SHARE CAPITAL (Detail) - Non-vested restricted stock units (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2011
Unvested at December 31, 2010 (in Shares)	1,317
Unvested at December 31, 2010	$ 18.61
Granted (in Shares)	1,318
Granted	$ 22.78
Vested (in Shares)	(447)
Vested	$ 18.80
Forfeited (in Shares)	(125)
Forfeited	$ 20.28
Unvested at December 31, 2011 (in Shares)	2,063
Unvested at December 31, 2011	$ 21.13

The number of non-vested restricted stock units forfeited.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockUnitsForfeited
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The number of grants made during the period on non-vested restricted stock units.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockUnitsGranted
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The number of non-vested restricted stock units outstanding as of the balance sheet date.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockUnitsNumber
Namespace Prefix:	xlgroup_
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Period Type:	instant

The number of restricted stock units vested.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockUnitsVested
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The weighted average fair value at grant date for non-vested restricted stock units.

No definition available.

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Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
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Period Type:	instant

The weighted average fair value granted on non-vested restricted stock units.

No definition available.

Name:	xlgroup_ShareBasedCompensationArrangementNonvestedRestrictedStockUnitsWeightedAverageGrantDateFairValueForfeited
Namespace Prefix:	xlgroup_
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The weighted average fair value granted on non-vested restricted stock units.

No definition available.

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Namespace Prefix:	xlgroup_
Data Type:	num:perShareItemType
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The weighted average fair value vested on non-vested restricted stock units.

No definition available.

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