<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.3.1.900</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1715</ContextCount>
  <ElementCount>817</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>469</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>16</UnitCount>
  <MyReports>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedStatementsOfChangesInEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004501 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ConsolidatedStatementsOfCashFlowsParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Significant Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SignificantTransactions</Role>
      <ShortName>Significant Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Reserves for Unpaid Losses and Loss Adjustment Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ReservesForUnpaidLossesAndLossAdjustmentExpenses</Role>
      <ShortName>Reserves for Unpaid Losses and Loss Adjustment Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Third Party Reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ThirdPartyReinsurance</Role>
      <ShortName>Third Party Reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Debt and Standby Letter of Credit Facilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DebtAndStandbyLetterOfCreditFacilities</Role>
      <ShortName>Debt and Standby Letter of Credit Facilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/GoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/Derivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Municipal Bond Guarantee Insurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/MunicipalBondGuaranteeInsurance</Role>
      <ShortName>Municipal Bond Guarantee Insurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Retirement and Postretirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlans</Role>
      <ShortName>Retirement and Postretirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Employee Share-Based Incentive Compensation Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlans</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Common Shareholders??? Equity and Non-controlling Interests</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommonShareholdersEquityAndNonControllingInterests</Role>
      <ShortName>Common Shareholders??? Equity and Non-controlling Interests</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Statutory Capital and Surplus</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/StatutoryCapitalAndSurplus</Role>
      <ShortName>Statutory Capital and Surplus</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Investments in Unconsolidated Affiliates</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliates</Role>
      <ShortName>Investments in Unconsolidated Affiliates</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Variable Interest Entities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/VariableInterestEntities</Role>
      <ShortName>Variable Interest Entities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - Fair Value of Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/FairValueOfFinancialInstruments</Role>
      <ShortName>Fair Value of Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2120100 - Disclosure - Transactions with Related Persons</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/TransactionsWithRelatedPersons</Role>
      <ShortName>Transactions with Related Persons</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2123100 - Disclosure - Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperations</Role>
      <ShortName>Discontinued Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2124100 - Disclosure - Subsequent Event</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SubsequentEvent</Role>
      <ShortName>Subsequent Event</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2125100 - Schedule - Selected Quarterly Financial Data (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SelectedQuarterlyFinancialDataUnaudited</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2126100 - Disclosure - SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleISummaryOfInvestmentsOtherThanInvestmentsInRelatedParties</Role>
      <ShortName>SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2127100 - Disclosure - SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiCondensedFinancialInformationOfRegistrant</Role>
      <ShortName>SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2128100 - Disclosure - SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiiSupplementaryInsuranceInformation</Role>
      <ShortName>SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2129100 - Disclosure - SCHEDULE IV REINSURANCE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIvReinsurance</Role>
      <ShortName>SCHEDULE IV REINSURANCE</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2130100 - Disclosure - SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleVValuationAndQualifyingAccounts</Role>
      <ShortName>SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2131100 - Disclosure - SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleViSupplementalInformationForPropertyAndCasualtyInsuranceUnderwriters</Role>
      <ShortName>SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2301302 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Reserves for Unpaid Losses and Loss Adjustment Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ReservesForUnpaidLossesAndLossAdjustmentExpensesTables</Role>
      <ShortName>Reserves for Unpaid Losses and Loss Adjustment Expenses (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Third Party Reinsurance (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ThirdPartyReinsuranceTables</Role>
      <ShortName>Third Party Reinsurance (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Debt and Standby Letter of Credit Facilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DebtAndStandbyLetterOfCreditFacilitiesTables</Role>
      <ShortName>Debt and Standby Letter of Credit Facilities (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/GoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Municipal Bond Guarantee Insurance (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/MunicipalBondGuaranteeInsuranceTables</Role>
      <ShortName>Municipal Bond Guarantee Insurance (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Retirement and Postretirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansTables</Role>
      <ShortName>Retirement and Postretirement Plans (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Employee Share-Based Incentive Compensation Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansTables</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Common Shareholders??? Equity and Non-controlling Interests Common Shareholders??? Equity and Non-controlling Interests (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommonShareholdersEquityAndNonControllingInterestsCommonShareholdersEquityAndNonControllingInterestsTables</Role>
      <ShortName>Common Shareholders??? Equity and Non-controlling Interests Common Shareholders??? Equity and Non-controlling Interests (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2316301 - Disclosure - Investments in Unconsolidated Affiliates (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesTables</Role>
      <ShortName>Investments in Unconsolidated Affiliates (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2318301 - Disclosure - Fair Value of Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/FairValueOfFinancialInstrumentsTables</Role>
      <ShortName>Fair Value of Financial Instruments (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2323301 - Disclosure - Discontinued Operations (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsTables</Role>
      <ShortName>Discontinued Operations (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2325301 - Schedule - Selected Quarterly Financial Data (Unaudited) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SelectedQuarterlyFinancialDataUnauditedTables</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited) (Tables)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - Summary of Significant Accounting Policies (Basis of Presentation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesBasisOfPresentationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Basis of Presentation) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2401404 - Disclosure - Summary of Significant Accounting Policies (Investment Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesInvestmentSecuritiesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Investment Securities) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2401405 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Warrants) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesWarrantsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Warrants) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2401406 - Disclosure - Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesInsuranceAndReinsuranceOperationsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Insurance and Reinsurance Operations) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2401407 - Disclosure - Summary of Significant Accounting Policies (Deferred Softward Costs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesDeferredSoftwardCostsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Deferred Softward Costs) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2401408 - Disclosure - Summary of Significant Accounting Policies (Foreign Currency Exchange) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesForeignCurrencyExchangeDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Foreign Currency Exchange) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2401410 - Disclosure - Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesLihcDetails</Role>
      <ShortName>Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2402401 - Disclosure - Significant Transactions Significant Transactions (Acquisitions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SignificantTransactionsSignificantTransactionsAcquisitionsDetails</Role>
      <ShortName>Significant Transactions Significant Transactions (Acquisitions) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Significant Transactions (Dispositions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SignificantTransactionsDispositionsDetails</Role>
      <ShortName>Significant Transactions (Dispositions) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ReservesForUnpaidLossesAndLossAdjustmentExpensesNarrativeDetails</Role>
      <ShortName>Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ReservesForUnpaidLossesAndLossAdjustmentExpensesReserveSummaryDetails</Role>
      <ShortName>Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Summary) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2403404 - Disclosure - Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ReservesForUnpaidLossesAndLossAdjustmentExpensesReservesForUnpaidLossesAndLossAdjustmentExpensesReserveDevelopmentDetails</Role>
      <ShortName>Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Third Party Reinsurance (Written and Earned Premiums) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ThirdPartyReinsuranceWrittenAndEarnedPremiumsDetails</Role>
      <ShortName>Third Party Reinsurance (Written and Earned Premiums) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Third-Party Reinsurance (OneBeacon) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ThirdPartyReinsuranceOnebeaconDetails</Role>
      <ShortName>Third-Party Reinsurance (OneBeacon) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ThirdPartyReinsuranceRecoverableAmountsByReinsurerDetails</Role>
      <ShortName>Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Investment Securities (Net Investment Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNetInvestmentIncomeDetails</Role>
      <ShortName>Investment Securities (Net Investment Income) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNetRealizedAndUnrealizedInvestementGainsAndLossesDetails</Role>
      <ShortName>Investment Securities (Net Realized and Unrealized Investement Gains and Losses) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentSecuritiesNetRealizedInvestmentGainLossesDetails</Role>
      <ShortName>Investment Securities Investment Securities (Net Realized Investment Gain (losses) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2405405 - Disclosure - Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentSecuritiesNetUnrealizedInvestmentGainsLossesDetails</Role>
      <ShortName>Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - Investment Securities (Investment Gains (Losses) For Level 3) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentGainsLossesForLevel3Details</Role>
      <ShortName>Investment Securities (Investment Gains (Losses) For Level 3) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2405407 - Disclosure - Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNetRealizedAndUnrealizedGainsLossesAfterTaxDetails</Role>
      <ShortName>Investment Securities (Net Realized and Unrealized Gains (Losses) After Tax) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2405408 - Disclosure - Investment Securities (Investment Holdings) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentHoldingsDetails</Role>
      <ShortName>Investment Securities (Investment Holdings) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2405409 - Disclosure - Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesCostAndAmortizedCostMaturityScheduleDetails</Role>
      <ShortName>Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2405410 - Disclosure - Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesCostOrAmortizedCostGrossUnrealizedInvestmentGainsAndLossesDetails</Role>
      <ShortName>Investment Securities (Cost or Amortized Cost, Gross Unrealized Investment Gains and Losses) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2405411 - Disclosure - Investment Securities (Investments Held on Deposit or as Collateral) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentsHeldOnDepositOrAsCollateralDetails</Role>
      <ShortName>Investment Securities (Investments Held on Deposit or as Collateral) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2405412 - Disclosure - Investment Securities (Fair Value Measurement by Level) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesFairValueMeasurementByLevelDetails</Role>
      <ShortName>Investment Securities (Fair Value Measurement by Level) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2405413 - Disclosure - Investment Securities (Debt Securities Issued By Corporation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesDebtSecuritiesIssuedByCorporationDetails</Role>
      <ShortName>Investment Securities (Debt Securities Issued By Corporation) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2405414 - Disclosure - Investment Securities (Mortgage-backed Asset Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesMortgageBackedAssetSecuritiesDetails</Role>
      <ShortName>Investment Securities (Mortgage-backed Asset Securities) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2405415 - Disclosure - Investment Securities (Non-agency Mortgage-backed Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNonAgencyMortgageBackedSecuritiesDetails</Role>
      <ShortName>Investment Securities (Non-agency Mortgage-backed Securities) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2405416 - Disclosure - Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNonAgencyResidentialMortgageBackedSecuritiesDetails</Role>
      <ShortName>Investment Securities (Non-agency Residential Mortgage-backed Securities) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2405417 - Disclosure - Investment Securities (Other Long-Term Investments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesOtherLongTermInvestmentsDetails</Role>
      <ShortName>Investment Securities (Other Long-Term Investments) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2405418 - Disclosure - Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesNonAgencyCommercialMortgageBackedSecuritiesDetails</Role>
      <ShortName>Investment Securities (Non-agency Commercial Mortgage-backed Securities) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2405419 - Disclosure - Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesFairValueOfHedgeFundsSubjectToRestrictionsOnRedemptionFrequencyDetails</Role>
      <ShortName>Investment Securities (Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2405420 - Disclosure - Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesRestrictionsOnRedemptionFrequencyAndAdvanceNoticeRequirementsDetails</Role>
      <ShortName>Investment Securities (Restrictions on Redemption Frequency and Advance Notice Requirements) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2405421 - Disclosure - Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentsInPrivateEquityFundsSubjectToLockUpPeriodsDetails</Role>
      <ShortName>Investment Securities (Investments In Private Equity Funds Subject to Lock-Up Periods) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2405422 - Disclosure - Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesRollforwardOfFairValueMeasurementsByLevelDetails</Role>
      <ShortName>Investment Securities (Rollforward of Fair Value Measurements by Level) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2405423 - Disclosure - Investment Securities (Significant Unobservable Inputs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesSignificantUnobservableInputsDetails</Role>
      <ShortName>Investment Securities (Significant Unobservable Inputs) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2405424 - Disclosure - Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentSecuritiesInvestmentSecuritiesOnebeaconSurplusNotesDetails</Role>
      <ShortName>Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DebtAndStandbyLetterOfCreditFacilitiesDebtOutstandingDetails</Role>
      <ShortName>Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/GoodwillAndOtherIntangibleAssetsGoodwillAndIntangibleAssetsDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Goodwill and Intangible Assets) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Debt and Standby Letter of Credit Facilities (Schedule of Contractual Repayments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DebtAndStandbyLetterOfCreditFacilitiesScheduleOfContractualRepaymentsDetails</Role>
      <ShortName>Debt and Standby Letter of Credit Facilities (Schedule of Contractual Repayments) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/GoodwillAndOtherIntangibleAssetsGoodwillAndIntangibleAssetsRollforwardDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Goodwill and Intangible Assets Rollforward) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - Debt and Standby Letter of Credit Facilities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DebtAndStandbyLetterOfCreditFacilitiesNarrativeDetails</Role>
      <ShortName>Debt and Standby Letter of Credit Facilities (Narrative) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - Goodwill and Other Intangible Assets (Amortization Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/GoodwillAndOtherIntangibleAssetsAmortizationExpenseDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Amortization Expense) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Income Taxes (Income Tax (Benefit) Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesIncomeTaxBenefitExpenseDetails</Role>
      <ShortName>Income Taxes (Income Tax (Benefit) Expense (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Income Taxes (Effective Rate Reconciliation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesEffectiveRateReconciliationDetails</Role>
