Segment Information (Detail) - Statement of Earnings by Segment (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Gross premiums written	$ 1,862,724		$ 1,599,918		$ 4,059,503		$ 3,635,132
Net premiums written	1,398,253		1,200,698		3,202,207		2,901,889
Net premiums earned	1,398,339		1,302,761		2,759,722		2,671,246
Net losses and loss expenses	(961,000)		(870,540)		(2,303,096)		(1,886,483)
Acquisition costs	(215,099)		(180,560)		(403,589)		(381,697)
Operating expenses (1)	(212,884)	[1]	(209,432)	[1]	(424,775)	[1]	(407,704)	[1]
Underwriting profit (loss)	9,356		42,229		(371,738)		(4,638)
Net investment income	278,110		280,735		538,651		564,993
Net results from structured products (2)	4,916	[2]	11,802	[3]	12,390	[4]	23,570	[5]
Net fee income and other (3)	(3,131)	[6]	(3,648)	[6]	(7,608)	[6]	(7,075)	[6]
Net realized gains (losses) on investments	(9,544)		(61,386)		(75,981)		(97,562)
Contribution from P&C, Life Operations and Corporate	279,707		269,732		95,714		479,288
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(10,950)		(19,896)		(7,383)		(40,376)
Net income (loss) from investment fund affiliates and operating affiliates (4)	56,501	[7]	40,097	[7]	97,287	[7]	59,881	[7]
Exchange gains (losses)	8,498		32,276		(1,016)		53,359
Corporate operating expenses	(39,522)		(22,793)		(74,749)		(41,656)
Interest expense (5)	(41,599)	[8]	(38,551)	[8]	(83,498)	[8]	(75,451)	[8]
Non-controlling interests	(2,102)		80		(35,438)		81
Loss on termination of guarantee			(23,500)				(23,500)
Income taxes & other	(24,870)		(43,440)		7,462		(73,741)
Net income attributable to XL Group plc	225,663		194,005		(1,621)		337,885
Insurance Segment [Member]
Gross premiums written	1,290,030		1,085,740		2,512,379		2,217,630
Net premiums written	893,191		757,424		1,812,181		1,658,688
Net premiums earned	907,443		868,666		1,783,363		1,765,677
Net losses and loss expenses	(608,182)		(592,184)		(1,396,695)		(1,240,503)
Acquisition costs	(113,883)		(92,236)		(221,527)		(202,378)
Operating expenses (1)	(166,608)	[1]	(163,005)	[1]	(330,703)	[1]	(316,241)	[1]
Underwriting profit (loss)	18,770		21,241		(165,562)		6,555
Net results from structured products (2)	2,690	[2]	2,940	[3]	5,950	[4]	8,579	[5]
Net fee income and other (3)	(3,218)	[6]	(4,337)	[6]	(9,130)	[6]	(8,123)	[6]
Ratios – P&C operations: (6)
Loss and loss expense ratio	67.00%	[9]	68.20%	[9]	78.30%	[9]	70.30%	[9]
Underwriting expense ratio	30.90%	[9]	29.40%	[9]	31.00%	[9]	29.30%	[9]
Combined ratio	97.90%	[9]	97.60%	[9]	109.30%	[9]	99.60%	[9]
Reinsurance Segment [Member]
Gross premiums written	472,413		421,340		1,349,184		1,211,763
Net premiums written	412,868		357,180		1,208,160		1,052,441
Net premiums earned	398,682		347,647		794,458		714,237
Net losses and loss expenses	(215,402)		(154,981)		(635,754)		(398,862)
Acquisition costs	(91,448)		(75,572)		(164,974)		(148,177)
Operating expenses (1)	(43,553)	[1]	(43,662)	[1]	(89,183)	[1]	(85,690)	[1]
Underwriting profit (loss)	48,279		73,432		(95,453)		81,508
Net results from structured products (2)	2,226	[2]	5,776	[3]	6,440	[4]	8,252	[5]
Net fee income and other (3)	(9)	[6]	575	[6]	1,385	[6]	894	[6]
Ratios – P&C operations: (6)
Loss and loss expense ratio	54.00%	[9]	44.60%	[9]	80.00%	[9]	55.80%	[9]
Underwriting expense ratio	33.90%	[9]	34.30%	[9]	32.00%	[9]	32.80%	[9]
Combined ratio	87.90%	[9]	78.90%	[9]	112.00%	[9]	88.60%	[9]
Total PandC [Member]
Gross premiums written	1,762,443		1,507,080		3,861,563		3,429,393
Net premiums written	1,306,059		1,114,604		3,020,341		2,711,129
