<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>237</ContextCount>
  <ElementCount>391</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>68</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - Consolidated Statements of Operations and Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedStatementsofOperationsandComprehensiveLoss</Role>
      <ShortName>Consolidated Statements of Operations and Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedStatementsofStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - Consolidated Statements of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedStatementsofStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - Disclosure - Nature of business and basis of presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Natureofbusinessandbasisofpresentation</Role>
      <ShortName>Nature of business and basis of presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Disclosure - Summary of significant accounting policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Summaryofsignificantaccountingpolicies</Role>
      <ShortName>Summary of significant accounting policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - Disclosure - Collaboration agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Collaborationagreements</Role>
      <ShortName>Collaboration agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - Disclosure - Fair value measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Fairvaluemeasurements</Role>
      <ShortName>Fair value measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - Cash, cash equivalents, and short-term marketable securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Cashcashequivalentsandshorttermmarketablesecurities</Role>
      <ShortName>Cash, cash equivalents, and short-term marketable securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - Property and equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Propertyandequipment</Role>
      <ShortName>Property and equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - Accrued expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Accruedexpenses</Role>
      <ShortName>Accrued expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - Disclosure - Stockholders??? equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Stockholdersequity</Role>
      <ShortName>Stockholders??? equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - Stock-based compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Stockbasedcompensation</Role>
      <ShortName>Stock-based compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>0000019 - Disclosure - Net loss per share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Netlosspershare</Role>
      <ShortName>Net loss per share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>0000020 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>0000021 - Disclosure - Income taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Incometaxes</Role>
      <ShortName>Income taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>0000022 - Disclosure - Employee benefit plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Employeebenefitplan</Role>
      <ShortName>Employee benefit plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>0000023 - Disclosure - Commitments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Commitments</Role>
      <ShortName>Commitments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>0000024 - Disclosure - Restructuring</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Restructuring</Role>
      <ShortName>Restructuring</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.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>25</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.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>26</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9954471 - Disclosure - Summary of significant accounting policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesPolicies</Role>
      <ShortName>Summary of significant accounting policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9954472 - Disclosure - Summary of significant accounting policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesTables</Role>
      <ShortName>Summary of significant accounting policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Summaryofsignificantaccountingpolicies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9954473 - Disclosure - Collaboration agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsTables</Role>
      <ShortName>Collaboration agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Collaborationagreements</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9954474 - Disclosure - Fair value measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/FairvaluemeasurementsTables</Role>
      <ShortName>Fair value measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Fairvaluemeasurements</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9954475 - Disclosure - Cash, cash equivalents, and short-term marketable securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CashcashequivalentsandshorttermmarketablesecuritiesTables</Role>
      <ShortName>Cash, cash equivalents, and short-term marketable securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Cashcashequivalentsandshorttermmarketablesecurities</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954476 - Disclosure - Property and equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/PropertyandequipmentTables</Role>
      <ShortName>Property and equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Propertyandequipment</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954477 - Disclosure - Accrued expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/AccruedexpensesTables</Role>
      <ShortName>Accrued expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Accruedexpenses</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954478 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Debt</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954479 - Disclosure - Stockholders??? equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockholdersequityTables</Role>
      <ShortName>Stockholders??? equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Stockholdersequity</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954480 - Disclosure - Stock-based compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationTables</Role>
      <ShortName>Stock-based compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Stockbasedcompensation</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954481 - Disclosure - Net loss per share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/NetlosspershareTables</Role>
