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(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund)  
Prospectus: rr_ProspectusTable  
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The Fund’s investment objective is above-average total return through a combination of current income and capital appreciation.
Expense [Heading] rr_ExpenseHeading Fund Fees & Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock
The following table describes the fees and expenses that you may pay if you buy, hold, and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in this table. You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the Natixis Funds Complex. More information about these and other discounts is available from your financial professional and in the section “How Sales Charges Are Calculated” on page 48 of the Prospectus, in Appendix A to the Prospectus and on page 111 in the section “Reduced Sales Charges” of the Statement of Additional Information (“SAI”).
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $100,000 in the Natixis Funds Complex.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder Fees(fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Fund Operating Expenses(expenses that you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination January 31, 2025
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock A 1.00% contingent deferred sales charge (“CDSC”) may apply to certain purchases of Class A shares of $1,000,000 or more that are redeemed within eighteen months of the date of purchase.
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Other expenses for Class T shares are estimated for the current fiscal year.
Expense Example [Heading] rr_ExpenseExampleHeading Example
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock
This example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example assumes that you invest $10,000 in the Fund for the time periods indicated and then redeem all of your shares at the end of those periods (except where indicated). The example also assumes that your investment has a 5% return each year and that the Fund’s operating expenses remain the same, except that the example is based on the Total Annual Fund Operating Expenses After Fee Waiver and/or Expense Reimbursement assuming that such waiver and/or reimbursement will only be in place through the date noted above and on the Total Annual Fund Operating Expenses for the remaining periods. The example for Class C shares for the ten-year period reflects the conversion to Class A shares after eight years. The example does not take into account brokerage commissions and other fees to financial intermediaries that you may pay on your purchases and sales of shares of the Fund. Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption If shares are redeemed:
Expense Example, No Redemption, By Year, Caption [Text] rr_ExpenseExampleNoRedemptionByYearCaption If shares are not redeemed:
Portfolio Turnover [Heading] rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover [Text Block] rr_PortfolioTurnoverTextBlock
The Fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes for you if your Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During its most recently ended fiscal year, the Fund’s portfolio turnover rate was 138% of the average value of its portfolio. 
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 138.00%
Strategy [Heading] rr_StrategyHeading Investments, Risks and PerformancePrincipal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
Under normal circumstances, the Fund will invest at least 80% of its net assets (plus any borrowings made for investment purposes) in fixed-income securities (for example, bonds and other investments that Loomis Sayles believes have similar economic characteristics, such as notes, debentures and loans). It is anticipated that the Fund’s weighted average duration will generally be between two and five years. Duration is a measure of the expected life of a fixed-income security that is used to determine the sensitivity of a security’s price to changes in interest rates. A fund with a longer average portfolio duration will be more sensitive to changes in interest rates than a fund with a shorter average portfolio duration. By way of example, the price of a bond fund with an average duration of five years would be expected to fall approximately 5% if interest rates rose by one percentage point.
The Fund will purchase only investment-grade fixed-income securities, which are those securities that are rated in one of the top four rating categories at the time of purchase by at least one of the three major ratings agencies (Moody’s Investors Service, Inc., Fitch Investor Services, Inc. or S&P Global Ratings) or, if unrated, are determined by Loomis Sayles to be of comparable quality. The Fund may continue to hold up to 10% of its net assets in securities that are downgraded to a rating below investment-grade subsequent to their purchase if Loomis Sayles believes it is appropriate to do so.
In deciding which securities to buy and sell, Loomis Sayles may consider a number of factors related to the bond issue and the current bond market including, for example, the stability and volatility of a country’s bond markets, the financial strength of the issuer, current interest rates, current valuations, Loomis Sayles’ expectations regarding general trends in interest rates and currency considerations. Loomis Sayles will also consider how purchasing or selling a bond would impact the overall portfolio’s risk profile (for example, its sensitivity to currency risk, interest rate risk and sector-specific risk) and potential return (income and capital gains).
Three themes typically drive the Fund’s investment approach. First, Loomis Sayles generally seeks fixed-income securities that are attractively valued relative to the Loomis Sayles’ credit research team’s assessment of credit risk. The broad coverage combined with the objective of identifying attractive investment opportunities makes this an important component of the investment approach. Second, the Fund may invest significantly in securities the prices of which Loomis Sayles believes are more sensitive to events related to the underlying issuer than to changes in general interest rates or overall market default rates. These securities may not have a direct correlation with changes in interest rates, thus helping to manage interest rate risk and to offer diversified sources for return. Third, Loomis Sayles analyzes different sectors of the economy and differences in the yields (“spreads”) of various fixed-income securities in an effort to find securities that it believes may produce attractive returns for the Fund in comparison to their risk.
The Fund may invest up to 20% of its assets in U.S. dollar-denominated foreign securities, including emerging market securities. The Fund may invest without limit in obligations of supranational entities (e.g., the World Bank). Although certain securities purchased by the Fund may be issued by domestic companies incorporated outside of the United States, the Adviser does not consider these securities to be foreign if the issuer is included in the U.S. fixed-income indices published by Bloomberg. The Fund may also invest in mortgage-related securities, including mortgage dollar rolls. The Fund may also engage in futures transactions for hedging and investment purposes.
The fixed-income securities in which the Fund may invest include, among other things, corporate bond and other debt securities (including junior and senior bonds), U.S. government securities, zero-coupon securities, collateralized loan obligations, mortgage-backed securities and other asset-backed securities, real estate investment trusts (“REITs”), securities issued pursuant to Rule 144A under the Securities Act of 1933 (“Rule 144A securities”).
The Fund may also engage in active and frequent trading of securities. Frequent trading may produce a high level of taxable gains, including short-term capital gains taxable as ordinary income, as well as increased trading costs, which may lower the Fund’s return.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Risk/Return Bar Chart and Table
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
The bar chart and table shown below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year-to-year and by showing how the Fund’s average annual returns for the one-year, five-year, ten-year and life-of-class periods (as applicable) compare to those of a broad-based securities market index that reflects the performance of the overall market applicable to the Fund and an additional index that represents the market sectors in which the Fund primarily invests. Performance for Class C shares includes the automatic conversion to Class A shares after eight years. The Fund’s past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future. Updated performance information is available online at im.natixis.com and/or by calling the Fund toll-free at 800-225-5478.
The chart does not reflect any sales charge that you may be required to pay when you buy or redeem the Fund’s shares. A sales charge will reduce your return. To the extent that a class of shares was subject to the waiver or reimbursement of certain expenses during a period, had such expenses not been waived or reimbursed during the period, total returns would have been lower.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and table shown below provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year-to-year and by showing how the Fund’s average annual returns for the one-year, five-year, ten-year and life-of-class periods (as applicable) compare to those of a broad-based securities market index that reflects the performance of the overall market applicable to the Fund and an additional index that represents the market sectors in which the Fund primarily invests.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 800-225-5478
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress im.natixis.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture The Fund’s past performance (before and after taxes) does not necessarily indicate how the Fund will perform in the future.
Bar Chart [Heading] rr_BarChartHeading Total Returns for Class Y Shares
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads The chart does not reflect any sales charge that you may be required to pay when you buy or redeem the Fund’s shares. A sales charge will reduce your return.
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Index performance reflects no deduction for fees, expenses or taxes
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans, qualified plans, education savings accounts, such as 529 plans, or individual retirement accounts.
