<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.1.u1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>144</ContextCount>
  <ElementCount>224</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>46</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="mrsn-20240331.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-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedStatementsofOperationsandComprehensiveLoss</Role>
      <ShortName>Condensed Consolidated Statements of Operations and Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - Condensed Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedStatementsofStockholdersEquity</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - Condensed Consolidated Statements of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedStatementsofStockholdersEquityParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CondensedConsolidatedStatementsofCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - 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>8</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - 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>9</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Disclosure - Collaboration agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Collaborationagreements</Role>
      <ShortName>Collaboration agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - 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>11</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - 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>12</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - Accrued expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Accruedexpenses</Role>
      <ShortName>Accrued expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - Stockholders' equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Stockholdersequity</Role>
      <ShortName>Stockholders' equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - 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>16</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - 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>17</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - Commitments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/Commitments</Role>
      <ShortName>Commitments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.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>19</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.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>20</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.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>21</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9954472 - 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>22</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9954473 - 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>23</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9954474 - 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>24</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9954475 - 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>25</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9954476 - 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>26</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9954477 - 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>27</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9954478 - 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>28</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9954479 - 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>29</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9954480 - 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>30</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9954481 - Disclosure - Summary of significant accounting policies - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/SummaryofsignificantaccountingpoliciesNarrativeDetails</Role>
      <ShortName>Summary of significant accounting policies - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954482 - 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>32</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954483 - 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>33</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954484 - Disclosure - Collaboration agreements - Merck Immunosythen Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsMerckImmunosythenAgreementDetails</Role>
      <ShortName>Collaboration agreements - Merck Immunosythen Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954485 - Disclosure - Collaboration agreements - Contract Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CollaborationagreementsContractAssetsandLiabilitiesDetails</Role>
      <ShortName>Collaboration agreements - Contract Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954486 - 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>36</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954487 - Disclosure - Fair value measurements - Fair Value, Assets Measured on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/FairvaluemeasurementsFairValueAssetsMeasuredonRecurringBasisDetails</Role>
      <ShortName>Fair value measurements - Fair Value, Assets Measured on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954488 - 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>38</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954489 - 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>39</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954490 - 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>40</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954491 - 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>41</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954492 - 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>42</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954493 - 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>43</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954494 - 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>44</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954495 - 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>45</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954496 - Disclosure - Stock-based compensation - Plans Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationPlansNarrativeDetails</Role>
      <ShortName>Stock-based compensation - Plans Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954497 - 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>47</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954498 - Disclosure - Stock-based compensation - Restricted Stock Units and Other Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationRestrictedStockUnitsandOtherAwardsDetails</Role>
      <ShortName>Stock-based compensation - Restricted Stock Units and Other Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954499 - Disclosure - Stock-based compensation - Stock Based Compensation Expense by Award Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationStockBasedCompensationExpensebyAwardTypeDetails</Role>
      <ShortName>Stock-based compensation - Stock Based Compensation Expense by Award Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954500 - Disclosure - Stock-based compensation - Schedule of Stock-Based Compensation Expense as Reflected in the Company's Condensed Consolidated Statements of Operations and Comprehensive Loss (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/StockbasedcompensationScheduleofStockBasedCompensationExpenseasReflectedintheCompanysCondensedConsolidatedStatementsofOperationsandComprehensiveLossDetails</Role>
      <ShortName>Stock-based compensation - Schedule of Stock-Based Compensation Expense as Reflected in the Company's Condensed Consolidated Statements of Operations and Comprehensive Loss (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954501 - 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>51</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954502 - 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>52</Position>
    </Report>
    <Report instance="mrsn-20240331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954503 - Disclosure - Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mersana.com/role/CommitmentsDetails</Role>
      <ShortName>Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mersana.com/role/Commitments</ParentRole>
      <Position>53</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Warning">[ix-0514-Hidden-Fact-Eligible-For-Transform] WARN: 2 fact(s) appearing in ix:hidden were eligible for transformation: us-gaap:NumberOfOperatingSegments, us-gaap:RevenueFromContractWithCustomerExcludingAssessedTax -  mrsn-20240331.htm 4</Log>
  </Logs>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="mrsn-20240331.htm">mrsn-20240331.htm</File>
    <File>mrsn-20240331.xsd</File>
    <File>mrsn-20240331_cal.xml</File>
    <File>mrsn-20240331_def.xml</File>
    <File>mrsn-20240331_lab.xml</File>
    <File>mrsn-20240331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="444">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
