<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.4.0.6</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>87</ContextCount>
  <ElementCount>94</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>68</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>2</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentDocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000011 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Global Equity and Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesGlobalEquityandIncomeFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Global Equity and Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>000019 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Global Equity and Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesGlobalEquityandIncomeFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Global Equity and Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>000021 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesGrowthFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>000029 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesGrowthFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>000031 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Mid Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesMidCapGrowthFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Mid Cap Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>000039 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Mid Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesMidCapGrowthFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Mid Cap Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>000041 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesValueFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Value Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>000049 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Value Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesValueFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Value Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>000051 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles High Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesHighIncomeFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles High Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>000059 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles High Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesHighIncomeFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles High Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>000061 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles International Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesInternationalBondFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles International Bond Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>000069 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles International Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesInternationalBondFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles International Bond Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>000071 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Investment Grade Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesInvestmentGradeBondFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Investment Grade Bond Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>000079 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Investment Grade Bond Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesInvestmentGradeBondFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Investment Grade Bond Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>000081 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Limited Term Government and Agency Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesLimitedTermGovernmentandAgencyFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Limited Term Government and Agency Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>000089 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Limited Term Government and Agency Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesLimitedTermGovernmentandAgencyFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Limited Term Government and Agency Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>000091 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Strategic Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesStrategicIncomeFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Strategic Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>000099 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Strategic Income Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesStrategicIncomeFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Strategic Income Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>true</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>000101 - Document - Risk/Return Summary {Unlabeled} - Loomis Sayles Small Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DocumentRiskReturnSummaryUnlabeledLoomisSaylesSmallCapGrowthFund</Role>
      <ShortName>Risk/Return Summary  - Loomis Sayles Small Cap Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>000109 - Disclosure - Risk/Return Detail Data {Elements} - Loomis Sayles Small Cap Growth Fund</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.loomissayles.com/role/DisclosureRiskReturnDetailDataElementsLoomisSaylesSmallCapGrowthFund</Role>
      <ShortName>Risk/Return Detail Data  - Loomis Sayles Small Cap Growth Fund</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.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>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs />
  <InputFiles>
    <File>lsf1-20130128.xml</File>
    <File>lsf1-20130128.xsd</File>
    <File>lsf1-20130128_cal.xml</File>
    <File>lsf1-20130128_def.xml</File>
    <File>lsf1-20130128_lab.xml</File>
    <File>lsf1-20130128_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>BarChart1.jpg</File>
    <File>BarChart2.jpg</File>
    <File>BarChart3.jpg</File>
    <File>BarChart4.jpg</File>
    <File>BarChart5.jpg</File>
    <File>BarChart6.jpg</File>
    <File>BarChart7.jpg</File>
    <File>BarChart8.jpg</File>
    <File>BarChart9.jpg</File>
    <File>BarChart10.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
