<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>366</ContextCount>
  <ElementCount>511</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>101</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>8</UnitCount>
  <MyReports>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100030 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100050 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100060 - Statement - Consolidated Statements of Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/StatementConsolidatedStatementsOfComprehensiveIncomeLoss</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100070 - Statement - Consolidated Statements of Comprehensive Income (Loss) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/StatementConsolidatedStatementsOfComprehensiveIncomeLossParenthetical</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Loss) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100080 - Statement - Consolidated Statement of Changes in Stockholders' Equity (Deficit)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_StatementConsolidatedStatementOfChangesInStockholdersEquityDeficit</Role>
      <ShortName>Consolidated Statement of Changes in Stockholders' Equity (Deficit)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100090 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Description of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureDescriptionOfBusiness</Role>
      <ShortName>Description of Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Revenue from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRevenueFromContractsWithCustomers</Role>
      <ShortName>Revenue from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAcquisitions</Role>
      <ShortName>Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Supplementary Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSupplementaryInformation1</Role>
      <ShortName>Supplementary Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureDebt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureNetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Retirement Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefits</Role>
      <ShortName>Retirement Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Accumulated Other Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAccumulatedOtherComprehensiveLoss</Role>
      <ShortName>Accumulated Other Comprehensive Loss</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Revenue from Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRevenueFromContractsWithCustomersTables</Role>
      <ShortName>Revenue from Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRevenueFromContractsWithCustomers</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Supplementary Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationTables</Role>
      <ShortName>Supplementary Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/DisclosureSupplementaryInformation1</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureDebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureDebt</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeases</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensation</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxes</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureNetLossPerShare</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Retirement Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefitsTables</Role>
      <ShortName>Retirement Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefits</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Accumulated Other Comprehensive Loss (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAccumulatedOtherComprehensiveLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Loss (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAccumulatedOtherComprehensiveLoss</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformation</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995785 - Disclosure - Summary of Significant Accounting Policies - Summary of Changes Made to Consolidated Statement of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesSummaryOfChangesMadeToConsolidatedStatementOfOperationsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Summary of Changes Made to Consolidated Statement of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRevenueFromContractsWithCustomersScheduleOfDisaggregationOfRevenueDetail</Role>
      <ShortName>Revenue from Contracts with Customers - Schedule of Disaggregation of Revenue (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Revenue from Contracts with Customers - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRevenueFromContractsWithCustomersAdditionalInformationDetail</Role>
      <ShortName>Revenue from Contracts with Customers - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Revenue from Contracts with Customers - Additional Information (Detail)1</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureRevenueFromContractsWithCustomersAdditionalInformationDetail1</Role>
      <ShortName>Revenue from Contracts with Customers - Additional Information (Detail)1</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Acquisitions - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAcquisitionsAdditionalInformationDetail</Role>
      <ShortName>Acquisitions - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Supplementary Information - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationReconciliationOfCashCashEquivalentsAndRestrictedCashDetail</Role>
      <ShortName>Supplementary Information - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Supplementary Information - Schedule of Accounts Receivable Net (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationScheduleOfAccountsReceivableNetDetail</Role>
      <ShortName>Supplementary Information - Schedule of Accounts Receivable Net (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Supplementary Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationAdditionalInformationDetail</Role>
      <ShortName>Supplementary Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Supplementary Information - Schedule of Provision for Credit Loss (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationScheduleOfProvisionForCreditLossDetail</Role>
      <ShortName>Supplementary Information - Schedule of Provision for Credit Loss (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Supplementary Information - Summary of Property and Equipment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationSummaryOfPropertyAndEquipmentDetail</Role>
      <ShortName>Supplementary Information - Summary of Property and Equipment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Supplementary Information - Summary of Property and Equipment (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationSummaryOfPropertyAndEquipmentParentheticalDetail</Role>
      <ShortName>Supplementary Information - Summary of Property and Equipment (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Supplementary Information - Summary of Other Accrued Expenses and Current Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationSummaryOfOtherAccruedExpensesAndCurrentLiabilitiesDetail</Role>
