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Derivative Instruments - Summary of contingent credit features (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Derivative [Line Items]    
Collateral posted to counterparty $ 0us-gaap_DerivativeCollateralRightToReclaimCash $ 19,847us-gaap_DerivativeCollateralRightToReclaimCash
Contracts with downgrade provisions [Member]    
Derivative [Line Items]    
Aggregate fair value of derivative agreements with downgrade provisions in a net liability position 5,770us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= xlgroup_ContractsWithDowngradeProvisionsMember		 47,703us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
/ us-gaap_DerivativeByNatureAxis
= xlgroup_ContractsWithDowngradeProvisionsMember
Collateral posted to counterparty $ 0us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_DerivativeByNatureAxis
= xlgroup_ContractsWithDowngradeProvisionsMember		 $ 13,260us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_DerivativeByNatureAxis
= xlgroup_ContractsWithDowngradeProvisionsMember