Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting [Line Items]
Gross premiums written	$ 1,163,243,000	$ 1,082,205,000	$ 2,730,726,000	$ 2,639,766,000
Net premiums written	1,136,046,000	1,056,467,000	2,609,331,000	2,526,887,000
Decrease (increase) in unearned premiums	(45,168,000)	50,978,000	(528,623,000)	(354,853,000)
Net premiums earned	1,090,878,000	1,107,445,000	2,080,708,000	2,172,034,000
Losses and loss expenses and life policy benefits	(706,000,000)	(814,000,000)	(1,283,000,000)	(2,422,000,000)
Acquisition costs	(232,723,000)	(229,251,000)	(444,330,000)	(437,100,000)
Technical result	152,000,000	64,000,000	354,000,000	(687,000,000)
Other income	2,654,000	1,596,000	5,400,000	3,408,000
Other operating expenses	(106,184,000)	(113,694,000)	(204,358,000)	(217,991,000)
Underwriting result	49,000,000	(49,000,000)	155,000,000	(902,000,000)
Net investment income	153,506,000	158,328,000	300,402,000	309,962,000
Net realized and unrealized investment gains (losses)	38,132,000	78,199,000	230,867,000	(34,000,000)
Interest expense	(12,223,000)	(12,214,000)	(24,443,000)	(24,514,000)
Amortization of intangible assets	(8,893,000)	(9,165,000)	(17,786,000)	(17,992,000)
Net foreign exchange gains	7,770,000	8,737,000	5,181,000	9,433,000
Income tax expense	(50,136,000)	(50,085,000)	(117,310,000)	(23,828,000)
Interest in (losses) earnings of equity investments	(498,000)	(1,188,000)	4,579,000	(443,000)
Net income (loss)	176,146,000	124,185,000	536,287,000	(682,771,000)
[LifeMember]
Segment Reporting [Line Items]
Gross premiums written	200,000,000	195,000,000	417,000,000	403,000,000
Net premiums written	199,000,000	195,000,000	414,000,000	402,000,000
Decrease (increase) in unearned premiums	1,000,000	6,000,000	(20,000,000)	(17,000,000)
Net premiums earned	200,000,000	201,000,000	394,000,000	385,000,000
Losses and loss expenses and life policy benefits	(173,000,000)	(166,000,000)	(322,000,000)	(311,000,000)
Acquisition costs	(26,000,000)	(26,000,000)	(55,000,000)	(56,000,000)
Technical result	1,000,000	9,000,000	17,000,000	18,000,000
Other income	1,000,000	0	2,000,000	0
Other operating expenses	(13,000,000)	(13,000,000)	(26,000,000)	(25,000,000)
Underwriting result	(11,000,000)	(4,000,000)	(7,000,000)	(7,000,000)
Net investment income	17,000,000	16,000,000	33,000,000	31,000,000
Allocated underwriting result	6,000,000	12,000,000	26,000,000	24,000,000
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written	3,000,000	4,000,000	6,000,000	6,000,000
Net premiums written	3,000,000	4,000,000	6,000,000	6,000,000
Decrease (increase) in unearned premiums	(2,000,000)	(3,000,000)	(4,000,000)	(5,000,000)
Net premiums earned	1,000,000	1,000,000	2,000,000	1,000,000
Losses and loss expenses and life policy benefits	0	(2,000,000)	0	(2,000,000)
Acquisition costs	0	0	0	0
Technical result	1,000,000	(1,000,000)	2,000,000	(1,000,000)
Other income	2,000,000	1,000,000	2,000,000	1,000,000
Other operating expenses	(27,000,000)	(30,000,000)	(49,000,000)	(56,000,000)
Net investment income	136,000,000	142,000,000	267,000,000	279,000,000
Net realized and unrealized investment gains (losses)	38,000,000	78,000,000	231,000,000	(34,000,000)
Interest expense	(12,000,000)	(12,000,000)	(24,000,000)	(24,000,000)
Amortization of intangible assets	(9,000,000)	(9,000,000)	(18,000,000)	(18,000,000)
Net foreign exchange gains	8,000,000	9,000,000	5,000,000	9,000,000
Income tax expense	(50,000,000)	(50,000,000)	(117,000,000)	(24,000,000)
Interest in (losses) earnings of equity investments	(1,000,000)	(1,000,000)	4,000,000	0
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written	960,000,000	883,000,000	2,308,000,000	2,231,000,000
Net premiums written	934,000,000	857,000,000	2,189,000,000	2,119,000,000
Decrease (increase) in unearned premiums	(44,000,000)	48,000,000	(504,000,000)	(333,000,000)
