Segment Information (Details) - Statement of earnings by segment (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Gross premiums written									$ 7,530,884		$ 7,292,839		$ 6,673,269
Net premiums written									6,281,453		5,795,750		5,381,663
Net premiums earned									6,090,441		5,690,130		5,414,061
Net losses and loss expenses									(4,251,680)		(4,613,465)		(3,725,633)
Acquisition costs									(913,492)		(826,411)		(788,258)
Operating expenses									(921,299)	[1]	(869,292)	[1]	(828,159)	[1]
Underwriting profit (loss)	(90,677)	114,203	129,372	63,240	(114,138)	(22,200)	67,049	(328,064)	3,970		(619,038)		72,011
Net investment income									940,679		1,063,199		1,116,530
Net results from structured products									5,098	[2]	23,029	[2]	27,575	[3]
Net fee income and other									1,848	[4]	(12,248)	[5]	(12,825)	[5]
Net realized gains (losses) on investments									14,098		(188,359)		(270,803)
Contribution from P&C and life operations									965,693		266,583		932,488
Corporate & other:
Net realized and unrealized (gains) losses on derivative instruments									5,221		(10,738)		(33,843)
Net income (loss) from investment fund affiliates and operating affiliates									114,314		103,039		172,474
Exchange gains (losses)									(10,546)		40,640		10,161
Corporate operating expenses									(204,500)		(160,596)		(92,544)
Interest expense									(105,766)	[6]	(154,084)	[6]	(159,118)
Non-controlling interests									(79,254)		(70,877)		(39,831)
Loss on termination of guarantee									0		0		(23,500)
Impairment of goodwill									0		(429,020)		0
Income tax									(34,028)		(59,707)		(162,737)
Net income (loss) attributable to XL Group plc and ordinary shareholders									651,134		(474,760)		603,550
Total P And C Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									7,175,130		6,898,284		6,261,331
Net premiums written									5,957,021		5,433,388		4,999,588
Net premiums earned	1,538,559	1,468,553	1,400,972	1,357,898	1,386,759	1,362,532	1,306,125	1,271,696	5,765,982		5,327,112		5,031,137
Net losses and loss expenses									(3,765,482)		(4,078,391)		(3,211,800)
Acquisition costs									(872,399)		(786,093)		(739,154)
Operating expenses									(911,963)	[1]	(859,981)	[1]	(817,689)	[1]
Underwriting profit (loss)									216,138		(397,353)		262,494
Net investment income									641,237		745,138		803,358
Net results from structured products									5,098	[2]	23,029	[2]	17,771	[3]
Net fee income and other									1,422	[4]	(12,467)	[5]	(13,076)	[5]
Net realized gains (losses) on investments									35,101		(98,360)		(206,877)
Contribution from P&C and life operations									898,996		259,987		863,670
Ratios - P&C operations: (6)
Loss and loss expense ratio									65.30%	[7]	76.60%	[7]	63.80%	[7]
Underwriting expense ratio									31.00%	[7]	30.90%	[7]	31.00%	[7]
Combined ratio									96.30%	[7]	107.50%	[7]	94.80%	[7]
Insurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									5,166,974		4,824,665		4,418,380
Net premiums written									4,072,513		3,707,664		3,461,150
Net premiums earned									3,924,640		3,663,727		3,529,138
Net losses and loss expenses									(2,691,056)		(2,951,413)		(2,505,502)
Acquisition costs									(504,227)		(461,965)		(418,146)
Operating expenses									(754,306)	[1]	(683,814)	[1]	(642,103)	[1]
Underwriting profit (loss)									(24,949)		(433,465)		(36,613)
Net results from structured products									20,980	[2]	10,976	[2]	14,696	[3]
Net fee income and other									(1,071)	[4]	(16,370)	[5]	(15,564)	[5]
Ratios - P&C operations: (6)
Loss and loss expense ratio									68.60%	[7]	80.60%	[7]	71.00%	[7]
Underwriting expense ratio									32.00%	[7]	31.20%	[7]	30.00%	[7]
Combined ratio									100.60%	[7]	111.80%	[7]	101.00%	[7]
Reinsurance Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									2,008,156		2,073,619		1,842,951
Net premiums written									1,884,508		1,725,724		1,538,438
Net premiums earned									1,841,342		1,663,385		1,501,999
Net losses and loss expenses									(1,074,426)		(1,126,978)		(706,298)
Acquisition costs									(368,172)		(324,128)		(321,008)
Operating expenses									(157,657)	[1]	(176,167)	[1]	(175,586)	[1]
Underwriting profit (loss)									241,087		36,112		299,107
Net results from structured products									(15,882)	[2]	12,053	[2]	3,075	[3]
Net fee income and other									2,493	[4]	3,903	[5]	2,488	[5]
Ratios - P&C operations: (6)
Loss and loss expense ratio									58.40%	[7]	67.80%	[7]	47.00%	[7]
Underwriting expense ratio									28.50%	[7]	30.00%	[7]	33.10%	[7]
Combined ratio									86.90%	[7]	97.80%	[7]	80.10%	[7]
Life Operations Segment [Member]
Segment Reporting Information [Line Items]
Gross premiums written									355,754		394,555		411,938
Net premiums written									324,432		362,362		382,075
Net premiums earned	80,829	79,487	85,623	78,520	90,323	90,794	92,214	89,687	324,459		363,018		382,924
Net losses and loss expenses									(486,198)		(535,074)		(513,833)
Acquisition costs									(41,093)		(40,318)		(49,104)
Operating expenses									(9,336)	[1]	(9,311)	[1]	(10,470)	[1]
Underwriting profit (loss)									(212,168)		(221,685)		(190,483)
Net investment income									299,442		318,061		313,172
Net fee income and other									426	[4]	219	[5]	249	[5]
Net realized gains (losses) on investments									(21,003)		(89,999)		(54,444)
Contribution from P&C and life operations									66,697		6,596		68,494
Corporate And Other [Member]
Segment Reporting Information [Line Items]
Net results from structured products													9,804	[3]
Net fee income and other													2	[5]
Net realized gains (losses) on investments													(9,482)
Contribution from P&C and life operations													324
Corporate & other:
Net realized and unrealized (gains) losses on derivative instruments													(33,843)
Net income (loss) from investment fund affiliates and operating affiliates													172,474	[8]
Exchange gains (losses)													10,161
Corporate operating expenses													(92,544)
Interest expense													(159,118)	[6]
Non-controlling interests													(39,831)
Loss on termination of guarantee													(23,500)
Income tax													$ (162,737)

[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] The net results from P&C structured products include net investment income, interest expense and operating expenses.

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

[5] 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.

[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.

[8] 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.