Net premiums earned	1,306,125		1,216,313		2,577,821		2,479,914
Net losses and loss expenses	(823,584)		(747,165)		(2,032,449)		(1,639,365)
Acquisition costs	(205,331)		(167,808)		(386,501)		(350,555)
Operating expenses (1)	(210,161)	[1]	(206,667)	[1]	(419,886)	[1]	(401,931)	[1]
Underwriting profit (loss)	67,049		94,673		(261,015)		88,063
Net investment income	196,053		205,346		379,618		409,260
Net results from structured products (2)	4,916	[2]	8,716	[3]	12,390	[4]	16,831	[5]
Net fee income and other (3)	(3,227)	[6]	(3,762)	[6]	(7,745)	[6]	(7,229)	[6]
Net realized gains (losses) on investments	(10,248)		(55,876)		(37,134)		(84,453)
Contribution from P&C, Life Operations and Corporate	254,543		249,097		86,114		422,472
Ratios – P&C operations: (6)
Loss and loss expense ratio	63.10%	[9]	61.40%	[9]	78.80%	[9]	66.10%	[9]
Underwriting expense ratio	31.80%	[9]	30.80%	[9]	31.30%	[9]	30.30%	[9]
Combined ratio	94.90%	[9]	92.20%	[9]	110.10%	[9]	96.40%	[9]
Life Operations [Member]
Gross premiums written	100,281		92,838		197,940		205,739
Net premiums written	92,194		86,094		181,866		190,760
Net premiums earned	92,214		86,448		181,901		191,332
Net losses and loss expenses	(137,416)		(123,375)		(270,647)		(247,118)
Acquisition costs	(9,768)		(12,752)		(17,088)		(31,142)
Operating expenses (1)	(2,723)	[1]	(2,765)	[1]	(4,889)	[1]	(5,773)	[1]
Underwriting profit (loss)	(57,693)		(52,444)		(110,723)		(92,701)
Net investment income	82,057		75,389		159,033		155,733
Net results from structured products (2)		[2]		[3]		[4]		[5]
Net fee income and other (3)	96	[6]	114	[6]	137	[6]	154	[6]
Net realized gains (losses) on investments	704		(5,445)		(38,847)		(9,858)
Contribution from P&C, Life Operations and Corporate	25,164		17,614		9,600		53,328
Corporate [Member]
Operating expenses (1)		[1]		[1]		[1]		[1]
Net results from structured products (2)		[2]	3,086	[3]		[4]	6,739	[5]
Net fee income and other (3)		[6]		[6]		[6]		[6]
Net realized gains (losses) on investments			(65)				(3,251)
Contribution from P&C, Life Operations and Corporate			3,021				3,488
Corporate & other:
Net realized & unrealized gains (losses) on derivative instruments	(10,950)		(19,896)		(7,383)		(40,376)
Net income (loss) from investment fund affiliates and operating affiliates (4)	56,501	[7]	40,097	[7]	97,287	[7]	59,881	[7]
Exchange gains (losses)	8,498		32,276		(1,016)		53,359
Corporate operating expenses	(39,522)		(22,793)		(74,749)		(41,656)
Interest expense (5)	(41,599)	[8]	(38,551)	[8]	(83,498)	[8]	(75,451)	[8]
Non-controlling interests	(2,102)		80		(35,438)		81
Loss on termination of guarantee			(23,500)				(23,500)
Income taxes & other	$ (24,870)		$ (43,440)		$ 7,462		$ (73,741)

[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 $18.4 million and $13.5 million.

[3] The net results from P&C structured products and Corporate structured products include net investment income, interest expense and operating expenses of $17.2 million, $9.9 million and $1.4 million (credit) and $4.7 million, $0.7 million and $0.9 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $38.1 million and $25.7 million.

[5] The net results from P&C structured products and Corporate structured products include net investment income, interest expense and operating expenses of $36.9 million, $21.4 million and $1.3 million (credit) and $9.0 million, $1.3 million and $0.9 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] Ratios are based on net premiums earned from P&C operations.