<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>126</ContextCount>
  <ElementCount>262</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>32</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/DocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001002 - Statement - Consolidated Balance Sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Consolidated Balance Sheets (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002003 - Statement - Consolidated Balance Sheets (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003004 - Statement - Consolidated Statements of Operations (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedStatementsofOperationsUnaudited</Role>
      <ShortName>Consolidated Statements of Operations (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004005 - Statement - Consolidated Statements of Total Equity (Unaudited) Statement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedStatementsofTotalEquityUnauditedStatement</Role>
      <ShortName>Consolidated Statements of Total Equity (Unaudited) Statement</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1006006 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedStatementsofCashFlowsUnaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1405401 - Statement - Consolidated Statements of Total Equity (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/ConsolidatedStatementsofTotalEquityUnauditedParenthetical</Role>
      <ShortName>Consolidated Statements of Total Equity (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/BasisofPresentation</Role>
      <ShortName>Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2104102 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2105103 - Disclosure - Real Estate (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/RealEstateNotes</Role>
      <ShortName>Real Estate (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2108104 - Disclosure - Revenues (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/RevenuesNotes</Role>
      <ShortName>Revenues (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2111105 - Disclosure - Capitalized Interest (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/CapitalizedInterestNotes</Role>
      <ShortName>Capitalized Interest (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2114106 - Disclosure - Other Assets, Accrued Expenses and Other Liabilities (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/OtherAssetsAccruedExpensesandOtherLiabilitiesNotes</Role>
      <ShortName>Other Assets, Accrued Expenses and Other Liabilities (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2117107 - Disclosure - Debt, net (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/DebtnetNotes</Role>
      <ShortName>Debt, net (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2120108 - Disclosure - Earnings per Share (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/EarningsperShareNotes</Role>
      <ShortName>Earnings per Share (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2123109 - Disclosure - Income Taxes (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/IncomeTaxesNotes</Role>
      <ShortName>Income Taxes (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2125110 - Disclosure - Stockholders' Equity (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/StockholdersEquityNotes</Role>
      <ShortName>Stockholders' Equity (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2128111 - Disclosure - Commitments and Contingencies (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/CommitmentsandContingenciesNotes</Role>
      <ShortName>Commitments and Contingencies (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2130112 - Disclosure - Related Party Transactions (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/RelatedPartyTransactionsNotes</Role>
      <ShortName>Related Party Transactions (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2133113 - Disclosure - Fair Value (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.forestargroup.com/role/FairValueNotes</Role>
      <ShortName>Fair Value (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2202201 - Disclosure - Basis of Presentation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/BasisofPresentationPolicies</Role>
      <ShortName>Basis of Presentation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - Real Estate (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RealEstateTables</Role>
      <ShortName>Real Estate (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/RealEstateNotes</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2309302 - Disclosure - Revenues (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RevenuesTables</Role>
      <ShortName>Revenues (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/RevenuesNotes</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2312303 - Disclosure - Capitalized Interest (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/CapitalizedInterestTables</Role>
      <ShortName>Capitalized Interest (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/CapitalizedInterestNotes</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2315304 - Disclosure - Other Assets, Accrued Expenses and Other Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/OtherAssetsAccruedExpensesandOtherLiabilitiesTables</Role>
      <ShortName>Other Assets, Accrued Expenses and Other Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/OtherAssetsAccruedExpensesandOtherLiabilitiesNotes</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2318305 - Disclosure - Debt, net - Schedule of Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/DebtnetScheduleofDebtTables</Role>
      <ShortName>Debt, net - Schedule of Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2321306 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2331308 - Disclosure - Related Party Transactions Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RelatedPartyTransactionsRelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2334309 - Disclosure - Fair Value Measures and Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/FairValueMeasuresandDisclosuresTables</Role>
      <ShortName>Fair Value Measures and Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Basis of Presentation Details (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/BasisofPresentationDetailsDetails</Role>
      <ShortName>Basis of Presentation Details (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Real Estate - Real Estate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RealEstateRealEstateDetails</Role>
      <ShortName>Real Estate - Real Estate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Revenues (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RevenuesDetails</Role>
      <ShortName>Revenues (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/RevenuesTables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2413405 - Disclosure - Capitalized Interest (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/CapitalizedInterestDetails</Role>
      <ShortName>Capitalized Interest (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/CapitalizedInterestTables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2416406 - Disclosure - Other Assets, Accrued Expenses and Other Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/OtherAssetsAccruedExpensesandOtherLiabilitiesDetails</Role>
      <ShortName>Other Assets, Accrued Expenses and Other Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/OtherAssetsAccruedExpensesandOtherLiabilitiesTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2419407 - Disclosure - Debt, net - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/DebtnetAdditionalInformationDetails</Role>
      <ShortName>Debt, net - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2422408 - Disclosure - Earnings per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/EarningsperShareDetails</Role>
      <ShortName>Earnings per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/EarningsperShareNotes</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2424409 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/IncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2427410 - Disclosure - Stockholders' Equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/StockholdersEquityDetails</Role>
      <ShortName>Stockholders' Equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/StockholdersEquityNotes</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2429411 - Disclosure - Commitments and Contingencies Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/CommitmentsandContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2432412 - Disclosure - Related Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/RelatedPartyTransactionsDetails</Role>
      <ShortName>Related Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.forestargroup.com/role/RelatedPartyTransactionsRelatedPartyTransactionsTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="for-20210331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2435413 - Disclosure - Fair Value Measurements, Not Measured at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.forestargroup.com/role/FairValueMeasurementsNotMeasuredatFairValueDetails</Role>
      <ShortName>Fair Value Measurements, Not Measured at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</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="10-Q" original="for-20210331.htm">for-20210331.htm</File>
    <File>a3312021exhibit311.htm</File>
    <File>a3312021exhibit312.htm</File>
    <File>a3312021exhibit321.htm</File>
    <File>a3312021exhibit322.htm</File>
    <File>for-20210331.xsd</File>
    <File>for-20210331_cal.xml</File>
    <File>for-20210331_def.xml</File>
    <File>for-20210331_lab.xml</File>
    <File>for-20210331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
