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Commitments and Contingencies - Broker credit risk (Details) (Gross written premium, Property and casualty operations, Credit concentration risk)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Concentration Risk [Line Items]      
Percentage of gross written premiums 10.00%us-gaap_ConcentrationRiskPercentage1    
AON Corporation and subsidiaries
      
Concentration Risk [Line Items]      
Percentage of gross written premiums 18.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerOneMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 21.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerOneMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 22.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerOneMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember
Marsh & McLennan Companies
      
Concentration Risk [Line Items]      
Percentage of gross written premiums 20.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerTwoMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 23.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerTwoMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 21.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerTwoMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember
Willis Group and subsidiaries
      
Concentration Risk [Line Items]      
Percentage of gross written premiums 11.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerThreeMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 13.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerThreeMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember		 11.00%us-gaap_ConcentrationRiskPercentage1
/ us-gaap_ConcentrationRiskByBenchmarkAxis
= xlgroup_GrossPremiumWrittenMember
/ us-gaap_ConcentrationRiskByTypeAxis
= us-gaap_CreditConcentrationRiskMember
/ us-gaap_MajorCustomersAxis
= xlgroup_ProducerThreeMember
/ us-gaap_StatementBusinessSegmentsAxis
= us-gaap_PropertyLiabilityAndCasualtyInsuranceSegmentMember