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Investments - Cost (amortized cost for fixed maturities and short-term investments), fair value, gross unrealized gains, gross unrealized (losses), and OTTI recorded in AOCI (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Fixed maturities - AFS    
Amortized cost of Fixed maturities $ 27,728,771us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 27,111,874us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Available for sale fixed income securities, gross unrealized gains   807,684us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available for sale fixed income securities, gross unrealized losses   (419,422)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fixed maturities, AFS, at fair value 29,359,034us-gaap_AvailableForSaleSecuritiesDebtSecurities 27,500,136us-gaap_AvailableForSaleSecuritiesDebtSecurities
Gross Unrealized Losses Non-credit Related OTTI (76,720)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome [1] (86,882)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome [1]
Short Term Investments    
Short-term investments, cost or amortized cost 257,221us-gaap_AvailableForSaleSecuritiesShortTermInvestmentsAmortizedCost 455,470us-gaap_AvailableForSaleSecuritiesShortTermInvestmentsAmortizedCost
Short -term investments, Gross unrealized gains   962xlgroup_ShortTermInvestmentsGrossUnrealizedGains
Short -term investments, Gross unrealized losses   (144)xlgroup_ShortTermInvestmentsGrossUnrealizedLossesRelatedtoChangesinEstimatedFairValue
Short-term investments, AFS, at fair value   456,288us-gaap_OtherShortTermInvestments
Equity securities    
Cost of equity securities 763,833us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis 903,201us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
Equity securities, gross unrealized gains   154,506us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Equity securities, gross unrealized losses   (17,470)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Equity securities, AFS, at fair value 868,292us-gaap_AvailableForSaleSecuritiesEquitySecurities 1,040,237us-gaap_AvailableForSaleSecuritiesEquitySecurities
Total investments - AFS    
Cost or Amortized cost of total investments - AFS 28,749,825us-gaap_AvailableForSaleSecuritiesAmortizedCost 28,470,545us-gaap_AvailableForSaleSecuritiesAmortizedCost
Available for sale securities, gross unrealized gain 1,947,804us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 963,152us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Available for sale securities, gross unrealized loss (213,576)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (437,036)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Total investments available for sale 30,484,053us-gaap_AvailableForSaleSecurities 28,996,661us-gaap_AvailableForSaleSecurities
HTM securities    
Fixed maturities, held to maturity 0us-gaap_HeldToMaturitySecurities 2,858,695us-gaap_HeldToMaturitySecurities
Fixed maturities, held to maturity, gross unrealized losses 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss  
Fair value of held to maturity securities 0us-gaap_HeldToMaturitySecuritiesFairValue 3,131,235us-gaap_HeldToMaturitySecuritiesFairValue
Fixed maturities, trading, at fair value 1,171us-gaap_TradingSecuritiesDebt 0us-gaap_TradingSecuritiesDebt
US Government And Government Related [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   2,484,193us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available for sale fixed income securities, gross unrealized gains   51,701us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Available for sale fixed income securities, gross unrealized losses   (34,043)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fixed maturities, AFS, at fair value   2,501,851us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Losses Non-credit Related OTTI   0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]
HTM securities    
Fixed maturities, held to maturity   10,993us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fixed maturities, held to maturity, gross unrealized gains   629us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fixed maturities, held to maturity, gross unrealized losses   0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fair value of held to maturity securities   11,622us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Corporate    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   10,802,332us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Available for sale fixed income securities, gross unrealized gains   433,097us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [2]
Available for sale fixed income securities, gross unrealized losses   (109,599)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [2]
Fixed maturities, AFS, at fair value   11,125,830us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Losses Non-credit Related OTTI   (4,758)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]
HTM securities    
Fixed maturities, held to maturity   1,386,863us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fixed maturities, held to maturity, gross unrealized gains   113,179us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fixed maturities, held to maturity, gross unrealized losses   (968)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair value of held to maturity securities   1,499,074us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
RMBS - Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   3,540,101us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available for sale fixed income securities, gross unrealized gains   68,098us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Available for sale fixed income securities, gross unrealized losses   (62,077)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Fixed maturities, AFS, at fair value   3,546,122us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Gross Unrealized Losses Non-credit Related OTTI   0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]
RMBS - Non-Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   396,798us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Available for sale fixed income securities, gross unrealized gains   33,096us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Available for sale fixed income securities, gross unrealized losses   (31,126)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Fixed maturities, AFS, at fair value   398,768us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Gross Unrealized Losses Non-credit Related OTTI   (74,528)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 [1]
HTM securities    
Fixed maturities, held to maturity   66,987us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Fixed maturities, held to maturity, gross unrealized gains   4,985us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Fixed maturities, held to maturity, gross unrealized losses   0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Fair value of held to maturity securities   71,972us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
CMBS    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   1,223,313us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Available for sale fixed income securities, gross unrealized gains   39,255us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Available for sale fixed income securities, gross unrealized losses   (15,773)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fixed maturities, AFS, at fair value   1,246,795us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Gross Unrealized Losses Non-credit Related OTTI   (2,753)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]
HTM securities    
Fixed maturities, held to maturity   144,924us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fixed maturities, held to maturity, gross unrealized gains   11,864us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fixed maturities, held to maturity, gross unrealized losses   0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Fair value of held to maturity securities   156,788us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Collateralized debt obligations ('CDO') [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   754,414us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Available for sale fixed income securities, gross unrealized gains   5,833us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Available for sale fixed income securities, gross unrealized losses   (42,934)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Fixed maturities, AFS, at fair value   717,313us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember
Gross Unrealized Losses Non-credit Related OTTI   (2,036)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		 [1]
Other asset-backed securities    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   1,210,384us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Available for sale fixed income securities, gross unrealized gains   40,560us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [2]
Available for sale fixed income securities, gross unrealized losses   (8,840)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [2]
Fixed maturities, AFS, at fair value   1,242,104us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Gross Unrealized Losses Non-credit Related OTTI   (2,807)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [1]
HTM securities    
Fixed maturities, held to maturity   106,540us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Fixed maturities, held to maturity, gross unrealized gains   6,908us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Fixed maturities, held to maturity, gross unrealized losses   0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
Fair value of held to maturity securities   113,448us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember
U.S. States and political subdivisions of the States [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   1,821,499us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available for sale fixed income securities, gross unrealized gains   55,083us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available for sale fixed income securities, gross unrealized losses   (30,770)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Fixed maturities, AFS, at fair value   1,845,812us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Losses Non-credit Related OTTI   0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1]
Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities   4,878,840us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Available for sale fixed income securities, gross unrealized gains   80,961us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Available for sale fixed income securities, gross unrealized losses   (84,260)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fixed maturities, AFS, at fair value   4,875,541us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Gross Unrealized Losses Non-credit Related OTTI   0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 [1]
HTM securities    
Fixed maturities, held to maturity   1,142,388us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fixed maturities, held to maturity, gross unrealized gains   136,585us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fixed maturities, held to maturity, gross unrealized losses   (642)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
Fair value of held to maturity securities   1,278,331us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
HTM    
HTM securities    
Fixed maturities, held to maturity 0us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember		 2,858,695us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember
Fixed maturities, held to maturity, gross unrealized gains 0us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember		 274,150us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember
Fixed maturities, held to maturity, gross unrealized losses   (1,610)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember
Fair value of held to maturity securities 0us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember		 3,131,235us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HeldtomaturitySecuritiesMember
Exclude Life Funds Withheld Assets [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 23,427,432us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		 27,111,874us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
Available for sale fixed income securities, gross unrealized gains 936,840us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Available for sale fixed income securities, gross unrealized losses (186,803)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Fixed maturities, AFS, at fair value 24,177,469us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		 27,500,136us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
Gross Unrealized Losses Non-credit Related OTTI (76,720)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		 [1]  
Short Term Investments    
Short-term investments, cost or amortized cost 257,221us-gaap_AvailableForSaleSecuritiesShortTermInvestmentsAmortizedCost
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Short -term investments, Gross unrealized gains 49xlgroup_ShortTermInvestmentsGrossUnrealizedGains
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Short -term investments, Gross unrealized losses (543)xlgroup_ShortTermInvestmentsGrossUnrealizedLossesRelatedtoChangesinEstimatedFairValue
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Short-term investments, AFS, at fair value 256,727us-gaap_OtherShortTermInvestments
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Equity securities    
Cost of equity securities 763,833us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Equity securities, gross unrealized gains 130,689us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Equity securities, gross unrealized losses (26,230)us-gaap_AvailableForSaleEquitySecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Equity securities, AFS, at fair value 868,292us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Total investments - AFS    
Cost or Amortized cost of total investments - AFS 24,448,486us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Available for sale securities, gross unrealized gain 1,067,578us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Available for sale securities, gross unrealized loss (213,576)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Total investments available for sale 25,302,488us-gaap_AvailableForSaleSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember		  
Exclude Life Funds Withheld Assets [Member] | US Government And Government Related [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 2,100,851us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Available for sale fixed income securities, gross unrealized gains 77,889us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Available for sale fixed income securities, gross unrealized losses (6,787)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Fixed maturities, AFS, at fair value 2,171,953us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | Corporate    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 8,462,130us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized gains 365,805us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized losses (49,562)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Fixed maturities, AFS, at fair value 8,778,373us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Gross Unrealized Losses Non-credit Related OTTI (3,309)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | RMBS - Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 3,625,171us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Available for sale fixed income securities, gross unrealized gains 114,188us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Available for sale fixed income securities, gross unrealized losses (10,783)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Fixed maturities, AFS, at fair value 3,728,576us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | RMBS - Non-Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 404,398us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Available for sale fixed income securities, gross unrealized gains 41,108us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Available for sale fixed income securities, gross unrealized losses (18,155)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Fixed maturities, AFS, at fair value 427,351us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Gross Unrealized Losses Non-credit Related OTTI (67,918)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | CMBS    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 1,033,819us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Available for sale fixed income securities, gross unrealized gains 23,987us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Available for sale fixed income securities, gross unrealized losses (5,262)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Fixed maturities, AFS, at fair value 1,052,544us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Gross Unrealized Losses Non-credit Related OTTI (2,033)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | Collateralized debt obligations ('CDO') [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 717,544us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Available for sale fixed income securities, gross unrealized gains 1,659us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Available for sale fixed income securities, gross unrealized losses (27,169)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Fixed maturities, AFS, at fair value 692,034us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		  
