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Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Private equity funds |  |
Investment securities |  |
Number of Investments Held | 32 wtm_NumberOfInvestmentsHeld / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember |
Trading Securities, Other | $ 150.9 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember |
Hedge funds |  |
Investment securities |  |
Number of Investments Held | 12 wtm_NumberOfInvestmentsHeld / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember |
Trading Securities, Other | 92.0 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember |
Total hedge and private equity funds included in other long-term investments |  |
Investment securities |  |
Trading Securities, Other | 242.9 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember | 239.0 us-gaap_TradingSecuritiesOther / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember
Fair value unfunded commitments | 81.7 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember | 110.6 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember
Other Long-term Investments Largest Single Investment | 22.0 wtm_OtherLongTermInvestmentsLargestSingleInvestment / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember | 18.3 wtm_OtherLongTermInvestmentsLargestSingleInvestment / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsAndPrivateEquityFundsMember
Private equity funds | Private equity funds |  |
Investment securities |  |
Trading Securities, Other | 150.9 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember | 118.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember
Fair value unfunded commitments | 81.7 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember | 110.6 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PrivateEquityFundsMember
Private equity funds | Multi-sector |  |
Investment securities |  |
Trading Securities, Other | 24.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityMultiSectorMember | 23.8 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityMultiSectorMember
Fair value unfunded commitments | 5.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityMultiSectorMember | 6.5 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityMultiSectorMember
Private equity funds | Energy infrastructure & services |  |
Investment securities |  |
Trading Securities, Other | 59.6 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityEnergyInfrastructureAndServicesMember | 45.9 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityEnergyInfrastructureAndServicesMember
Fair value unfunded commitments | 11.0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityEnergyInfrastructureAndServicesMember | 13.1 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityEnergyInfrastructureAndServicesMember
Private equity funds | Distressed residential real estate |  |
Investment securities |  |
Trading Securities, Other | 0 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityDistressedResidentialRealEstateMember | 0.4 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityDistressedResidentialRealEstateMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityDistressedResidentialRealEstateMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityDistressedResidentialRealEstateMember
Private equity funds | Real estate |  |
Investment securities |  |
Trading Securities, Other | 3.6 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_RealEstateFundsMember | 8.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_RealEstateFundsMember
Fair value unfunded commitments | 3.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_RealEstateFundsMember | 3.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_RealEstateFundsMember
Private equity funds | Private equity secondaries |  |
Investment securities |  |
Trading Securities, Other | 8.5 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquitySecondariesMember | 9.5 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquitySecondariesMember
Fair value unfunded commitments | 3.1 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquitySecondariesMember | 3.1 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquitySecondariesMember
Private equity funds | International multi-sector, Europe |  |
Investment securities |  |
Trading Securities, Other | 1.5 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorEuropeMember | 3.9 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorEuropeMember
Fair value unfunded commitments | 2.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorEuropeMember | 2.8 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorEuropeMember
Private equity funds | Manufacturing/Industrial |  |
Investment securities |  |
Trading Securities, Other | 23.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsManufacturingAndIndustrialMember | 11.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsManufacturingAndIndustrialMember
Fair value unfunded commitments | 7.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsManufacturingAndIndustrialMember | 15.5 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsManufacturingAndIndustrialMember
Private equity funds | Healthcare |  |
Investment securities |  |
Trading Securities, Other | 6.1 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityHealthcareMember | 5.6 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityHealthcareMember
Fair value unfunded commitments | 2.8 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityHealthcareMember | 2.8 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityHealthcareMember
Private equity funds | International multi-sector, Asia |  |
Investment securities |  |
Trading Securities, Other | 0 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorAsiaMember | 0 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorAsiaMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorAsiaMember | 2.7 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityFundsMultiSectorAsiaMember
Private equity funds | Insurance |  |
Investment securities |  |
Trading Securities, Other | 2.1 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityInsuranceMember | 2.3 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityInsuranceMember
Fair value unfunded commitments | 41.2 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityInsuranceMember | 41.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_PrivateEquityInsuranceMember
Private equity funds | Aerospace/Defense/Government |  |
Investment securities |  |
Trading Securities, Other | 20.7 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_AerospaceDefenseGovernmentMember | 5.8 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_AerospaceDefenseGovernmentMember
Fair value unfunded commitments | 5.1 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_AerospaceDefenseGovernmentMember | 19.2 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_AerospaceDefenseGovernmentMember
Private equity funds | Venture capital |  |
Investment securities |  |
Trading Securities, Other | 1.4 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_VentureCapitalFundsMember | 1.6 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_VentureCapitalFundsMember
Fair value unfunded commitments | 0.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_VentureCapitalFundsMember | 0.3 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_VentureCapitalFundsMember
Hedge funds | Hedge funds |  |
Investment securities |  |
Trading Securities, Other | 92.0 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember | 120.8 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsMember
Hedge funds | Long/short equity |  |
Investment securities |  |
Trading Securities, Other | 43.9 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsEquityLongShortMember | 62.6 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsEquityLongShortMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsEquityLongShortMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_HedgeFundsEquityLongShortMember
Hedge funds | Long/short credit & distressed |  |
Investment securities |  |
Trading Securities, Other | 21.4 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsCreditAndDistressedMember | 22.8 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsCreditAndDistressedMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsCreditAndDistressedMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsCreditAndDistressedMember
Hedge funds | Long diversified strategies |  |
Investment securities |  |
Trading Securities, Other | 0 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsDiversifiedStrategiesMember | 0.1 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsDiversifiedStrategiesMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsDiversifiedStrategiesMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsDiversifiedStrategiesMember
Hedge funds | Long/short equity REIT |  |
Investment securities |  |
Trading Securities, Other | 20.3 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityREITMember | 18.3 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityREITMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityREITMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityREITMember
Hedge funds | Long/short equity activist |  |
Investment securities |  |
Trading Securities, Other | 6.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityActivistMember | 16.8 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityActivistMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityActivistMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsEquityActivistMember
Hedge funds | Long bank loan |  |
Investment securities |  |
Trading Securities, Other | 0.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsBankLoanMember | 0.2 us-gaap_TradingSecuritiesOther / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsBankLoanMember
Fair value unfunded commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsBankLoanMember | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = wtm_HedgeFundsBankLoanMember
DavidShield [Member] |  |
Investment securities |  |
Fair value unfunded commitments | $ 21.0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / invest_InvestmentAxis = wtm_DavidShieldMember |

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