Segment Information (Detail) - Statement of Earnings by Segment (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Gross premiums written	$ 2,403,667		$ 2,196,779
Net premiums written	2,041,724		1,803,954
Net premiums earned	1,436,418		1,361,383
Net losses and loss expenses	(975,372)		(1,342,096)
Acquisition costs	(224,151)		(188,490)
Operating expenses	(226,700)	[1]	(211,891)	[1]
Underwriting profit (loss)	10,195		(381,094)
Net investment income	247,994		260,541
Net results from structured products	5,317	[2]	7,474	[3]
Net fee income and other	(1,639)	[4]	(4,477)	[4]
Net realized gains (losses) on investments	20,803		(66,437)
Contribution from P&C, Life Operations and Corporate	282,670		(183,993)
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	702		3,567
Net income (loss) from investment fund affiliates and operating affiliates	35,661	[5]	40,786	[5]
Exchange gains (losses)	(12,718)		(9,514)
Corporate operating expenses	(44,260)		(35,692)
Interest expense	(27,320)	[6]	(41,899)	[6]
Non-controlling interest	(36,557)		(33,336)
Income taxes & other	(21,550)		32,797
Net income (loss) attributable to XL Group plc	176,628		(227,284)
Insurance Segment [Member]
Gross premiums written	1,336,634		1,222,349
Net premiums written	1,036,526		918,990
Net premiums earned	934,056		875,920
Net losses and loss expenses	(631,685)		(788,513)
Acquisition costs	(128,256)		(107,644)
Operating expenses	(185,346)	[1]	(164,095)	[1]
Underwriting profit (loss)	(11,231)		(184,332)
Net investment income	0		0
Net results from structured products	2,819	[2]	3,260	[3]
Net fee income and other	(2,020)	[4]	(5,912)	[4]
Ratios – P&C operations: (6)
Loss and loss expense ratio	67.60%	[7]	90.00%	[7]
Underwriting expense ratio	33.60%	[7]	31.00%	[7]
Combined ratio	101.20%	[7]	121.00%	[7]
Reinsurance Segment [Member]
Gross premiums written	980,350		876,771
Net premiums written	926,702		795,292
Net premiums earned	423,842		395,776
Net losses and loss expenses	(222,380)		(420,352)
Acquisition costs	(88,244)		(73,526)
Operating expenses	(38,747)	[1]	(45,630)	[1]
Underwriting profit (loss)	74,471		(143,732)
Net investment income	0		0
Net results from structured products	2,498	[2]	4,214	[3]
Net fee income and other	333	[4]	1,394	[4]
Ratios – P&C operations: (6)
Loss and loss expense ratio	52.50%	[7]	106.20%	[7]
Underwriting expense ratio	29.90%	[7]	30.10%	[7]
Combined ratio	82.40%	[7]	136.30%	[7]
Total P And C Segment [Member]
Gross premiums written	2,316,984		2,099,120
Net premiums written	1,963,228		1,714,282
Net premiums earned	1,357,898		1,271,696
Net losses and loss expenses	(854,065)		(1,208,865)
Acquisition costs	(216,500)		(181,170)
Operating expenses	(224,093)	[1]	(209,725)	[1]
Underwriting profit (loss)	63,240		(328,064)
Net investment income	172,968		183,565
Net results from structured products	5,317	[2]	7,474	[3]
Net fee income and other	(1,687)	[4]	(4,518)	[4]
Net realized gains (losses) on investments	24,967		(26,886)
Contribution from P&C, Life Operations and Corporate	264,805		(168,429)
Ratios – P&C operations: (6)
Loss and loss expense ratio	62.90%	[7]	95.10%	[7]
Underwriting expense ratio	32.40%	[7]	30.70%	[7]
Combined ratio	95.30%	[7]	125.80%	[7]
Life Operations Segment [Member]
Gross premiums written	86,683		97,659
Net premiums written	78,496		89,672
Net premiums earned	78,520		89,687
Net losses and loss expenses	(121,307)		(133,231)
Acquisition costs	(7,651)		(7,320)
Operating expenses	(2,607)	[1]	(2,166)	[1]
Underwriting profit (loss)	(53,045)		(53,030)
Net investment income	75,026		76,976
Net results from structured products	0	[2]	0	[3]
Net fee income and other	48	[4]	41	[4]
Net realized gains (losses) on investments	(4,164)		(39,551)
Contribution from P&C, Life Operations and Corporate	$ 17,865		$ (15,564)

[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 $17.2 million and $12.0 million, respectively.

[3] The net results from P&C structured products include net investment income and interest expense of $19.7 million and $12.2 million, respectively.

[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.