<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1489</ContextCount>
  <ElementCount>679</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>252</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000010001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010002 - Statement - Consolidated Statement of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofIncome</Role>
      <ShortName>Consolidated Statement of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100020003 - Statement - Consolidated Statement of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofComprehensiveIncome</Role>
      <ShortName>Consolidated Statement of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100030004 - Statement - Consolidated Balance Sheet</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedBalanceSheet</Role>
      <ShortName>Consolidated Balance Sheet</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100040005 - Statement - Consolidated Balance Sheet (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedBalanceSheetParenthetical</Role>
      <ShortName>Consolidated Balance Sheet (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100050006 - Statement - Consolidated Statement of Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofEquity</Role>
      <ShortName>Consolidated Statement of Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100060007 - Statement - Consolidated Statement of Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofEquityParenthetical</Role>
      <ShortName>Consolidated Statement of Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100070008 - Statement - Consolidated Statement of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofCashFlows</Role>
      <ShortName>Consolidated Statement of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100080009 - Statement - Consolidated Statement of Cash Flows (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ConsolidatedStatementofCashFlowsParenthetical</Role>
      <ShortName>Consolidated Statement of Cash Flows (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>210011001 - Disclosure - Nature of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/NatureofOperations</Role>
      <ShortName>Nature of Operations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>210031002 - Disclosure - Summary of Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SummaryofAccountingPolicies</Role>
      <ShortName>Summary of Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>210061003 - Disclosure - Acquisitions, Divestitures, Research Collaborations and License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreements</Role>
      <ShortName>Acquisitions, Divestitures, Research Collaborations and License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>210111004 - Disclosure - Collaborative Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/CollaborativeArrangements</Role>
      <ShortName>Collaborative Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>210151005 - Disclosure - Restructuring</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/Restructuring</Role>
      <ShortName>Restructuring</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>210201006 - Disclosure - Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstruments</Role>
      <ShortName>Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>210331007 - Disclosure - Inventories</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/Inventories</Role>
      <ShortName>Inventories</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>210371008 - Disclosure - Goodwill and Other Intangibles</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/GoodwillandOtherIntangibles</Role>
      <ShortName>Goodwill and Other Intangibles</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>210421009 - Disclosure - Loans Payable, Long-Term Debt and Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeases</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>210481010 - Disclosure - Contingencies and Environmental Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ContingenciesandEnvironmentalLiabilities</Role>
      <ShortName>Contingencies and Environmental Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>210521011 - Disclosure - Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/Equity</Role>
      <ShortName>Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>210561012 - Disclosure - Share-Based Compensation Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlans</Role>
      <ShortName>Share-Based Compensation Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>210631013 - Disclosure - Pension and Other Postretirement Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlans</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>210761014 - Disclosure - Other (Income) Expense, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherIncomeExpenseNet</Role>
      <ShortName>Other (Income) Expense, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>210801015 - Disclosure - Taxes on Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncome</Role>
      <ShortName>Taxes on Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>210881016 - Disclosure - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EarningsperShare</Role>
      <ShortName>Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>210921017 - Disclosure - Other Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherComprehensiveIncomeLoss</Role>
      <ShortName>Other Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>210951018 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>220042001 - Disclosure - Summary of Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SummaryofAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.merck.com/role/SummaryofAccountingPolicies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>230073001 - Disclosure - Acquisitions, Divestitures, Research Collaborations and License Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreementsTables</Role>
      <ShortName>Acquisitions, Divestitures, Research Collaborations and License Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreements</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>230123002 - Disclosure - Collaborative Arrangements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/CollaborativeArrangementsTables</Role>
      <ShortName>Collaborative Arrangements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/CollaborativeArrangements</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>230163003 - Disclosure - Restructuring (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/RestructuringTables</Role>
