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Investments in Affiliates - Narrative (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Schedule of Equity Method Investments [Line Items]    
Investments in affiliates $ 1,637,620,000us-gaap_EquityMethodInvestments $ 1,370,943,000us-gaap_EquityMethodInvestments
Number of interests in operating affiliates purchased   1xlgroup_NumberOfInterestsPurchasedOperatingAffiliates
Cost of operating affiliate interests acquired   30,000,000xlgroup_EquityMethodInvestmentsCostOfInterestAcquired
Operating affiliates    
Schedule of Equity Method Investments [Line Items]    
Alternative investments, timelag for reporting 3 months  
Investments in affiliates 418,482,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= us-gaap_OtherAffiliatesMember		 328,871,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= us-gaap_OtherAffiliatesMember
Other strategic operating affiliates    
Schedule of Equity Method Investments [Line Items]    
Investments in affiliates 317,216,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_OtherStrategicOperatingAffiliatesMember		 266,796,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_OtherStrategicOperatingAffiliatesMember
ARX Holding Corporation [Member]    
Schedule of Equity Method Investments [Line Items]    
Ownership percentage remaining 40.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_ArxHoldingCorporationMember		  
Investment manager affiliates    
Schedule of Equity Method Investments [Line Items]    
Investments in affiliates 101,266,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_InvestmentManagerAffiliatesMember		 62,075,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_InvestmentManagerAffiliatesMember		 [1]
Distributions received from investment manager affiliates 60,300,000us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_InvestmentManagerAffiliatesMember		 28,300,000us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= xlgroup_InvestmentManagerAffiliatesMember
Subject To Redemption Restrictions [Member]    
Schedule of Equity Method Investments [Line Items]    
Investments in affiliates 543,000,000us-gaap_EquityMethodInvestments
/ invest_InvestmentAxis
= xlgroup_SubjectToRedemptionRestrictionsMember		 488,500,000us-gaap_EquityMethodInvestments
/ invest_InvestmentAxis
= xlgroup_SubjectToRedemptionRestrictionsMember
Funds held in side pockets    
Schedule of Equity Method Investments [Line Items]    
Investments in affiliates $ 22,200,000us-gaap_EquityMethodInvestments
/ invest_InvestmentAxis
= xlgroup_RedemptionRestrictionSidePocketMember		 $ 42,300,000us-gaap_EquityMethodInvestments
/ invest_InvestmentAxis
= xlgroup_RedemptionRestrictionSidePocketMember
Investments in the common stock of a corporation    
Schedule of Equity Method Investments [Line Items]    
Threshold for signicant influence, percentage 20.00%xlgroup_SignificantInfluenceRecognitionPercentage
/ invest_InvestmentAxis
= xlgroup_InvestmentsInCommonStockOfCorporationsMember		  
Investments in closed end funds, limited partnerships, LLCs or similar investment vehicles    
Schedule of Equity Method Investments [Line Items]    
Threshold for signicant influence, percentage 3.00%xlgroup_SignificantInfluenceRecognitionPercentage
/ invest_InvestmentAxis
= xlgroup_InvestmentsInOtherTypesOfLegalStructuresMember		  
Alternative funds    
Schedule of Equity Method Investments [Line Items]    
Alternative investments, timelag for reporting 1 month  
Private investment funds    
Schedule of Equity Method Investments [Line Items]    
Alternative investments, timelag for reporting 3 months  
[1] During the years ended December 31, 2014 and 2013, the Company received distributions from its Investment Manager Affiliates of approximately $60.3 million and $28.3 million, respectively.