Segment Information (Detail) - Statement of Earnings by Segment (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Gross premiums written	$ 1,876,725		$ 1,629,468		$ 5,936,228		$ 5,264,600
Net premiums written	1,383,475		1,262,516		4,585,682		4,164,405
Net premiums earned	1,453,326		1,365,327		4,213,048		4,036,573
Net losses and loss expenses	(1,127,608)		(956,338)		(3,430,704)		(2,842,821)
Acquisition costs	(207,553)		(198,173)		(611,142)		(579,870)
Operating expenses	(200,665)	[1]	(202,408)	[1]	(625,440)	[1]	(610,112)	[1]
Underwriting profit (loss)	(82,500)		8,408		(454,238)		3,770
Net investment income	271,235		279,416		809,886		844,409
Net results from structured products	5,845	[2]	(5,248)	[3]	18,235	[4]	18,322	[5]
Net fee income and other	(2,323)	[6]	(2,642)	[6]	(9,931)	[6]	(9,717)	[6]
Net realized gains (losses) on investments	(62,354)		(68,755)		(138,335)		(166,317)
Contribution from P&C, Life Operations and Corporate	129,903		211,179		225,617		690,467
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(26,916)		12,473		(34,299)		(27,903)
Net income (loss) from investment fund affiliates and operating affiliates	28,429	[7]	17,140	[7]	125,716	[7]	77,021	[7]
Exchange gains (losses)	39,425		(44,540)		38,409		8,819
Corporate operating expenses	(32,962)		(19,782)		(107,711)		(61,438)
Interest expense	(35,885)	[8]	(36,146)	[8]	(119,383)	[8]	(111,597)	[8]
Non-controlling interests	(33,993)		(32,506)		(69,431)		(32,425)
Loss on termination of guarantee							(23,500)
Income taxes & other	(25,603)		(30,275)		(18,141)		(104,016)
Net income attributable to XL Group plc	42,398	[9]	77,543	[9]	40,777	[9]	415,428	[9]
Insurance Segment [Member]
Gross premiums written	1,156,901		991,781		3,669,280		3,209,411
Net premiums written	872,757		776,869		2,684,938		2,435,557
Net premiums earned	930,067		872,148	[10]	2,713,430		2,637,825	[10]
Net losses and loss expenses	(767,091)		(640,999)		(2,163,786)		(1,881,502)
Acquisition costs	(118,521)		(109,788)		(340,048)		(312,166)
Operating expenses	(158,134)	[1]	(155,816)	[1]	(488,837)	[1]	(472,057)	[1]
Underwriting profit (loss)	(113,679)		(34,455)		(279,241)		(27,900)
Net results from structured products	2,457	[2]	3,015	[3]	8,407	[4]	11,594	[5]
Net fee income and other	(3,484)	[6]	(3,713)	[6]	(12,614)	[6]	(11,836)	[6]
Ratios – P&C operations: (6)
Loss and loss expense ratio	82.50%	[11]	73.50%	[11]	79.70%	[11]	71.30%	[11]
Underwriting expense ratio	29.70%	[11]	30.50%	[11]	30.60%	[11]	29.80%	[11]
Combined ratio	112.20%	[11]	104.00%	[11]	110.30%	[11]	101.10%	[11]
Reinsurance Segment [Member]
Gross premiums written	619,260		533,730		1,968,444		1,745,493
Net premiums written	419,949		389,203		1,628,109		1,441,644
Net premiums earned	432,465		396,593	[10]	1,226,923		1,110,830	[10]
Net losses and loss expenses	(220,968)		(170,981)		(856,722)		(569,843)
Acquisition costs	(79,839)		(82,271)		(244,813)		(230,448)
Operating expenses	(40,179)	[1]	(44,220)	[1]	(129,362)	[1]	(129,910)	[1]
Underwriting profit (loss)	91,479		99,121		(3,974)		180,629
Net results from structured products	3,388	[2]	(9,829)	[3]	9,828	[4]	(1,577)	[5]
Net fee income and other	1,120	[6]	1,017	[6]	2,505	[6]	1,911	[6]
Ratios – P&C operations: (6)
Loss and loss expense ratio	51.10%	[11]	43.10%	[11]	69.80%	[11]	51.30%	[11]
Underwriting expense ratio	27.70%	[11]	31.90%	[11]	30.50%	[11]	32.40%	[11]
Combined ratio	78.80%	[11]	75.00%	[11]	100.30%	[11]	83.70%	[11]
Total P and C [Member]
Gross premiums written	1,776,161		1,525,511		5,637,724		4,954,904
Net premiums written	1,292,706		1,166,072		4,313,047		3,877,201
Net premiums earned	1,362,532		1,268,741		3,940,353		3,748,655
Net losses and loss expenses	(988,059)		(811,980)		(3,020,508)		(2,451,345)
Acquisition costs	(198,360)		(192,059)		(584,861)		(542,614)
Operating expenses	(198,313)	[1]	(200,036)	[1]	(618,199)	[1]	(601,967)	[1]
Underwriting profit (loss)	(22,200)		64,666		(283,215)		152,729
Net investment income	190,886		201,029		570,504		610,289
Net results from structured products	5,845	[2]	(6,814)	[3]	18,235	[4]	10,017	[5]
Net fee income and other	(2,364)	[6]	(2,696)	[6]	(10,109)	[6]	(9,925)	[6]
Net realized gains (losses) on investments	(39,047)		(69,770)		(76,181)		(154,223)
Contribution from P&C, Life Operations and Corporate	133,120		186,415		219,234		608,887
Ratios – P&C operations: (6)
Loss and loss expense ratio	72.50%	[11]	64.00%	[11]	76.70%	[11]	65.40%	[11]
Underwriting expense ratio	29.10%	[11]	30.90%	[11]	30.50%	[11]	30.50%	[11]
Combined ratio	101.60%	[11]	94.90%	[11]	107.20%	[11]	95.90%	[11]
Life Operations [Member]
Gross premiums written	100,564		103,957		298,504		309,696
Net premiums written	90,769		96,444		272,635		287,204
Net premiums earned	90,794		96,586		272,695		287,918
Net losses and loss expenses	(139,549)		(144,358)		(410,196)		(391,476)
Acquisition costs	(9,193)		(6,114)		(26,281)		(37,256)
Operating expenses	(2,352)	[1]	(2,372)	[1]	(7,241)	[1]	(8,145)	[1]
Underwriting profit (loss)	(60,300)		(56,258)		(171,023)		(148,959)
Net investment income	80,349		78,387		239,382		234,120
Net results from structured products		[2]		[3]		[4]		[5]
Net fee income and other	41	[6]	54	[6]	178	[6]	208	[6]
Net realized gains (losses) on investments	(23,307)		(1,512)		(62,154)		(11,370)
Contribution from P&C, Life Operations and Corporate	(3,217)		20,671		6,383		73,999
Corporate [Member]
Operating expenses		[1]		[1]		[1]		[1]
Net results from structured products		[2]	1,566	[3]		[4]	8,305	[5]
Net fee income and other		[6]		[6]		[6]		[6]
Net realized gains (losses) on investments			2,527				(724)
Contribution from P&C, Life Operations and Corporate			4,093				7,581
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(26,916)		12,473		(34,299)		(27,903)
Net income (loss) from investment fund affiliates and operating affiliates	28,429	[7]	17,140	[7]	125,716	[7]	77,021	[7]
Exchange gains (losses)	39,425		(44,540)		38,409		8,819
Corporate operating expenses	(32,962)		(19,782)		(107,711)		(61,438)
Interest expense	(35,885)	[8]	(36,146)	[8]	(119,383)	[8]	(111,597)	[8]
Non-controlling interests	(33,993)		(32,506)		(69,431)		(32,425)
Loss on termination of guarantee							(23,500)
Income taxes & other	$ (25,603)		$ (30,275)		$ (18,141)		$ (104,016)