      <ShortName>Restructuring (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/Restructuring</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>230213004 - Disclosure - Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsTables</Role>
      <ShortName>Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/FinancialInstruments</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>230343005 - Disclosure - Inventories (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/InventoriesTables</Role>
      <ShortName>Inventories (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/Inventories</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>230383006 - Disclosure - Goodwill and Other Intangibles (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/GoodwillandOtherIntangiblesTables</Role>
      <ShortName>Goodwill and Other Intangibles (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/GoodwillandOtherIntangibles</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>230433007 - Disclosure - Loans Payable, Long-Term Debt and Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeasesTables</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/LoansPayableLongTermDebtandLeases</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>230533008 - Disclosure - Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EquityTables</Role>
      <ShortName>Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/Equity</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>230573009 - Disclosure - Share-Based Compensation Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansTables</Role>
      <ShortName>Share-Based Compensation Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/ShareBasedCompensationPlans</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>230643010 - Disclosure - Pension and Other Postretirement Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansTables</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlans</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>230773011 - Disclosure - Other (Income) Expense, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherIncomeExpenseNetTables</Role>
      <ShortName>Other (Income) Expense, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/OtherIncomeExpenseNet</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>230813012 - Disclosure - Taxes on Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeTables</Role>
      <ShortName>Taxes on Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/TaxesonIncome</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>230893013 - Disclosure - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EarningsperShareTables</Role>
      <ShortName>Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/EarningsperShare</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>230933014 - Disclosure - Other Comprehensive Income (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherComprehensiveIncomeLossTables</Role>
      <ShortName>Other Comprehensive Income (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/OtherComprehensiveIncomeLoss</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>230963015 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.merck.com/role/SegmentReporting</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>240024001 - Disclosure - Nature of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/NatureofOperationsDetails</Role>
      <ShortName>Nature of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.merck.com/role/NatureofOperations</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>240054002 - Disclosure - Summary of Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SummaryofAccountingPoliciesDetails</Role>
      <ShortName>Summary of Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.merck.com/role/SummaryofAccountingPoliciesPolicies</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>240084003 - Disclosure - Acquisitions, Divestitures, Research Collaborations and License Agreements - Acquisitions Narrative (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreementsAcquisitionsNarrativeDetail</Role>
      <ShortName>Acquisitions, Divestitures, Research Collaborations and License Agreements - Acquisitions Narrative (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>240094004 - Disclosure - Acquisitions, Divestitures, Research Collaborations and License Agreements - Divestitures Narrative (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreementsDivestituresNarrativeDetail</Role>
      <ShortName>Acquisitions, Divestitures, Research Collaborations and License Agreements - Divestitures Narrative (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>240104005 - Disclosure - Acquisitions, Divestitures, Research Collaborations and License Agreements - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/AcquisitionsDivestituresResearchCollaborationsandLicenseAgreementsEstimatedFairValueofAssetsAcquiredandLiabilitiesAssumedDetails</Role>
      <ShortName>Acquisitions, Divestitures, Research Collaborations and License Agreements - Estimated Fair Value of Assets Acquired and Liabilities Assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>240134006 - Disclosure - Collaborative Arrangements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/CollaborativeArrangementsNarrativeDetails</Role>
      <ShortName>Collaborative Arrangements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>240144007 - Disclosure - Collaborative Arrangements - Schedule of Collaborative Arrangement Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/CollaborativeArrangementsScheduleofCollaborativeArrangementTransactionsDetails</Role>
      <ShortName>Collaborative Arrangements - Schedule of Collaborative Arrangement Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>240174008 - Disclosure - Restructuring - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/RestructuringNarrativeDetails</Role>
      <ShortName>Restructuring - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>240184009 - Disclosure - Restructuring - Charges Related to Restructuring Program Activities by Type of Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/RestructuringChargesRelatedtoRestructuringProgramActivitiesbyTypeofCostDetails</Role>
      <ShortName>Restructuring - Charges Related to Restructuring Program Activities by Type of Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>240194010 - Disclosure - Restructuring - Charges and Spending Relating to Restructuring Activities by Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/RestructuringChargesandSpendingRelatingtoRestructuringActivitiesbyProgramDetails</Role>
