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v2.4.1.9

Municipal Bond Guarantee (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 1 Months Ended
--- | --- | --- | ---
Mar. 31, 2015 Y Contract | Mar. 31, 2014 | Dec. 31, 2014 Contract | Jul. 31, 2012
BAM |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Municipal bond contract | 2,029 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember |  | 1,750 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember |
Insured Financial Obligations, Remaining Weighted Average Contract Period | 12 years 10 months 0 days |  | 12 years 10 months 0 days |
Municipal bond outstanding principal | $ 14,421.6 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember |  | $ 12,362.5 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember |
Municipal bond outstanding interest | 8,044.7 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember |  | 7,086.9 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember |
Municipal bond debt service outstanding | 22,466.3 wtm_Municipalbonddebtserviceoutstanding / dei_LegalEntityAxis = wtm_BamMember |  | 19,449.4 wtm_Municipalbonddebtserviceoutstanding / dei_LegalEntityAxis = wtm_BamMember |
Interest income (expense), surplus notes | 4.0 wtm_InterestIncomeExpenseSurplusNotes / dei_LegalEntityAxis = wtm_BamMember | 3.9 wtm_InterestIncomeExpenseSurplusNotes / dei_LegalEntityAxis = wtm_BamMember |  |
Financial Guarantee Insurance Contracts, Unearned Premium Revenue | 31.2 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember |  | 27.6 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember |
Year of U.S. Treasury Rate | 1 wtm_YearofU.S.TreasuryRate / dei_LegalEntityAxis = wtm_BamMember |  |  |
HG Global |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Partners Capital Contribution |  |  |  | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_HgGlobalMember
Surplus Notes |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember
Income (Loss) from Continuing Operations before Income Taxes, Domestic | 5.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic / dei_LegalEntityAxis = wtm_HgGlobalMember | 4.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic / dei_LegalEntityAxis = wtm_HgGlobalMember |  |
HG Global | Preferred Stock |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Ownership interest (as a percent) | 96.90%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |  |
HG Global | Common Stock |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Ownership interest (as a percent) | 88.40%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |  |
Build America Mutual |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Income (Loss) from Continuing Operations before Income Taxes, Domestic | (8.7)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | (8.6)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesDomestic / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember |  |
Interest income (expense), surplus notes | (4.0)wtm_InterestIncomeExpenseSurplusNotes / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember | (3.9)wtm_InterestIncomeExpenseSurplusNotes / dei_LegalEntityAxis = wtm_BuildAmericaMutualMember |  |
HG Re |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Statutory Accounting Practices, Statutory Capital and Surplus, Balance | 453.0 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |  |  |
BAM Management |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Partners Capital Contribution |  |  |  | $ 14.5 us-gaap_PartnersCapitalAccountContributions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember
Surplus Note | BAM |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Basis points | 3.00%wtm_Basispoints / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_BamMember |  |  |
Derivative, Fixed Interest Rate | 3.15%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_BamMember |  | 3.13%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_BamMember |
Combination One | Surplus Note | BAM |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Number of years of change in interest rate | 5 wtm_Numberofyearsofchangeininterestrate / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / us-gaap_DebtInstrumentRedemptionPeriodAxis = wtm_CombinationOneMember / dei_LegalEntityAxis = wtm_BamMember |  |  |
Combination Two | Surplus Note | BAM |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Number of years of change in interest rate | 3 wtm_Numberofyearsofchangeininterestrate / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / us-gaap_DebtInstrumentRedemptionPeriodAxis = wtm_CombinationTwoMember / dei_LegalEntityAxis = wtm_BamMember |  |  |
Financial Guarantee [Member] |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Percentage of par value of policy reinsured | 15.00%us-gaap_ReinsuranceRetentionPolicyReinsuredRiskPercentage / us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis = us-gaap_FinancialGuaranteeMember |  |  |
Fixed interest rate [Member] | Surplus Note | BAM |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Derivative, Fixed Interest Rate | 8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_BamMember / us-gaap_VariableRateAxis = wtm_FixedinterestrateMember |  |  |

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- Definition Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525 + Details
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