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Investment Securities (Significant Unobservable Inputs) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 Investment | Dec. 31, 2013
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value, Assets Measured on Recurring Basis, Transfers out | $ 97,800,000 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut | $ 217,600,000 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut
Number of Investments with Significant Unobseravble Inputs | 1 wtm_NumberOfInvestmentsWithSignificantUnobseravbleInputs |
Standard Poors NR Rating [Member] | Redeemable preferred stocks | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 71,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 71,000,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Standard Poors NR Rating [Member] | Redeemable preferred stocks | Discount Yield |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Discount Yield (as a percent) | 7.10%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember | 7.40%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember
Standard Poors NR Rating [Member] | Private equity funds | Multiple of GAAP book value |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Book Value Multiple to Calculate Fair Value | 110.00%wtm_BookValueMultipleToCalculateFairValue / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember | 100.00%wtm_BookValueMultipleToCalculateFairValue / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember
Assets, Fair Value Disclosure | 40,200,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember | 35,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember
Standard Poors NR Rating [Member] | Private equity funds | Recent transaction [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 20,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember | 10,500,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember
Standard Poors NR Rating [Member] | Private equity funds | Average Share Price [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Average Share Price Used As Unobservable Input | 1.06 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_AverageSharePriceMember | 1.10 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_AverageSharePriceMember
Standard Poors NR Rating [Member] | Private equity securities | Recent transaction [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Average Share Price Used As Unobservable Input |  | 0.71 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember
Assets, Fair Value Disclosure | 4,500,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember | 3,000,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember
Standard Poors NR Rating [Member] | Private equity securities | Multiple of EBITDA [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Unobservable Inputs | 6.00 wtm_UnobservableInputs / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember |
Assets, Fair Value Disclosure | 3,800,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember |
Standard Poors NR Rating [Member] | Mortgage-backed and asset-backed securities | Broker Pricing |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure |  | 22,000,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ValuationTechniqueAxis = wtm_BrokerPricingMember
Standard Poors NR Rating [Member] | Seller Priority Surplus Note [Member] | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Timing of Interest Payments | 5 years |
Discount Yield (as a percent) | 9.30%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Assets, Fair Value Disclosure | 44,000,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Timing of principal payments | 10 years |
Standard Poors NR Rating [Member] | Pari Passu Surplus Note [Member] | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Timing of Interest Payments | 5 years |
Discount Yield (as a percent) | 13.50%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Assets, Fair Value Disclosure | 21,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Timing of principal payments | 10 years |
Level 2 |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value, Assets Measured on Recurring Basis, Transfers out | 0 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 97,200,000 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Level 3 Inputs | Fixed maturity investments |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value, Assets Measured on Recurring Basis, Transfers out | 97,600,000 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember | 119,400,000 wtm_FairValueAssetsMeasuredOnRecurringBasisTransfersOut / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_FixedMaturitiesMember
durchblicker.at [Member] | Standard Poors NR Rating [Member] | Private equity funds | Recent Transaction Two [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Average Share Price Used As Unobservable Input | 290.96 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_Durchblicker.atMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionTwoMember |
Assets, Fair Value Disclosure | 10,400,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_Durchblicker.atMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionTwoMember |
BE Reinsurance [Member] | Standard Poors NR Rating [Member] | Private equity funds | Recent Transaction Three [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Average Share Price Used As Unobservable Input | 0.13 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_BEReinsuranceMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionThreeMember |
Assets, Fair Value Disclosure | 15,800,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_BEReinsuranceMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionThreeMember |
Centrus [Member] | Standard Poors NR Rating [Member] | Mortgage-backed and asset-backed securities | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Unobservable Inputs | 0.10 wtm_UnobservableInputs / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / invest_InvestmentAxis = wtm_CentrusMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Assets, Fair Value Disclosure | $ 5,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_AssetBackedSecuritiesMember / invest_InvestmentAxis = wtm_CentrusMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
Pari Passu Surplus Note [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
basis points | 2.50%wtm_Basispoints / us-gaap_InvestmentTypeAxis = wtm_PariPassuSurplusNoteMember |
Seller Priority Surplus Note [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
basis points | 2.50%wtm_Basispoints / us-gaap_InvestmentTypeAxis = wtm_SellerPrioritySurplusNoteMember |

X
- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Details
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Balance Type: | na
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X
- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Average Share Price Used As Unobservable Input + References No definition available. + Details
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- Definition basis points + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Book Value Multiple to Calculate Fair Value + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition This element represents transfers out of one of the levels in the fair value hierarchy for assets measured at fair value on a recurring basis. + References No definition available. + Details
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- Definition Number of Investments with Significant Unobseravble Inputs + References No definition available. + Details
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- Definition Timing of interest payments + References No definition available. + Details
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Period Type: | duration

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- Definition Timing of principal payments + References No definition available. + Details
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Period Type: | duration

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- Definition Unobservable Inputs + References No definition available. + Details
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- Details
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- Details
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