Segment Information (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Segment Reporting [Line Items]
Gross premiums written	$ 1,082,205,000	$ 1,140,175,000	$ 2,639,766,000	$ 3,049,501,000
Net premiums written	1,056,467,000	1,112,735,000	2,526,887,000	2,896,899,000
Decrease (increase) in unearned premiums	50,978,000	(8,104,000)	(354,853,000)	(638,489,000)
Net premiums earned	1,107,445,000	1,104,631,000	2,172,034,000	2,258,410,000
Losses and loss expenses and life policy benefits	(814,000,000)	(705,000,000)	(2,422,000,000)	(1,717,000,000)
Acquisition costs	(229,251,000)	(244,144,000)	(437,100,000)	(464,251,000)
Technical result	64,000,000	156,000,000	(687,000,000)	77,000,000
Other income	1,596,000	753,000	3,408,000	2,028,000
Other operating expenses	(113,694,000)	(160,151,000)	(217,991,000)	(288,286,000)
Underwriting result	(49,000,000)	(3,000,000)	(902,000,000)	(209,000,000)
Net investment income	158,328,000	174,454,000	309,962,000	347,576,000
Net realized and unrealized investment gains (losses)	78,199,000	46,046,000	(34,000,000)	191,519,000
Interest expense	(12,214,000)	(12,803,000)	(24,514,000)	(19,935,000)
Amortization of intangible assets	(9,165,000)	(7,833,000)	(17,992,000)	(12,636,000)
Net foreign exchange gains	8,737,000	11,021,000	9,433,000	14,648,000
Income tax expense	(50,085,000)	(17,762,000)	(23,828,000)	(45,316,000)
Interest in (losses) earnings of equity investments	(1,188,000)	1,346,000	(443,000)	3,791,000
Net income (loss)	124,185,000	190,927,000	(682,771,000)	270,581,000
Life [Member]
Segment Reporting [Line Items]
Gross premiums written	195,000,000	167,000,000	403,000,000	354,000,000
Net premiums written	195,000,000	167,000,000	402,000,000	349,000,000
Decrease (increase) in unearned premiums	6,000,000	6,000,000	(17,000,000)	(12,000,000)
Net premiums earned	201,000,000	173,000,000	385,000,000	337,000,000
Losses and loss expenses and life policy benefits	(166,000,000)	(169,000,000)	(311,000,000)	(299,000,000)
Acquisition costs	(26,000,000)	(25,000,000)	(56,000,000)	(49,000,000)
Technical result	9,000,000	(21,000,000)	18,000,000	(11,000,000)
Other income	0	1,000,000	0	1,000,000
Other operating expenses	(13,000,000)	(13,000,000)	(25,000,000)	(27,000,000)
Underwriting result	(4,000,000)	(33,000,000)	(7,000,000)	(37,000,000)
Net investment income	16,000,000	20,000,000	31,000,000	37,000,000
Allocated underwriting result	12,000,000	(13,000,000)	24,000,000	0
Corporate And Other [Member]
Segment Reporting [Line Items]
Gross premiums written	4,000,000	0	6,000,000	3,000,000
Net premiums written	4,000,000	7,000,000	6,000,000	2,000,000
Decrease (increase) in unearned premiums	(3,000,000)	(6,000,000)	(5,000,000)	(1,000,000)
Net premiums earned	1,000,000	1,000,000	1,000,000	1,000,000
Losses and loss expenses and life policy benefits	(2,000,000)	0	(2,000,000)	0
Acquisition costs	0	0	0	0
Technical result	(1,000,000)	1,000,000	(1,000,000)	1,000,000
Other income	1,000,000	(1,000,000)	1,000,000	(1,000,000)
Other operating expenses	(30,000,000)	(65,000,000)	(56,000,000)	(101,000,000)
Net investment income	142,000,000	154,000,000	279,000,000	311,000,000
Net realized and unrealized investment gains (losses)	78,000,000	46,000,000	(34,000,000)	191,000,000
Interest expense	(12,000,000)	(13,000,000)	(24,000,000)	(20,000,000)
Amortization of intangible assets	(9,000,000)	(8,000,000)	(18,000,000)	(13,000,000)
Net foreign exchange gains	9,000,000	11,000,000	9,000,000	15,000,000
Income tax expense	(50,000,000)	(17,000,000)	(24,000,000)	(45,000,000)
Interest in (losses) earnings of equity investments	(1,000,000)	1,000,000	0	4,000,000
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written	883,000,000	973,000,000	2,231,000,000	2,692,000,000
Net premiums written	857,000,000	939,000,000	2,119,000,000	2,546,000,000
Decrease (increase) in unearned premiums	48,000,000	(8,000,000)	(333,000,000)	(626,000,000)