      <ShortName>Income Taxes (Effective Rate Reconciliation) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2407404 - Disclosure - Income Taxes (Deferred Tax Inventory) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesDeferredTaxInventoryDetails</Role>
      <ShortName>Income Taxes (Deferred Tax Inventory) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2407405 - Disclosure - Income Taxes (Valuation Allowance) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesValuationAllowanceDetails</Role>
      <ShortName>Income Taxes (Valuation Allowance) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2407406 - Disclosure - Income Taxes (Net Operating and Capital Loss Carryforwards) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesNetOperatingAndCapitalLossCarryforwardsDetails</Role>
      <ShortName>Income Taxes (Net Operating and Capital Loss Carryforwards) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2407407 - Disclosure - Income Taxes (ASC 740-10) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesAsc74010Details</Role>
      <ShortName>Income Taxes (ASC 740-10) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2407408 - Disclosure - Income Taxes (Tax Examinations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/IncomeTaxesTaxExaminationsDetails</Role>
      <ShortName>Income Taxes (Tax Examinations) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesDetails</Role>
      <ShortName>Derivatives (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Derivatives (Pre-Tax Operating Results of WM Life Re) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesPreTaxOperatingResultsOfWmLifeReDetails</Role>
      <ShortName>Derivatives (Pre-Tax Operating Results of WM Life Re) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Derivatives (Realized and Unrealized Gains (Losses)) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesRealizedAndUnrealizedGainsLossesDetails</Role>
      <ShortName>Derivatives (Realized and Unrealized Gains (Losses)) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesVariableAnnuityReinsuranceLiabilitiesAndDerivativeInstrumentsDetails</Role>
      <ShortName>Derivatives (Variable Annuity Reinsurance Liabilities and Derivative Instruments) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - Derivatives (Additional Collateral) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesAdditionalCollateralDetails</Role>
      <ShortName>Derivatives (Additional Collateral) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Derivatives (Level 3 Derivative Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesLevel3DerivativeInstrumentsDetails</Role>
      <ShortName>Derivatives (Level 3 Derivative Instruments) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>2408408 - Disclosure - Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesDerivativesGrossAssetGrossLiabilityAmountsOffsetUnderMasterNettingAgreementsDetails</Role>
      <ShortName>Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>2408409 - Disclosure - Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesUncollateralizedAmountsDueUnderWmLifeResOtcDerivativeContractsDetails</Role>
      <ShortName>Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>2408411 - Disclosure - Derivatives Derivative (Tranzact Interest Rate Swap) (Details) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DerivativesDerivativeTranzactInterestRateSwapDetailsDetails</Role>
      <ShortName>Derivatives Derivative (Tranzact Interest Rate Swap) (Details) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Municipal Bond Guarantee Insurance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/MunicipalBondGuaranteeInsuranceDetails</Role>
      <ShortName>Municipal Bond Guarantee Insurance (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>117</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Retirement and Postretirement Plans (Benefit Obligations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansBenefitObligationsDetails</Role>
      <ShortName>Retirement and Postretirement Plans (Benefit Obligations) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>119</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansQualifedNonQualifiedAndAccumulatedBenefitObligationsDetails</Role>
      <ShortName>Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>120</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>2411404 - Disclosure - Retirement and Postretirement Plans (Periodic Benefit Cost) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansPeriodicBenefitCostDetails</Role>
      <ShortName>Retirement and Postretirement Plans (Periodic Benefit Cost) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>2411405 - Disclosure - Retirement and Postretirement Plans (Qualified Plan's Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansQualifiedPlansAssetsDetails</Role>
      <ShortName>Retirement and Postretirement Plans (Qualified Plan's Assets) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>122</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>2411406 - Disclosure - Retirement and Postretirement Plans Retirement and Postretirment Plans (Assumptions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansRetirementAndPostretirmentPlansAssumptionsDetails</Role>
      <ShortName>Retirement and Postretirement Plans Retirement and Postretirment Plans (Assumptions) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>2411407 - Disclosure - Retirement and Postretirement Plans (Cash Flow) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/RetirementAndPostretirementPlansCashFlowDetails</Role>
      <ShortName>Retirement and Postretirement Plans (Cash Flow) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R125.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Employee Share-Based Incentive Compensation Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlanDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plan (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>125</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R126.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansWtmPerformanceSharesDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>126</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R127.htm</HtmlFileName>
      <LongName>2412404 - Disclosure - Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansWtmPerformanceSharesGrantedUnderWtmIncentivePlanDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (WTM Performance Shares Granted Under the WTM Incentive Plan) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>127</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R128.htm</HtmlFileName>
      <LongName>2412405 - Disclosure - Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansWtmRestrictedSharesDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>128</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R129.htm</HtmlFileName>
      <LongName>2412406 - Disclosure - Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansWtmNonQualifiedAndQualifiedOptionsDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (WTM Non-Qualified and Qualified Options) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>129</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R130.htm</HtmlFileName>
      <LongName>2412407 - Disclosure - Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansOnebeasonPeformanceSharesDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>130</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R131.htm</HtmlFileName>
      <LongName>2412408 - Disclosure - Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansOnebeaconPerformanceSharesOutstandingDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>131</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R132.htm</HtmlFileName>
      <LongName>2412409 - Disclosure - Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansOnebeaconRestrictedSharesDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>132</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R133.htm</HtmlFileName>
      <LongName>2412410 - Disclosure - Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/EmployeeShareBasedIncentiveCompensationPlansUnderQualifiedRetirementPlansDetails</Role>
      <ShortName>Employee Share-Based Incentive Compensation Plans (Under Qualified Retirement Plans) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>133</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R134.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Common Shareholders??? Equity and Non-controlling Interests (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommonShareholdersEquityAndNonControllingInterestsDetails</Role>
      <ShortName>Common Shareholders??? Equity and Non-controlling Interests (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>134</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R135.htm</HtmlFileName>
      <LongName>2414401 - Disclosure - Statutory Capital and Surplus (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/StatutoryCapitalAndSurplusDetails</Role>
      <ShortName>Statutory Capital and Surplus (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>135</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R136.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SegmentInformationDetails</Role>
      <ShortName>Segment Information (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>136</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R137.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Segment Information (Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SegmentInformationAssetsDetails</Role>
      <ShortName>Segment Information (Assets) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>137</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R138.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - Investments in Unconsolidated Affiliates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesDetails</Role>
      <ShortName>Investments in Unconsolidated Affiliates (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>138</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R139.htm</HtmlFileName>
      <LongName>2416403 - Disclosure - Investments in Unconsolidated Affiliates (Symetra Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesSymetraNarrativeDetails</Role>
      <ShortName>Investments in Unconsolidated Affiliates (Symetra Narrative) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>139</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R140.htm</HtmlFileName>
      <LongName>2416404 - Disclosure - Investments in Unconsolidated Affiliates (Investments in Symetra) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesInvestmentsInSymetraDetails</Role>
      <ShortName>Investments in Unconsolidated Affiliates (Investments in Symetra) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>140</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R141.htm</HtmlFileName>
      <LongName>2416405 - Disclosure - Investments in Unconsolidated Affiliates (Financial Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesFinancialInformationDetails</Role>
      <ShortName>Investments in Unconsolidated Affiliates (Financial Information) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>141</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R142.htm</HtmlFileName>
      <LongName>2416406 - Disclosure - Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (Hamer and Bri-Mar) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/InvestmentsInUnconsolidatedAffiliatesInvestmentsInUnconsolidatedAffiliatesHamerAndBriMarDetails</Role>
      <ShortName>Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (Hamer and Bri-Mar) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>142</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R143.htm</HtmlFileName>
      <LongName>2417401 - Disclosure - Variable Interest Entities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/VariableInterestEntitiesDetails</Role>
      <ShortName>Variable Interest Entities (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>143</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R144.htm</HtmlFileName>
      <LongName>2418402 - Disclosure - Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/FairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>144</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R145.htm</HtmlFileName>
      <LongName>2420401 - Disclosure - Transactions with Related Persons (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/TransactionsWithRelatedPersonsDetails</Role>
      <ShortName>Transactions with Related Persons (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>145</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R146.htm</HtmlFileName>
      <LongName>2421401 - Disclosure - Commitments and Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommitmentsAndContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>146</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R147.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - Commitments and Contingencies (Legal Contingencies) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/CommitmentsAndContingenciesLegalContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Legal Contingencies) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>147</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R148.htm</HtmlFileName>
