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Municipal Bond Guarantee Insurance (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured |  |
Pre-tax loss | $ 46.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 34.4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 109.0 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 112.1 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 134.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 55.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 3.6 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 151.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 301.7 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 344.9 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments | $ 262.8 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments
Net investment income (loss) - surplus note interest |  |  |  |  |  |  |  |  | 0 wtm_NetInvestmentIncomeSurplusNotes | 0 wtm_NetInvestmentIncomeSurplusNotes | 0 wtm_NetInvestmentIncomeSurplusNotes
BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Municipal bond contract | 1,750 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 701 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 1,750 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember | 701 wtm_MunicipalBondContract / dei_LegalEntityAxis = wtm_BamMember |
Insured Financial Obligations with Credit Deterioration, Remaining Weighted Average Contract Period |  |  |  |  |  |  |  |  | 12 years 10 months 0 days | 13 years 9 months 21 days |
Municipal bond outstanding principal | 12,362.5 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 4,703.7 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 12,362.5 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember | 4,703.7 wtm_MunicipalBondOutstandingPrincipal / dei_LegalEntityAxis = wtm_BamMember |
Municipal bond outstanding interest | 7,086.9 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 3,264.4 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 7,086.9 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember | 3,264.4 wtm_MunicipalBondOutstandingInterest / dei_LegalEntityAxis = wtm_BamMember |
Financial Guarantee Insurance Contracts, Unearned Premium Revenue | 27.6 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 13.2 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember |  |  |  | 27.6 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember | 13.2 us-gaap_FinancialGuaranteeInsuranceContractsFutureExpectedPremiumRevenueToBeRecognized / dei_LegalEntityAxis = wtm_BamMember |
White Mountains |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_WhiteMountainsMember | 594.5 us-gaap_PartnersCapitalAccountContributions / dei_LegalEntityAxis = wtm_WhiteMountainsMember
HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |  |
BAM Management |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  | 14.5 us-gaap_PartnersCapitalAccountContributions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember | 14.5 us-gaap_PartnersCapitalAccountContributions / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = wtm_BamManagementMember
Preferred stocks | HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 96.90%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |  |  | 97.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |  |  | 96.90%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember | 97.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |
Common Stock | HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) | 88.40%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |  |  | 88.70%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |  |  | 88.40%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 88.70%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |
BAM |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Surplus notes | 503.0 us-gaap_SurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |  |  |  |  |  |  | 503.0 us-gaap_SurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember |  |
Pre-tax loss |  |  |  |  |  |  |  |  | (40.5)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (78.6)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (36.3)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember
Net investment income (loss) - surplus note interest |  |  |  |  |  |  |  |  | (15.7)wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (40.2)wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember | (18.4)wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_BamMember
HG Re |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Statutory capital and surplus | 449.1 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |  |  |  | 436.9 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |  |  |  | 449.1 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember | 436.9 us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |
Collateral Held in Supplement Trust | 120,000,000 wtm_CollateralHeldInSupplementTrust / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |  |  |  | 105,400,000 wtm_CollateralHeldInSupplementTrust / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |  |  |  | 120,000,000 wtm_CollateralHeldInSupplementTrust / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember | 105,400,000 wtm_CollateralHeldInSupplementTrust / us-gaap_StatementBusinessSegmentsAxis = wtm_HgReMember |
HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember |  |
Pre-tax loss |  |  |  |  |  |  |  |  | 18.3 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 38.1 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 14.2 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember
Net investment income (loss) - surplus note interest |  |  |  |  |  |  |  |  | 15.7 wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 40.2 wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember | 18.4 wtm_NetInvestmentIncomeSurplusNotes / us-gaap_StatementBusinessSegmentsAxis = wtm_HgGlobalMember
Surplus Note | HG Global |  |  |  |  |  |  |  |  |  |  |
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items] |  |  |  |  |  |  |  |  |  |  |
Interest Receivable | $ 74.4 us-gaap_InterestReceivable / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |  |  |  |  |  | $ 74.4 us-gaap_InterestReceivable / us-gaap_DebtInstrumentAxis = wtm_SurplusNoteMember / dei_LegalEntityAxis = wtm_HgGlobalMember |  |

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