<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>4</ContextCount>
  <ElementCount>34</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>1</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Disclosure - Basic Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/BasicFinancialInformation</Role>
      <ShortName>Basic Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Disclosure - Schedule I (Form 5500)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIForm5500</Role>
      <ShortName>Schedule I (Form 5500)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Disclosure - Schedule I (Form 5500) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIForm5500Tables</Role>
      <ShortName>Schedule I (Form 5500) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.whitemountains.com/role/ScheduleIForm5500</ParentRole>
      <Position>4</Position>
    </Report>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Disclosure - Basic Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/BasicFinancialInformationDetails</Role>
      <ShortName>Basic Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.whitemountains.com/role/BasicFinancialInformation</ParentRole>
      <Position>5</Position>
    </Report>
    <Report instance="wtm-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Disclosure - Schedule I (Form 5500) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.whitemountains.com/role/ScheduleIForm5500Details</Role>
      <ShortName>Schedule I (Form 5500) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.whitemountains.com/role/ScheduleIForm5500Tables</ParentRole>
      <Position>6</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="11-K" isUsgaap="true" original="wtm-20260630.htm">wtm-20260630.htm</File>
    <File>wtm-20260630.xsd</File>
    <File>wtm-20260630_cal.xml</File>
    <File>wtm-20260630_def.xml</File>
    <File>wtm-20260630_lab.xml</File>
    <File>wtm-20260630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>wtm-20260630_g1.jpg</File>
    <File>wtm-20260630_g2.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="31">http://fasb.org/us-gaap-ebp/2026</BaseTaxonomy>
    <BaseTaxonomy items="9">http://xbrl.sec.gov/dei/2026</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
