<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.23.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>54</ContextCount>
  <ElementCount>27</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>11</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d450201ddef14a.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>995100 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/dei/role/document/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d450201ddef14a.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.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>2</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="DEF 14A" original="d450201ddef14a.htm">d450201ddef14a.htm</File>
    <File>mmc-20221231.xsd</File>
    <File>mmc-20221231_def.xml</File>
    <File>mmc-20221231_lab.xml</File>
    <File>mmc-20221231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g450201g00a04.jpg</File>
    <File>g450201g01a10.jpg</File>
    <File>g450201g01k89.jpg</File>
    <File>g450201g02a10.jpg</File>
    <File>g450201g02b89.jpg</File>
    <File>g450201g03a10.jpg</File>
    <File>g450201g03h00.jpg</File>
    <File>g450201g04a10.jpg</File>
    <File>g450201g08a27.jpg</File>
    <File>g450201g08d68.jpg</File>
    <File>g450201g09a94.jpg</File>
    <File>g450201g10v12.jpg</File>
    <File>g450201g11a94.jpg</File>
    <File>g450201g11d05.jpg</File>
    <File>g450201g11e04.jpg</File>
    <File>g450201g14a85.jpg</File>
    <File>g450201g17d72.jpg</File>
    <File>g450201g23a10.jpg</File>
    <File>g450201g23n16.jpg</File>
    <File>g450201g24u95.jpg</File>
    <File>g450201g24v37.jpg</File>
    <File>g450201g24v79.jpg</File>
    <File>g450201g24w51.jpg</File>
    <File>g450201g24x38.jpg</File>
    <File>g450201g24z58.jpg</File>
    <File>g450201g25a53.jpg</File>
    <File>g450201g25d78.jpg</File>
    <File>g450201g25d94.jpg</File>
    <File>g450201g25f34.jpg</File>
    <File>g450201g25g30.jpg</File>
    <File>g450201g25k62.jpg</File>
    <File>g450201g25l21.jpg</File>
    <File>g450201g25o56.jpg</File>
    <File>g450201g25p31.jpg</File>
    <File>g450201g25p47.jpg</File>
    <File>g450201g25r70.jpg</File>
    <File>g450201g25r98.jpg</File>
    <File>g450201g26q64.jpg</File>
    <File>g450201g27f57.jpg</File>
    <File>g450201g27f89.jpg</File>
    <File>g450201g27g27.jpg</File>
    <File>g450201g27h01.jpg</File>
    <File>g450201g27h29.jpg</File>
    <File>g450201g27i29.jpg</File>
    <File>g450201g27i61.jpg</File>
    <File>g450201g28j29.jpg</File>
    <File>g450201g28k19.jpg</File>
    <File>g450201g31a52.jpg</File>
    <File>g450201g31b53.jpg</File>
    <File>g450201g32p11.jpg</File>
    <File>g450201g32r14.jpg</File>
    <File>g450201g32s11.jpg</File>
    <File>g450201g35b12.jpg</File>
    <File>g450201g40a10.jpg</File>
    <File>g450201g42a10.jpg</File>
    <File>g450201g45x71.jpg</File>
    <File>g450201g47s47.jpg</File>
    <File>g450201g48z48.jpg</File>
    <File>g450201g55m04.jpg</File>
    <File>g450201g58b64.jpg</File>
    <File>g450201g60j19.jpg</File>
    <File>g450201g60w91.jpg</File>
    <File>g450201g63b88.jpg</File>
    <File>g450201g63c88.jpg</File>
    <File>g450201g70k70.jpg</File>
    <File>g450201g72a85.jpg</File>
    <File>g450201g73a51.jpg</File>
    <File>g450201g73b52.jpg</File>
    <File>g450201g74w85.jpg</File>
    <File>g450201g76m25.jpg</File>
    <File>g450201g82y90.jpg</File>
    <File>g450201g83a83.jpg</File>
    <File>g450201g83a84.jpg</File>
    <File>g450201g83a85.jpg</File>
    <File>g450201g84q36.jpg</File>
    <File>g450201g85d86.jpg</File>
    <File>g450201g95v03.jpg</File>
    <File>g450201g96t14.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="3">http://fasb.org/us-gaap/2022</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/dei/2022q4</BaseTaxonomy>
    <BaseTaxonomy items="97">http://xbrl.sec.gov/ecd/2022q4</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