      <ShortName>Restructuring - Charges and Spending Relating to Restructuring Activities by Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>240224011 - Disclosure - Financial Instruments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsNarrativeDetails</Role>
      <ShortName>Financial Instruments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>240234012 - Disclosure - Financial Instruments - Effect of Net Investment Hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsEffectofNetInvestmentHedgesDetails</Role>
      <ShortName>Financial Instruments - Effect of Net Investment Hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>240244013 - Disclosure - Financial Instruments - Information About Interest Rate Swaps (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsInformationAboutInterestRateSwapsDetails</Role>
      <ShortName>Financial Instruments - Information About Interest Rate Swaps (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>240254014 - Disclosure - Financial Instruments - Fair Value of Hedges (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsFairValueofHedgesDetails</Role>
      <ShortName>Financial Instruments - Fair Value of Hedges (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>240264015 - Disclosure - Financial Instruments - Fair Value of Derivatives Segregated Between Those Derivatives That are Designated as Hedging Instruments and Those That are Not Designated as Hedging Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsFairValueofDerivativesSegregatedBetweenThoseDerivativesThatareDesignatedasHedgingInstrumentsandThoseThatareNotDesignatedasHedgingInstrumentsDetails</Role>
      <ShortName>Financial Instruments - Fair Value of Derivatives Segregated Between Those Derivatives That are Designated as Hedging Instruments and Those That are Not Designated as Hedging Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>240274016 - Disclosure - Financial Instruments - Information on Derivative Positions Subject to Master Netting Arrangements as if they were Presented on a Net Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsInformationonDerivativePositionsSubjecttoMasterNettingArrangementsasiftheywerePresentedonaNetBasisDetails</Role>
      <ShortName>Financial Instruments - Information on Derivative Positions Subject to Master Netting Arrangements as if they were Presented on a Net Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>240284017 - Disclosure - Financial Instruments - Location and Pretax (Gains) or Loss Amounts for Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsLocationandPretaxGainsorLossAmountsforDerivativesDetails</Role>
      <ShortName>Financial Instruments - Location and Pretax (Gains) or Loss Amounts for Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>240294018 - Disclosure - Financial Instruments - Effects of Derivatives Not Designated as Hedging Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsEffectsofDerivativesNotDesignatedasHedgingInstrumentsDetails</Role>
      <ShortName>Financial Instruments - Effects of Derivatives Not Designated as Hedging Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>240304019 - Disclosure - Financial Instruments - Information on Debt and Equity Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsInformationonDebtandEquitySecuritiesDetails</Role>
      <ShortName>Financial Instruments - Information on Debt and Equity Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>240314020 - Disclosure - Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails</Role>
      <ShortName>Financial Instruments - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>240324021 - Disclosure - Financial Instruments - Summarized Information About Changes in Liabilities for Contingent Consideration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/FinancialInstrumentsSummarizedInformationAboutChangesinLiabilitiesforContingentConsiderationDetails</Role>
      <ShortName>Financial Instruments - Summarized Information About Changes in Liabilities for Contingent Consideration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>240354022 - Disclosure - Inventories - Inventories (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/InventoriesInventoriesDetails</Role>
      <ShortName>Inventories - Inventories (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>240364023 - Disclosure - Inventories - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/InventoriesNarrativeDetails</Role>
      <ShortName>Inventories - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>240394024 - Disclosure - Goodwill and Other Intangibles - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/GoodwillandOtherIntangiblesNarrativeDetails</Role>
      <ShortName>Goodwill and Other Intangibles - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>240404025 - Disclosure - Goodwill and Other Intangibles - Goodwill Activity by Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/GoodwillandOtherIntangiblesGoodwillActivitybySegmentDetails</Role>
      <ShortName>Goodwill and Other Intangibles - Goodwill Activity by Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>240414026 - Disclosure - Goodwill and Other Intangibles - Other Intangibles (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/GoodwillandOtherIntangiblesOtherIntangiblesDetails</Role>
      <ShortName>Goodwill and Other Intangibles - Other Intangibles (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>240444027 - Disclosure - Loans Payable, Long-Term Debt and Leases - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeasesNarrativeDetails</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>240454028 - Disclosure - Loans Payable, Long-Term Debt and Leases - Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeasesLongTermDebtDetails</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases - Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>240464029 - Disclosure - Loans Payable, Long-Term Debt and Leases - Balance Sheet Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeasesBalanceSheetInformationDetails</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases - Balance Sheet Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>240474030 - Disclosure - Loans Payable, Long-Term Debt and Leases - Maturity Schedule (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/LoansPayableLongTermDebtandLeasesMaturityScheduleDetails</Role>
      <ShortName>Loans Payable, Long-Term Debt and Leases - Maturity Schedule (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>240494031 - Disclosure - Contingencies and Environmental Liabilities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ContingenciesandEnvironmentalLiabilitiesNarrativeDetails</Role>