      <LongName>2423402 - Disclosure - Discontinued Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>148</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R149.htm</HtmlFileName>
      <LongName>2423403 - Disclosure - Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsAssetsAndLiabilitiesClassifiedAsHeldForSaleDetails</Role>
      <ShortName>Discontinued Operations (Assets and Liabilities Classified as Held for Sale) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>149</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R150.htm</HtmlFileName>
      <LongName>2423404 - Disclosure - Discontinued Operations (Loss from Discontinued Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsLossFromDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Loss from Discontinued Operations) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>150</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R151.htm</HtmlFileName>
      <LongName>2423405 - Disclosure - Discontinued Operations (Earnings Per Share) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsEarningsPerShareDetails</Role>
      <ShortName>Discontinued Operations (Earnings Per Share) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>151</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R152.htm</HtmlFileName>
      <LongName>2423406 - Disclosure - Discontinued Operations Discontinued Operations (Surplus Note Table) (Details) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsDiscontinuedOperationsSurplusNoteTableDetailsDetails</Role>
      <ShortName>Discontinued Operations Discontinued Operations (Surplus Note Table) (Details) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>152</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R153.htm</HtmlFileName>
      <LongName>2423407 - Disclosure - Discontinued Operations Discontinued Operations (Interest Rate Cap) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsDiscontinuedOperationsInterestRateCapDetails</Role>
      <ShortName>Discontinued Operations Discontinued Operations (Interest Rate Cap) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>153</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R154.htm</HtmlFileName>
      <LongName>2423408 - Disclosure - Discontinued Operations Discontinued Operations (Fair Value) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/DiscontinuedOperationsDiscontinuedOperationsFairValueDetails</Role>
      <ShortName>Discontinued Operations Discontinued Operations (Fair Value) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>154</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R155.htm</HtmlFileName>
      <LongName>2424401 - Disclosure - Subsequent Event (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SubsequentEventDetails</Role>
      <ShortName>Subsequent Event (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>155</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R156.htm</HtmlFileName>
      <LongName>2425402 - Schedule - Selected Quarterly Financial Data (Unaudited) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/SelectedQuarterlyFinancialDataUnauditedDetails</Role>
      <ShortName>Selected Quarterly Financial Data (Unaudited) (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>156</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R157.htm</HtmlFileName>
      <LongName>2426401 - Disclosure - SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleISummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesDetails</Role>
      <ShortName>SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>157</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R158.htm</HtmlFileName>
      <LongName>2427401 - Disclosure - SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiCondensedFinancialInformationOfRegistrantDetails</Role>
      <ShortName>SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>158</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R159.htm</HtmlFileName>
      <LongName>2427402 - Disclosure - SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiCondensedFinancialInformationOfRegistrantDetails2</Role>
      <ShortName>SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 2)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>159</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R160.htm</HtmlFileName>
      <LongName>2427403 - Disclosure - SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiCondensedFinancialInformationOfRegistrantDetails3</Role>
      <ShortName>SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT (Details 3)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>160</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R161.htm</HtmlFileName>
      <LongName>2428401 - Disclosure - SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIiiSupplementaryInsuranceInformationDetails</Role>
      <ShortName>SCHEDULE III SUPPLEMENTARY INSURANCE INFORMATION (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>161</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R162.htm</HtmlFileName>
      <LongName>2429401 - Disclosure - SCHEDULE IV REINSURANCE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIvReinsuranceDetails</Role>
      <ShortName>SCHEDULE IV REINSURANCE (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>162</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R163.htm</HtmlFileName>
      <LongName>2430401 - Disclosure - SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleVValuationAndQualifyingAccountsDetails</Role>
      <ShortName>SCHEDULE V VALUATION AND QUALIFYING ACCOUNTS (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>163</Position>
    </Report>
    <Report instance="wtm-20151231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R164.htm</HtmlFileName>
      <LongName>2431401 - Disclosure - SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleViSupplementalInformationForPropertyAndCasualtyInsuranceUnderwritersDetails</Role>
      <ShortName>SCHEDULE VI SUPPLEMENTAL INFORMATION FOR PROPERTY AND CASUALTY INSURANCE UNDERWRITERS (Details)</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>164</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>wtm-20151231.xml</File>
    <File>wtm-20151231.xsd</File>
    <File>wtm-20151231_cal.xml</File>
    <File>wtm-20151231_def.xml</File>
    <File>wtm-20151231_lab.xml</File>
    <File>wtm-20151231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
