<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>719</ContextCount>
  <ElementCount>494</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>185</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Statements of Financial Position</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofFinancialPosition</Role>
      <ShortName>Consolidated Statements of Financial Position</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statements of Comprehensive Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofComprehensiveIncomeParenthetical</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952157 - Statement - Consolidated Statements of Changes in Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofChangesinEquity</Role>
      <ShortName>Consolidated Statements of Changes in Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952158 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - CORPORATE INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/CORPORATEINFORMATION</Role>
      <ShortName>CORPORATE INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - BASIS OF PREPARATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BASISOFPREPARATION</Role>
      <ShortName>BASIS OF PREPARATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - SUMMARY OF MATERIAL ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIES</Role>
      <ShortName>SUMMARY OF MATERIAL ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - CRITICAL ACCOUNTING ESTIMATES AND ASSUMPTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/CRITICALACCOUNTINGESTIMATESANDASSUMPTIONS</Role>
      <ShortName>CRITICAL ACCOUNTING ESTIMATES AND ASSUMPTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - ACQUISITIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ACQUISITIONS</Role>
      <ShortName>ACQUISITIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - INVENTORIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INVENTORIES</Role>
      <ShortName>INVENTORIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - DEPOSITS, PREPAIDS AND OTHER ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/DEPOSITSPREPAIDSANDOTHERASSETS</Role>
      <ShortName>DEPOSITS, PREPAIDS AND OTHER ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIES</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - OTHER ASSETS AND LIABILITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OTHERASSETSANDLIABILITIES</Role>
      <ShortName>OTHER ASSETS AND LIABILITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - PROPERTY, PLANT AND EQUIPMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROPERTYPLANTANDEQUIPMENT</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - GOODWILL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/GOODWILL</Role>
      <ShortName>GOODWILL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - INTANGIBLE ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INTANGIBLEASSETS</Role>
      <ShortName>INTANGIBLE ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENT</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - PROVISIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROVISIONS</Role>
      <ShortName>PROVISIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952173 - Disclosure - EMPLOYEE BENEFITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITS</Role>
      <ShortName>EMPLOYEE BENEFITS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9952174 - Disclosure - BANK INDEBTEDNESS AND LONG-TERM DEBT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBT</Role>
      <ShortName>BANK INDEBTEDNESS AND LONG-TERM DEBT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9952175 - Disclosure - SHARE CAPITAL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SHARECAPITAL</Role>
      <ShortName>SHARE CAPITAL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9952176 - Disclosure - TAXATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATION</Role>
      <ShortName>TAXATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9952177 - Disclosure - STOCK-BASED COMPENSATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATION</Role>
      <ShortName>STOCK-BASED COMPENSATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9952178 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/COMMITMENTSANDCONTINGENCIES</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9952179 - Disclosure - SEGMENTED DISCLOSURE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SEGMENTEDDISCLOSURE</Role>
      <ShortName>SEGMENTED DISCLOSURE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9952180 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERS</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9952181 - Disclosure - OPERATING COSTS AND EXPENSES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OPERATINGCOSTSANDEXPENSES</Role>
      <ShortName>OPERATING COSTS AND EXPENSES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9952182 - Disclosure - NET FINANCE COSTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/NETFINANCECOSTS</Role>
      <ShortName>NET FINANCE COSTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9952183 - Disclosure - EARNINGS (LOSS) PER SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EARNINGSLOSSPERSHARE</Role>
      <ShortName>EARNINGS (LOSS) PER SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9952185 - Disclosure - CAPITAL MANAGEMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/CAPITALMANAGEMENT</Role>
      <ShortName>CAPITAL MANAGEMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9952186 - Disclosure - RELATED PARTY DISCLOSURE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RELATEDPARTYDISCLOSURE</Role>
      <ShortName>RELATED PARTY DISCLOSURE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - SUMMARY OF MATERIAL ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIESPolicies</Role>
      <ShortName>SUMMARY OF MATERIAL ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - SUMMARY OF MATERIAL ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIESTables</Role>
      <ShortName>SUMMARY OF MATERIAL ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIES</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - ACQUISITIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ACQUISITIONSTables</Role>
      <ShortName>ACQUISITIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/ACQUISITIONS</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - INVENTORIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INVENTORIESTables</Role>
      <ShortName>INVENTORIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/INVENTORIES</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - DEPOSITS, PREPAIDS AND OTHER ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/DEPOSITSPREPAIDSANDOTHERASSETSTables</Role>
