000,000)		(1,000,000)
Net premiums earned	5,000,000		4,000,000		6,000,000		5,000,000
Losses and loss expenses and life policy benefits	1,000,000		(3,000,000)		2,000,000		(3,000,000)
Acquisition costs	0		(1,000,000)		0		0
Technical result	6,000,000		0		8,000,000		2,000,000
Other income	0		1,000,000		1,000,000		3,000,000
Other operating expenses	(29,000,000)		(25,000,000)		(128,000,000)		(74,000,000)
Net investment income	107,000,000		120,000,000		325,000,000		387,000,000
Net realized and unrealized investment gains (losses)	16,000,000		257,000,000		(260,000,000)		488,000,000
Interest expense	(12,000,000)		(12,000,000)		(37,000,000)		(37,000,000)
Amortization of intangible assets	(7,000,000)		(9,000,000)		(21,000,000)		(27,000,000)
Net foreign exchange losses (gains)	1,000,000		2,000,000		10,000,000		(3,000,000)
Income tax expense	(70,000,000)		(64,000,000)		(37,000,000)		(181,000,000)
Interest in earnings of equity investments	$ 6,000,000		$ 5,000,000		$ 10,000,000		$ 9,000,000

[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.