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Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | Mar. 31, 2015 Rating | Dec. 31, 2014
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Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | $ 47.0 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet | $ 56.4 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet
Collertal provided to counterparty in cash | 1.6 us-gaap_DerivativeCollateralRightToReclaimCash | 3.5 us-gaap_DerivativeCollateralRightToReclaimCash
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities
Net amount of exposure after effect of collateral | 48.6 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral | 59.9 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral
Excess collateral provided to counterparty in cash | 27.4 wtm_Excesscollateralprovidedtocounterpartyincash | 17.7 wtm_Excesscollateralprovidedtocounterpartyincash
Derivative, Collateral, Right to Reclaim Securities | 4.5 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments | 9.5 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments
Counter-party collateral held by WMLife Re - Cash | 7.4 us-gaap_DerivativeCollateralObligationToReturnCash | 9.9 us-gaap_DerivativeCollateralObligationToReturnCash
Net amount of exposure to counter-party | 73.1 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments | 77.2 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments
S and P Credit Ratings | 23 wtm_SAndPCreditRatings |
Standard & Poor's, A Rating | Bank of America [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 4.9 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 5.6 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 4.9 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 5.6 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 4.9 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 5.6 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_BankOfAmericaMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, A Rating | Barclays |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 0 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0.1 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0.1 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 0 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0.1 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_BarclaysMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, A Rating | Citigroup - OTC |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 18.6 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 22.2 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 18.6 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 22.2 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 0.7 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 1.1 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 19.3 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 21.1 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupOtcMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, A Rating | Citigroup Exchange Traded [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | (1.2)us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 3.7 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 1.2 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 3.7 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 21.1 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 16.0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 21.1 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 19.7 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_CitigroupExchangeTradedMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsARatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, A plus | JP Morgan |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 21.3 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 24.3 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 21.3 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 24.3 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 7.4 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 8.8 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 13.9 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 15.5 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_JPMorganChaseMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, BBB+ Rating [Member] | Nomura [Member] |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | (0.4)us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (3.5)us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0.4 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 3.5 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 5.6 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 1.7 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 4.5 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 9.5 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | 10.1 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 11.2 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_NomuraMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBBBPlusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Standard & Poor's, A- Rating | Royal Bank of Scotland |  |
Credit risk on derivative instruments |  |
Excess collateral provided - Financial Instruments | 3.8 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 4.0 us-gaap_DerivativeFairValueAmountOffsetAgainstCollateralNet / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collertal provided to counterparty in cash | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimCash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Collateral provided to counterparty in financial instruments | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralRightToReclaimSecurities / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure after effect of collateral | 3.8 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 4.0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Excess collateral provided to counterparty in cash | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyincash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative, Collateral, Right to Reclaim Securities | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_Excesscollateralprovidedtocounterpartyinfinancialinstruments / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Counter-party collateral held by WMLife Re - Cash | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 us-gaap_DerivativeCollateralObligationToReturnCash / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Net amount of exposure to counter-party | $ 3.8 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 4.0 wtm_NetExposureOnFairValueOfOverCounterDerivativeInstruments / us-gaap_CounterpartyNameAxis = wtm_RoyalBankOfScotlandMember / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsAMinusRatingMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember

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