XML 25 R112.htm IDEA: XBRL DOCUMENT v2.4.1.9
Derivatives (Additional Collateral) (Details) (White Mountains Life Re, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Credit risk on derivative instruments      
Collateral Cash and Investments $ 33.2wtm_CollateralCashAndInvestments $ 81.3wtm_CollateralCashAndInvestments $ 393.6wtm_CollateralCashAndInvestments
Cash
      
Credit risk on derivative instruments      
Derivative Collateral Right to Reclaim 23.7wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashAndCashEquivalentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		 56.1wtm_DerivativeCollateralRightToReclaim
/ us-gaap_InvestmentTypeAxis
= us-gaap_CashAndCashEquivalentsMember
/ dei_LegalEntityAxis
= wtm_WhiteMountainsLifeReMember		  
Short-term investments
      
Credit risk on derivative instruments      
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		  
Fixed maturity investments
      
Credit risk on derivative instruments      
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