Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Segment Reporting Information [Line Items]
Gross premiums written									$ 8,094,926		$ 7,741,135		$ 7,530,883
Net premiums written									5,944,841		6,199,404		6,281,451
Net premiums earned									5,895,070		6,309,521		6,090,437
Net losses and loss expenses									3,501,356		4,197,166		4,251,677
Acquisition costs									738,118		909,323		913,492
Operating expenses									1,069,689		957,841		921,300
Underwriting profit (loss)									585,907		245,191		3,968
Net investment income											885,789		940,680
Net results from structured products									54,209		23,791		5,257
Net fee income and other									(6,891)		(5,692)		3,768
Loss on sale of life reinsurance subsidiary									666,423		0		0
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									107,462		87,777		14,098
Net realized and unrealized (gains) losses on derivative instruments									29,886		7,798		5,221
Net income (loss) from investment fund affiliates and operating affiliates									203,034		258,195		112,391
Exchange (gains) losses									(37,568)		(28,243)		10,545
Corporate operating expenses									220,165		209,454		204,502
Contribution from P&C and Corporate and Other									486,956		1,321,638		870,336
Interest expense									121,221		107,486		105,925
Non-controlling interests									80,498		76,731		79,255
Provision (benefit) for income tax									96,897		77,505		34,028
Net income (loss) attributable to ordinary shareholders									188,340		1,059,916		651,128
Exclude Life Funds Withheld Assets [Member]
Segment Reporting Information [Line Items]
Net investment income									721,016
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									122,991
Life Funds Withheld Assets [Member]
Segment Reporting Information [Line Items]
Net investment income									129,575
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									(15,529)
Net realized and unrealized (gains) losses on derivative instruments									(488,222)		0		0
Corporate and other
Segment Reporting Information [Line Items]
Gross premiums written									333,436	[1]	324,343	[1]	355,753	[1]
Net premiums written									177,632	[1]	295,422	[1]	324,432	[1]
Net premiums earned									177,632	[1]	295,422	[1]	324,459	[1]
Net losses and loss expenses									242,963	[1]	465,702	[1]	486,195	[1]
Acquisition costs									14,115	[1]	26,665	[1]	41,093	[1]
Operating expenses									10,693	[1],[2]	8,926	[1],[2]	9,335	[1],[2]
Underwriting profit (loss)									(90,139)	[1]	(205,871)	[1]	(212,164)	[1]
Net investment income											286,645	[1],[3]	299,443	[1],[3]
Net results from structured products									0	[1],[4]	0	[1],[5]	0	[1],[6]
Net fee income and other									360	[1],[7]	1,305	[1],[7]	426	[1],[7]
Loss on sale of life reinsurance subsidiary									666,423	[1]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments											1,985	[1]	(21,003)	[1]
Net realized and unrealized (gains) losses on derivative instruments									29,886	[1]	7,798	[1]	5,221	[1]
Net income (loss) from investment fund affiliates and operating affiliates									203,034	[1],[8]	258,195	[1],[8]	112,391	[1],[8]
Exchange (gains) losses									(37,568)	[1]	(28,243)	[1]	10,545	[1]
Corporate operating expenses									220,165	[1]	209,454	[1]	204,502	[1]
Contribution from P&C and Corporate and Other									(929,872)	[1]	168,846	[1]	(30,733)	[1]
Interest expense									121,221	[1],[9]	107,486	[1],[9]	105,925	[1],[9]
Non-controlling interests									80,498	[1]	76,731	[1]	79,255	[1]
Provision (benefit) for income tax									96,897	[1]	77,505	[1]	34,028	[1]
Corporate and other | Exclude Life Funds Withheld Assets [Member]
Segment Reporting Information [Line Items]
Net investment income									146,558	[1],[3]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									3,625	[1]
Corporate and other | Life Funds Withheld Assets [Member]
Segment Reporting Information [Line Items]
Net investment income									129,575	[1]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									(15,529)	[1]
Net realized and unrealized (gains) losses on derivative instruments									(488,222)	[1]