      <ShortName>DEPOSITS, PREPAIDS AND OTHER ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/DEPOSITSPREPAIDSANDOTHERASSETS</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIESTables</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIES</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - OTHER ASSETS AND LIABILITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OTHERASSETSANDLIABILITIESTables</Role>
      <ShortName>OTHER ASSETS AND LIABILITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/OTHERASSETSANDLIABILITIES</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - PROPERTY, PLANT AND EQUIPMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROPERTYPLANTANDEQUIPMENTTables</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/PROPERTYPLANTANDEQUIPMENT</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - GOODWILL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/GOODWILLTables</Role>
      <ShortName>GOODWILL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/GOODWILL</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - INTANGIBLE ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INTANGIBLEASSETSTables</Role>
      <ShortName>INTANGIBLE ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/INTANGIBLEASSETS</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTTables</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENT</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - PROVISIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROVISIONSTables</Role>
      <ShortName>PROVISIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/PROVISIONS</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - EMPLOYEE BENEFITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSTables</Role>
      <ShortName>EMPLOYEE BENEFITS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/EMPLOYEEBENEFITS</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - BANK INDEBTEDNESS AND LONG-TERM DEBT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBTTables</Role>
      <ShortName>BANK INDEBTEDNESS AND LONG-TERM DEBT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBT</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - SHARE CAPITAL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SHARECAPITALTables</Role>
      <ShortName>SHARE CAPITAL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/SHARECAPITAL</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - TAXATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONTables</Role>
      <ShortName>TAXATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/TAXATION</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - STOCK-BASED COMPENSATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONTables</Role>
      <ShortName>STOCK-BASED COMPENSATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/STOCKBASEDCOMPENSATION</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - COMMITMENTS AND CONTINGENCIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/COMMITMENTSANDCONTINGENCIESTables</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/COMMITMENTSANDCONTINGENCIES</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - SEGMENTED DISCLOSURE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SEGMENTEDDISCLOSURETables</Role>
      <ShortName>SEGMENTED DISCLOSURE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/SEGMENTEDDISCLOSURE</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSTables</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERS</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - OPERATING COSTS AND EXPENSES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OPERATINGCOSTSANDEXPENSESTables</Role>
      <ShortName>OPERATING COSTS AND EXPENSES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/OPERATINGCOSTSANDEXPENSES</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - NET FINANCE COSTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/NETFINANCECOSTSTables</Role>
      <ShortName>NET FINANCE COSTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/NETFINANCECOSTS</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - EARNINGS (LOSS) PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EARNINGSLOSSPERSHARETables</Role>
      <ShortName>EARNINGS (LOSS) PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/EARNINGSLOSSPERSHARE</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - CAPITAL MANAGEMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/CAPITALMANAGEMENTTables</Role>
      <ShortName>CAPITAL MANAGEMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/CAPITALMANAGEMENT</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - RELATED PARTY DISCLOSURE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RELATEDPARTYDISCLOSURETables</Role>
      <ShortName>RELATED PARTY DISCLOSURE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://atscorp.com/role/RELATEDPARTYDISCLOSURE</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - SUMMARY OF MATERIAL ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIESDetails</Role>
      <ShortName>SUMMARY OF MATERIAL ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/SUMMARYOFMATERIALACCOUNTINGPOLICIESTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - ACQUISITIONS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ACQUISITIONSNarrativeDetails</Role>
      <ShortName>ACQUISITIONS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - ACQUISITIONS - Assets acquired and liabilities assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/ACQUISITIONSAssetsacquiredandliabilitiesassumedDetails</Role>
      <ShortName>ACQUISITIONS - Assets acquired and liabilities assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - INVENTORIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INVENTORIESDetails</Role>
      <ShortName>INVENTORIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/INVENTORIESTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - DEPOSITS, PREPAIDS AND OTHER ASSETS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/DEPOSITSPREPAIDSANDOTHERASSETSDetails</Role>
      <ShortName>DEPOSITS, PREPAIDS AND OTHER ASSETS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/DEPOSITSPREPAIDSANDOTHERASSETSTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Right-of-use assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIESRightofuseassetsDetails</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Right-of-use assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Lease liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIESLeaseliabilitiesDetails</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Lease liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIESNarrativeDetails</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - RIGHT-OF-USE ASSETS AND LEASE LIABILITIES- Annual lease obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RIGHTOFUSEASSETSANDLEASELIABILITIESAnnualleaseobligationsDetails</Role>
      <ShortName>RIGHT-OF-USE ASSETS AND LEASE LIABILITIES- Annual lease obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - OTHER ASSETS AND LIABILITIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OTHERASSETSANDLIABILITIESDetails</Role>