      <ShortName>Supplementary Information - Summary of Other Accrued Expenses and Current Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995915 - Disclosure - Supplementary Information - Summary of Other Long-term Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSupplementaryInformationSummaryOfOtherLongTermLiabilitiesDetail</Role>
      <ShortName>Supplementary Information - Summary of Other Long-term Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Supplementary Information - Schedule of Other (Income) Expense, Net (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSupplementaryInformationScheduleOfOtherIncomeExpenseNetDetail</Role>
      <ShortName>Supplementary Information - Schedule of Other (Income) Expense, Net (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill Attributable to Software Reportable Segment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssetsChangesInCarryingAmountOfGoodwillAttributableToSoftwareReportableSegmentDetail</Role>
      <ShortName>Goodwill and Other Intangible Assets - Changes in Carrying Amount of Goodwill Attributable to Software Reportable Segment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Goodwill and Other Intangible Assets - Schedule of Other Intangible Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssetsScheduleOfOtherIntangibleAssetsDetail</Role>
      <ShortName>Goodwill and Other Intangible Assets - Schedule of Other Intangible Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995955 - Disclosure - Goodwill and Other Intangible Assets - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssetsAdditionalInformationDetail</Role>
      <ShortName>Goodwill and Other Intangible Assets - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995965 - Disclosure - Goodwill and Other Intangible Assets - Summary of Estimated Amortization Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureGoodwillAndOtherIntangibleAssetsSummaryOfEstimatedAmortizationExpenseDetail</Role>
      <ShortName>Goodwill and Other Intangible Assets - Summary of Estimated Amortization Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995975 - Disclosure - Debt - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureDebtAdditionalInformationDetail</Role>
      <ShortName>Debt - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995985 - Disclosure - Debt - Schedule of Net Carrying Value of 2027 and 2024 Notes (Detail)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureDebtScheduleOfNetCarryingValueOf2027And2024NotesDetail</Role>
      <ShortName>Debt - Schedule of Net Carrying Value of 2027 and 2024 Notes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995995 - Disclosure - Debt - Schedule of Interest Expense Related to 2027 and 2024 Notes (Detail)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureDebtScheduleOfInterestExpenseRelatedTo2027And2024NotesDetail</Role>
      <ShortName>Debt - Schedule of Interest Expense Related to 2027 and 2024 Notes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>996005 - Disclosure - Leases - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeasesAdditionalInformationDetail</Role>
      <ShortName>Leases - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>996015 - Disclosure - Leases - Schedule of Supplemental Balance Sheet Information Related to Lease Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeasesScheduleOfSupplementalBalanceSheetInformationRelatedToLeaseLiabilitiesDetail</Role>
      <ShortName>Leases - Schedule of Supplemental Balance Sheet Information Related to Lease Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>996025 - Disclosure - Leases - Schedule of Supplemental Cash Flow Information Related to Leases (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeasesScheduleOfSupplementalCashFlowInformationRelatedToLeasesDetail</Role>
      <ShortName>Leases - Schedule of Supplemental Cash Flow Information Related to Leases (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>996035 - Disclosure - Leases - Schedule of Maturities of Operating Lease Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureLeasesScheduleOfMaturitiesOfOperatingLeaseLiabilitiesDetail</Role>
      <ShortName>Leases - Schedule of Maturities of Operating Lease Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>996045 - Disclosure - Fair Value Measurements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureFairValueMeasurementsAdditionalInformationDetail</Role>
      <ShortName>Fair Value Measurements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>996055 - Disclosure - Stockholders' Equity - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockholdersEquityAdditionalInformationDetail</Role>
      <ShortName>Stockholders' Equity - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>996075 - Disclosure - Stock-based Compensation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationAdditionalInformationDetail</Role>
      <ShortName>Stock-based Compensation - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>996085 - Disclosure - Stock-based Compensation - Summary of Stock Option Activity under NSO Plan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfStockOptionActivityUnderNSOPlanDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Stock Option Activity under NSO Plan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>996095 - Disclosure - Stock-based Compensation - Summary of Stock Option Activity under 2012 Plan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfStockOptionActivityUnder2012PlanDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Stock Option Activity under 2012 Plan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>996105 - Disclosure - Stock-based Compensation - Summary of Restricted Stock Units Awarded (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfRestrictedStockUnitsAwardedDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Restricted Stock Units Awarded (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>996115 - Disclosure - Stock-based Compensation - Summary of Stock Option Activity under 2017 Plan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfStockOptionActivityUnder2017PlanDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Stock Option Activity under 2017 Plan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>996125 - Disclosure - Stock-based Compensation - Summary of Fair value of Company's Stock Options Granted (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfFairValueOfCompanySStockOptionsGrantedDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Fair value of Company's Stock Options Granted (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>996135 - Disclosure - Stock-based Compensation - Summary of Stock-Based Compensation Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureStockBasedCompensationSummaryOfStockBasedCompensationExpenseDetail</Role>
      <ShortName>Stock-based Compensation - Summary of Stock-Based Compensation Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>996145 - Disclosure - Income Taxes - Components of Income (Loss) Before Income Taxes (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesComponentsOfIncomeLossBeforeIncomeTaxesDetail</Role>
