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v2.4.1.9

Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases | $ 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases | $ 59.4 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases |
Changes in derivative instruments |  |  |
Beginning of period | 69.2 us-gaap_DerivativeFairValueOfDerivativeNet | 98.3 us-gaap_DerivativeFairValueOfDerivativeNet | 290.4 us-gaap_DerivativeFairValueOfDerivativeNet
Purchases |  |  | 6.1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Realized and unrealized gains (losses) | 72.4 us-gaap_DerivativeGainLossOnDerivativeNet | 402.0 us-gaap_DerivativeGainLossOnDerivativeNet | 339.0 us-gaap_DerivativeGainLossOnDerivativeNet
Transfers in (out) | 0 wtm_DerivativeTransfersInOutNet | 0 wtm_DerivativeTransfersInOutNet | 0 wtm_DerivativeTransfersInOutNet
Sales/settlements | 59.6 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements | 313.5 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements | 140.8 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements
Ending balance | 56.4 us-gaap_DerivativeFairValueOfDerivativeNet | 69.2 us-gaap_DerivativeFairValueOfDerivativeNet | 98.3 us-gaap_DerivativeFairValueOfDerivativeNet
Collateral Cash and Investments | 33.2 wtm_CollateralCashAndInvestments | 81.3 wtm_CollateralCashAndInvestments | 393.6 wtm_CollateralCashAndInvestments
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses | 0.6 wtm_GainLossOnLiabilityForDeathBenefit | 10.2 wtm_GainLossOnLiabilityForDeathBenefit | 14.2 wtm_GainLossOnLiabilityForDeathBenefit
Level 3 |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases |  | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Changes in variable annuity (liabilities) |  |  |
Purchases |  |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Level 2 |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Changes in derivative instruments |  |  |
Beginning of period | 4.7 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (20.5)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 39.2 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Purchases |  |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Realized and unrealized gains (losses) | 71.0 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 196.1 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 186.9 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Transfers in (out) | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Sales/settlements | 100.1 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 221.3 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 127.2 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Ending balance | 33.8 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 4.7 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (20.5)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Level 1 |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Changes in derivative instruments |  |  |
Beginning of period | 1.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (21.7)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 4.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Purchases |  |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Realized and unrealized gains (losses) | (37.2)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 69.4 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 68.1 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Transfers in (out) | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Sales/settlements | (34.6)wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 92.2 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 42.3 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Ending balance | 3.7 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 1.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (21.7)us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Variable Annuity [Member] | Level 3 |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |  |
Changes in variable annuity (liabilities) |  |  |
Beginning balance | (52.8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (441.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (768.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Realized and unrealized gains (losses) | (53.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (388.7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (327.0)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Transfers in (out) | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfers / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfers / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfers / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Sales/settlements | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySalesOrSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySalesOrSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySalesOrSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Ending balance | 0.7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (52.8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (441.5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_VariableAnnuityMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Derivative [Member] | Level 3 |  |  |
Variable Annuity Reinsurance [Line Items] |  |  |
Fair Value Derivatives Measured on Recurring Basis, Asset Purchases | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 59.4 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |
Changes in derivative instruments |  |  |
Beginning of period | 63.4 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 140.5 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 247.1 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Purchases |  |  | 6.1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Realized and unrealized gains (losses) | 38.6 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 136.5 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 84.0 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Transfers in (out) | 0 wtm_DerivativeTransfersInOutNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_DerivativeTransfersInOutNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Sales/settlements | (5.9)wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 0 wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (28.7)wtm_FairValueDerivativesMeasuredOnRecurringBasisAssetSalesAndSettlements / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Ending balance | $ 18.9 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 63.4 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 140.5 us-gaap_DerivativeFairValueOfDerivativeNet / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_DerivativeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember

X
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959 + Details
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- Definition Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized on the income statement for financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c)(2) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + 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 (c) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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