SEGMENT INFORMATION (Detail) - Statement of Earnings by Segment (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Gross premiums written	$ 7,292,839		$ 6,673,269		$ 6,687,473
Net premiums written	5,795,750		5,381,663		5,276,564
Net premiums earned	5,690,130		5,414,061		5,706,840
Net losses and loss expenses	(4,613,465)		(3,725,633)		(3,846,399)
Acquisition costs	(826,411)		(788,258)		(853,558)
Operating expenses	(869,292)	[1]	(828,159)	[1]	(895,736)	[1]
Underwriting profit (loss)	(619,038)		72,011		111,147
Net investment income	1,063,199		1,116,530		1,215,173
Net results from structured products	23,029	[2]	27,575	[3]	59,643	[4]
Net fee income and other	(12,248)	[5]	(12,825)	[5]	(7,742)	[5]
Net realized gains (losses) on investments	(188,359)		(270,803)		(921,437)
Contribution from P&C, Life Operations and Corporate	266,583		932,488		456,784
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(10,738)		(33,843)		(33,647)
Net income (loss) from investment fund affiliates and operating affiliates	103,039		172,474		139,347
Exchange gains (losses)	40,640		10,161		(84,813)
Corporate operating expenses	(159,158)		(90,686)		(107,877)
Interest expense	(154,084)	[6]	(159,118)	[6]	(172,764)	[6]
Non-controlling interest in net income (loss) of subsidiary	(70,877)		(39,831)		104
Loss on termination of guarantee			(23,500)		0
Impairment of goodwill (5)	(429,020)	[7]	0		0
Income taxes & other	(61,145)		(164,595)		(122,143)
Net income attributable to XL Group plc	(474,760)		603,550		74,991
Insurance Segment [Member]
Gross premiums written	4,824,665		4,418,380		4,251,888
Net premiums written	3,707,664		3,461,150		3,273,380
Net premiums earned	3,663,727		3,529,138		3,559,793
Net losses and loss expenses	(2,951,413)		(2,505,502)		(2,399,747)
Acquisition costs	(461,965)		(418,146)		(429,170)
Operating expenses	(683,814)	[1]	(642,103)	[1]	(689,131)	[1]
Underwriting profit (loss)	(433,465)		(36,613)		41,745
Net results from structured products	10,976	[2]	14,696	[3]	16,660	[4]
Net fee income and other	(16,370)	[5]	(15,564)	[5]	(14,241)	[5]
Corporate & other:
Impairment of goodwill (5)	(429,000)
Ratios – P&C operations: (6)
Loss and loss expense ratio	80.60%	[8]	71.00%	[8]	67.40%	[8]
Underwriting expense ratio	31.20%	[8]	30.00%	[8]	31.40%	[8]
Combined ratio	111.80%	[8]	101.00%	[8]	98.80%	[8]
Reinsurance Segment [Member]
Gross premiums written	2,073,619		1,842,951		1,859,423
Net premiums written	1,725,724		1,538,438		1,470,332
Net premiums earned	1,663,385		1,501,999		1,591,946
Net losses and loss expenses	(1,126,978)		(706,298)		(769,090)
Acquisition costs	(324,128)		(321,008)		(346,699)
Operating expenses	(176,167)	[1]	(175,586)	[1]	(190,596)	[1]
Underwriting profit (loss)	36,112		299,107		285,561
Net results from structured products	12,053	[2]	3,075	[3]	26,374	[4]
Net fee income and other	3,903	[5]	2,488	[5]	6,209	[5]
Ratios – P&C operations: (6)
Loss and loss expense ratio	67.80%	[8]	47.00%	[8]	48.30%	[8]
Underwriting expense ratio	30.00%	[8]	33.10%	[8]	33.80%	[8]
Combined ratio	97.80%	[8]	80.10%	[8]	82.10%	[8]
Total P And C Segment [Member]
Gross premiums written	6,898,284		6,261,331		6,111,311
Net premiums written	5,433,388		4,999,588		4,743,712
Net premiums earned	5,327,112		5,031,137		5,151,739
Net losses and loss expenses	(4,078,391)		(3,211,800)		(3,168,837)
Acquisition costs	(786,093)		(739,154)		(775,869)
Operating expenses	(859,981)	[1]	(817,689)	[1]	(879,727)	[1]
