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Investment Securities (Significant Unobservable Inputs) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Number of Investments with Significant Unobseravble Inputs | 1 wtm_NumberOfInvestmentsWithSignificantUnobseravbleInputs |
Conditional Prepayment Rate | 12.00%wtm_ConditionalPrepaymentRate |
loss severity [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value Inputs, Discount Rate | 50.00%us-gaap_FairValueInputsDiscountRate / us-gaap_ValuationTechniqueAxis = wtm_LossseverityMember |
Discount Yield [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value Inputs, Discount Rate | 11.30%us-gaap_FairValueInputsDiscountRate / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember | 11.00%us-gaap_FairValueInputsDiscountRate / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember
NR | Preferred Stock | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 71,400,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 71,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
NR | Preferred Stock | Discount Yield [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair Value Inputs, Discount Rate | 7.10%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember | 7.10%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountYieldMember
NR | Private equity funds | Multiple of GAAP book value [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 32,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember | 40,200,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember
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 | 110.00%wtm_BookValueMultipleToCalculateFairValue / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleOfGaapBookValueMember
NR | 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 | 20,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_RecenttransactionMember
NR | 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 | 1.06 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / us-gaap_ValuationTechniqueAxis = wtm_AverageSharePriceMember
NR | Convertible Preferred Stock [Member] | Multiple of EBITA [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 3,900,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember | 3,800,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember
Average Share Price Used As Unobservable Input | 6.00 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember | 6.00 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember
Unobservable Inputs | 0 wtm_UnobservableInputs / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_ConvertiblePreferredStockMember / us-gaap_ValuationTechniqueAxis = wtm_MultipleofEBITDAMember |
NR | Convertible Preferred Stock [Member] | Recent transaction [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
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
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
NR | Seller Priority Surplus Note [Member] | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 46,500,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 44,000,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Fair Value Inputs, Discount Rate | 9.40%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 9.30%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_SellerPrioritySurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Timing of interest payments | 5 years | 5 years
Timing of principal payments | 10 years | 10 years
NR | Pari Passu Surplus Note [Member] | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 21,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 21,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Fair Value Inputs, Discount Rate | 14.40%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember | 13.50%us-gaap_FairValueInputsDiscountRate / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_PariPassuSurplusNoteMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Timing of interest payments | 5 years | 5 years
Timing of principal payments | 15 years | 10 years
Standard & Poor's, A- Rating | Non-Agency Commercial Mortgage Backed Securities [Member] [Member] | Broker Pricing [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 20,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_NonAgencyCommercialMortgageBackedSecuritiesMemberMember / us-gaap_ValuationTechniqueAxis = wtm_BrokerPricingMember |
Standard & Poor's, A- Rating | Non-Agency Residential Mortgage [Member] | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 47,100,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = us-gaap_StandardPoorsAMinusRatingMember / us-gaap_FairValueByAssetClassAxis = wtm_NonAgencyResidentialMortgageMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember |
durchblicker.at [Member] | NR | Private equity funds | Recent Transaction Two [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 9,200,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_Durchblicker.atMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionTwoMember | 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
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 | 290.96 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_Durchblicker.atMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionTwoMember
BE Reinsurance [Member] | NR | Private equity funds | Recent Transaction Three [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure | 14,200,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_BEReinsuranceMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionThreeMember | 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
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 | 0.13 wtm_AverageSharePriceUsedAsUnobservableInput / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_PrivateEquityFundsMember / invest_InvestmentAxis = wtm_BEReinsuranceMember / us-gaap_ValuationTechniqueAxis = wtm_RecentTransactionThreeMember
Centrus [Member] | NR | Debt securities issued by corporations | Discounted cash flows |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Assets, Fair Value Disclosure |  | 5,600,000 us-gaap_AssetsFairValueDisclosure / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / invest_InvestmentAxis = wtm_CentrusMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Unobservable Inputs |  | 0.10 wtm_UnobservableInputs / invest_CreditRatingSourceAxis = wtm_StandardPoorsNrRatingMember / us-gaap_FairValueByAssetClassAxis = us-gaap_CorporateDebtSecuritiesMember / invest_InvestmentAxis = wtm_CentrusMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountedCashFlowValuationTechniqueMember
Minimum | discount spread [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Basis points | 0.45%wtm_Basispoints / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountspreadMember |
Minimum | Default Rate [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Constant Default Rate | 2.50%wtm_ConstantDefaultRate / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = wtm_DefaultRateMember |
Maximum | discount spread [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Basis points | 1.80%wtm_Basispoints / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DiscountspreadMember |
Maximum | Default Rate [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Constant Default Rate | 8.75%wtm_ConstantDefaultRate / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = wtm_DefaultRateMember |
Standard & Poor's, B Rating [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Basis points | 2.50%wtm_Basispoints / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsBRatingMember |
Standard & Poor's, CCC Rating [Member] |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Basis points | 2.50%wtm_Basispoints / us-gaap_CreditRatingStandardPoorsAxis = us-gaap_StandardPoorsCCCRatingMember |

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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- 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 Book Value Multiple to Calculate Fair Value + References No definition available. + Details
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- Definition An annualized percentage of the existing mortgage pool that is expected to be prepaid in a year. + References No definition available. + Details
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- Definition An annualized rate of default on a pool of mortgagr loans, expressed as a percentage. + 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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