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Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details) (Hedge funds, USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Investment securities  
Trading Securities, Other $ 92.0us-gaap_TradingSecuritiesOther
Distributions from inactive hedge funds 2.1wtm_DistributionsFromInactiveHedgeFunds
Outstanding redemptions 6.7wtm_Outstandingredemptions
30-59 days notice  
Investment securities  
Trading Securities, Other 36.1us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
60-89 days notice  
Investment securities  
Trading Securities, Other 43.5us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
90-119 days notice  
Investment securities  
Trading Securities, Other 4.4us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
120 days notice  
Investment securities  
Trading Securities, Other 8.0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
Monthly  
Investment securities  
Trading Securities, Other 4.3us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 30-59 days notice  
Investment securities  
Trading Securities, Other 4.3us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 60-89 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 90-119 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Monthly | 120 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyMonthlyMember
Quarterly  
Investment securities  
Trading Securities, Other 61.0us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 30-59 days notice  
Investment securities  
Trading Securities, Other 31.8us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 60-89 days notice  
Investment securities  
Trading Securities, Other 21.4us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 90-119 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Quarterly | 120 days notice  
Investment securities  
Trading Securities, Other 7.8us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyQuarterlyMember
Semi-annual  
Investment securities  
Trading Securities, Other 22.1us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 30-59 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 60-89 days notice  
Investment securities  
Trading Securities, Other 22.1us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 90-119 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Semi-annual | 120 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencySemiAnnualMember
Annual  
Investment securities  
Trading Securities, Other 4.6us-gaap_TradingSecuritiesOther
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 30-59 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod30To59DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 60-89 days notice  
Investment securities  
Trading Securities, Other 0us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod60To89DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 90-119 days notice  
Investment securities  
Trading Securities, Other 4.4us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod90To119DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember
Annual | 120 days notice  
Investment securities  
Trading Securities, Other $ 0.2us-gaap_TradingSecuritiesOther
/ wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriodOver120DaysMember
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_HedgeFundsMember
/ wtm_RedemptionFrequencyAxis
= wtm_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionFrequencyAnnualMember