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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended | 3 Months Ended |
--- | --- | --- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014 | Jul. 31, 2012
Basis of Presentation |  |  |  |
Assets held for sale | $ 58.0 us-gaap_AssetsHeldForSaleAtCarryingValue | $ 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue |  |
OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Cumulative Effect on Retained Earnings, Net of Tax | 1.2 us-gaap_CumulativeEffectOnRetainedEarningsNetOfTax1 / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 0.5 us-gaap_CumulativeEffectOnRetainedEarningsNetOfTax1 / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
Ownership interest (as a percent) | 75.30%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |  |
Noncontrolling equity percentage | 24.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 24.70%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Surplus Notes |  |  |  | 503.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_HgGlobalMember
Percentage of par value of policy reinsured | 15.00%wtm_PercentageOfParValueOfPolicyReinsured / dei_LegalEntityAxis = wtm_HgGlobalMember |  |  |
Noncontrolling equity percentage | 3.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember | 3.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember |  |
Star & Shield Insurance Exchange |  |  |  |
Basis of Presentation |  |  |  |
Surplus Notes | 19.0 us-gaap_SurplusNotes / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |  |
Noncontrolling equity percentage | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember | 100.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_StarShieldInsuranceExchangeMember |  |
White Mountains Advisors L L C [Member] |  |  |  |
Basis of Presentation |  |  |  |
Investment Advisory, Management and Administrative Fees | 2.0 us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember |  | 4.4 us-gaap_InvestmentAdvisoryManagementAndAdministrativeFees / dei_LegalEntityAxis = wtm_WhiteMountainsAdvisorsLLCMember |
Preferred Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 96.90%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |  |
Noncontrolling equity percentage | 96.90%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_PreferredStockMember |  |  |
Common Stock | HG Global |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 88.40%us-gaap_EquityMethodInvestmentOwnershipPercentage / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |  |
Noncontrolling equity percentage | 88.40%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners / dei_LegalEntityAxis = wtm_HgGlobalMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember |  |  |
Wholly Owned Properties [Member] | OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Assets held for sale | 58.0 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_FairValueByAssetClassAxis = us-gaap_WhollyOwnedPropertiesMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember | 58.1 us-gaap_AssetsHeldForSaleAtCarryingValue / us-gaap_FairValueByAssetClassAxis = us-gaap_WhollyOwnedPropertiesMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |
Investment Income [Member] | OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Cumulative-Effect Adjustment, Gross Gains | 0.4 us-gaap_CumulativeEffectAdjustmentGrossGains / us-gaap_IncomeStatementLocationAxis = us-gaap_InvestmentIncomeMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |  |
Tax Expense [Member] | OneBeacon Ltd. |  |  |  |
Basis of Presentation |  |  |  |
Cumulative-Effect Adjustment, Gross Gains | $ 0.6 us-gaap_CumulativeEffectAdjustmentGrossGains / us-gaap_IncomeStatementLocationAxis = wtm_TaxExpenseMember / dei_LegalEntityAxis = wtm_OneBeaconInsuranceGroupLtdMember |  |  |

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