<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>14</ContextCount>
  <ElementCount>164</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>10</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001 - Document - Document and Entity Information  {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentAndEntityInformationElements</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory></MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0002 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/RiskReturnSummaryUnlabeledLoomisSaylesBondFund</Role>
      <ShortName>Risk/Return Summary- Loomis Sayles Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0007 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/RiskReturnDetailDataElementsLoomisSaylesBondFund</Role>
      <ShortName>Risk/Return Detail Data- Loomis Sayles Bond Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0008 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Investment Grade Fixed Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/RiskReturnSummaryUnlabeledLoomisSaylesInvestmentGradeFixedIncomeFund</Role>
      <ShortName>Risk/Return Summary- Loomis Sayles Investment Grade Fixed Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0013 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Investment Grade Fixed Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/RiskReturnDetailDataElementsLoomisSaylesInvestmentGradeFixedIncomeFund</Role>
      <ShortName>Risk/Return Detail Data- Loomis Sayles Investment Grade Fixed Income Fund</ShortName>
      <MenuCategory></MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d131600d485bpos.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>040000 - Disclosure - Risk/Return Detail Data {Elements}</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/rr/role/RiskReturnDetailData</Role>
      <ShortName>Risk/Return Detail Data</ShortName>
      <MenuCategory></MenuCategory>
      <Position>6</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="485BPOS" original="d131600d485bpos.htm">d131600d485bpos.htm</File>
    <File>d131600dex99e5.htm</File>
    <File>d131600dex99h3.htm</File>
    <File>d131600dex99h4ii.htm</File>
    <File>d131600dex99ji.htm</File>
    <File>d131600dex99o2.htm</File>
    <File>d131600dex99o3.htm</File>
    <File>lsfi-20210501.xsd</File>
    <File>lsfi-20210501_cal.xml</File>
    <File>lsfi-20210501_def.xml</File>
    <File>lsfi-20210501_lab.xml</File>
    <File>lsfi-20210501_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>g122416dsp1.jpg</File>
    <File>g131600g1lo3902img001.jpg</File>
    <File>g131600g1lo3902img002.jpg</File>
    <File>g131600g1lo3902img003.jpg</File>
    <File>g131600g1lo3902img004.jpg</File>
    <File>g131600g1lo3902img006.jpg</File>
    <File>BarChart1.png</File>
    <File>BarChart2.png</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/rr/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
