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Reinsurance - Ceded Credit Risk (Details)
Dec. 31, 2014
Dec. 31, 2013
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 100.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal  
AAA    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 0.10%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAARatingMember		  
AA    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 42.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAARatingMember		  
A    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 50.40%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsARatingMember		  
BBB    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 0.40%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsBBBRatingMember		  
Captives    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 5.40%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= xlgroup_CaptivesMember		  
Other    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 1.70%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= xlgroup_RatingOtherMember		  
A- or better rating    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 93.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= xlgroup_AminusOrBetterRatingMember		 90.00%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CreditRatingStandardPoorsAxis
= xlgroup_AminusOrBetterRatingMember
Ceded Credit Risk, Concentrated Credit Risk [Member]    
Ceded Credit Risk [Line Items]    
Threshold percentage, concentration risk 3.00%xlgroup_ConcentrationRiskPercentageReinsuranceRecoverable
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Munich Reinsurance Company    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 10.50%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_MunichReinsuranceCompanyMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Swiss Reinsurance Company    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 8.80%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_SwissReinsuranceCompanyMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Swiss Re Europe S.A.    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 7.70%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_SwissReEuropeSaMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | Arch Reinsurance Co [Member]    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 6.30%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_ArchReinsuranceCoMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | AA- | LLoyds Syndicate [Member]    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 5.10%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_LloydsSyndicateMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAAMinusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Transatlantic Reinsurance Company [Member]    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 8.10%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_TransatlanticReinsuranceCompanyMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAPlusRatingMember		  
Ceded Credit Risk, Concentrated Credit Risk [Member] | A+ | Axis Reinsurance Co [Member]    
Ceded Credit Risk [Line Items]    
Percentage of total reinsurance recoverables 4.60%us-gaap_ReinsuranceRecoverablesPercentageOfTotal
/ us-gaap_CededCreditRiskAxis
= xlgroup_AxisReinsuranceCoMember
/ us-gaap_CededCreditRiskRiskClassificationAxis
= us-gaap_CededCreditRiskConcentratedCreditRiskMember
/ us-gaap_CreditRatingStandardPoorsAxis
= us-gaap_StandardPoorsAPlusRatingMember