      <ShortName>Net loss per share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Netlosspershare</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954482 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Leases</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954483 - Disclosure - Income taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/IncometaxesTables</Role>
      <ShortName>Income taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Incometaxes</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954484 - Disclosure - Restructuring (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/RestructuringTables</Role>
      <ShortName>Restructuring (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Restructuring</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954485 - Disclosure - Nature of business and basis of presentation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/NatureofbusinessandbasisofpresentationDetails</Role>
      <ShortName>Nature of business and basis of presentation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Natureofbusinessandbasisofpresentation</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954486 - Disclosure - Summary of significant accounting policies - Revenue Recognition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesRevenueRecognitionDetails</Role>
      <ShortName>Summary of significant accounting policies - Revenue Recognition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954487 - Disclosure - Summary of significant accounting policies - Stock-Based Compensation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesStockBasedCompensationDetails</Role>
      <ShortName>Summary of significant accounting policies - Stock-Based Compensation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954488 - Disclosure - Summary of significant accounting policies - Property and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesPropertyandEquipmentDetails</Role>
      <ShortName>Summary of significant accounting policies - Property and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954489 - Disclosure - Summary of significant accounting policies - Risks (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesRisksDetails</Role>
      <ShortName>Summary of significant accounting policies - Risks (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954490 - Disclosure - Collaboration agreements - GlaxoSmithKline (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsGlaxoSmithKlineDetails</Role>
      <ShortName>Collaboration agreements - GlaxoSmithKline (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954491 - Disclosure - Collaboration agreements - Johnson &amp; Johnson (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsJohnsonJohnsonDetails</Role>
      <ShortName>Collaboration agreements - Johnson &amp; Johnson (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954492 - Disclosure - Collaboration agreements - Merck Immunosynthen Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsMerckImmunosynthenAgreementDetails</Role>
      <ShortName>Collaboration agreements - Merck Immunosynthen Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954493 - Disclosure - Collaboration agreements - Merck Dolaflexin Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsMerckDolaflexinAgreementDetails</Role>
      <ShortName>Collaboration agreements - Merck Dolaflexin Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954494 - Disclosure - Collaboration agreements - Summary of Contract Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsSummaryofContractAssetsandLiabilitiesDetails</Role>
      <ShortName>Collaboration agreements - Summary of Contract Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954495 - Disclosure - Collaboration agreements - Other Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsOtherRevenueDetails</Role>
      <ShortName>Collaboration agreements - Other Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954496 - Disclosure - Fair value measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/FairvaluemeasurementsDetails</Role>
      <ShortName>Fair value measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/FairvaluemeasurementsTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954497 - Disclosure - Cash, cash equivalents, and short-term marketable securities - Schedule of Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CashcashequivalentsandshorttermmarketablesecuritiesScheduleofRestrictedCashDetails</Role>
      <ShortName>Cash, cash equivalents, and short-term marketable securities - Schedule of Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9954498 - Disclosure - Cash, cash equivalents, and short-term marketable securities - Debt Securities, Available-for-Sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CashcashequivalentsandshorttermmarketablesecuritiesDebtSecuritiesAvailableforSaleDetails</Role>
      <ShortName>Cash, cash equivalents, and short-term marketable securities - Debt Securities, Available-for-Sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9954499 - Disclosure - Cash, cash equivalents, and short-term marketable securities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CashcashequivalentsandshorttermmarketablesecuritiesNarrativeDetails</Role>
      <ShortName>Cash, cash equivalents, and short-term marketable securities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/CashcashequivalentsandshorttermmarketablesecuritiesTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9954500 - Disclosure - Property and equipment - Summary (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/PropertyandequipmentSummaryDetails</Role>
      <ShortName>Property and equipment - Summary (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9954501 - Disclosure - Property and equipment - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/PropertyandequipmentNarrativeDetails</Role>
      <ShortName>Property and equipment - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9954502 - Disclosure - Accrued expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/AccruedexpensesDetails</Role>
      <ShortName>Accrued expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/AccruedexpensesTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9954503 - Disclosure - Debt - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/DebtNarrativeDetails</Role>
      <ShortName>Debt - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9954504 - Disclosure - Debt - Schedule of Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/DebtScheduleofDebtDetails</Role>
      <ShortName>Debt - Schedule of Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9954505 - Disclosure - Debt - Schedule of Estimated Future Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/DebtScheduleofEstimatedFuturePaymentsDetails</Role>