Underwriting profit (loss)	(397,353)		262,494		327,306
Net investment income	745,138		803,358		882,748
Net results from structured products	23,029	[2]	17,771	[3]	43,034	[4]
Net fee income and other	(12,467)	[5]	(13,076)	[5]	(8,032)	[5]
Net realized gains (losses) on investments	(98,360)		(206,877)		(659,687)
Contribution from P&C, Life Operations and Corporate	259,987		863,670		585,369
Ratios – P&C operations: (6)
Loss and loss expense ratio	76.60%	[8]	63.80%	[8]	61.50%	[8]
Underwriting expense ratio	30.90%	[8]	31.00%	[8]	32.10%	[8]
Combined ratio	107.50%	[8]	94.80%	[8]	93.60%	[8]
Life Operations Segment [Member]
Gross premiums written	394,555		411,938		576,162
Net premiums written	362,362		382,075		532,852
Net premiums earned	363,018		382,924		555,101
Net losses and loss expenses	(535,074)		(513,833)		(677,562)
Acquisition costs	(40,318)		(49,104)		(77,689)
Operating expenses	(9,311)	[1]	(10,470)	[1]	(16,009)	[1]
Underwriting profit (loss)	(221,685)		(190,483)		(216,159)
Net investment income	318,061		313,172		332,425
Net results from structured products	0	[2]	0	[3]	0	[4]
Net fee income and other	219	[5]	249	[5]	290	[5]
Net realized gains (losses) on investments	(89,999)		(54,444)		(232,375)
Contribution from P&C, Life Operations and Corporate	6,596		68,494		(115,819)
Corporate Segment [Member]
Gross premiums written	0		0		0
Net premiums written	0		0		0
Net premiums earned	0		0		0
Net losses and loss expenses	0		0		0
Acquisition costs	0		0		0
Operating expenses	0	[1]	0	[1]	0	[1]
Underwriting profit (loss)	0		0		0
Net investment income	0		0		0
Net results from structured products	0	[2]	9,804	[3]	16,609	[4]
Net fee income and other	0	[5]	2	[5]	0	[5]
Net realized gains (losses) on investments	0		(9,482)		(29,375)
Contribution from P&C, Life Operations and Corporate	0		324		(12,766)
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(10,738)		(33,843)		(33,647)
Net income (loss) from investment fund affiliates and operating affiliates	103,039		172,474		139,347
Exchange gains (losses)	40,640		10,161		(84,813)
Corporate operating expenses	(159,158)		(90,686)		(107,877)
Interest expense	(154,084)	[6]	(159,118)	[6]	(172,764)	[6]
Non-controlling interest in net income (loss) of subsidiary	(70,877)		(39,831)		104
Loss on termination of guarantee			(23,500)		0
Impairment of goodwill (5)	(429,020)	[7]
Income taxes & other	$ (61,145)		$ (164,595)		$ (122,143)

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

[2] The net results from P&C structured products include net investment income, interest expense and operating expenses of $74.6 million, $51.5 million and nil, respectively.

[3] The net results from P&C and Corporate structured products include net investment income, interest expense and operating expenses of $69.0 million, $52.9 million and $1.6 million (credit) and $12.5 million, $1.7 million and $1.0 million, respectively.

[4] The net results from P&C and Corporate structured products include net investment income, interest expense and operating expenses of $79.3 million, $36.3 million and nil, and $25.3 million, $7.4 million and $1.3 million, respectively.

[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 and Corporate.

[7] Represents goodwill impairment charges recorded in the fourth quarter of 2011, associated with the Company's insurance operations.

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