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Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Credit risk on derivative instruments      
Collateral cash and investments $ 33.5wtm_CollateralCashAndInvestments $ 33.2wtm_CollateralCashAndInvestments $ 36.5wtm_CollateralCashAndInvestments
Cash
      
Credit risk on derivative instruments      
Derivative collateral right to reclaim 29.0wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashAndCashEquivalentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 23.7wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashAndCashEquivalentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 22.2wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashAndCashEquivalentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember
Fixed maturity investments.
      
Credit risk on derivative instruments      
Derivative collateral right to reclaim $ 4.5wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 $ 9.5wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 $ 14.3wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember