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Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Pre-tax operating results of WM Life Re |  |  |  |  |  |  |  |  |  |  |
Total revenues | $ 689.7 us-gaap_Revenues | $ 589.4 us-gaap_Revenues | $ 652.8 us-gaap_Revenues | $ 578.3 us-gaap_Revenues | $ 626.6 us-gaap_Revenues | $ 574.1 us-gaap_Revenues | $ 489.3 us-gaap_Revenues | $ 627.4 us-gaap_Revenues | $ 2,510.2 us-gaap_Revenues | $ 2,317.4 us-gaap_Revenues | $ 2,435.7 us-gaap_Revenues
General and administrative expenses |  |  |  |  |  |  |  |  | (288.2)us-gaap_GeneralAndAdministrativeExpense | (181.3)us-gaap_GeneralAndAdministrativeExpense | (182.2)us-gaap_GeneralAndAdministrativeExpense
Pre-tax income | 46.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 34.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 109.0 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 112.1 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 134.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 55.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 3.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 151.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 301.7 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 344.9 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | 262.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
White Mountains Life Re |  |  |  |  |  |  |  |  |  |  |
Pre-tax operating results of WM Life Re |  |  |  |  |  |  |  |  |  |  |
Fees, included in other revenue |  |  |  |  |  |  |  |  | 18.6 us-gaap_InsuranceCommissionsAndFees / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 25.0 us-gaap_InsuranceCommissionsAndFees / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 31.8 us-gaap_InsuranceCommissionsAndFees / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Change in Fair Value of Variable Annuity Liability, included in other revenues |  |  |  |  |  |  |  |  | 52.9 wtm_GainLossOnVariableAnnuityLiabilitiesFromChangesAssociatedWithProjectedSurrenderAssumptions / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 378.5 wtm_GainLossOnVariableAnnuityLiabilitiesFromChangesAssociatedWithProjectedSurrenderAssumptions / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 312.8 wtm_GainLossOnVariableAnnuityLiabilitiesFromChangesAssociatedWithProjectedSurrenderAssumptions / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Change in fair value of derivatives, included in other revenue |  |  |  |  |  |  |  |  | (72.4)us-gaap_DerivativeGainLossOnDerivativeNet / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (402.0)us-gaap_DerivativeGainLossOnDerivativeNet / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (339.0)us-gaap_DerivativeGainLossOnDerivativeNet / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Foreign exchange, included in other revenue |  |  |  |  |  |  |  |  | (3.2)wtm_ForeignExchangeDerivativesGainLoss / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (14.5)wtm_ForeignExchangeDerivativesGainLoss / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (30.3)wtm_ForeignExchangeDerivativesGainLoss / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Other investment income and (losses) gains |  |  |  |  |  |  |  |  | (1.4)wtm_OtherInvestmentIncomeAndGainLossOnInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (7.1)wtm_OtherInvestmentIncomeAndGainLossOnInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 2.5 wtm_OtherInvestmentIncomeAndGainLossOnInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Total revenues |  |  |  |  |  |  |  |  | (5.5)us-gaap_Revenues / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (20.1)us-gaap_Revenues / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (22.2)us-gaap_Revenues / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses |  |  |  |  |  |  |  |  | 0.6 wtm_GainLossOnLiabilityForDeathBenefit / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 10.2 wtm_GainLossOnLiabilityForDeathBenefit / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | 14.2 wtm_GainLossOnLiabilityForDeathBenefit / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Death benefit claims paid, included in general and administrative expenses |  |  |  |  |  |  |  |  | (0.1)us-gaap_LiabilityForFuturePolicyBenefitsBenefitsPaid / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (1.9)us-gaap_LiabilityForFuturePolicyBenefitsBenefitsPaid / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (5.7)us-gaap_LiabilityForFuturePolicyBenefitsBenefitsPaid / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
General and administrative expenses |  |  |  |  |  |  |  |  | (4.3)us-gaap_GeneralAndAdministrativeExpense / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (4.9)us-gaap_GeneralAndAdministrativeExpense / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (5.2)us-gaap_GeneralAndAdministrativeExpense / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Pre-tax income |  |  |  |  |  |  |  |  | (9.3)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (16.7)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | (18.9)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember
Level 3 | Projected Surrender Assumption [Member] | White Mountains Life Re |  |  |  |  |  |  |  |  |  |  |
Pre-tax operating results of WM Life Re |  |  |  |  |  |  |  |  |  |  |
Change in fair value of derivatives, included in other revenue |  |  |  |  |  |  |  |  | $ (0.2)us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = wtm_ProjectedSurrenderAssumptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember | $ 7.2 us-gaap_DerivativeGainLossOnDerivativeNet / us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis = wtm_ProjectedSurrenderAssumptionMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / dei_LegalEntityAxis = wtm_WhiteMountainsLifeReMember |

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- Definition Income from agency and brokerage operations (includes sales of annuities and supplemental contracts); service charges, commissions, and fees from the sale of insurance and related services; and management fees from separate accounts, deferred annuities, and universal life products. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.4) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(c),(d)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 13 -Subparagraph c, d -Article 9 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 4 -Article 7 + Details
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