Gross Unrealized Losses Non-credit Related OTTI (1,663)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedDebtObligationsMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | Other asset-backed securities    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 1,028,528us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		  
Available for sale fixed income securities, gross unrealized gains 42,810us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [3]  
Available for sale fixed income securities, gross unrealized losses (6,045)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [3]  
Fixed maturities, AFS, at fair value 1,065,293us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		  
Gross Unrealized Losses Non-credit Related OTTI (1,797)xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesSecuritizedLoansAndReceivablesMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | U.S. States and political subdivisions of the States [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 1,892,566us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Available for sale fixed income securities, gross unrealized gains 129,910us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Available for sale fixed income securities, gross unrealized losses (1,204)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Fixed maturities, AFS, at fair value 2,021,272us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 [1]  
Exclude Life Funds Withheld Assets [Member] | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 4,162,425us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized gains 139,484us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized losses (61,836)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		  
Fixed maturities, AFS, at fair value 4,240,073us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_ExcludeLifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember		 [1]  
Life Funds Withheld Assets [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 4,301,339us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		 0us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
Available for sale fixed income securities, gross unrealized gains 880,226us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		  
Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		  
Fixed maturities, AFS, at fair value 5,181,565us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		 0us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		 [1]  
HTM securities    
Trading Securities, Cost 1,180us-gaap_TradingSecuritiesCost
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		  
Fixed maturities, trading, at fair value 1,171us-gaap_TradingSecuritiesDebt
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember		  
Life Funds Withheld Assets [Member] | US Government And Government Related [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 14,866us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Available for sale fixed income securities, gross unrealized gains 3,858us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Fixed maturities, AFS, at fair value 18,724us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember		 [1]  
Life Funds Withheld Assets [Member] | Corporate    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 2,407,849us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized gains 410,131us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Fixed maturities, AFS, at fair value 2,817,980us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		 [1]  
HTM securities    
Trading Securities, Cost 1,180us-gaap_TradingSecuritiesCost
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Fixed maturities, trading, at fair value 1,171us-gaap_TradingSecuritiesDebt
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Life Funds Withheld Assets [Member] | RMBS - Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 3,301us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Available for sale fixed income securities, gross unrealized gains 481us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Fixed maturities, AFS, at fair value 3,782us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember		 [1]  
Life Funds Withheld Assets [Member] | RMBS - Non-Agency    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 71,075us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Available for sale fixed income securities, gross unrealized gains 14,260us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Fixed maturities, AFS, at fair value 85,335us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentTypeAxis
= xlgroup_LifeFundsWithheldAssetsMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember		  
Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
/ us-gaap_InvestmentTypeAxis
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Fixed maturities - AFS    
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Available for sale fixed income securities, gross unrealized gains 24,281us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fixed maturities, AFS, at fair value 193,167us-gaap_AvailableForSaleSecuritiesDebtSecurities
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Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
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Life Funds Withheld Assets [Member] | Collateralized debt obligations ('CDO') [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 0us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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Available for sale fixed income securities, gross unrealized gains 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fixed maturities, AFS, at fair value 0us-gaap_AvailableForSaleSecuritiesDebtSecurities
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Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
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= us-gaap_CollateralizedDebtObligationsMember		 [1]  
Life Funds Withheld Assets [Member] | Other asset-backed securities    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 238,168us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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Available for sale fixed income securities, gross unrealized gains 35,373us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fixed maturities, AFS, at fair value 273,541us-gaap_AvailableForSaleSecuritiesDebtSecurities
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Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
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Life Funds Withheld Assets [Member] | U.S. States and political subdivisions of the States [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 0us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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Available for sale fixed income securities, gross unrealized gains 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fixed maturities, AFS, at fair value 0us-gaap_AvailableForSaleSecuritiesDebtSecurities
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Gross Unrealized Losses Non-credit Related OTTI 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
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Life Funds Withheld Assets [Member] | Non-U.S. Sovereign Government, Provincial, Supranational, and Government-Related/Supported [Member]    
Fixed maturities - AFS    
Amortized cost of Fixed maturities 1,397,194us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
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Available for sale fixed income securities, gross unrealized gains 391,842us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Available for sale fixed income securities, gross unrealized losses 0us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Fixed maturities, AFS, at fair value 1,789,036us-gaap_AvailableForSaleSecuritiesDebtSecurities
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Gross Unrealized Losses Non-credit Related OTTI $ 0xlgroup_DebtSecuritiesOtherThanTemporaryImpairmentsIncludedInOtherComprehensiveIncome
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[1] Represents the non-credit component of OTTI losses, adjusted for subsequent sales of securities. It does not include the change in fair value subsequent to the impairment measurement date.
[2] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $154.6 million and an amortized cost of $147.7 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.
[3] Included are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $79.9 million and an amortized cost of $68.4 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.