Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting [Line Items]
Gross premiums written										$ 5,547,525	$ 5,932,003	$ 5,569,706
Net premiums written		$ 1,064,000	$ 1,190,000	$ 1,322,000	$ 1,653,000	$ 1,220,000	$ 1,343,000	$ 1,419,000	$ 1,738,000	5,229,548	5,719,884	5,396,526
Decrease (increase) in unearned premiums										39,630	(110,689)	(198,316)
Net premiums earned		1,294,000	1,412,000	1,328,000	1,235,000	1,446,000	1,557,000	1,353,000	1,254,000	5,269,178	5,609,195	5,198,210
Losses and loss expenses and life policy benefits										(3,157,000)	(3,463,000)	(3,158,000)
Acquisition costs		(311,000)	(347,000)	(283,000)	(276,000)	(325,000)	(322,000)	(303,000)	(265,000)	(1,217,003)	(1,213,822)	(1,077,628)
Technical result										895,000	932,000	962,000
Other income		1,000	3,000	0	4,000	4,000	2,000	9,000	0	9,144	16,190	16,565
Other expenses		(120,000)	(416,000)	(130,000)	(125,000)	(123,000)	(108,000)	(107,000)	(111,000)	(790,723)	(449,688)	(500,466)
Underwriting result										113,000	498,000	479,000
Net investment income		108,000	117,000	120,000	105,000	115,000	118,000	130,000	117,000	449,784	479,696	484,367
Net realized and unrealized investment (losses) gains		(24,000)	(133,000)	(256,000)	116,000	98,000	(34,000)	166,000	142,000	(297,479)	371,796	(160,735)
Interest expense		(12,000)	(12,000)	(12,000)	(12,000)	(12,000)	(12,000)	(12,000)	(12,000)	(48,988)	(48,963)	(48,929)
Amortization of intangible assets		(6,000)	(7,000)	(7,000)	(7,000)	(6,000)	(7,000)	(7,000)	(7,000)	(26,593)	(27,486)	(27,180)
Net foreign exchange (losses) gains		6,000	(22,000)	(6,000)	13,000	7,000	8,000	2,000	0	(9,461)	18,201	(18,203)
Income tax expense		3,000	(17,000)	14,000	(80,000)	(53,000)	(46,000)	(78,000)	(62,000)	(79,664)	(239,506)	(48,416)
Interest in earnings of equity method investments		5,000	(3,000)	8,000	(4,000)	(1,000)	5,000	5,000	6,000	6,375	15,270	13,665
Net income		$ 177,000	$ (229,000)	$ (89,000)	$ 248,000	$ 280,000	$ 201,000	$ 274,000	$ 313,000	107,150	1,068,113	673,442
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written										4,277,000	4,667,000	4,590,000
Net premiums written										4,022,000	4,500,000	4,427,000
Decrease (increase) in unearned premiums										38,000	(113,000)	(192,000)
Net premiums earned										4,060,000	4,387,000	4,235,000
Losses and loss expenses and life policy benefits										(2,193,000)	(2,463,000)	(2,400,000)
Acquisition costs										(1,064,000)	(1,065,000)	(953,000)
Technical result										803,000	859,000	882,000
Other income										0	3,000	3,000
Other expenses										(219,000)	(252,000)	(259,000)
Underwriting result										$ 584,000	$ 610,000	$ 626,000
Reinsurance Ratios [Abstract]
Loss ratio	[1]									54.00%	56.10%	56.70%
Acquisition ratio	[2]									26.20%	24.30%	22.50%
Technical ratio	[3]									80.20%	80.40%	79.20%
Other expense ratio	[4]									5.40%	5.80%	6.10%
Combined ratio	[5]									85.60%	86.20%	85.30%
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written										$ 1,604,000	$ 1,642,000	$ 1,601,000
Net premiums written										1,542,000	1,630,000	1,587,000
Decrease (increase) in unearned premiums										30,000	(33,000)	(54,000)
Net premiums earned										1,572,000	1,597,000	1,533,000
Losses and loss expenses and life policy benefits										(881,000)	(1,000,000)	(975,000)
Acquisition costs										(443,000)	(401,000)	(351,000)
Technical result										$ 248,000	$ 196,000	$ 207,000
Reinsurance Ratios [Abstract]
Loss ratio	[1]									56.00%	62.60%	63.60%
Acquisition ratio	[2]									28.20%	25.10%	22.90%
Technical ratio	[3]									84.20%	87.70%	86.50%
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written										$ 735,000	$ 803,000	$ 818,000
Net premiums written										726,000	794,000	811,000
Decrease (increase) in unearned premiums										(33,000)	(26,000)	(68,000)
Net premiums earned										693,000	768,000	743,000
Losses and loss expenses and life policy benefits										(473,000)	(438,000)	(373,000)
Acquisition costs										(189,000)	(222,000)	(196,000)
Technical result										$ 31,000	$ 108,000	$ 174,000
Reinsurance Ratios [Abstract]
Loss ratio	[1]									68.30%	57.00%	50.20%
Acquisition ratio	[2]									27.30%	28.90%	26.40%
Technical ratio	[3]									95.60%	85.90%	76.60%
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written										$ 1,556,000	$ 1,797,000	$ 1,676,000
Net premiums written										1,482,000	1,696,000	1,579,000
Decrease (increase) in unearned premiums										29,000	(58,000)	(73,000)
Net premiums earned										1,511,000	1,638,000	1,506,000
Losses and loss expenses and life policy benefits										(785,000)	(963,000)	(920,000)
Acquisition costs										(407,000)	(400,000)	(362,000)
Technical result										$ 319,000	$ 275,000	$ 224,000
Reinsurance Ratios [Abstract]
Loss ratio	[1]									52.00%	58.80%	61.10%
Acquisition ratio	[2]									26.90%	24.40%	24.00%
Technical ratio	[3]									78.90%	83.20%	85.10%
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written										$ 382,000	$ 425,000	$ 495,000
Net premiums written										272,000	380,000	450,000
Decrease (increase) in unearned premiums										12,000	4,000	3,000
Net premiums earned										284,000	384,000	453,000
Losses and loss expenses and life policy benefits										(54,000)	(62,000)	(132,000)
Acquisition costs										(25,000)	(42,000)	(44,000)
Technical result										$ 205,000	$ 280,000	$ 277,000
Reinsurance Ratios [Abstract]
Loss ratio	[1]									19.10%	16.10%	29.00%
Acquisition ratio	[2]									8.60%	11.00%	9.70%
Technical ratio	[3]									27.70%	27.10%	38.70%
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written										$ 1,271,000	$ 1,265,000	$ 972,000
Net premiums written										1,208,000	1,220,000	964,000
Decrease (increase) in unearned premiums										1,000	2,000	(7,000)
Net premiums earned										1,209,000	1,222,000	957,000
Losses and loss expenses and life policy benefits										(964,000)	(1,000,000)	(760,000)
Acquisition costs										(153,000)	(149,000)	(125,000)
Technical result										92,000	73,000	72,000
Other income										6,000	8,000	11,000
Other expenses										(63,000)	(68,000)	(71,000)
Underwriting result										35,000	13,000	12,000
Net investment income										59,000	60,000	61,000
Allocated underwriting result	[6]									94,000	73,000	73,000
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written										0	0	8,000
Net premiums written										0	0	6,000
Decrease (increase) in unearned premiums										0	0	0
Net premiums earned										0	0	6,000
Losses and loss expenses and life policy benefits										0	0	2,000
Acquisition costs										0	0	0
Technical result										0	0	8,000
Other income										3,000	5,000	3,000
Other expenses										(509,000)	(130,000)	(170,000)
Net investment income										391,000	420,000	423,000
Net realized and unrealized investment (losses) gains										(297,000)	372,000	(161,000)
Interest expense										(49,000)	(49,000)	(49,000)
Amortization of intangible assets										(27,000)	(27,000)	(27,000)
Net foreign exchange (losses) gains										(9,000)	18,000	(18,000)
Income tax expense										(80,000)	(239,000)	(49,000)
Interest in earnings of equity method investments										$ 6,000	$ 15,000	$ 14,000

[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.