Fair Value Measurements (Detail) - Additional information about assets and liabilities measured at fair value on a recurring basis (USD $) In Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Corporate [Member]
Balance	$ 4,786	$ 40,618	$ 36,866	$ 10,311
Realized gains (losses)	23	(3)	(257)	(4,315)
Movement in unrealized gains (losses)	(22)	338	173	393
Purchases and issuances		2,400	6,878	1,265
Sales and settlements		(1,282)	(10,049)	(519)
Transfers into Level 3				8,115
Transfers out of Level 3		(30,762)	(28,824)	(3,941)
Balance	4,787	11,309	4,787	11,309
Movement in total gains (losses) above relating to instruments still held at the reporting date	1	338	(52)	576
Residential Mortgage-Backed Securities Agency [Member]
Balance	15,385	1,226	30,255	7,894
Realized gains (losses)	(81)	(6)	(81)
Movement in unrealized gains (losses)	52	39	2	39
Purchases and issuances	33	6,740	33	7,949
Sales and settlements	(206)		(492)
Transfers out of Level 3			(14,534)	(7,883)
Balance	15,183	7,999	15,183	7,999
Movement in total gains (losses) above relating to instruments still held at the reporting date	(29)	40	(79)	39
Residential Mortgage-Backed Securities Non-Agency [Member]
Balance	3,194	10,008	4,964	42,190
Realized gains (losses)	(328)	(385)	(329)	(46)
Movement in unrealized gains (losses)	76	(41)	61	104
Sales and settlements	(143)	(6)	(517)	(2,294)
Transfers into Level 3				4,044
Transfers out of Level 3		(4,518)	(1,380)	(38,940)
Balance	2,799	5,058	2,799	5,058
Movement in total gains (losses) above relating to instruments still held at the reporting date	(252)	(37)	(268)	(56)
Commercial Mortgage Backed Securities [Member]
Balance	4,563	427	1,623	2,755
Realized gains (losses)	(501)		(1,179)	(209)
Movement in unrealized gains (losses)	564	3	1,078	29
Purchases and issuances			3,155
Sales and settlements	(232)		(283)	(707)
Transfers out of Level 3				(1,438)
Balance	4,394	430	4,394	430
Movement in total gains (losses) above relating to instruments still held at the reporting date	63	3	(855)	(136)
Collateralized Debt Obligations [Member]
Balance	727,239	690,906	721,572	197,149
Realized gains (losses)	(2,662)	(5,520)	(3,733)	(17,498)
Movement in unrealized gains (losses)	(46,520)	46,999	(10,104)	76,655
Purchases and issuances			2,379
Sales and settlements	(12,215)	(11,825)	(46,158)	(16,614)
Transfers into Level 3			1,886	475,548
Transfers out of Level 3		(28,651)		(23,331)
Balance	665,842	691,909	665,842	691,909
Movement in total gains (losses) above relating to instruments still held at the reporting date	(49,432)	46,685	(15,656)	72,239
Other Asset Backed Securities [Member]
Balance	17,048	22,521	24,650	38,179
Realized gains (losses)	(238)	(7,515)	(555)	(19,018)
Movement in unrealized gains (losses)	302	5,643	7,162	14,757
Purchases and issuances		10,714		12,978
Sales and settlements			(9,114)	(1,504)
Transfers into Level 3				204
Transfers out of Level 3		(1)	(5,031)	(14,234)
Balance	17,112	31,362	17,112	31,362
Movement in total gains (losses) above relating to instruments still held at the reporting date	64	(2,006)	6,287	7,642
Non US Sovereign Government Supranational And Government Related [Member]
Balance		3,182	3,667	3,217
Movement in unrealized gains (losses)		307		32
Transfers into Level 3				240
Transfers out of Level 3			(3,667)
Balance		3,489		3,489
Movement in total gains (losses) above relating to instruments still held at the reporting date		307		32
Other Investments [Member]
Balance	114,540	92,857	133,717	75,584
Realized gains (losses)	(244)	1,294	11,911	3,860
Movement in unrealized gains (losses)	2,642	234	10,134	5,785
Purchases and issuances	1,667	664	9,782	11,575
Sales and settlements	(4,243)	(12,489)	(51,182)	(14,244)
Balance	114,362	82,560	114,362	82,560
Movement in total gains (losses) above relating to instruments still held at the reporting date	2,398	234	19,735	5,785
Derivative Contracts [Member]
Balance	(49,645)	162,911	(39,195)	100,515
Movement in unrealized gains (losses)	(2,344)	32,567	(12,620)	84,202
Purchases and issuances				10,985
Sales and settlements		(1,516)	(174)	(1,740)
Balance	(51,989)	193,962	(51,989)	193,962
Movement in total gains (losses) above relating to instruments still held at the reporting date	$ (2,344)	$ 32,567	$ (12,620)	$ 84,202

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in earnings or resulted in a change in net asset value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents total gains or losses for the period (realized and unrealized), arising from assets measured at fair value on a recurring basis using unobservable inputs (Level 3), which are included in other comprehensive income (a separate component of shareholders' equity).

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncome
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents purchases and issuances,which have taken place during the period in relation to assets measured at fair value on a recurring basis using unobservable inputs (Level 3).

+ Details

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Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents sales, and settlements which have taken place during the period in relation to assets measured at fair value on a recurring basis using unobservable inputs (Level 3).

+ Details

Name:	xlgroup_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSalesSettlements
Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents [net] transfers out of net assets measured at fair value on a recurring basis using unobservable inputs (Level 3) which have taken place during the period.

+ Details

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Namespace Prefix:	xlgroup
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Movement in total (losses) relating to instruments still held at the reporting date.

+ Details

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Data Type:	xbrli:monetaryItemType
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