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Derivatives (Level 3 Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Discounted cash flows |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 0.7 us-gaap_LiabilitiesFairValueDisclosure / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Equity Index Options | Counterparty valuations [Member] |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 11.7 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = wtm_EquityIndexOptionsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Equity Index Options | Counterparty valuations [Member] | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Adjustment to counterparty valuations | 0.70%wtm_FairValueInputsSignificantUnobservableInputsForEquityIndexOptions / us-gaap_FairValueByLiabilityClassAxis = wtm_EquityIndexOptionsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Equity Index Options | Counterparty valuations [Member] | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Adjustment to counterparty valuations | 6.50%wtm_FairValueInputsSignificantUnobservableInputsForEquityIndexOptions / us-gaap_FairValueByLiabilityClassAxis = wtm_EquityIndexOptionsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Equity Index Options | Counterparty valuations [Member] | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Adjustment to counterparty valuations | 2.00%wtm_FairValueInputsSignificantUnobservableInputsForEquityIndexOptions / us-gaap_FairValueByLiabilityClassAxis = wtm_EquityIndexOptionsMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Mortality | 0.00%wtm_FairValueInputsMortality / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Mortality | 5.90%wtm_FairValueInputsMortality / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Mortality | 1.00%wtm_FairValueInputsMortality / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Foreign Exchange Option [Member] | Counterparty valuations [Member] |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Fair value Liabilities | 7.2 us-gaap_LiabilitiesFairValueDisclosure / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ForeignExchangeOptionMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities | (0.50%)wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ForeignExchangeOptionMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities | 11.60%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ForeignExchangeOptionMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Foreign Exchange Option [Member] | Counterparty valuations [Member] | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Foreign exchange volatilities | 3.60%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_ForeignExchangeOptionMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_CounterpartyvaluationsMember
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 0.10%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Foreign exchange volatilities | 11.70%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Index volatilities | 19.40%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 40.00%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Foreign exchange volatilities | 13.80%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Index volatilities | 23.10%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Year Two [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 32.10%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Foreign exchange volatilities | 12.60%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Index volatilities | 21.80%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearTwoMember
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Minimum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 0.20%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Foreign exchange volatilities | 12.60%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Index volatilities | 24.00%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Maximum |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 40.00%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Foreign exchange volatilities | 15.10%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Index volatilities | 26.60%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Year One [Member] | Variable annuity benefit guarantee liabilities | Discounted cash flows | Weighted Average |
Quantitative information for Level 3 Fair Value Measurements Liabilities |
Surrenders | 40.00%wtm_FairValueInputsSurrenders / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Foreign exchange volatilities | 13.30%wtm_FairValueInputsForeignExchangeVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember
Index volatilities | 25.60%wtm_FairValueInputsIndexVolatilities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_VariableAnnuityMember / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember / wtm_VolatilityYearAxis = wtm_YearOneMember

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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Represents the foreign exchange volatilities, used as an input to measure fair value. + References No definition available. + Details
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- Definition Represents the mortality, used as an input to measure fair value. + References No definition available. + Details
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- Definition Significant unobservable inputs for equity index options + References No definition available. + Details
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