Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 2,198,239		$ 2,025,472		$ 4,711,175		$ 4,503,358
Net premiums written	1,491,844		1,533,868		3,487,695		3,641,522
Net premiums earned	1,496,594		1,562,393		2,985,433		3,092,084
Net losses and loss expenses	913,179		1,054,373		1,858,271		2,008,410
Acquisition costs	184,619		231,886		384,033		452,145
Operating expenses	263,722		236,342		510,824		463,355
Underwriting profit (loss)	135,074		39,792		232,305		168,174
Net investment income			214,410				443,137
Net results from structured products	34,885		6,053		42,838		11,229
Net fee income and other	(2,858)		(791)		(3,352)		(1,055)
Loss on sale of life reinsurance subsidiary	666,423		0		666,423		0
Net realized gains (losses) on investments	72,697		40,968		91,926		77,477
Net realized and unrealized gains (losses) on derivative instruments	11,599		(5,105)		13,409		2,780
Net income (loss) from investment fund affiliates and operating affiliates	45,421		79,368		125,009		142,587
Exchange (gains) losses	21,141		(11,331)		31,582		(44,766)
Corporate operating expenses	56,495		55,155		107,833		102,077
Contribution from P&C and Corporate and Other	(248,949)		330,871		109,008		787,018
Interest expense	32,284		26,121		64,444		52,257
Non-controlling interests	3,682		3,180		39,441		38,922
Income tax	(5,654)		28,872		28,667		72,351
Net income (loss) attributable to ordinary shareholders	(279,261)		272,698		(23,544)		623,488
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Gross premiums written	86,710	[1]	81,302	[1]	171,007	[1]	154,272	[1]
Net premiums written	58,518	[1]	73,896	[1]	134,829	[1]	139,341	[1]
Net premiums earned	58,518	[1]	73,896	[1]	134,829	[1]	139,341	[1]
Net losses and loss expenses	85,299	[1]	116,767	[1]	198,886	[1]	227,720	[1]
Acquisition costs	3,882	[1]	7,255	[1]	11,061	[1]	11,358	[1]
Operating expenses	2,209	[1],[2]	2,206	[1],[2]	5,397	[1],[2]	4,801	[1],[2]
Underwriting profit (loss)	(32,872)	[1]	(52,332)	[1]	(80,515)	[1]	(104,538)	[1]
Net investment income			70,592	[1]			142,280	[1]
Net results from structured products	0	[1],[3]	0	[1],[4]	0	[1],[5]	0	[1],[6]
Net fee income and other	45	[1],[7]	44	[1],[7]	90	[1],[7]	1,087	[1],[7]
Loss on sale of life reinsurance subsidiary	666,423	[1]			666,423	[1]
Net realized and unrealized gains (losses) on derivative instruments	11,599	[1]	(5,105)	[1]	13,409	[1]	2,780	[1]
Net income (loss) from investment fund affiliates and operating affiliates	45,421	[1],[8]	79,368	[1]	125,009	[1]	142,587	[1]
Exchange (gains) losses	21,141	[1]	(11,331)	[1]	31,582	[1]	(44,766)	[1]
Corporate operating expenses	56,495	[1]	55,155	[1]	107,833	[1]	102,077	[1]
Contribution from P&C and Corporate and Other	(671,937)	[1]	60,342	[1]	(627,461)	[1]	129,415	[1]
Interest expense	32,284	[1],[9]	26,121	[1]	64,444	[1]	52,257	[1]
Non-controlling interests	3,682	[1]	3,180	[1]	39,441	[1]	38,922	[1]
Income tax	(5,654)	[1]	28,872	[1]	28,667	[1]	72,351	[1]
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	2,111,529		1,944,170		4,540,168		4,349,086
Net premiums written	1,433,326		1,459,972		3,352,866		3,502,181
Net premiums earned	1,438,076		1,488,497		2,850,604		2,952,743
Net losses and loss expenses	827,880		937,606		1,659,385		1,780,690
Acquisition costs	180,737		224,631		372,972		440,787
Operating expenses	261,513	[2]	234,136	[2]	505,427	[2]	458,554	[2]
Underwriting profit (loss)	167,946		92,124		312,820		272,712
Net investment income			143,818				300,857
Net results from structured products	34,885	[3]	6,053	[4]	42,838	[5]	11,229	[6]
Net fee income and other	(2,903)	[7]	(835)	[7]	(3,442)	[7]	(2,142)	[7]
Contribution from P&C and Corporate and Other	422,988		270,529		736,469		657,603
Ratios - P&C operations
Loss and loss expense ratio	57.60%	[10]	63.00%	[10]	58.20%	[10]	60.30%	[10]
Underwriting expense ratio	30.70%	[10]	30.80%	[10]	30.80%	[10]	30.50%	[10]
Combined ratio	88.30%	[10]	93.80%	[10]	89.00%	[10]	90.80%	[10]
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,618,405		1,472,388		3,189,331		2,970,175
Net premiums written	996,880		1,049,163		2,124,247		2,238,351
Net premiums earned	1,003,990		1,058,542		1,996,430		2,097,634
Net losses and loss expenses	627,627		721,323		1,266,820		1,371,840
Acquisition costs	99,863		138,032		204,863		268,654
Operating expenses	213,931	[2]	194,514	[2]	416,984	[2]	378,511	[2]
Underwriting profit (loss)	62,569		4,673		107,763		78,629
Net results from structured products	31,645	[3]	3,532	[4]	36,535	[5]	7,717	[6]
Net fee income and other	(3,567)	[7]	(1,355)	[7]	(4,779)	[7]	(3,294)	[7]
Ratios - P&C operations
Loss and loss expense ratio	62.50%	[10]	68.10%	[10]	63.50%	[10]	65.40%	[10]
Underwriting expense ratio	31.30%	[10]	31.50%	[10]	31.10%	[10]	30.90%	[10]
Combined ratio	93.80%	[10]	99.60%	[10]	94.60%	[10]	96.30%	[10]
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	493,124		471,782		1,350,837		1,378,911
Net premiums written	436,446		410,809		1,228,619		1,263,830
Net premiums earned	434,086		429,955		854,174		855,109
Net losses and loss expenses	200,253		216,283		392,565		408,850
Acquisition costs	80,874		86,599		168,109		172,133
Operating expenses	47,582	[2]	39,622	[2]	88,443	[2]	80,043	[2]
Underwriting profit (loss)	105,377		87,451		205,057		194,083
Net results from structured products	3,240	[3]	2,521	[4]	6,303	[5]	3,512	[6]
Net fee income and other	664	[7]	520	[7]	1,337	[7]	1,152	[7]
Ratios - P&C operations
Loss and loss expense ratio	46.10%	[10]	50.30%	[10]	46.00%	[10]	47.80%	[10]
Underwriting expense ratio	29.60%	[10]	29.40%	[10]	30.00%	[10]	29.50%	[10]
Combined ratio	75.70%	[10]	79.70%	[10]	76.00%	[10]	77.30%	[10]
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	196,673				411,092
Net realized gains (losses) on investments	80,844		40,968		100,073		77,477
Excluding Life Funds Withheld Assets | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	52,118	[1]			123,010	[1]
Net realized gains (losses) on investments	2,339	[1]	11,599	[1]	3,902	[1]	2,530	[1]
Excluding Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net investment income	144,555				288,082
Net realized gains (losses) on investments	78,505		29,369		96,171		74,947
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	19,165				19,165
Net realized gains (losses) on investments	(8,147)		0		(8,147)		0
Net realized and unrealized gains (losses) on derivative instruments	(17,546)		0		(17,546)		0
Life Funds Withheld Assets | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	19,165	[1]			19,165	[1]
Net realized gains (losses) on investments	(8,147)	[1]	0	[1]	(8,147)	[1]	0	[1]
Net realized and unrealized gains (losses) on derivative instruments	$ (17,546)	[1]			$ (17,546)	[1]

[1] Corporate and Other includes the Company's run-off life operations.

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

[3] The net results from structured products include net investment income and interest expense (credit) of $16.9 million and $(18.2) million, respectively. Net results from structured products includes a benefit of $28.7 million from a negotiated termination of one of the Company's larger structured indemnity contracts during the second quarter of 2014, producing a net interest expense credit.

[4] The net results from structured products include net investment income and interest expense of $18.1 million and $12.1 million, respectively.

[5] The net results from structured products include net investment income and net interest expense (credit) of $35.7 million and $(7.4) million, respectively. Net results from structured products includes a benefit of $28.7 million from a negotiated termination of one of the Company's larger structured indemnity contracts during the second quarter of 2014, producing a net interest expense credit.

[6] The net results from structured products include net investment income and interest expense of $35.9 million and $24.6 million, respectively.

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

[8] The Company 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] Ratios are based on net premiums earned from P&C operations.