Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$ 2,025,472		$ 1,856,355		$ 4,503,358		$ 4,260,022
Net premiums written	1,533,868		1,432,900		3,641,522		3,474,624
Net premiums earned	1,562,393		1,486,593		3,092,084		2,923,010
Net losses and loss expenses	1,054,373		957,503		2,008,410		1,932,875
Acquisition costs	231,886		218,937		452,145		443,087
Operating expenses	236,342	[1]	237,066	[1]	463,355	[1]	463,767	[1]
Underwriting profit (loss)	39,792		73,087		168,174		83,281
Net investment income	214,410		245,225		443,137		493,219
Net results from structured products	6,053	[2]	(13,759)	[3]	11,229	[4]	(8,502)	[5]
Net fee income and other	(791)	[6]	1,288	[6]	(1,055)	[6]	(351)	[6]
Net realized gains (losses) on investments	40,968		(12,393)		77,477		8,409
Contribution from P&C and Life Operations	300,432		293,448		698,962		576,056
Corporate & other:
Net realized and unrealized gains (losses) on derivative instruments	(5,105)		(4,301)		2,780		(3,598)
Net income (loss) from investment fund affiliates and operating affiliates	79,368		23,554		142,587	[7]	59,216
Exchange (gains) losses	(11,331)		(17,976)		(44,766)		(5,258)
Corporate operating expenses	55,155		50,062		102,078		94,322
Interest expense	26,121		26,194		52,257		53,454
Non-controlling interests	3,180		3,455		38,922		40,012
Income tax	28,872		29,812		72,351		51,362
Net income (loss) attributable to ordinary shareholders	272,698		221,154		623,487		397,782
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	1,944,170		1,763,452		4,349,086		4,080,436
Net premiums written	1,459,972		1,347,277		3,502,181		3,310,505
Net premiums earned	1,488,497		1,400,970		2,952,743		2,758,867
Net losses and loss expenses	937,606		826,354		1,780,690		1,680,420
Acquisition costs	224,631		211,007		440,787		427,506
Operating expenses	234,136	[1]	234,237	[1]	458,554	[1]	458,331	[1]
Underwriting profit (loss)	92,124		129,372		272,712		192,610
Net investment income	143,818		170,579		300,857		343,547
Net results from structured products	6,053	[2]	(13,759)	[3]	11,229	[4]	(8,502)	[5]
Net fee income and other	(835)	[6]	1,246	[6]	(2,142)	[6]	(441)	[6]
Net realized gains (losses) on investments	29,369		(2,154)		74,947		22,812
Contribution from P&C and Life Operations	270,529		285,284		657,603		550,026
Ratios - P&C operations
Loss and loss expense ratio	63.00%	[8]	59.00%	[8]	60.30%	[8]	60.90%	[8]
Underwriting expense ratio	30.80%	[8]	31.80%	[8]	30.50%	[8]	32.10%	[8]
Combined ratio	93.80%	[8]	90.80%	[8]	90.80%	[8]	93.00%	[8]
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,472,388		1,311,035		2,970,175		2,647,669
Net premiums written	1,049,163		944,266		2,238,351		1,980,792
Net premiums earned	1,058,542		959,293		2,097,634		1,893,348
Net losses and loss expenses	721,323		635,282		1,371,840		1,266,968
Acquisition costs	138,032		123,285		268,654		251,540
Operating expenses	194,514	[1]	192,246	[1]	378,511	[1]	377,593	[1]
Underwriting profit (loss)	4,673		8,480		78,629		(2,753)
Net results from structured products	3,532	[2]	9,047	[3]	7,717	[4]	11,866	[5]
Net fee income and other	(1,355)	[6]	256	[6]	(3,294)	[6]	(1,764)	[6]
Ratios - P&C operations
Loss and loss expense ratio	68.10%	[8]	66.20%	[8]	65.40%	[8]	66.90%	[8]
Underwriting expense ratio	31.50%	[8]	32.90%	[8]	30.90%	[8]	33.20%	[8]
Combined ratio	99.60%	[8]	99.10%	[8]	96.30%	[8]	100.10%	[8]
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	471,782		452,417		1,378,911		1,432,767
Net premiums written	410,809		403,011		1,263,830		1,329,713
Net premiums earned	429,955		441,677		855,109		865,519
Net losses and loss expenses	216,283		191,072		408,850		413,452
Acquisition costs	86,599		87,722		172,133		175,966
Operating expenses	39,622	[1]	41,991	[1]	80,043	[1]	80,738	[1]
Underwriting profit (loss)	87,451		120,892		194,083		195,363
Net results from structured products	2,521	[2]	(22,806)	[3]	3,512	[4]	(20,368)	[5]
Net fee income and other	520	[6]	990	[6]	1,152	[6]	1,323	[6]
Ratios - P&C operations
Loss and loss expense ratio	50.30%	[8]	43.30%	[8]	47.80%	[8]	47.80%	[8]
Underwriting expense ratio	29.40%	[8]	29.30%	[8]	29.50%	[8]	29.60%	[8]
Combined ratio	79.70%	[8]	72.60%	[8]	77.30%	[8]	77.40%	[8]
Life Operations Segment
Segment Reporting Information [Line Items]
Gross premiums written	81,302		92,903		154,272		179,586
Net premiums written	73,896		85,623		139,341		164,119
Net premiums earned	73,896		85,623		139,341		164,143
Net losses and loss expenses	116,767		131,149		227,720		252,455
Acquisition costs	7,255		7,930		11,358		15,581
Operating expenses	2,206	[1]	2,829	[1]	4,801	[1]	5,436	[1]
Underwriting profit (loss)	(52,332)		(56,285)		(104,538)		(109,329)
Net investment income	70,592		74,646		142,280		149,672
Net results from structured products	0	[2]	0	[3]	0	[4]	0	[5]
Net fee income and other	44	[6]	42	[6]	1,087	[6]	90	[6]
Net realized gains (losses) on investments	11,599		(10,239)		2,530		(14,403)
Contribution from P&C and Life Operations	$ 29,903		$ 8,164		$ 41,359		$ 26,030

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

[2] The net results from P&C structured products include net investment income and interest expense of $18.1 million and $12.1 million, respectively

[3] The net results from P&C structured products include net investment income and interest expense of $17.4 million and $31.2 million, respectively.

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

[5] The net results from P&C structured products include net investment income and interest expense of $34.7 million and $43.2 million, respectively.

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

[7] 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.

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