      <ShortName>Contingencies and Environmental Liabilities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>240504032 - Disclosure - Contingencies and Environmental Liabilities - Legal Defense Reserves - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ContingenciesandEnvironmentalLiabilitiesLegalDefenseReservesNarrativeDetails</Role>
      <ShortName>Contingencies and Environmental Liabilities - Legal Defense Reserves - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>240514033 - Disclosure - Contingencies and Environmental Liabilities - Environmental Litigation and Matters - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ContingenciesandEnvironmentalLiabilitiesEnvironmentalLitigationandMattersNarrativeDetails</Role>
      <ShortName>Contingencies and Environmental Liabilities - Environmental Litigation and Matters - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>240544034 - Disclosure - Equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EquityNarrativeDetails</Role>
      <ShortName>Equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>240554035 - Disclosure - Equity - Shareholders' Equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EquityShareholdersEquityDetails</Role>
      <ShortName>Equity - Shareholders' Equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>240584036 - Disclosure - Share-Based Compensation Plans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansNarrativeDetails</Role>
      <ShortName>Share-Based Compensation Plans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>240594037 - Disclosure - Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansAssumptionsUsedtoDetermineWeightedAverageFairValueofOptionsGrantedDetails</Role>
      <ShortName>Share-Based Compensation Plans - Assumptions Used to Determine Weighted-Average Fair Value of Options Granted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>240604038 - Disclosure - Share-Based Compensation Plans - Summary of Information Relative to Stock Option Plan Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansSummaryofInformationRelativetoStockOptionPlanActivityDetails</Role>
      <ShortName>Share-Based Compensation Plans - Summary of Information Relative to Stock Option Plan Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>240614039 - Disclosure - Share-Based Compensation Plans - Additional Information Pertaining to Stock Option Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansAdditionalInformationPertainingtoStockOptionPlansDetails</Role>
      <ShortName>Share-Based Compensation Plans - Additional Information Pertaining to Stock Option Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>240624040 - Disclosure - Share-Based Compensation Plans - Summary of Nonvested RSU and PSU Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/ShareBasedCompensationPlansSummaryofNonvestedRSUandPSUActivityDetails</Role>
      <ShortName>Share-Based Compensation Plans - Summary of Nonvested RSU and PSU Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>240654041 - Disclosure - Pension and Other Postretirement Benefit Plans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansNarrativeDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>240664042 - Disclosure - Pension and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansComponentsofNetPeriodicBenefitCostDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>240674043 - Disclosure - Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansObligationandFundedStatusDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>240684044 - Disclosure - Pension and Other Postretirement Benefit Plans - Accumulated and Projected Benefit Obligation in Excess of Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansAccumulatedandProjectedBenefitObligationinExcessofPlanAssetsDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Accumulated and Projected Benefit Obligation in Excess of Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>240694045 - Disclosure - Pension and Other Postretirement Benefit Plans - Fair Values of Pension Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansFairValuesofPensionPlanAssetsDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Fair Values of Pension Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>240704046 - Disclosure - Pension and Other Postretirement Benefit Plans - Summary of Changes in Fair Value of Company's Level 3 Pension Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansSummaryofChangesinFairValueofCompanysLevel3PensionPlanAssetsDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Summary of Changes in Fair Value of Company's Level 3 Pension Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>240714047 - Disclosure - Pension and Other Postretirement Benefit Plans - Fair Values of Other Postretirement Benefit Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansFairValuesofOtherPostretirementBenefitPlanAssetsDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Fair Values of Other Postretirement Benefit Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>240724048 - Disclosure - Pension and Other Postretirement Benefit Plans - Summary of Expected Benefit Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansSummaryofExpectedBenefitPaymentsDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Summary of Expected Benefit Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>240734049 - Disclosure - Pension and Other Postretirement Benefit Plans - Components of Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansComponentsofOtherComprehensiveIncomeDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Components of Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>240744050 - Disclosure - Pension and Other Postretirement Benefit Plans - Summary of Weighted Average Assumptions Used in Determining Pension Plan and U.S. Pension and Other Postretirement Benefit Plan Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansSummaryofWeightedAverageAssumptionsUsedinDeterminingPensionPlanandUSPensionandOtherPostretirementBenefitPlanInformationDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Summary of Weighted Average Assumptions Used in Determining Pension Plan and U.S. Pension and Other Postretirement Benefit Plan Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>240754051 - Disclosure - Pension and Other Postretirement Benefit Plans - Summary of Health Care Cost Trend Rate Assumptions for Other Postretirement Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/PensionandOtherPostretirementBenefitPlansSummaryofHealthCareCostTrendRateAssumptionsforOtherPostretirementBenefitPlansDetails</Role>
