Segment Information (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting [Line Items]
Gross premiums written		$ 1,629,009	$ 1,748,933
Net premiums written		1,500,718	1,653,215
(Increase) decrease in unearned premiums		(359,002)	(418,493)
Net premiums earned		1,141,716	1,234,722
Losses and loss expenses and life policy benefits		(714,000)	(721,000)
Acquisition costs		(282,974)	(275,791)
Technical result		145,000	238,000
Other income		4,840	4,292
Other expenses		(152,674)	(124,750)
Underwriting result		(3,000)	117,000
Net investment income		102,987	104,631
Net realized and unrealized investment gains		167,193	115,645
Interest expense		(12,259)	(12,245)
Amortization of intangible assets		(6,588)	(6,768)
Net foreign exchange gains		2,074	13,147
Income tax expense		(30,954)	(79,665)
Interest in losses of equity method investments		(3,467)	(3,838)
Net income		215,626	248,099
Non Life [Member]
Segment Reporting [Line Items]
Gross premiums written		1,336,000	1,425,000
Net premiums written		1,224,000	1,340,000
(Increase) decrease in unearned premiums		(352,000)	(404,000)
Net premiums earned		872,000	936,000
Losses and loss expenses and life policy benefits		(509,000)	(481,000)
Acquisition costs		(245,000)	(242,000)
Technical result		118,000	213,000
Other income		2,000	0
Other expenses		(68,000)	(52,000)
Underwriting result		$ 52,000	$ 161,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	58.50%	51.40%
Acquisition ratio	[2]	28.00%	25.90%
Technical ratio	[3]	86.50%	77.30%
Other expense Ratio	[4]	7.80%	5.50%
Combined Ratio	[5]	94.30%	82.80%
Non Life [Member] | North America Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 494,000	$ 473,000
Net premiums written		481,000	471,000
(Increase) decrease in unearned premiums		(129,000)	(132,000)
Net premiums earned		352,000	339,000
Losses and loss expenses and life policy benefits		(199,000)	(172,000)
Acquisition costs		(107,000)	(93,000)
Technical result		$ 46,000	$ 74,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	56.50%	50.70%
Acquisition ratio	[2]	30.40%	27.50%
Technical ratio	[3]	86.90%	78.20%
Non Life [Member] | Global (Non-US) P&C [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 274,000	$ 334,000
Net premiums written		269,000	331,000
(Increase) decrease in unearned premiums		(119,000)	(157,000)
Net premiums earned		150,000	174,000
Losses and loss expenses and life policy benefits		(123,000)	(119,000)
Acquisition costs		(47,000)	(52,000)
Technical result		$ (20,000)	$ 3,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	82.10%	68.40%
Acquisition ratio	[2]	31.10%	30.10%
Technical ratio	[3]	113.20%	98.50%
Non Life [Member] | Global Specialty [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 398,000	$ 427,000
Net premiums written		333,000	362,000
(Increase) decrease in unearned premiums		(11,000)	3,000
Net premiums earned		322,000	365,000
Losses and loss expenses and life policy benefits		(184,000)	(170,000)
Acquisition costs		(90,000)	(93,000)
Technical result		$ 48,000	$ 102,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	57.20%	46.70%
Acquisition ratio	[2]	27.80%	25.40%
Technical ratio	[3]	85.00%	72.10%
Non Life [Member] | Catastrophe Segment [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 170,000	$ 191,000
Net premiums written		141,000	176,000
(Increase) decrease in unearned premiums		(93,000)	(118,000)
Net premiums earned		48,000	58,000
Losses and loss expenses and life policy benefits		(3,000)	(20,000)
Acquisition costs		(1,000)	(4,000)
Technical result		$ 44,000	$ 34,000
Reinsurance Ratios [Abstract]
Loss Ratio	[1]	7.20%	33.70%
Acquisition ratio	[2]	3.10%	7.00%
Technical ratio	[3]	10.30%	40.70%
Life and Health [Member]
Segment Reporting [Line Items]
Gross premiums written		$ 293,000	$ 324,000
Net premiums written		277,000	313,000
(Increase) decrease in unearned premiums		(7,000)	(14,000)
Net premiums earned		270,000	299,000
Losses and loss expenses and life policy benefits		(205,000)	(240,000)
Acquisition costs		(38,000)	(34,000)
Technical result		27,000	25,000
Other income		2,000	1,000
Other expenses		(18,000)	(15,000)
Underwriting result		11,000	11,000
Net investment income		13,000	14,000
Allocated underwriting result	[6]	24,000	25,000
Corporate and Other [Member]
Segment Reporting [Line Items]
Gross premiums written		0	0
Net premiums written		0	0
(Increase) decrease in unearned premiums		0	0
Net premiums earned		0	0
Losses and loss expenses and life policy benefits		0	0
Acquisition costs		0	0
Technical result		0	0
Other income		1,000	3,000
Other expenses		(67,000)	(58,000)
Net investment income		90,000	91,000
Net realized and unrealized investment gains		167,000	116,000
Interest expense		(12,000)	(12,000)
Amortization of intangible assets		(7,000)	(7,000)
Net foreign exchange gains		2,000	13,000
Income tax expense		(31,000)	(80,000)
Interest in losses of equity method investments		$ (3,000)	$ (4,000)

[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.