      <ShortName>Debt - Schedule of Estimated Future Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9954506 - Disclosure - Stockholders??? equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockholdersequityNarrativeDetails</Role>
      <ShortName>Stockholders??? equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9954507 - Disclosure - Stockholders??? equity - Common Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockholdersequityCommonStockDetails</Role>
      <ShortName>Stockholders??? equity - Common Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9954508 - Disclosure - Stock-based compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationNarrativeDetails</Role>
      <ShortName>Stock-based compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9954509 - Disclosure - Stock-based compensation - Activity Under Stock Option Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationActivityUnderStockOptionPlanDetails</Role>
      <ShortName>Stock-based compensation - Activity Under Stock Option Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9954510 - Disclosure - Stock-based compensation - Restricted Stock Units and Other Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationRestrictedStockUnitsandOtherStockAwardsDetails</Role>
      <ShortName>Stock-based compensation - Restricted Stock Units and Other Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9954511 - Disclosure - Stock-based compensation - Stock Based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationStockBasedCompensationExpenseDetails</Role>
      <ShortName>Stock-based compensation - Stock Based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9954512 - Disclosure - Stock-based compensation - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationFairValueAssumptionsDetails</Role>
      <ShortName>Stock-based compensation - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9954513 - Disclosure - Net loss per share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/NetlosspershareDetails</Role>
      <ShortName>Net loss per share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/NetlosspershareTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9954514 - Disclosure - Leases - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesNarrativeDetails</Role>
      <ShortName>Leases - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9954515 - Disclosure - Leases - Components of Lease Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesComponentsofLeaseExpenseDetails</Role>
      <ShortName>Leases - Components of Lease Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9954516 - Disclosure - Leases - Supplemental Balance Sheet Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesSupplementalBalanceSheetInformationDetails</Role>
      <ShortName>Leases - Supplemental Balance Sheet Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9954517 - Disclosure - Leases - Supplemental Cash Flow Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesSupplementalCashFlowInformationDetails</Role>
      <ShortName>Leases - Supplemental Cash Flow Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9954518 - Disclosure - Leases - Future Minimum Lease Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/LeasesFutureMinimumLeasePaymentsDetails</Role>
      <ShortName>Leases - Future Minimum Lease Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9954519 - Disclosure - Income taxes - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/IncometaxesNarrativeDetails</Role>
      <ShortName>Income taxes - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9954520 - Disclosure - Income taxes - Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/IncometaxesReconciliationDetails</Role>
      <ShortName>Income taxes - Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9954521 - Disclosure - Income taxes - Components of Deferred Tax Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/IncometaxesComponentsofDeferredTaxAssetsDetails</Role>
      <ShortName>Income taxes - Components of Deferred Tax Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9954522 - Disclosure - Employee benefit plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/EmployeebenefitplanDetails</Role>
      <ShortName>Employee benefit plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Employeebenefitplan</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9954523 - Disclosure - Commitments - License agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CommitmentsLicenseagreementsDetails</Role>
      <ShortName>Commitments - License agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9954524 - Disclosure - Restructuring - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/RestructuringNarrativeDetails</Role>
      <ShortName>Restructuring - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9954525 - Disclosure - Restructuring - Charges Incurred (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/RestructuringChargesIncurredDetails</Role>
      <ShortName>Restructuring - Charges Incurred (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9954526 - Disclosure - Restructuring - Restructuring Related to Research and Development Activities and General and Administrative Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/RestructuringRestructuringRelatedtoResearchandDevelopmentActivitiesandGeneralandAdministrativeActivitiesDetails</Role>
      <ShortName>Restructuring - Restructuring Related to Research and Development Activities and General and Administrative Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="mrsn-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9954527 - Disclosure - Restructuring - Accrued Restructuring Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/RestructuringAccruedRestructuringCostsDetails</Role>
      <ShortName>Restructuring - Accrued Restructuring Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</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="mrsn-20231231.htm">mrsn-20231231.htm</File>
    <File>mrsn-20231231.xsd</File>
    <File>mrsn-20231231_cal.xml</File>
    <File>mrsn-20231231_def.xml</File>
    <File>mrsn-20231231_lab.xml</File>
    <File>mrsn-20231231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>mrsn-20231231_g1.jpg</File>
    <File>mrsn-20231231_g2.jpg</File>
    <File>mrsn-20231231_g3.jpg</File>
    <File>mrsn-20231231_g4.jpg</File>
    <File>mrsn-20231231_g5.jpg</File>
    <File>mrsn-20231231_g6.jpg</File>
    <File>mrsn-20231231_g7.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="832">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="8">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