Operating Segments [Member] | Total P And C Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									7,761,490		7,416,792		7,175,130
Net premiums written									5,767,209		5,903,982		5,957,019
Net premiums earned	1,413,161	1,453,673	1,438,076	1,412,528	1,510,727	1,550,629	1,488,497	1,464,246	5,717,438		6,014,099		5,765,978
Net losses and loss expenses									3,258,393		3,731,464		3,765,482
Acquisition costs									724,003		882,658		872,399
Operating expenses									1,058,996	[2]	948,915	[2]	911,965	[2]
Underwriting profit (loss)	218,851	144,375	167,946	144,874	101,450	76,900	92,124	180,588	676,046		451,062		216,132
Net investment income											599,144	[3]	641,237	[3]
Net results from structured products									54,209	[4]	23,791	[5]	5,257	[6]
Net fee income and other									(7,251)	[7]	(6,997)	[7]	3,342	[7]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments											85,792		35,101
Net realized and unrealized (gains) losses on derivative instruments									0				0
Contribution from P&C and Corporate and Other									1,416,828		1,152,792		901,069
Ratios - P&C operations:
Loss and loss expense ratio									57.00%	[10]	62.00%	[10]	65.30%	[10]
Underwriting expense ratio									31.20%	[10]	30.50%	[10]	31.00%	[10]
Combined ratio									88.20%	[10]	92.50%	[10]	96.30%	[10]
Operating Segments [Member] | Total P And C Segment [Member] | Exclude Life Funds Withheld Assets [Member]
Segment Reporting Information [Line Items]
Net investment income									574,458	[3]
Total net realized gains (losses) on investments, and net unrealized gains (losses) on investments									119,366
Operating Segments [Member] | Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									5,976,011		5,523,181		5,166,973
Net premiums written									4,134,151		4,154,093		4,072,511
Net premiums earned									4,026,713		4,267,677		3,924,636
Net losses and loss expenses									2,543,108		2,829,999		2,691,056
Acquisition costs									393,319		529,270		504,227
Operating expenses									865,592	[2]	782,677	[2]	754,308	[2]
Underwriting profit (loss)									224,694		125,731		(24,955)
Net results from structured products									43,710	[4]	15,562	[5]	20,978	[6]
Net fee income and other									(10,051)	[7]	(9,317)	[7]	850	[7]
Ratios - P&C operations:
Loss and loss expense ratio									63.20%	[10]	66.30%	[10]	68.60%	[10]
Underwriting expense ratio									31.20%	[10]	30.80%	[10]	32.00%	[10]
Combined ratio									94.40%	[10]	97.10%	[10]	100.60%	[10]
Operating Segments [Member] | Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									1,785,479		1,893,611		2,008,157
Net premiums written									1,633,058		1,749,889		1,884,508
Net premiums earned									1,690,725		1,746,422		1,841,342
Net losses and loss expenses									715,285		901,465		1,074,426
Acquisition costs									330,684		353,388		368,172
Operating expenses									193,404	[2]	166,238	[2]	157,657	[2]
Underwriting profit (loss)									451,352		325,331		241,087
Net results from structured products									10,499	[4]	8,229	[5]	(15,721)	[6]
Net fee income and other									$ 2,800	[7]	$ 2,320	[7]	$ 2,492	[7]
Ratios - P&C operations:
Loss and loss expense ratio									42.30%	[10]	51.60%	[10]	58.40%	[10]
Underwriting expense ratio									31.00%	[10]	29.80%	[10]	28.50%	[10]
Combined ratio									73.30%	[10]	81.40%	[10]	86.90%	[10]

[1] 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.

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

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

[4] The net results from P&C structured products include net investment income and interest expense of $68.0 million and $12.9 million, respectively.

[5] The net results from P&C structured products include net investment income and interest expense of $71.9 million and $48.0 million, respectively.

[6] The net results from P&C structured products include net investment income and interest expense of $71.7 million and $66.3 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] Ratios are based on net premiums earned from P&C operations.