SHARE CAPITAL (Detail) - Non-vested performance units (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Unvested at December 31, 2010 (in Shares)	1,517
Unvested at December 31, 2010	$ 17.20
Granted (in Shares)	1,289
Granted	$ 22.14
Forfeited (in Shares)	(213)
Forfeited	$ 19.43
Performance driven reduction (in Shares)	(858)
Performance driven reduction	$ 18.37
Unvested at December 31, 2011 (in Shares)	1,735
Unvested at December 31, 2011	$ 19.47

The number of non-vested restricted performance units forfeited.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsForfeited
Namespace Prefix:	xlgroup_
Data Type:	xbrli:sharesItemType
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Period Type:	duration

The number of non-vested restricted performance units granted.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsGranted
Namespace Prefix:	xlgroup_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of shares non-vested restricted stock performance units as of the balance sheet date.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsNumber
Namespace Prefix:	xlgroup_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

The number of non-vested restricted performance units performance driven reduction..

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsPerformanceDrivenReduction
Namespace Prefix:	xlgroup_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The weighted average fair value on non-vested restricted performance units weighted average grant date fair value as of the balance sheet date.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsWeightedAverageGrantDateFairValue
Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

The weighted average fair value forfeited on non-vested restricted performance units during the period.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsWeightedAverageGrantDateFairValueForfeited
Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

The weighted average fair value granted during the period.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsWeightedAverageGrantDateFairValueGranted
Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

The weighted average fair value performance driven reduction during the period.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockPerformanceUnitsWeightedAverageGrantDateFairValuePerformanceDrivenReduction
Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration