Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,684,198		$ 1,692,131		$ 6,395,373		$ 6,195,489
Net premiums written	1,233,985		1,402,062		4,721,680		5,043,584
Net premiums earned	1,473,412		1,625,312		4,458,845		4,717,396
Net losses and loss expenses	879,689		1,123,069		2,737,960		3,131,479
Acquisition costs	182,882		238,328		566,915		690,473
Operating expenses	271,703		237,650		782,527		701,005
Underwriting profit (loss)	139,138		26,265		371,443		194,439
Net results from structured products	5,291		6,463		48,129		17,692
Net fee income and other	(3,058)		(1,547)		(6,410)		(2,602)
Loss on sale of life reinsurance subsidiary	0		0		666,423		0
Net realized gains (losses) on investments	4,341		(1,958)		96,267		75,519
Net realized and unrealized gains (losses) on derivative instruments	5,131		880		18,540		3,660
Net income (loss) from investment fund affiliates and operating affiliates	44,521		29,669		169,530		172,256
Exchange (gains) losses	(23,348)		20,303		8,234		(24,463)
Corporate operating expenses	55,322		61,427		163,155		163,504
Contribution from P&C and Corporate and Other	171,727		197,909		280,735		984,927
Interest expense	31,703		26,342		96,147		78,599
Non-controlling interests	37,583		35,022		77,024		73,944
Income tax	30,057		897		58,724		73,248
Net income (loss) attributable to ordinary shareholders	72,384		135,648		48,840		759,136
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Gross premiums written	83,496	[1]	82,291	[1]	254,503	[1]	236,563	[1]
Net premiums written	19,739	[1]	74,683	[1]	154,568	[1]	214,024	[1]
Net premiums earned	19,739	[1]	74,683	[1]	154,568	[1]	214,024	[1]
Net losses and loss expenses	20,101	[1]	116,549	[1]	218,987	[1]	344,269	[1]
Acquisition costs	2,090	[1]	6,110	[1]	13,151	[1]	17,468	[1]
Operating expenses	2,785	[1],[2]	2,659	[1],[2]	8,182	[1],[2]	7,460	[1],[2]
Underwriting profit (loss)	(5,237)	[1]	(50,635)	[1]	(85,752)	[1]	(155,173)	[1]
Net results from structured products	0	[1],[3]	0	[1],[4]	0	[1],[5]	0	[1],[6]
Net fee income and other	167	[1],[7]	43	[1],[7]	257	[1],[7]	1,130	[1],[7]
Loss on sale of life reinsurance subsidiary					666,423	[1]
Net realized and unrealized gains (losses) on derivative instruments	5,131	[1]	880	[1]	18,540	[1]	3,660	[1]
Net income (loss) from investment fund affiliates and operating affiliates	44,521	[1],[8]	29,669	[1],[8]	169,530	[1],[8]	172,256	[1],[8]
Exchange (gains) losses	(23,348)	[1]	20,303	[1]	8,234	[1]	(24,463)	[1]
Corporate operating expenses	55,322	[1]	61,427	[1]	163,155	[1]	163,504	[1]
Contribution from P&C and Corporate and Other	(125,707)	[1]	(29,516)	[1]	(753,168)	[1]	99,899	[1]
Interest expense	31,703	[1],[9]	26,342	[1],[9]	96,147	[1],[9]	78,599	[1],[9]
Non-controlling interests	37,583	[1]	35,022	[1]	77,024	[1]	73,944	[1]
Income tax	30,057	[1]	897	[1]	58,724	[1]	73,248	[1]
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	1,600,702		1,609,840		6,140,870		5,958,926
Net premiums written	1,214,246		1,327,379		4,567,112		4,829,560
Net premiums earned	1,453,673		1,550,629		4,304,277		4,503,372
Net losses and loss expenses	859,588		1,006,520		2,518,973		2,787,210
Acquisition costs	180,792		232,218		553,764		673,005
Operating expenses	268,918	[2]	234,991	[2]	774,345	[2]	693,545	[2]
Underwriting profit (loss)	144,375		76,900		457,195		349,612
Net results from structured products	5,291	[3]	6,463	[4]	48,129	[5]	17,692	[6]
Net fee income and other	(3,225)	[7]	(1,590)	[7]	(6,667)	[7]	(3,732)	[7]
Contribution from P&C and Corporate and Other	297,434		227,425		1,033,903		885,028
Ratios - P&C operations
Loss and loss expense ratio	59.10%	[10]	64.90%	[10]	58.50%	[10]	61.90%	[10]
Underwriting expense ratio	31.00%	[10]	30.10%	[10]	30.90%	[10]	30.30%	[10]