Net premiums earned	905,000,000	931,000,000	1,786,000,000	1,920,000,000
Losses and loss expenses and life policy benefits	(646,000,000)	(536,000,000)	(2,109,000,000)	(1,418,000,000)
Acquisition costs	(203,000,000)	(219,000,000)	(381,000,000)	(415,000,000)
Technical result	56,000,000	176,000,000	(704,000,000)	87,000,000
Other income	0	1,000,000	2,000,000	2,000,000
Other operating expenses	(71,000,000)	(82,000,000)	(137,000,000)	(160,000,000)
Underwriting result	(15,000,000)	95,000,000	(839,000,000)	(71,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	71.40%	57.60%	118.00%	73.90%
Acquisition ratio	22.40%	23.40%	21.40%	21.60%
Technical ratio	93.80%	81.00%	139.40%	95.50%
Other operating expense ratio	7.90%	8.80%	7.70%	8.30%
Combined ratio	101.70%	89.80%	147.10%	103.80%
Non Life [Member] | North America [Member]
Segment Reporting [Line Items]
Gross premiums written	242,000,000	207,000,000	581,000,000	564,000,000
Net premiums written	242,000,000	207,000,000	581,000,000	564,000,000
Decrease (increase) in unearned premiums	19,000,000	16,000,000	(60,000,000)	(73,000,000)
Net premiums earned	261,000,000	223,000,000	521,000,000	491,000,000
Losses and loss expenses and life policy benefits	(190,000,000)	(116,000,000)	(365,000,000)	(296,000,000)
Acquisition costs	(63,000,000)	(69,000,000)	(129,000,000)	(137,000,000)
Technical result	8,000,000	38,000,000	27,000,000	58,000,000
Reinsurance Ratios [Abstract]
Loss ratio	72.80%	52.20%	70.00%	60.20%
Acquisition ratio	24.10%	30.70%	24.80%	28.00%
Technical ratio	96.90%	82.90%	94.80%	88.20%
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written	122,000,000	189,000,000	440,000,000	633,000,000
Net premiums written	121,000,000	186,000,000	437,000,000	621,000,000
Decrease (increase) in unearned premiums	72,000,000	45,000,000	(63,000,000)	(160,000,000)
Net premiums earned	193,000,000	231,000,000	374,000,000	461,000,000
Losses and loss expenses and life policy benefits	(127,000,000)	(141,000,000)	(277,000,000)	(387,000,000)
Acquisition costs	(53,000,000)	(61,000,000)	(93,000,000)	(114,000,000)
Technical result	13,000,000	29,000,000	4,000,000	(40,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	65.90%	61.10%	74.00%	84.00%
Acquisition ratio	27.60%	26.40%	25.00%	24.60%
Technical ratio	93.50%	87.50%	99.00%	108.60%
Non Life [Member] | Global (Non-US) Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written	350,000,000	371,000,000	724,000,000	880,000,000
Net premiums written	333,000,000	355,000,000	648,000,000	811,000,000
Decrease (increase) in unearned premiums	8,000,000	21,000,000	10,000,000	(98,000,000)
Net premiums earned	341,000,000	376,000,000	658,000,000	713,000,000
Losses and loss expenses and life policy benefits	(206,000,000)	(292,000,000)	(427,000,000)	(595,000,000)
Acquisition costs	(78,000,000)	(82,000,000)	(158,000,000)	(146,000,000)
Technical result	57,000,000	2,000,000	73,000,000	(28,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	60.30%	77.60%	64.90%	83.40%
Acquisition ratio	22.90%	21.90%	24.00%	20.50%
Technical ratio	83.20%	99.50%	88.90%	103.90%
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written	169,000,000	206,000,000	486,000,000	615,000,000
Net premiums written	161,000,000	191,000,000	453,000,000	550,000,000
Decrease (increase) in unearned premiums	(51,000,000)	(90,000,000)	(220,000,000)	(295,000,000)
Net premiums earned	110,000,000	101,000,000	233,000,000	255,000,000
Losses and loss expenses and life policy benefits	(123,000,000)	13,000,000	(1,040,000,000)	(140,000,000)
Acquisition costs	(9,000,000)	(7,000,000)	(1,000,000)	(18,000,000)
Technical result	$ (22,000,000)	$ 107,000,000	$ (808,000,000)	$ 97,000,000
Reinsurance Ratios [Abstract]
Loss ratio	111.70%	(12.80%)	446.30%	54.60%
Acquisition ratio	8.10%	6.30%	0.30%	7.30%
Technical ratio	119.80%	(6.50%)	446.60%	61.90%