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Investment Securities (Investments Held on Deposit or as Collateral) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Investment securities    
Investments held in trusts $ 171.5wtm_InvestmentsHeldInTrusts $ 170.4wtm_InvestmentsHeldInTrusts
Assets held by insurance regulators 130.3us-gaap_AssetsHeldByInsuranceRegulators 290.7us-gaap_AssetsHeldByInsuranceRegulators
Surety Product Line [Member] | OneBeacon    
Investment securities    
Assets Held in Collateral 81.0wtm_AssetsHeldinCollateral
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= us-gaap_SuretySegmentMember		 63.3wtm_AssetsHeldinCollateral
/ dei_LegalEntityAxis
= wtm_OnebeaconMember
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= us-gaap_SuretySegmentMember
Fixed maturity investments | White Mountains Life Re    
Investment securities    
Derivative Collateral Right to Reclaim 9.5wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 23.2wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember
Short-term investments | White Mountains Life Re    
Investment securities    
Derivative Collateral Right to Reclaim 0wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 2.0wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember
Interest Rate Cap    
Investment securities    
Derivative Collateral Right to Reclaim $ 4.3wtm_DerivativeCollateralRightToReclaim
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateCapMember