Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting [Line Items]
Gross premiums written		$ 1,267,961	$ 1,361,280	$ 4,448,907	$ 4,695,327
Net premiums written		1,190,393	1,342,690	4,165,912	4,499,849
Decrease (increase) in unearned premiums		221,737	213,924	(191,235)	(336,384)
Net premiums earned		1,412,130	1,556,614	3,974,677	4,163,465
Losses and loss expenses and life policy benefits		(804,000)	(960,000)	(2,390,000)	(2,593,000)
Acquisition costs		(346,520)	(321,756)	(905,774)	(888,937)
Technical result		261,000	275,000	679,000	681,000
Other (loss) income		3,056	2,223	7,584	11,892
Other expenses		(415,818)	(108,615)	(670,334)	(327,149)
Underwriting result		(152,000)	169,000	16,000	366,000
Net investment income		117,054	118,176	341,877	365,010
Net realized and unrealized investment (losses) gains		(133,017)	(34,420)	(273,107)	273,468
Interest expense		(12,249)	(12,241)	(36,742)	(36,719)
Amortization of intangible assets		(6,768)	(7,003)	(20,303)	(21,007)
Net foreign exchange (losses) gains		(22,413)	8,206	(15,657)	10,900
Income tax expense		(17,170)	(45,617)	(82,990)	(186,363)
Interest in (losses) earnings of equity method investments		(3,231)	5,294	1,564	16,283
Net (loss) income		(229,142)	201,318	(69,599)	787,996
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written		954,000	1,025,000	3,477,000	3,744,000
Net premiums written		892,000	1,019,000	3,242,000	3,583,000
Decrease (increase) in unearned premiums		218,000	208,000	(184,000)	(323,000)
Net premiums earned		1,110,000	1,227,000	3,058,000	3,260,000
Losses and loss expenses and life policy benefits		(556,000)	(688,000)	(1,643,000)	(1,853,000)
Acquisition costs		(309,000)	(284,000)	(804,000)	(778,000)
Technical result		245,000	255,000	611,000	629,000
Other (loss) income		0	(1,000)	0	1,000
Other expenses		(55,000)	(62,000)	(162,000)	(187,000)
Underwriting result		$ 190,000	$ 192,000	$ 449,000	$ 443,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	50.10%	56.10%	53.70%	56.80%
Acquisition ratio	[2]	27.80%	23.10%	26.30%	23.90%
Technical ratio	[3]	77.90%	79.20%	80.00%	80.70%
Other expense Ratio	[4]	4.90%	5.00%	5.30%	5.70%
Combined Ratio	[5]	82.80%	84.20%	85.30%	86.40%
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 351,000	$ 372,000	$ 1,251,000	$ 1,302,000
Net premiums written		335,000	372,000	1,207,000	1,291,000
Decrease (increase) in unearned premiums		73,000	52,000	(25,000)	(99,000)
Net premiums earned		408,000	424,000	1,182,000	1,192,000
Losses and loss expenses and life policy benefits		(182,000)	(247,000)	(650,000)	(747,000)
Acquisition costs		(137,000)	(106,000)	(341,000)	(299,000)
Technical result		$ 89,000	$ 71,000	$ 191,000	$ 146,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	44.70%	58.20%	55.00%	62.60%
Acquisition ratio	[2]	33.40%	24.90%	28.80%	25.10%
Technical ratio	[3]	78.10%	83.10%	83.80%	87.70%
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 153,000	$ 162,000	$ 630,000	$ 682,000
Net premiums written		153,000	164,000	621,000	672,000
Decrease (increase) in unearned premiums		33,000	38,000	(102,000)	(104,000)
Net premiums earned		186,000	202,000	519,000	568,000
Losses and loss expenses and life policy benefits		(122,000)	(123,000)	(362,000)	(319,000)
Acquisition costs		(50,000)	(56,000)	(139,000)	(162,000)
Technical result		$ 14,000	$ 23,000	$ 18,000	$ 87,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	65.80%	61.10%	69.80%	56.20%
Acquisition ratio	[2]	26.90%	27.60%	26.80%	28.50%
Technical ratio	[3]	92.70%	88.70%	96.60%	84.70%
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 393,000	$ 432,000	$ 1,226,000	$ 1,348,000
Net premiums written		391,000	428,000	1,153,000	1,250,000
Decrease (increase) in unearned premiums		13,000	20,000	(10,000)	(42,000)
Net premiums earned		404,000	448,000	1,143,000	1,208,000
Losses and loss expenses and life policy benefits		(224,000)	(279,000)	(593,000)	(749,000)
Acquisition costs		(112,000)	(105,000)	(307,000)	(283,000)
Technical result		$ 68,000	$ 64,000	$ 243,000	$ 176,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	55.50%	62.30%	51.90%	62.10%
Acquisition ratio	[2]	27.70%	23.50%	26.80%	23.40%
Technical ratio	[3]	83.20%	85.80%	78.70%	85.50%
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 57,000	$ 59,000	$ 370,000	$ 412,000
Net premiums written		13,000	55,000	261,000	370,000
Decrease (increase) in unearned premiums		99,000	98,000	(47,000)	(78,000)
Net premiums earned		112,000	153,000	214,000	292,000
Losses and loss expenses and life policy benefits		(28,000)	(39,000)	(38,000)	(38,000)
Acquisition costs		(10,000)	(17,000)	(17,000)	(34,000)
Technical result		$ 74,000	$ 97,000	$ 159,000	$ 220,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	24.90%	25.20%	17.80%	12.90%
Acquisition ratio	[2]	8.70%	11.70%	8.10%	11.50%
Technical ratio	[3]	33.60%	36.90%	25.90%	24.40%
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 314,000	$ 336,000	$ 972,000	$ 951,000
Net premiums written		298,000	325,000	924,000	918,000
Decrease (increase) in unearned premiums		4,000	6,000	(7,000)	(14,000)
Net premiums earned		302,000	331,000	917,000	904,000
Losses and loss expenses and life policy benefits		(248,000)	(272,000)	(748,000)	(740,000)
Acquisition costs		(38,000)	(38,000)	(102,000)	(111,000)
Technical result		16,000	21,000	67,000	53,000
Other (loss) income		3,000	2,000	4,000	6,000
Other expenses		(16,000)	(17,000)	(47,000)	(52,000)
Underwriting result		3,000	6,000	24,000	7,000
Net investment income		15,000	14,000	45,000	45,000
Allocated underwriting result	[6]	18,000	20,000	69,000	52,000
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written		0	0	0	0
Net premiums written		0	(1,000)	0	(1,000)
Decrease (increase) in unearned premiums		0	0	0	0
Net premiums earned		0	(1,000)	0	(1,000)
Losses and loss expenses and life policy benefits		0	0	1,000	0
Acquisition costs		0	0	0	0
Technical result		0	(1,000)	1,000	(1,000)
Other (loss) income		0	1,000	3,000	5,000
Other expenses		(345,000)	(29,000)	(461,000)	(88,000)
Net investment income		102,000	104,000	297,000	320,000
Net realized and unrealized investment (losses) gains		(133,000)	(34,000)	(273,000)	273,000
Interest expense		(12,000)	(12,000)	(37,000)	(36,000)
Amortization of intangible assets		(7,000)	(7,000)	(20,000)	(21,000)
Net foreign exchange (losses) gains		(22,000)	8,000	(16,000)	11,000
Income tax expense		(17,000)	(46,000)	(83,000)	(186,000)
Interest in (losses) earnings of equity method investments		$ (3,000)	$ 5,000	$ 1,000	$ 16,000

[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.