[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 $18.9 million and $13.1 million.

[3] The net results from P&C structured products and Corporate structured products include net investment income, interest expense and operating expenses of $15.3 million, $22.1 million and nil and $2.0 million, $0.3 million and $0.1 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $57.0 million and $38.8 million.

[5] The net results from P&C structured products and Corporate structured products include net investment income, interest expense and operating expenses of $52.2 million, $43.6 million and $1.3 million (credit) and $11.0 million, $1.7 million and $1.0 million, respectively.

[6] 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.

[7] The Company records the income related to the alternative funds and to the private investment fund and operating affiliates on a one month and three month lag, respectively.

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

[9] During the first quarter of 2011, the Redeemable Series C preference ordinary shares were reclassified as Non-controlling interest - Redeemable Series C preference ordinary shares and Series E preference ordinary shares were reclassified as Non-controlling interest in equity of consolidated subsidiaries on the Company's consolidated balance sheet as a result of changes in ownership structure arising as part of the Company's redomestication of the ultimate parent holding company to Ireland as of July 1, 2010. Accordingly, preference share dividends declared are recorded as Non-controlling interests rather than as Preference share dividends within the consolidated statements of income from July 1, 2010 onwards. See Note 1, "Basis of Preparation and Consolidation" for additional details. During the third quarter of 2011, all outstanding Redeemable Series C preference ordinary shares were repurchased and canceled.

[10] Certain reclassifications have been made to conform to current year presentation.

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