      <ShortName>Pension and Other Postretirement Benefit Plans - Summary of Health Care Cost Trend Rate Assumptions for Other Postretirement Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>240784052 - Disclosure - Other (Income) Expense, Net - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherIncomeExpenseNetNarrativeDetails</Role>
      <ShortName>Other (Income) Expense, Net - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.merck.com/role/OtherIncomeExpenseNetTables</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>240794053 - Disclosure - Other (Income) Expense, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherIncomeExpenseNetDetails</Role>
      <ShortName>Other (Income) Expense, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.merck.com/role/OtherIncomeExpenseNetTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>240824054 - Disclosure - Taxes on Income - Reconciliation Between Effective Tax Rate and US Statutory Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeReconciliationBetweenEffectiveTaxRateandUSStatutoryRateDetails</Role>
      <ShortName>Taxes on Income - Reconciliation Between Effective Tax Rate and US Statutory Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>240834055 - Disclosure - Taxes on Income - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeNarrativeDetails</Role>
      <ShortName>Taxes on Income - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>240844056 - Disclosure - Taxes on Income - Income Before Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeIncomeBeforeTaxesDetails</Role>
      <ShortName>Taxes on Income - Income Before Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>240854057 - Disclosure - Taxes on Income - Taxes on Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeTaxesonIncomeDetails</Role>
      <ShortName>Taxes on Income - Taxes on Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>240864058 - Disclosure - Taxes on Income - Deferred Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeDeferredIncomeTaxesDetails</Role>
      <ShortName>Taxes on Income - Deferred Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>240874059 - Disclosure - Taxes on Income - Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/TaxesonIncomeUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Taxes on Income - Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>240904060 - Disclosure - Earnings Per Share - Calculations of Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EarningsPerShareCalculationsofEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share - Calculations of Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>240914061 - Disclosure - Earnings Per Share - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/EarningsPerShareNarrativeDetails</Role>
      <ShortName>Earnings Per Share - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>240944062 - Disclosure - Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/OtherComprehensiveIncomeLossDetails</Role>
      <ShortName>Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.merck.com/role/OtherComprehensiveIncomeLossTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>240974063 - Disclosure - Segment Reporting - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingNarrativeDetails</Role>
      <ShortName>Segment Reporting - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>240984064 - Disclosure - Segment Reporting - Sales of Company's Products (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingSalesofCompanysProductsDetails</Role>
      <ShortName>Segment Reporting - Sales of Company's Products (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>107</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>240994065 - Disclosure - Segment Reporting - Consolidated Revenues by Geographic Area (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingConsolidatedRevenuesbyGeographicAreaDetails</Role>
      <ShortName>Segment Reporting - Consolidated Revenues by Geographic Area (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>241004066 - Disclosure - Segment Reporting - Reconciliation of Segment Profits to Income Before Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingReconciliationofSegmentProfitstoIncomeBeforeTaxesDetails</Role>
      <ShortName>Segment Reporting - Reconciliation of Segment Profits to Income Before Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>241014067 - Disclosure - Segment Reporting - Schedule of Equity Income from Affiliates and Depreciation and Amortization Included in Segment Profits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingScheduleofEquityIncomefromAffiliatesandDepreciationandAmortizationIncludedinSegmentProfitsDetails</Role>
      <ShortName>Segment Reporting - Schedule of Equity Income from Affiliates and Depreciation and Amortization Included in Segment Profits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="mrk-20201231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>241024068 - Disclosure - Segment Reporting - Property, Plant and Equipment, Net by Geographic Area (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.merck.com/role/SegmentReportingPropertyPlantandEquipmentNetbyGeographicAreaDetails</Role>
      <ShortName>Segment Reporting - Property, Plant and Equipment, Net by Geographic Area (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</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" original="mrk-20201231.htm">mrk-20201231.htm</File>
    <File>ex10-24x2021annualrsuterms.htm</File>
    <File>ex1023-2021annualnqsoterms.htm</File>
    <File>ex21-subsidiarylistasof12x.htm</File>
    <File>ex23-consentofindependentr.htm</File>
    <File>ex241-powerofattorney.htm</File>
    <File>ex242-certificationofboard.htm</File>
    <File>ex311-rule13ax14a15dx14ace.htm</File>
    <File>ex312-rule13ax14a15dx14ace.htm</File>
    <File>ex321-section1350certifica.htm</File>
    <File>ex322-section1350certifica.htm</File>
    <File>exhibit410-merckdescriptio.htm</File>
    <File>exhibit411-merckdescriptio.htm</File>
    <File>exhibit412-merckdescriptio.htm</File>
    <File>mrk-20201231.xsd</File>
    <File>mrk-20201231_cal.xml</File>
    <File>mrk-20201231_def.xml</File>
    <File>mrk-20201231_lab.xml</File>
    <File>mrk-20201231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>mrk-20201231_g1.jpg</File>
    <File>mrk-20201231_g2.jpg</File>
    <File>mrk-20201231_g3.jpg</File>
    <File>mrk-20201231_g4.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
