Segment Information (Details) - Statement of earnings by segment (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,663,644		$ 1,876,725		$ 5,923,666		$ 5,936,228
Net premiums written	1,387,751		1,383,475		4,862,376		4,585,682
Net premiums earned	1,548,040		1,453,326		4,471,053		4,213,048
Net losses and loss expenses	(1,013,515)		(1,127,608)		(2,946,392)		(3,430,704)
Acquisition costs	(236,347)		(207,553)		(679,435)		(611,142)
Operating expenses	(236,373)	[1]	(200,665)	[1]	(700,140)	[1]	(625,440)	[1]
Underwriting profit (loss)	61,805		(82,500)		145,086		(454,238)
Net investment income	221,961		271,235		715,181		809,886
Net results from structured products	5,701	[2]	5,845	[2]	(2,848)	[2]	18,235	[2]
Net fee income and other	1,488	[3]	(2,323)	[4]	(966)	[3]	(9,931)	[4]
Net realized gains (losses) on investments	(5,493)		(62,354)		2,917		(138,335)
Contribution from P&C and life operations	285,462		129,903		859,370		225,617
Corporate & other:
Net realized & unrealized (gains) losses on derivative instruments	6,254		(26,916)		2,656		(34,299)
Net income (loss) from investment fund affiliates and operating affiliates	18,967	[5]	28,429	[5]	80,286	[5]	125,716	[5]
Exchange gains (losses)	(18,690)		39,425		(13,432)		38,409
Corporate operating expenses	(57,604)		(33,847)		(151,925)		(109,105)
Interest expense	(26,192)	[6]	(35,885)	[6]	(79,601)	[6]	(119,383)	[6]
Non-controlling interests	(35,844)		(33,993)		(75,855)		(69,431)
Income tax	(451)		(24,718)		(51,813)		(16,747)
Net income (loss) attributable to XL Group plc and ordinary shareholders	171,902		42,398		569,686		40,777
Total P And C Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	1,576,231		1,776,161		5,656,666		5,637,724
Net premiums written	1,308,264		1,292,706		4,618,770		4,313,047
Net premiums earned	1,468,553		1,362,532		4,227,423		3,940,353
Net losses and loss expenses	(894,076)		(988,059)		(2,574,496)		(3,020,508)
Acquisition costs	(226,253)		(198,360)		(653,760)		(584,861)
Operating expenses	(234,021)	[1]	(198,313)	[1]	(692,352)	[1]	(618,199)	[1]
Underwriting profit (loss)	114,203		(22,200)		306,815		(283,215)
Net investment income	148,482		190,886		492,031		570,504
Net results from structured products	5,701	[2]	5,845	[2]	(2,848)	[2]	18,235	[2]
Net fee income and other	1,248	[3]	(2,364)	[4]	(1,296)	[3]	(10,109)	[4]
Net realized gains (losses) on investments	2,257		(39,047)		25,070		(76,181)
Contribution from P&C and life operations	271,891		133,120		819,772		219,234
Ratios - P&C operations: (6)
Loss and loss expense ratio	60.90%	[7]	72.50%	[7]	60.90%	[7]	76.70%	[7]
Underwriting expense ratio	31.30%	[7]	29.10%	[7]	31.80%	[7]	30.50%	[7]
Combined ratio	92.20%	[7]	101.60%	[7]	92.70%	[7]	107.20%	[7]
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	1,158,683		1,156,901		3,806,351		3,669,280
Net premiums written	905,054		872,757		2,885,847		2,684,938
Net premiums earned	985,623		930,067		2,878,973		2,713,430
Net losses and loss expenses	(623,543)		(767,091)		(1,890,512)		(2,163,786)
Acquisition costs	(128,514)		(118,521)		(380,054)		(340,048)
Operating expenses	(186,794)	[1]	(158,134)	[1]	(564,387)	[1]	(488,837)	[1]
Underwriting profit (loss)	46,772		(113,679)		44,020		(279,241)
Net results from structured products	3,087	[2]	2,457	[2]	14,953	[2]	8,407	[2]
Net fee income and other	591	[3]	(3,484)	[4]	(3,276)	[3]	(12,614)	[4]
Ratios - P&C operations: (6)
Loss and loss expense ratio	63.30%	[7]	82.50%	[7]	65.70%	[7]	79.70%	[7]
Underwriting expense ratio	32.00%	[7]	29.70%	[7]	32.80%	[7]	30.60%	[7]
Combined ratio	95.30%	[7]	112.20%	[7]	98.50%	[7]	110.30%	[7]
Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	417,548		619,260		1,850,315		1,968,444
Net premiums written	403,210		419,949		1,732,923		1,628,109
Net premiums earned	482,930		432,465		1,348,450		1,226,923
Net losses and loss expenses	(270,533)		(220,968)		(683,984)		(856,722)
Acquisition costs	(97,739)		(79,839)		(273,706)		(244,813)
Operating expenses	(47,227)	[1]	(40,179)	[1]	(127,965)	[1]	(129,362)	[1]
Underwriting profit (loss)	67,431		91,479		262,795		(3,974)
Net results from structured products	2,614	[2]	3,388	[2]	(17,801)	[2]	9,828	[2]
Net fee income and other	657	[3]	1,120	[4]	1,980	[3]	2,505	[4]
Ratios - P&C operations: (6)
Loss and loss expense ratio	56.00%	[7]	51.10%	[7]	50.70%	[7]	69.80%	[7]
Underwriting expense ratio	30.00%	[7]	27.70%	[7]	29.80%	[7]	30.50%	[7]
Combined ratio	86.00%	[7]	78.80%	[7]	80.50%	[7]	100.30%	[7]
Life Operations Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	87,413		100,564		267,000		298,504
Net premiums written	79,487		90,769		243,606		272,635
Net premiums earned	79,487		90,794		243,630		272,695
Net losses and loss expenses	(119,439)		(139,549)		(371,896)		(410,196)
Acquisition costs	(10,094)		(9,193)		(25,675)		(26,281)
Operating expenses	(2,352)	[1]	(2,352)	[1]	(7,788)	[1]	(7,241)	[1]
Underwriting profit (loss)	(52,398)		(60,300)		(161,729)		(171,023)
Net investment income	73,479		80,349		223,150		239,382
Net fee income and other	240	[3]	41	[4]	330	[3]	178	[4]
Net realized gains (losses) on investments	(7,750)		(23,307)		(22,153)		(62,154)
Contribution from P&C and life operations	$ 13,571		$ (3,217)		$ 39,598		$ 6,383

[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.

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

[4] Net fee income and other includes operating expenses from the Company's loss prevention consulting services business and expenses related to the cost of an endorsement facility with National Indemnity Company.

[5] 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.

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

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