Segment Information (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Segment Reporting [Line Items]
Gross premiums written	$ 1,567,483,000	$ 1,557,561,000
Net premiums written	1,473,286,000	1,470,419,000
(Increase) decrease in unearned premiums	(483,456,000)	(405,830,000)
Net premiums earned	989,830,000	1,064,589,000
Losses and loss expenses and life policy benefits	(576,000,000)	(1,608,000,000)
Acquisition costs	(211,608,000)	(207,849,000)
Technical result	202,000,000	(751,000,000)
Other income	2,746,000	1,813,000
Other operating expenses	(98,174,000)	(104,297,000)
Underwriting result	106,000,000	(853,000,000)
Net investment income	146,896,000	151,633,000
Net realized and unrealized investment gains (losses)	192,735,000	(112,199,000)
Interest expense	(12,220,000)	(12,300,000)
Amortization of intangible assets	(8,893,000)	(8,827,000)
Net foreign exchange (losses) gains	(2,589,000)	695,000
Income tax expense (benefit)	(67,174,000)	26,258,000
Interest in earnings of equity investments	5,078,000	745,000
Net income (loss)	360,141,000	(806,956,000)
[LifeMember]
Segment Reporting [Line Items]
Gross premiums written	217,000,000	208,000,000
Net premiums written	215,000,000	206,000,000
(Increase) decrease in unearned premiums	(21,000,000)	(22,000,000)
Net premiums earned	194,000,000	184,000,000
Losses and loss expenses and life policy benefits	(149,000,000)	(145,000,000)
Acquisition costs	(29,000,000)	(30,000,000)
Technical result	16,000,000	9,000,000
Other income	0	0
Other operating expenses	(12,000,000)	(12,000,000)
Underwriting result	4,000,000	(3,000,000)
Net investment income	17,000,000	15,000,000
Allocated underwriting result	21,000,000	12,000,000
[CorporateAndOtherMember]
Segment Reporting [Line Items]
Gross premiums written	3,000,000	2,000,000
Net premiums written	3,000,000	2,000,000
(Increase) decrease in unearned premiums	(2,000,000)	(2,000,000)
Net premiums earned	1,000,000	0
Losses and loss expenses and life policy benefits	0	0
Acquisition costs	0	0
Technical result	1,000,000	0
Other income	1,000,000	1,000,000
Other operating expenses	(23,000,000)	(26,000,000)
Net investment income	130,000,000	137,000,000
Net realized and unrealized investment gains (losses)	193,000,000	(112,000,000)
Interest expense	(12,000,000)	(12,000,000)
Amortization of intangible assets	(9,000,000)	(9,000,000)
Net foreign exchange (losses) gains	(3,000,000)	0
Income tax expense (benefit)	(67,000,000)	26,000,000
Interest in earnings of equity investments	5,000,000	1,000,000
[NonLifeMember]
Segment Reporting [Line Items]
Gross premiums written	1,347,000,000	1,348,000,000
Net premiums written	1,255,000,000	1,262,000,000
(Increase) decrease in unearned premiums	(460,000,000)	(381,000,000)
Net premiums earned	795,000,000	881,000,000
Losses and loss expenses and life policy benefits	(427,000,000)	(1,463,000,000)
Acquisition costs	(183,000,000)	(178,000,000)
Technical result	185,000,000	(760,000,000)
Other income	1,000,000	1,000,000
Other operating expenses	(63,000,000)	(66,000,000)
Underwriting result	123,000,000	(825,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	53.80%	166.00%
Acquisition ratio	23.00%	20.30%
Technical ratio	76.80%	186.30%
Other operating expense ratio	7.90%	7.40%
Combined ratio	84.70%	193.70%
[NonLifeMember] | [NorthAmericaMember]
Segment Reporting [Line Items]
Gross premiums written	341,000,000	338,000,000
Net premiums written	341,000,000	338,000,000
(Increase) decrease in unearned premiums	(103,000,000)	(79,000,000)
Net premiums earned	238,000,000	259,000,000
Losses and loss expenses and life policy benefits	(133,000,000)	(174,000,000)
Acquisition costs	(66,000,000)	(66,000,000)
Technical result	39,000,000	19,000,000
Reinsurance Ratios [Abstract]
Loss ratio	55.90%	67.00%
Acquisition ratio	27.50%	25.60%
Technical ratio	83.40%	92.60%
[NonLifeMember] | [GlobalNonUsPcMember]
Segment Reporting [Line Items]
Gross premiums written	347,000,000	318,000,000
Net premiums written	346,000,000	317,000,000
(Increase) decrease in unearned premiums	(187,000,000)	(136,000,000)
Net premiums earned	159,000,000	181,000,000
Losses and loss expenses and life policy benefits	(98,000,000)	(150,000,000)
Acquisition costs	(38,000,000)	(40,000,000)
Technical result	23,000,000	(9,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	61.60%	82.80%
Acquisition ratio	23.90%	22.10%
Technical ratio	85.50%	104.90%
[NonLifeMember] | [GlobalNonUsSpecialtyMember]
Segment Reporting [Line Items]
Gross premiums written	417,000,000	375,000,000
Net premiums written	353,000,000	315,000,000
(Increase) decrease in unearned premiums	(45,000,000)	2,000,000
Net premiums earned	308,000,000	317,000,000
Losses and loss expenses and life policy benefits	(194,000,000)	(221,000,000)
Acquisition costs	(70,000,000)	(80,000,000)
Technical result	44,000,000	16,000,000
Reinsurance Ratios [Abstract]
Loss ratio	63.20%	69.70%
Acquisition ratio	22.60%	25.30%
Technical ratio	85.80%	95.00%
[NonLifeMember] | [CatastropheSegmentMember]
Segment Reporting [Line Items]
Gross premiums written	242,000,000	317,000,000
Net premiums written	215,000,000	292,000,000
(Increase) decrease in unearned premiums	(125,000,000)	(168,000,000)
Net premiums earned	90,000,000	124,000,000
Losses and loss expenses and life policy benefits	(2,000,000)	(918,000,000)
Acquisition costs	(9,000,000)	8,000,000
Technical result	$ 79,000,000	$ (786,000,000)
Reinsurance Ratios [Abstract]
Loss ratio	2.10%	743.00%
Acquisition ratio	10.50%	(6.70%)
Technical ratio	12.60%	736.30%