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Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Unrealized gains (losses) on investments | $ (5.2)wtm_UnrealizedGainsLossesOnInvestments | $ 7.1 wtm_UnrealizedGainsLossesOnInvestments
Fixed maturity investments. |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Unrealized gains (losses) on investments | (0.7)wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 0.2 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
Common equity securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Unrealized gains (losses) on investments | (1.8)wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0.8 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Convertible Debt Securities |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Unrealized gains (losses) on investments | 0.1 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember | 0 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ConvertibleDebtSecuritiesMember
Other long-term investments |  |
Schedule of Trading Securities and Other Trading Assets [Line Items] |  |
Unrealized gains (losses) on investments | $ (2.8)wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember | $ 6.1 wtm_UnrealizedGainsLossesOnInvestments / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherLongTermInvestmentsMember

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- Details
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- Definition The aggregate increase (decrease) in the market value of unsold investments whose gains (losses) are included in earnings. + References No definition available. + Details
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