Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Gross premiums written	$ 2,555,360		$ 2,512,936
Net premiums written	1,851,249		1,995,851
Net premiums earned	1,334,000		1,488,839
Net losses and loss expenses	789,214		945,092
Acquisition costs	153,696		199,414
Operating expenses	251,883	[1]	247,102	[1]
Underwriting profit (loss)	139,207		97,231
Net results from structured products	5,025	[2]	7,953	[3]
Net fee income and other	(6,508)	[4]	(494)	[4]
Net realized gains (losses) on investments	52,891		19,229
Net realized and unrealized gains (losses) on derivative instruments	16,521		1,810
Net income (loss) from investment fund affiliates and operating affiliates	57,997	[5]	79,588	[5]
Exchange (gains) losses	27,390		10,441
Corporate operating expenses	62,443		51,338
Contribution from P&C and Corporate and Other	139,370		357,957
Interest expense	41,481	[6]	32,160	[6]
Non-controlling interests	37,390		35,759
Income tax	24,218		34,321
Net income (loss) attributable to ordinary shareholders	36,281		255,717
Corporate and Other
Segment Reporting Information [Line Items]
Gross premiums written	74,951	[7]	84,297	[7]
Net premiums written	14,506	[7]	76,311	[7]
Net premiums earned	14,506	[7]	76,311	[7]
Net losses and loss expenses	19,387	[7]	113,587	[7]
Acquisition costs	1,817	[7]	7,179	[7]
Operating expenses	931	[1],[7]	3,188	[1],[7]
Underwriting profit (loss)	(7,629)	[7]	(47,643)	[7]
Net results from structured products	0	[2],[7]	0	[3],[7]
Net fee income and other	132	[4],[7]	45	[4],[7]
Net realized and unrealized gains (losses) on derivative instruments	16,521	[7]	1,810	[7]
Net income (loss) from investment fund affiliates and operating affiliates	57,997	[5],[7]	79,588	[5],[7]
Exchange (gains) losses	27,390	[7]	10,441	[7]
Corporate operating expenses	62,443	[7]	51,338	[7]
Contribution from P&C and Corporate and Other	(144,466)	[7]	44,476	[7]
Interest expense	41,481	[6],[7]	32,160	[6],[7]
Non-controlling interests	37,390	[7]	35,759	[7]
Income tax	24,218	[7]	34,321	[7]
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	2,480,409		2,428,639
Net premiums written	1,836,743		1,919,540
Net premiums earned	1,319,494		1,412,528
Net losses and loss expenses	769,827		831,505
Acquisition costs	151,879		192,235
Operating expenses	250,952	[1]	243,914	[1]
Underwriting profit (loss)	146,836		144,874
Net results from structured products	5,025	[2]	7,953	[3]
Net fee income and other	(6,640)	[4]	(539)	[4]
Contribution from P&C and Corporate and Other	283,836		313,481
Ratios - P&C operations
Loss and loss expense ratio	58.30%	[8]	58.90%	[8]
Underwriting expense ratio	30.60%	[8]	30.80%	[8]
Combined ratio	88.90%	[8]	89.70%	[8]
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,654,747		1,570,926
Net premiums written	1,089,108		1,127,367
Net premiums earned	962,306		992,440
Net losses and loss expenses	616,947		639,193
Acquisition costs	80,387		105,000
Operating expenses	208,457	[1]	203,053	[1]
Underwriting profit (loss)	56,515		45,194
Net results from structured products	2,905	[2]	4,890	[3]
Net fee income and other	(7,465)	[4]	(1,212)	[4]
Ratios - P&C operations
Loss and loss expense ratio	64.10%	[8]	64.40%	[8]
Underwriting expense ratio	30.00%	[8]	31.00%	[8]
Combined ratio	94.10%	[8]	95.40%	[8]
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	825,662		857,713
Net premiums written	747,635		792,173
Net premiums earned	357,188		420,088
Net losses and loss expenses	152,880		192,312
Acquisition costs	71,492		87,235
Operating expenses	42,495	[1]	40,861	[1]
Underwriting profit (loss)	90,321		99,680
Net results from structured products	2,120	[2]	3,063	[3]
Net fee income and other	825	[4]	673	[4]
Ratios - P&C operations
Loss and loss expense ratio	42.80%	[8]	45.80%	[8]
Underwriting expense ratio	31.90%	[8]	30.50%	[8]
Combined ratio	74.70%	[8]	76.30%	[8]
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	143,018	[9]	214,419	[9]
Net realized gains (losses) on investments	4,602		19,229
Excluding Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	10,210	[7],[9]	70,892	[7],[9]
Net realized gains (losses) on investments	(1,205)	[7]	1,563	[7]
Excluding Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net investment income	132,808	[9]	143,527	[9]
Net realized gains (losses) on investments	5,807		17,666
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	50,419
Net realized gains (losses) on investments	48,289		0
Net realized and unrealized gains (losses) on derivative instruments	(229,367)		0
Life Funds Withheld Assets | Corporate and Other
Segment Reporting Information [Line Items]
Net investment income	50,419	[7]
Net realized gains (losses) on investments	48,289	[7]
Net realized and unrealized gains (losses) on derivative instruments	$ (229,367)	[7]

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

[2] The net results from P&C structured products include net investment income and interest expense of $15.1 million and $10.0 million, respectively.

[3] The net results from P&C structured products include net investment income and interest expense of $18.8 million and $10.8 million, respectively.

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

[5] 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.

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

[7] 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.

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

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