Combined ratio	90.10%	[10]	95.00%	[10]	89.40%	[10]	92.20%	[10]
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,324,418		1,259,986		4,513,749		4,230,161
Net premiums written	956,185		993,659		3,080,432		3,232,010
Net premiums earned	1,018,416		1,104,012		3,014,846		3,201,646
Net losses and loss expenses	650,256		762,782		1,917,076		2,134,622
Acquisition costs	95,992		136,421		300,855		405,075
Operating expenses	218,281	[2]	192,936	[2]	635,265	[2]	571,447	[2]
Underwriting profit (loss)	53,887		11,873		161,650		90,502
Net results from structured products	3,571	[3]	4,104	[4]	40,106	[5]	11,821	[6]
Net fee income and other	(3,826)	[7]	(2,187)	[7]	(8,605)	[7]	(5,481)	[7]
Ratios - P&C operations
Loss and loss expense ratio	63.80%	[10]	69.10%	[10]	63.60%	[10]	66.70%	[10]
Underwriting expense ratio	30.90%	[10]	29.80%	[10]	31.00%	[10]	30.50%	[10]
Combined ratio	94.70%	[10]	98.90%	[10]	94.60%	[10]	97.20%	[10]
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	276,284		349,854		1,627,121		1,728,765
Net premiums written	258,061		333,720		1,486,680		1,597,550
Net premiums earned	435,257		446,617		1,289,431		1,301,726
Net losses and loss expenses	209,332		243,738		601,897		652,588
Acquisition costs	84,800		95,797		252,909		267,930
Operating expenses	50,637	[2]	42,055	[2]	139,080	[2]	122,098	[2]
Underwriting profit (loss)	90,488		65,027		295,545		259,110
Net results from structured products	1,720	[3]	2,359	[4]	8,023	[5]	5,871	[6]
Net fee income and other	601	[7]	597	[7]	1,938	[7]	1,749	[7]
Ratios - P&C operations
Loss and loss expense ratio	48.10%	[10]	54.60%	[10]	46.70%	[10]	50.10%	[10]
Underwriting expense ratio	31.10%	[10]	30.80%	[10]	30.40%	[10]	30.00%	[10]
Combined ratio	79.20%	[10]	85.40%	[10]	77.10%	[10]	80.10%	[10]
Excluding Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	153,127		219,867		564,219		663,004
Net realized gains (losses) on investments	9,813		(1,958)		109,886		75,519
Excluding Life Funds Withheld Assets | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	11,552	[1],[11]	72,128	[1],[11]	134,562	[1],[11]	214,408	[1],[11]
Net realized gains (losses) on investments	395	[1]	129	[1]	4,297	[1]	2,659	[1]
Excluding Life Funds Withheld Assets | Total P&C
Segment Reporting Information [Line Items]
Net investment income	141,575	[11]	147,739	[11]	429,657	[11]	448,596	[11]
Net realized gains (losses) on investments	9,418		(2,087)		105,589		72,860
Life Funds Withheld Assets
Segment Reporting Information [Line Items]
Net investment income	56,474				75,639
Net realized gains (losses) on investments	(5,472)		0		(13,619)		0
Net realized and unrealized gains (losses) on derivative instruments	(201,264)		0		(218,810)		0
Life Funds Withheld Assets | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income	56,474	[1]			75,639	[1]
Net realized gains (losses) on investments	(5,472)	[1]			(13,619)	[1]
Net realized and unrealized gains (losses) on derivative instruments	$ (201,264)	[1]			$ (218,810)	[1]

[1] Corporate and Other includes other items of our revenue and expenditure 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] The net results from structured products include net investment income and interest expense of $16.8 million and $11.1 million, respectively.

[4] The net results from structured products include net investment income and interest expense of $18.1 million and $11.6 million, respectively.

[5] The net results from structured products include net investment income and net interest expense of $52.5 million and $3.7 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 $53.9 million and $36.2 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.

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