XML 162 R92.htm IDEA: XBRL DOCUMENT v2.4.1.9
Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details) (Private equity funds, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Investment securities  
Trading Securities, Other $ 150.9us-gaap_TradingSecuritiesOther
1-3 years  
Investment securities  
Trading Securities, Other 12.7us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PrivateEquityFundsMember
/ wtm_PrivateEquityFundsRemainingLockUpPeriodAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod1To3YearsMember
3 – 5 years  
Investment securities  
Trading Securities, Other 33.5us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PrivateEquityFundsMember
/ wtm_PrivateEquityFundsRemainingLockUpPeriodAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod3To5YearsMember
5 – 10 years  
Investment securities  
Trading Securities, Other 79.0us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PrivateEquityFundsMember
/ wtm_PrivateEquityFundsRemainingLockUpPeriodAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod5To10YearsMember
10 years  
Investment securities  
Trading Securities, Other $ 25.7us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PrivateEquityFundsMember
/ wtm_PrivateEquityFundsRemainingLockUpPeriodAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriodOver10YearsMember