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Other Investments - Analysis of net realized and unrealized gains on other investments (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Other Investments [Line Items]      
Net unrealized gains (losses) $ 231,278xlgroup_AccumulatedUnrealizedGainLossOnInvestments $ 202,852xlgroup_AccumulatedUnrealizedGainLossOnInvestments  
Net realized gains (losses) 19,960us-gaap_GainLossOnSaleOfOtherAssets 35,189us-gaap_GainLossOnSaleOfOtherAssets (1,896)us-gaap_GainLossOnSaleOfOtherAssets
Alternative funds      
Other Investments [Line Items]      
Net unrealized gains (losses) 188,674xlgroup_AccumulatedUnrealizedGainLossOnInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_HedgeFundsMember		 158,825xlgroup_AccumulatedUnrealizedGainLossOnInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_HedgeFundsMember		  
Net realized gains (losses) 6,685us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_HedgeFundsMember		 30,858us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_HedgeFundsMember		 (2,153)us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_HedgeFundsMember
Private investment funds      
Other Investments [Line Items]      
Net unrealized gains (losses) 42,604xlgroup_AccumulatedUnrealizedGainLossOnInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 44,027xlgroup_AccumulatedUnrealizedGainLossOnInvestments
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		  
Net realized gains (losses) $ 13,275us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 $ 4,331us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember		 $ 257us-gaap_GainLossOnSaleOfOtherAssets
/ us-gaap_InvestmentTypeAxis
= us-gaap_PrivateEquityFundsMember