Segment Information - Statement of earnings by segment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Segment Reporting Information [Line Items]
Gross premiums written	$ 2,477,886		$ 2,403,667
Net premiums written	2,107,654		2,041,724
Net premiums earned	1,529,691		1,436,418
Net losses and loss expenses	954,038		975,372
Acquisition costs	220,259		224,151
Operating expenses	227,013	[1]	226,700	[1]
Underwriting profit (loss)	128,381		10,195
Net investment income	228,728		247,994
Net results from structured products	5,176	[2]	5,257	[3]
Net fee income and other	(264)	[4]	(1,639)	[4]
Net realized gains (losses) on investments	36,510		20,803
Contribution from P&C and Life Operations	398,531		282,610
Corporate & other:
Net realized and unrealized gains (losses) on derivative instruments	7,885		702
Net income (loss) from investment fund affiliates and operating affiliates	63,219	[5]	35,661
Exchange gains (losses)	33,435		(12,718)
Corporate operating expenses	46,923		44,260
Interest expense	26,136	[6]	27,260
Non-controlling interests	35,742		36,557
Income tax	43,479		21,550
Net income (loss) attributable to XL Group plc and ordinary shareholders	350,790		176,628
Total P&C
Segment Reporting Information [Line Items]
Gross premiums written	2,404,916		2,316,984
Net premiums written	2,042,209		1,963,228
Net premiums earned	1,464,246		1,357,898
Net losses and loss expenses	843,084		854,065
Acquisition costs	216,156		216,500
Operating expenses	224,418	[1]	224,093	[1]
Underwriting profit (loss)	180,588		63,240
Net investment income	157,040		172,968
Net results from structured products	5,176	[2]	5,257	[3]
Net fee income and other	(1,307)	[4]	(1,687)	[4]
Net realized gains (losses) on investments	45,579		24,967
Contribution from P&C and Life Operations	387,076		264,745
Ratios - P&C operations
Loss and loss expense ratio	57.60%	[7]	62.90%	[7]
Underwriting expense ratio	30.10%	[7]	32.40%	[7]
Combined ratio	87.70%	[7]	95.30%	[7]
Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	1,497,787		1,336,634
Net premiums written	1,189,188		1,036,526
Net premiums earned	1,039,092		934,056
Net losses and loss expenses	650,517		631,685
Acquisition costs	130,622		128,256
Operating expenses	183,997	[1]	185,346	[1]
Underwriting profit (loss)	73,956		(11,231)
Net results from structured products	4,185	[2]	2,819	[3]
Net fee income and other	(1,939)	[4]	(2,020)	[4]
Ratios - P&C operations
Loss and loss expense ratio	62.60%	[7]	67.60%	[7]
Underwriting expense ratio	30.30%	[7]	33.60%	[7]
Combined ratio	92.90%	[7]	101.20%	[7]
Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written	907,129		980,350
Net premiums written	853,021		926,702
Net premiums earned	425,154		423,842
Net losses and loss expenses	192,567		222,380
Acquisition costs	85,534		88,244
Operating expenses	40,421	[1]	38,747	[1]
Underwriting profit (loss)	106,632		74,471
Net results from structured products	991	[2]	2,438	[3]
Net fee income and other	632	[4]	333	[4]
Ratios - P&C operations
Loss and loss expense ratio	45.30%	[7]	52.50%	[7]
Underwriting expense ratio	29.60%	[7]	29.90%	[7]
Combined ratio	74.90%	[7]	82.40%	[7]
Life Operations Segment
Segment Reporting Information [Line Items]
Gross premiums written	72,970		86,683
Net premiums written	65,445		78,496
Net premiums earned	65,445		78,520
Net losses and loss expenses	110,954		121,307
Acquisition costs	4,103		7,651
Operating expenses	2,595	[1]	2,607	[1]
Underwriting profit (loss)	(52,207)		(53,045)
Net investment income	71,688		75,026
Net results from structured products	0	[2]	0	[3]
Net fee income and other	1,043	[4]	48	[4]
Net realized gains (losses) on investments	(9,069)		(4,164)
Contribution from P&C and Life Operations	$ 11,455		$ 17,865

[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 $17.7 million and $12.6 million, respectively

[3] The net results from P&C structured products include net investment income and interest expense of $17.2 million and $12.0 million, respectively.

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

[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.