      <ShortName>Income Taxes - Components of Income (Loss) Before Income Taxes (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>996155 - Disclosure - Income Taxes - Significant Components of Income Tax Expense (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesSignificantComponentsOfIncomeTaxExpenseDetail</Role>
      <ShortName>Income Taxes - Significant Components of Income Tax Expense (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>996165 - Disclosure - Income Taxes - Reconciliation of Income Taxes Calculated at U.S. Federal Statutory Income Tax Rate (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesReconciliationOfIncomeTaxesCalculatedAtUSFederalStatutoryIncomeTaxRateDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Income Taxes Calculated at U.S. Federal Statutory Income Tax Rate (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>996175 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>996185 - Disclosure - Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesScheduleOfDeferredTaxAssetsAndLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>996195 - Disclosure - Income Taxes - Summary of Changes to Valuation Allowance Balance (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesSummaryOfChangesToValuationAllowanceBalanceDetail</Role>
      <ShortName>Income Taxes - Summary of Changes to Valuation Allowance Balance (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>996205 - Disclosure - Income Taxes - Operating Loss and Tax Credit Carryforwards (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesOperatingLossAndTaxCreditCarryforwardsDetail</Role>
      <ShortName>Income Taxes - Operating Loss and Tax Credit Carryforwards (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>996215 - Disclosure - Income Taxes - Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureIncomeTaxesReconciliationOfBeginningAndEndingAmountsOfUnrecognizedTaxBenefitsDetail</Role>
      <ShortName>Income Taxes - Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>996225 - Disclosure - Net Loss Per Share - Computation of Numerators and Denominators Used in Basic and Diluted Net (Loss) Income Per Share Amounts (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureNetLossPerShareComputationOfNumeratorsAndDenominatorsUsedInBasicAndDilutedNetLossIncomePerShareAmountsDetail</Role>
      <ShortName>Net Loss Per Share - Computation of Numerators and Denominators Used in Basic and Diluted Net (Loss) Income Per Share Amounts (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>996235 - Disclosure - Net Loss Per Share - Schedule of Anti-dilutive Shares Excluded from Computation of Diluted Net Loss Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureNetLossPerShareScheduleOfAntiDilutiveSharesExcludedFromComputationOfDilutedNetLossPerShareDetail</Role>
      <ShortName>Net Loss Per Share - Schedule of Anti-dilutive Shares Excluded from Computation of Diluted Net Loss Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>996245 - Disclosure - Retirement Benefits - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefitsAdditionalInformationDetail</Role>
      <ShortName>Retirement Benefits - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>996255 - Disclosure - Retirement Benefits - Schedule of Pension Benefits Obligation (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefitsScheduleOfPensionBenefitsObligationDetail</Role>
      <ShortName>Retirement Benefits - Schedule of Pension Benefits Obligation (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>996265 - Disclosure - Retirement Benefits - Schedule of Estimated Future Benefit Payments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureRetirementBenefitsScheduleOfEstimatedFutureBenefitPaymentsDetail</Role>
      <ShortName>Retirement Benefits - Schedule of Estimated Future Benefit Payments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>996275 - Disclosure - Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureAccumulatedOtherComprehensiveLossComponentsOfAccumulatedOtherComprehensiveLossDetail</Role>
      <ShortName>Accumulated Other Comprehensive Loss - Components of Accumulated Other Comprehensive Loss (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>996285 - Disclosure - Commitments and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetail</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>996295 - Disclosure - Commitments and Contingencies - Future Purchase Obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesFuturePurchaseObligationsDetails</Role>
      <ShortName>Commitments and Contingencies - Future Purchase Obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>996305 - Disclosure - Segment Information - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformationAdditionalInformationDetail</Role>
      <ShortName>Segment Information - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>996315 - Disclosure - Segment Information - Schedule of Segment Reporting Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformationScheduleOfSegmentReportingInformationDetail</Role>
      <ShortName>Segment Information - Schedule of Segment Reporting Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>996325 - Disclosure - Segment Information - Reconciliation of U.S. GAAP (Loss) Income Before Income Taxes to Adjusted EBITDA (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSegmentInformationReconciliationOfUsGaapLossIncomeBeforeIncomeTaxesToAdjustedEbitdaDetail</Role>
      <ShortName>Segment Information - Reconciliation of U.S. GAAP (Loss) Income Before Income Taxes to Adjusted EBITDA (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>996335 - Disclosure - Segment Information - Reconciliation of U.S. GAAP (Loss) Income Before Income Taxes to Adjusted EBITDA (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/DisclosureSegmentInformationReconciliationOfUsGaapLossIncomeBeforeIncomeTaxesToAdjustedEbitdaParentheticalDetail</Role>
      <ShortName>Segment Information - Reconciliation of U.S. GAAP (Loss) Income Before Income Taxes to Adjusted EBITDA (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="altr-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>996345 - Disclosure - Segment Information - Summary of Sales to External Customers and Long-Lived Assets by Geographical Areas (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.altair.com/20231231/taxonomy/role/Role_DisclosureSegmentInformationSummaryOfSalesToExternalCustomersAndLongLivedAssetsByGeographicalAreasDetail</Role>
      <ShortName>Segment Information - Summary of Sales to External Customers and Long-Lived Assets by Geographical Areas (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</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-K" isDefinitelyFs="true" isUsgaap="true" original="altr-20231231.htm">altr-20231231.htm</File>
    <File>altr-20231231.xsd</File>
  </InputFiles>
  <SupplementalFiles>
    <File>img206701346_0.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="1269">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="39">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="8">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
