Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting [Line Items]
Gross premiums written		$ 1,432,012	$ 1,462,307	$ 3,180,946	$ 3,334,047
Net premiums written		1,322,304	1,418,665	2,975,519	3,157,159
Decrease (increase) in unearned premiums		5,522	(65,596)	(412,972)	(550,308)
Net premiums earned		1,327,826	1,353,069	2,562,547	2,606,851
Losses and loss expenses and life policy benefits		(865,000)	(884,000)	(1,586,000)	(1,633,000)
Acquisition costs		(283,463)	(302,573)	(559,254)	(567,181)
Technical result		180,000	166,000	417,000	407,000
Other income		236	9,265	4,528	9,669
Other expenses		(129,766)	(107,072)	(254,516)	(218,534)
Underwriting result		50,000	68,000	167,000	198,000
Net investment income		120,192	129,967	224,823	246,834
Net realized and unrealized investment (losses) gains		(255,734)	165,717	(140,089)	307,888
Interest expense		(12,248)	(12,240)	(24,493)	(24,477)
Amortization of intangible assets		(6,767)	(7,003)	(13,535)	(14,005)
Net foreign exchange gains		(6,391)	2,023	6,756	2,693
Income tax benefit (expense)		13,844	(78,440)	(65,821)	(140,746)
Interest in earnings of equity method investments		8,633	4,925	4,795	10,989
Net (loss) income		(88,555)	273,792	159,543	586,678
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written		1,098,000	1,136,000	2,522,000	2,719,000
Net premiums written		1,009,000	1,108,000	2,349,000	2,564,000
Decrease (increase) in unearned premiums		3,000	(66,000)	(401,000)	(530,000)
Net premiums earned		1,012,000	1,042,000	1,948,000	2,034,000
Losses and loss expenses and life policy benefits		(605,000)	(632,000)	(1,087,000)	(1,165,000)
Acquisition costs		(253,000)	(260,000)	(495,000)	(494,000)
Technical result		154,000	150,000	366,000	375,000
Other income		0	1,000	0	2,000
Other expenses		(55,000)	(61,000)	(107,000)	(126,000)
Underwriting result		$ 99,000	$ 90,000	$ 259,000	$ 251,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	59.80%	60.60%	55.80%	57.30%
Acquisition ratio	[2]	25.00%	25.00%	25.40%	24.30%
Technical ratio	[3]	84.80%	85.60%	81.20%	81.60%
Other expense Ratio	[4]	5.50%	5.90%	5.50%	6.20%
Combined Ratio	[5]	90.30%	91.50%	86.70%	87.80%
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 427,000	$ 400,000	$ 900,000	$ 930,000
Net premiums written		401,000	392,000	872,000	919,000
Decrease (increase) in unearned premiums		34,000	(2,000)	(98,000)	(151,000)
Net premiums earned		435,000	390,000	774,000	768,000
Losses and loss expenses and life policy benefits		(296,000)	(240,000)	(468,000)	(499,000)
Acquisition costs		(111,000)	(102,000)	(204,000)	(194,000)
Technical result		$ 28,000	$ 48,000	$ 102,000	$ 75,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	68.10%	61.50%	60.40%	65.00%
Acquisition ratio	[2]	25.40%	26.10%	26.40%	25.20%
Technical ratio	[3]	93.50%	87.60%	86.80%	90.20%
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 143,000	$ 155,000	$ 477,000	$ 519,000
Net premiums written		137,000	148,000	468,000	508,000
Decrease (increase) in unearned premiums		22,000	39,000	(135,000)	(141,000)
Net premiums earned		159,000	187,000	333,000	367,000
Losses and loss expenses and life policy benefits		(121,000)	(103,000)	(240,000)	(196,000)
Acquisition costs		(36,000)	(52,000)	(89,000)	(107,000)
Technical result		$ 2,000	$ 32,000	$ 4,000	$ 64,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	75.90%	54.60%	72.00%	53.50%
Acquisition ratio	[2]	23.20%	27.90%	26.80%	29.00%
Technical ratio	[3]	99.10%	82.50%	98.80%	82.50%
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 406,000	$ 438,000	$ 833,000	$ 917,000
Net premiums written		400,000	432,000	762,000	822,000
Decrease (increase) in unearned premiums		(26,000)	(26,000)	(23,000)	(61,000)
Net premiums earned		374,000	406,000	739,000	761,000
Losses and loss expenses and life policy benefits		(198,000)	(270,000)	(369,000)	(471,000)
Acquisition costs		(102,000)	(98,000)	(194,000)	(178,000)
Technical result		$ 74,000	$ 38,000	$ 176,000	$ 112,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	53.00%	66.50%	49.90%	61.90%
Acquisition ratio	[2]	27.20%	24.20%	26.30%	23.40%
Technical ratio	[3]	80.20%	90.70%	76.20%	85.30%
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 122,000	$ 143,000	$ 312,000	$ 353,000
Net premiums written		71,000	136,000	247,000	315,000
Decrease (increase) in unearned premiums		(27,000)	(77,000)	(145,000)	(177,000)
Net premiums earned		44,000	59,000	102,000	138,000
Losses and loss expenses and life policy benefits		10,000	(19,000)	(10,000)	1,000
Acquisition costs		(4,000)	(8,000)	(8,000)	(15,000)
Technical result		$ 50,000	$ 32,000	$ 84,000	$ 124,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	(21.50%)	33.40%	9.90%	(0.90%)
Acquisition ratio	[2]	8.20%	13.00%	7.60%	11.40%
Technical ratio	[3]	(13.30%)	46.40%	17.50%	10.50%
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 334,000	$ 326,000	$ 659,000	$ 615,000
Net premiums written		313,000	311,000	626,000	593,000
Decrease (increase) in unearned premiums		3,000	0	(12,000)	(20,000)
Net premiums earned		316,000	311,000	614,000	573,000
Losses and loss expenses and life policy benefits		(260,000)	(252,000)	(499,000)	(468,000)
Acquisition costs		(30,000)	(43,000)	(64,000)	(73,000)
Technical result		26,000	16,000	51,000	32,000
Other income		0	3,000	1,000	4,000
Other expenses		(16,000)	(16,000)	(31,000)	(34,000)
Underwriting result		10,000	3,000	21,000	2,000
Net investment income		16,000	15,000	30,000	30,000
Allocated underwriting result	[6]	26,000	18,000	51,000	32,000
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written		0	0	0	0
Net premiums written		0	0	0	0
Decrease (increase) in unearned premiums		0	0	0	0
Net premiums earned		0	0	0	0
Losses and loss expenses and life policy benefits		0	0	0	0
Acquisition costs		0	0	0	0
Technical result		0	0	0	0
Other income		0	5,000	3,000	4,000
Other expenses		(59,000)	(30,000)	(116,000)	(59,000)
Net investment income		104,000	115,000	195,000	217,000
Net realized and unrealized investment (losses) gains		(256,000)	166,000	(140,000)	308,000
Interest expense		(12,000)	(12,000)	(25,000)	(25,000)
Amortization of intangible assets		(7,000)	(7,000)	(14,000)	(14,000)
Net foreign exchange gains		(6,000)	2,000	7,000	3,000
Income tax benefit (expense)		14,000	(78,000)	(65,000)	(141,000)
Interest in earnings of equity method investments		$ 8,000	$ 5,000	$ 5,000	$ 11,000

[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 expense ratio is obtained by dividing other expenses by net premiums earned.

[5] Combined ratio is defined as the sum of the technical ratio and the other 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 expenses.