Segment Information - Statement of earnings by segment (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Segment Reporting Information [Line Items]
Gross premiums written		$ 3,081,135		$ 2,198,239		$ 5,636,495		$ 4,711,175
Net premiums written		2,118,331		1,491,844		3,969,580		3,487,695
Net premiums earned		2,082,053		1,496,594		3,416,053		2,985,433
Net losses and loss expenses	[1]	1,173,276		913,179		1,962,490		1,858,271
Acquisition costs	[1]	341,617		184,619		495,313		384,033
Operating expenses	[2]	363,829		263,721		615,712		510,823
Underwriting profit (loss)		203,331		135,075		342,538		232,306
Net results from structured products		5,257	[3]	34,885	[4]	10,282	[5]	42,838	[6]
Net fee income and other	[7]	(1,364)		(2,858)		(7,872)		(3,352)
Loss on sale of life reinsurance subsidiary		0		666,423		0		666,423
Net realized gains (losses) on investments		49,967		72,697		102,858		91,926
Net realized and unrealized gains (losses) on derivative instruments		48,509		11,599		65,030		13,409
Net income (loss) from investment fund affiliates and operating affiliates	[8]	40,839		45,421		98,836		125,009
Gain on sale of operating affiliate		340,407		0		340,407		0
Exchange (gains) losses		10,374		21,141		37,764		31,582
Corporate operating expenses		130,987		56,495		193,430		107,833
Contribution from P&C and Corporate and Other		991,915		(248,948)		1,131,285		109,009
Interest expense	[9]	39,038		32,284		80,519		64,444
Non-controlling interests		4,879		3,682		42,269		39,441
Income tax		32,959		(5,654)		57,177		28,667
Net income (loss) attributable to ordinary shareholders		915,039		(279,260)		951,320		(23,543)
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	[10]	79,443		86,710		154,394		171,007
Net premiums written	[10]	18,258		58,518		32,764		134,829
Net premiums earned	[10]	18,258		58,518		32,764		134,829
Net losses and loss expenses	[1],[10]	22,081		85,299		41,468		198,886
Acquisition costs	[1],[10]	1,684		3,882		3,501		11,061
Operating expenses	[2],[10]	(59)		2,209		872		5,397
Underwriting profit (loss)	[10]	(5,448)		(32,872)		(13,077)		(80,515)
Net results from structured products	[10]	0	[3]	0	[4]	0	[5]	0	[6]
Net fee income and other	[7],[10]	46		45		178		90
Loss on sale of life reinsurance subsidiary				666,423				666,423	[10]
Net realized and unrealized gains (losses) on derivative instruments	[10]	48,509		11,599		65,030		13,409
Net income (loss) from investment fund affiliates and operating affiliates	[8],[10]	40,839		45,421		98,836		125,009
Gain on sale of operating affiliate		340,407				340,407	[10]
Exchange (gains) losses	[10]	10,374		21,141		37,764		31,582
Corporate operating expenses	[10]	130,987		56,495		193,430		107,833
Contribution from P&C and Corporate and Other	[10]	625,605		(671,937)		481,139		(627,461)
Interest expense	[9],[10]	39,038		32,284		80,519		64,444
Non-controlling interests	[10]	4,879		3,682		42,269		39,441
Income tax	[10]	32,959		(5,654)		57,177		28,667
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	[11]	160,292		196,673		303,310		411,092
Net realized gains (losses) on investments		4,351		80,844		8,953		100,073
Excluding Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	[10],[11]	10,831		52,118		21,041		123,010
Net realized gains (losses) on investments	[10]	128		2,339		(1,077)		3,902
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income		46,864		19,165		97,283		19,165
Net realized gains (losses) on investments		45,616		(8,147)		93,905		(8,147)
Net realized and unrealized gains (losses) on derivative instruments		239,174		(17,546)		9,807		(17,546)
Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	[10]	46,864		19,165		97,283		19,165
Net realized gains (losses) on investments	[10]	45,616		(8,147)		93,905		(8,147)
Net realized and unrealized gains (losses) on derivative instruments	[10]	239,174		(17,546)		9,807		(17,546)
Operating Segments [Member] | Total P&C
Segment Reporting Information [Line Items]
Gross premiums written		3,001,692		2,111,529		5,482,101		4,540,168
Net premiums written		2,100,073		1,433,326		3,936,816		3,352,866
Net premiums earned		2,063,795		1,438,076		3,383,289		2,850,604
Net losses and loss expenses	[1]	1,151,195		827,880		1,921,022		1,659,385
Acquisition costs	[1]	339,933		180,737		491,812		372,972
Operating expenses	[2]	363,888		261,512		614,840		505,426
Underwriting profit (loss)		208,779		167,947		355,615		312,821
Net results from structured products		5,257	[3]	34,885	[4]	10,282	[5]	42,838	[6]
Net fee income and other	[7]	(1,410)		(2,903)		(8,050)		(3,442)
Net realized and unrealized gains (losses) on derivative instruments						0
Contribution from P&C and Corporate and Other		$ 366,310		$ 422,989		$ 650,146		$ 736,470
Ratios - P&C operations
Loss and loss expense ratio	[12]	55.80%		57.60%		56.80%		58.20%
Underwriting expense ratio	[12]	34.10%		30.70%		32.70%		30.80%
Combined ratio	[12]	89.90%		88.30%		89.50%		89.00%
Operating Segments [Member] | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,219,444		$ 1,618,405		$ 3,874,191		$ 3,189,331
Net premiums written		1,401,772		996,880		2,490,880		2,124,247
Net premiums earned		1,412,906		1,003,990		2,375,212		1,996,430
Net losses and loss expenses	[1]	896,370		627,627		1,513,317		1,266,820
Acquisition costs	[1]	181,716		99,863		262,103		204,863
Operating expenses	[2]	292,161		213,930		500,618		416,983
Underwriting profit (loss)		42,659		62,570		99,174		107,764
Net results from structured products		3,401	[3]	31,645	[4]	6,306	[5]	36,535	[6]
Net fee income and other	[7]	$ (2,033)		$ (3,567)		$ (9,498)		$ (4,779)
Ratios - P&C operations
Loss and loss expense ratio	[12]	63.40%		62.50%		63.70%		63.50%
Underwriting expense ratio	[12]	33.60%		31.30%		32.10%		31.10%
Combined ratio	[12]	97.00%		93.80%		95.80%		94.60%
Operating Segments [Member] | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		$ 782,248		$ 493,124		$ 1,607,910		$ 1,350,837
Net premiums written		698,301		436,446		1,445,936		1,228,619
Net premiums earned		650,889		434,086		1,008,077		854,174
Net losses and loss expenses	[1]	254,825		200,253		407,705		392,565
Acquisition costs	[1]	158,217		80,874		229,709		168,109
Operating expenses	[2]	71,727		47,582		114,222		88,443
Underwriting profit (loss)		166,120		105,377		256,441		205,057
Net results from structured products		1,856	[3]	3,240	[4]	3,976	[5]	6,303	[6]
Net fee income and other	[7]	$ 623		$ 664		$ 1,448		$ 1,337
Ratios - P&C operations
Loss and loss expense ratio	[12]	39.20%		46.10%		40.40%		46.00%
Underwriting expense ratio	[12]	35.30%		29.60%		34.20%		30.00%
Combined ratio	[12]	74.50%		75.70%		74.60%		76.00%
Operating Segments [Member] | Excluding Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net investment income	[11]	$ 149,461		$ 144,555		$ 282,269		$ 288,082
Net realized gains (losses) on investments		$ 4,223		$ 78,505		$ 10,030		$ 96,171

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Catlin Acquisition within the respective segments.

[2] Operating expenses of the segments exclude Corporate operating expenses, shown separately.

[3] The net results from P&C structured products include net investment income and interest expense of $16.0 million and $10.6 million, respectively.

[4] The net results from P&C structured products include net investment income and interest credit of $16.9 million and $18.2 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $31.1 million and $20.6 million, respectively.

[6] The net results from P&C structured products include net investment income and interest credit of $35.7 million and $7.4 million, respectively.

[7] Net fee income and other includes operating expenses from the Company's loss prevention consulting services business.

[8] The Company generally records the income related to the alternative funds and to the private investment and operating fund affiliates on a one-month and three-month lag, respectively.

[9] Interest expense excludes interest expense related to deposit liabilities recorded in the Insurance and Reinsurance segments

[10] Corporate and Other includes other items of our revenue and expenditures that are not evaluated at the segment level for reporting purposes, as well as the Company's Run-Off Life Operations.

[11] Net investment income - excluding Life Funds Withheld Assets does not include net investment income related to the net results from structured products.

[12] Ratios are based on net premiums earned from P&C operations.