Segment Information (Details) - Statement of earnings by segment (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Segment Reporting Information [Line Items]
Gross premiums written	$ 1,856,355		$ 1,862,724		$ 4,260,022		$ 4,059,503
Net premiums written	1,432,901		1,398,253		3,474,625		3,202,207
Net premiums earned	1,486,595		1,398,339		2,923,013		2,759,722
Net losses and loss expenses	(957,505)		(961,000)		(1,932,877)		(2,303,096)
Acquisition costs	(218,937)		(215,099)		(443,088)		(403,589)
Operating expenses	(237,067)	[1]	(212,884)	[1]	(463,767)	[1]	(424,775)	[1]
Underwriting profit (loss)	73,086		9,356		83,281		(371,738)
Net investment income	245,226		278,110		493,220		538,651
Net results from structured products	(13,866)	[2]	4,916	[2]	(8,549)	[2]	12,390	[2]
Net fee income and other	(815)	[3]	(3,131)	[4]	(2,454)	[3]	(7,608)	[4]
Net realized gains (losses) on investments	(12,393)		(9,544)		8,410		(75,981)
Contribution from P&C and life operations	291,238		279,707		573,908		95,714
Corporate & other:
Derivative Gain Loss On Derivative Net	(4,300)		(10,950)		(3,598)		(7,383)
Net income (loss) from investment fund affiliates and operating affiliates	25,658	[5]	56,501	[5]	61,319	[5]	97,287	[5]
Exchange gains (losses)	17,976		8,498		5,258		(1,016)
Corporate operating expenses	(50,061)		(39,566)		(94,321)		(75,258)
Interest expense	(26,089)	[6]	(41,599)	[6]	(53,409)	[6]	(83,498)	[6]
Non-controlling interest in net income (loss) of subsidiary	(3,454)		(2,102)		(40,011)		(35,438)
Income taxes & other	(29,812)		(24,826)		(51,362)		7,971
Net income (loss) attributable to XL Group plc and ordinary shareholders	221,156		225,663		397,784		(1,621)
Total P And C Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	1,763,451		1,762,443		4,080,435		3,861,563
Net premiums written	1,347,278		1,306,059		3,310,506		3,020,341
Net premiums earned	1,400,972		1,306,125		2,758,870		2,577,821
Net losses and loss expenses	(826,355)		(823,584)		(1,680,420)		(2,032,449)
Acquisition costs	(211,007)		(205,331)		(427,507)		(386,501)
Operating expenses	(234,238)	[1]	(210,161)	[1]	(458,331)	[1]	(419,886)	[1]
Underwriting profit (loss)	129,372		67,049		192,612		(261,015)
Net investment income	170,581		196,053		343,549		379,618
Net results from structured products	(13,866)	[2]	4,916	[2]	(8,549)	[2]	12,390	[2]
Net fee income and other	(857)	[3]	(3,227)	[4]	(2,544)	[3]	(7,745)	[4]
Net realized gains (losses) on investments	(2,154)		(10,248)		22,813		(37,134)
Contribution from P&C and life operations	283,076		254,543		547,881		86,114
Ratios - P&C operations: (6)
Loss and loss expense ratio	59.00%	[7]	63.10%	[7]	60.90%	[7]	78.80%	[7]
Underwriting expense ratio	31.80%	[7]	31.80%	[7]	32.10%	[7]	31.30%	[7]
Combined ratio	90.80%	[7]	94.90%	[7]	93.00%	[7]	110.10%	[7]
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	1,311,034		1,290,030		2,647,668		2,512,379
Net premiums written	944,267		893,191		1,980,793		1,812,181
Net premiums earned	959,294		907,443		1,893,350		1,783,363
Net losses and loss expenses	(635,284)		(608,182)		(1,266,969)		(1,396,695)
Acquisition costs	(123,284)		(113,883)		(251,540)		(221,527)
Operating expenses	(192,247)	[1]	(166,608)	[1]	(377,593)	[1]	(330,703)	[1]
Underwriting profit (loss)	8,479		18,770		(2,752)		(165,562)
Net investment income	0		0		0		0
Net results from structured products	9,047	[2]	2,690	[2]	11,866	[2]	5,950	[2]
Net fee income and other	(1,847)	[3]	(3,218)	[4]	(3,867)	[3]	(9,130)	[4]
Ratios - P&C operations: (6)
Loss and loss expense ratio	66.20%	[7]	67.00%	[7]	66.90%	[7]	78.30%	[7]
Underwriting expense ratio	32.90%	[7]	30.90%	[7]	33.20%	[7]	31.00%	[7]
Combined ratio	99.10%	[7]	97.90%	[7]	100.10%	[7]	109.30%	[7]
Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	452,417		472,413		1,432,767		1,349,184
Net premiums written	403,011		412,868		1,329,713		1,208,160
Net premiums earned	441,678		398,682		865,520		794,458
Net losses and loss expenses	(191,071)		(215,402)		(413,451)		(635,754)
Acquisition costs	(87,723)		(91,448)		(175,967)		(164,974)
Operating expenses	(41,991)	[1]	(43,553)	[1]	(80,738)	[1]	(89,183)	[1]
Underwriting profit (loss)	120,893		48,279		195,364		(95,453)
Net investment income	0		0		0		0
Net results from structured products	(22,913)	[2]	2,226	[2]	(20,415)	[2]	6,440	[2]
Net fee income and other	990	[3]	(9)	[4]	1,323	[3]	1,385	[4]
Ratios - P&C operations: (6)
Loss and loss expense ratio	43.30%	[7]	54.00%	[7]	47.80%	[7]	80.00%	[7]
Underwriting expense ratio	29.30%	[7]	33.90%	[7]	29.60%	[7]	32.00%	[7]
Combined ratio	72.60%	[7]	87.90%	[7]	77.40%	[7]	112.00%	[7]
Life Operations Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written	92,904		100,281		179,587		197,940
Net premiums written	85,623		92,194		164,119		181,866
Net premiums earned	85,623		92,214		164,143		181,901
Net losses and loss expenses	(131,150)		(137,416)		(252,457)		(270,647)
Acquisition costs	(7,930)		(9,768)		(15,581)		(17,088)
Operating expenses	(2,829)	[1]	(2,723)	[1]	(5,436)	[1]	(4,889)	[1]
Underwriting profit (loss)	(56,286)		(57,693)		(109,331)		(110,723)
Net investment income	74,645		82,057		149,671		159,033
Net fee income and other	42	[3]	96	[4]	90	[3]	137	[4]
Net realized gains (losses) on investments	(10,239)		704		(14,403)		(38,847)
Contribution from P&C and life operations	$ 8,162		$ 25,164		$ 26,027		$ 9,600

[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.