      <ShortName>OTHER ASSETS AND LIABILITIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/OTHERASSETSANDLIABILITIESTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - PROPERTY, PLANT AND EQUIPMENT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROPERTYPLANTANDEQUIPMENTDetails</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/PROPERTYPLANTANDEQUIPMENTTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - GOODWILL (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/GOODWILLDetails</Role>
      <ShortName>GOODWILL (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/GOODWILLTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - INTANGIBLE ASSETS - Components of Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INTANGIBLEASSETSComponentsofIntangibleAssetsDetails</Role>
      <ShortName>INTANGIBLE ASSETS - Components of Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - INTANGIBLE ASSETS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/INTANGIBLEASSETSNarrativeDetails</Role>
      <ShortName>INTANGIBLE ASSETS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Categories of financial instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTCategoriesoffinancialinstrumentsDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Categories of financial instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Fair value measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTFairvaluemeasurementsDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Fair value measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTNarrativeDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Credit risk (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTCreditriskDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Credit risk (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Allowance for doubtful accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTAllowancefordoubtfulaccountsDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Allowance for doubtful accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Trade payables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTTradepayablesDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Trade payables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Outstanding cash flow hedge positions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTOutstandingcashflowhedgepositionsDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Outstanding cash flow hedge positions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTMaturityDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Cash flow hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/FINANCIALINSTRUMENTSANDRISKMANAGEMENTCashflowhedgesDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS AND RISK MANAGEMENT - Cash flow hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - PROVISIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/PROVISIONSDetails</Role>
      <ShortName>PROVISIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/PROVISIONSTables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - EMPLOYEE BENEFITS - Change in plan assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSChangeinplanassetsDetails</Role>
      <ShortName>EMPLOYEE BENEFITS - Change in plan assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - EMPLOYEE BENEFITS - Comprehensive income (before tax) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSComprehensiveincomebeforetaxDetails</Role>
      <ShortName>EMPLOYEE BENEFITS - Comprehensive income (before tax) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>9955563 - Disclosure - EMPLOYEE BENEFITS - Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSAssumptionsDetails</Role>
      <ShortName>EMPLOYEE BENEFITS - Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>9955564 - Disclosure - EMPLOYEE BENEFITS - Net employee benefits expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSNetemployeebenefitsexpenseDetails</Role>
      <ShortName>EMPLOYEE BENEFITS - Net employee benefits expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>9955565 - Disclosure - EMPLOYEE BENEFITS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EMPLOYEEBENEFITSNarrativeDetails</Role>
      <ShortName>EMPLOYEE BENEFITS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>9955566 - Disclosure - BANK INDEBTEDNESS AND LONG-TERM DEBT - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBTNarrativeDetails</Role>
      <ShortName>BANK INDEBTEDNESS AND LONG-TERM DEBT - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>9955567 - Disclosure - BANK INDEBTEDNESS AND LONG-TERM DEBT - Schedule of borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBTScheduleofborrowingsDetails</Role>
      <ShortName>BANK INDEBTEDNESS AND LONG-TERM DEBT - Schedule of borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>9955568 - Disclosure - BANK INDEBTEDNESS AND LONG-TERM DEBT - Principal repayments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/BANKINDEBTEDNESSANDLONGTERMDEBTPrincipalrepaymentsDetails</Role>
      <ShortName>BANK INDEBTEDNESS AND LONG-TERM DEBT - Principal repayments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>9955569 - Disclosure - SHARE CAPITAL - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SHARECAPITALNarrativeDetails</Role>
      <ShortName>SHARE CAPITAL - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>9955570 - Disclosure - SHARE CAPITAL - Common shares issued (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SHARECAPITALCommonsharesissuedDetails</Role>
      <ShortName>SHARE CAPITAL - Common shares issued (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>9955571 - Disclosure - TAXATION - Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONReconciliationDetails</Role>
      <ShortName>TAXATION - Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>9955572 - Disclosure - TAXATION - Deferred tax assets and liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONDeferredtaxassetsandliabilitiesDetails</Role>
      <ShortName>TAXATION - Deferred tax assets and liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>9955573 - Disclosure - TAXATION - Unrecognized deferred income tax assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONUnrecognizeddeferredincometaxassetsDetails</Role>
      <ShortName>TAXATION - Unrecognized deferred income tax assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>9955574 - Disclosure - TAXATION - Operating loss carryforwards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONOperatinglosscarryforwardsDetails</Role>
      <ShortName>TAXATION - Operating loss carryforwards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>9955575 - Disclosure - TAXATION - Investment tax credits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONInvestmenttaxcreditsDetails</Role>
      <ShortName>TAXATION - Investment tax credits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>9955576 - Disclosure - TAXATION - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/TAXATIONNarrativeDetails</Role>
      <ShortName>TAXATION - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>9955577 - Disclosure - STOCK-BASED COMPENSATION - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONNarrativeDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>9955578 - Disclosure - STOCK-BASED COMPENSATION - Stock option activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONStockoptionactivityDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION - Stock option activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>9955579 - Disclosure - STOCK-BASED COMPENSATION - Exercise price range (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONExercisepricerangeDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION - Exercise price range (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>9955580 - Disclosure - STOCK-BASED COMPENSATION - Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONAssumptionsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION - Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>9955581 - Disclosure - STOCK-BASED COMPENSATION - Compensation expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/STOCKBASEDCOMPENSATIONCompensationexpenseDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION - Compensation expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>9955582 - Disclosure - COMMITMENTS AND CONTINGENCIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/COMMITMENTSANDCONTINGENCIESDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/COMMITMENTSANDCONTINGENCIESTables</ParentRole>
      <Position>106</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>9955583 - Disclosure - SEGMENTED DISCLOSURE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SEGMENTEDDISCLOSUREDetails</Role>
      <ShortName>SEGMENTED DISCLOSURE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/SEGMENTEDDISCLOSURETables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>9955584 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Revenue by type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSRevenuebytypeDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Revenue by type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>9955585 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSDisaggregationofrevenueDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>9955586 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Backlog (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSBacklogDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Backlog (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>9955587 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Accounts receivable continuity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSAccountsreceivablecontinuityDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Accounts receivable continuity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>9955588 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSContractbalancesDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Contract balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>9955589 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS - Net contract assets and liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/REVENUEFROMCONTRACTSWITHCUSTOMERSNetcontractassetsandliabilitiesDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS - Net contract assets and liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>9955590 - Disclosure - OPERATING COSTS AND EXPENSES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/OPERATINGCOSTSANDEXPENSESDetails</Role>
      <ShortName>OPERATING COSTS AND EXPENSES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/OPERATINGCOSTSANDEXPENSESTables</ParentRole>
      <Position>114</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>9955591 - Disclosure - NET FINANCE COSTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/NETFINANCECOSTSDetails</Role>
      <ShortName>NET FINANCE COSTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/NETFINANCECOSTSTables</ParentRole>
      <Position>115</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>9955592 - Disclosure - EARNINGS (LOSS) PER SHARE - Earnings per share computation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EARNINGSLOSSPERSHAREEarningspersharecomputationDetails</Role>
      <ShortName>EARNINGS (LOSS) PER SHARE - Earnings per share computation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/EARNINGSLOSSPERSHARETables</ParentRole>
      <Position>116</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>9955593 - Disclosure - EARNINGS (LOSS) PER SHARE - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/EARNINGSLOSSPERSHARENarrativeDetails</Role>
      <ShortName>EARNINGS (LOSS) PER SHARE - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/EARNINGSLOSSPERSHARETables</ParentRole>
      <Position>117</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>9955594 - Disclosure - SUPPLEMENTAL CASH FLOW INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/SUPPLEMENTALCASHFLOWINFORMATIONDetails</Role>
      <ShortName>SUPPLEMENTAL CASH FLOW INFORMATION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>9955595 - Disclosure - CAPITAL MANAGEMENT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/CAPITALMANAGEMENTDetails</Role>
      <ShortName>CAPITAL MANAGEMENT (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/CAPITALMANAGEMENTTables</ParentRole>
      <Position>119</Position>
    </Report>
    <Report instance="ats-20260331_d2.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>9955596 - Disclosure - RELATED PARTY DISCLOSURE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://atscorp.com/role/RELATEDPARTYDISCLOSUREDetails</Role>
      <ShortName>RELATED PARTY DISCLOSURE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://atscorp.com/role/RELATEDPARTYDISCLOSURETables</ParentRole>
      <Position>120</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="40-F" isDefinitelyFs="true" isIfrs="true" original="ats-20260331_d2.htm">ats-20260331_d2.htm</File>
    <File doctype="40-F" isDefinitelyFs="true" isIfrs="true" original="ats-20260331.htm">ats-20260331.htm</File>
    <File>ats-20260331.xsd</File>
    <File>ats-20260331_cal.xml</File>
    <File>ats-20260331_def.xml</File>
    <File>ats-20260331_lab.xml</File>
    <File>ats-20260331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>ats-20260331_g1.jpg</File>
    <File>ats-20260331_g2.jpg</File>
    <File>ats-20260331_g3.jpg</File>
    <File>ats-20260331_g4.jpg</File>
    <File>ats-20260331_g5.jpg</File>
    <File>ats-20260331_g6.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="40">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="1718">https://xbrl.ifrs.org/taxonomy/2025-03-27/ifrs-full</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