Net premiums earned	890,000,000	905,000,000	1,685,000,000	1,786,000,000
Losses and loss expenses and life policy benefits	(533,000,000)	(646,000,000)	(961,000,000)	(2,109,000,000)
Acquisition costs	(207,000,000)	(203,000,000)	(389,000,000)	(381,000,000)
Technical result	150,000,000	56,000,000	335,000,000	(704,000,000)
Other income	0	0	1,000,000	2,000,000
Other operating expenses	(66,000,000)	(71,000,000)	(129,000,000)	(137,000,000)
Underwriting result	84,000,000	(15,000,000)	207,000,000	(839,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	59.90%	71.40%	57.00%	118.00%
Acquisition ratio	23.20%	22.40%	23.10%	21.40%
Technical ratio	83.10%	93.80%	80.10%	139.40%
Other operating expense ratio	7.50%	7.90%	7.70%	7.70%
Combined ratio	90.60%	101.70%	87.80%	147.10%
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written	271,000,000	242,000,000	613,000,000	581,000,000
Net premiums written	270,000,000	242,000,000	611,000,000	581,000,000
Decrease (increase) in unearned premiums	20,000,000	19,000,000	(84,000,000)	(60,000,000)
Net premiums earned	290,000,000	261,000,000	527,000,000	521,000,000
Losses and loss expenses and life policy benefits	(185,000,000)	(190,000,000)	(317,000,000)	(365,000,000)
Acquisition costs	(69,000,000)	(63,000,000)	(134,000,000)	(129,000,000)
Technical result	36,000,000	8,000,000	76,000,000	27,000,000
Reinsurance Ratios [Abstract]
Loss ratio	63.70%	72.80%	60.10%	70.00%
Acquisition ratio	23.80%	24.10%	25.50%	24.80%
Technical ratio	87.50%	96.90%	85.60%	94.80%
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written	130,000,000	122,000,000	477,000,000	440,000,000
Net premiums written	128,000,000	121,000,000	474,000,000	437,000,000
Decrease (increase) in unearned premiums	36,000,000	72,000,000	(150,000,000)	(63,000,000)
Net premiums earned	164,000,000	193,000,000	324,000,000	374,000,000
Losses and loss expenses and life policy benefits	(119,000,000)	(127,000,000)	(217,000,000)	(277,000,000)
Acquisition costs	(39,000,000)	(53,000,000)	(78,000,000)	(93,000,000)
Technical result	6,000,000	13,000,000	29,000,000	4,000,000
Reinsurance Ratios [Abstract]
Loss ratio	72.30%	65.90%	67.00%	74.00%
Acquisition ratio	23.90%	27.60%	23.90%	25.00%
Technical ratio	96.20%	93.50%	90.90%	99.00%
[NonLifeMember] | [GlobalNonUsSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written	400,000,000	350,000,000	817,000,000	724,000,000
Net premiums written	391,000,000	333,000,000	744,000,000	648,000,000
Decrease (increase) in unearned premiums	(28,000,000)	8,000,000	(73,000,000)	10,000,000
Net premiums earned	363,000,000	341,000,000	671,000,000	658,000,000
Losses and loss expenses and life policy benefits	(213,000,000)	(206,000,000)	(408,000,000)	(427,000,000)
Acquisition costs	(93,000,000)	(78,000,000)	(162,000,000)	(158,000,000)
Technical result	57,000,000	57,000,000	101,000,000	73,000,000
Reinsurance Ratios [Abstract]
Loss ratio	58.80%	60.30%	60.80%	64.90%
Acquisition ratio	25.50%	22.90%	24.20%	24.00%
Technical ratio	84.30%	83.20%	85.00%	88.90%
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	159,000,000	169,000,000	401,000,000	486,000,000
Net premiums written	145,000,000	161,000,000	360,000,000	453,000,000
Decrease (increase) in unearned premiums	(72,000,000)	(51,000,000)	(197,000,000)	(220,000,000)
Net premiums earned	73,000,000	110,000,000	163,000,000	233,000,000
Losses and loss expenses and life policy benefits	(16,000,000)	(123,000,000)	(19,000,000)	(1,040,000,000)
Acquisition costs	(6,000,000)	(9,000,000)	(15,000,000)	(1,000,000)
Technical result	$ 51,000,000	$ (22,000,000)	$ 129,000,000	$ (808,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	22.50%	111.70%	11.30%	446.30%
Acquisition ratio	8.30%	8.10%	9.50%	0.30%
Technical ratio	30.80%	119.80%	20.80%	446.60%