Performance Table One Class of after Tax Shown [Text] rr_PerformanceTableOneClassOfAfterTaxShown The after-tax returns are shown for only one class of the Fund. After-tax returns for the other classes of the Fund will vary.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Highest Quarterly Return:

Second Quarter 2020, 5.03%

Lowest Quarterly Return:

First Quarter 2022, -4.85%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns(for the periods ended December 31, 2023)
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock
Effective August 31, 2016, the Fund’s Retail Class shares and Institutional Class shares were redesignated as Class A shares and Class Y shares, respectively. Accordingly, the returns shown in the table for Class A shares prior to August 31, 2016 are those of Retail Class shares, restated to reflect the sales loads of Class A shares, and the returns in the table for Class Y shares prior to August 31, 2016 are those of Institutional Class shares. Prior to the inception of Class C shares (August 31, 2016), performance is that of Retail Class shares, restated to reflect the higher net expenses and sales loads of Class C shares.
The Fund did not have Class T shares outstanding during the periods shown above. The returns of Class T shares would have been substantially similar to the returns of the Fund’s other share classes because they would have been invested in the same portfolio of securities and would only differ to the extent the other share classes did not have the same expenses. Performance of Class T shares shown above is that of Class A shares, which have the same expenses as Class T shares, restated to reflect the different sales load applicable to Class T shares.
After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on an investor’s tax situation and may differ from those shown. After-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans, qualified plans, education savings accounts, such as 529 plans, or individual retirement accounts. The after-tax returns are shown for only one class of the Fund. After-tax returns for the other classes of the Fund will vary. Index performance reflects no deduction for fees, expenses or taxes
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Interest Rate Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Interest Rate Risk: Interest rate risk is the risk that the value of the Fund’s investments will fall if interest rates rise. Generally, the value of fixed-income securities rises when prevailing interest rates fall and falls when interest rates rise. Interest rate risk generally is greater for funds that invest in fixed-income securities with relatively longer durations than for funds that invest in fixed-income securities with shorter durations. The values of zero-coupon bonds may be more sensitive to fluctuations in interest rates than other fixed-income securities. In addition, an economic downturn or period of rising interest rates could adversely affect the market for these securities and reduce the Fund’s ability to sell them, negatively impacting the performance of the Fund. Potential future changes in government and/or central bank monetary policy and action may also affect the level of interest rates. Recently, there have been inflationary price movements, which have caused the fixed income securities markets to experience heightened levels of interest volatility and liquidity risk. Monetary policy measures have in the past, and may in the future, exacerbate risks associated with rising interest rates.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Credit/Counterparty Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Credit/Counterparty Risk: Credit/counterparty risk is the risk that the issuer or guarantor of a fixed-income security, or the counterparty to a derivative or other transaction, will be unable or unwilling to make timely payments of interest or principal or to otherwise honor its obligations. As a result, the Fund may sustain losses or be unable or delayed in its ability to realize gains. The Fund will be subject to credit/counterparty risk with respect to the counterparties to its derivatives transactions. This risk will be heightened to the extent the Fund enters into derivative transactions with a single counterparty (or affiliated counterparties that are part of the same organization), causing the Fund to have significant exposure to such counterparty. Many of the protections afforded to participants on organized exchanges and clearing houses, such as the performance guarantee given by a central clearing house, are not available in connection with over-the-counter (“OTC”) derivatives transactions, such as foreign currency transactions. For centrally cleared derivatives, such as cleared swaps, futures and many options, the primary credit/counterparty risk is the creditworthiness of the Fund’s clearing broker and the central clearing house itself. 
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Mortgage-Related and Asset-Backed Securities Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Mortgage-Related and Asset-Backed Securities Risk: In addition to the risks associated with investments in fixed-income securities generally (for example, credit, liquidity, inflation and valuation risk), mortgage-related and asset-backed securities are subject to the risks of the mortgages and assets underlying the securities as well as prepayment risk, the risk that the securities may be prepaid and result in the reinvestment of the prepaid amounts in securities with lower yields than the prepaid obligations. Conversely, there is a risk that a rise in interest rates will extend the life of a mortgage-related or asset-backed security beyond the expected prepayment time, typically reducing the security’s value, which is called extension risk. The Fund also may incur a loss when there is a prepayment of securities that were purchased at a premium. The Fund’s investments in other asset-backed securities are subject to risks similar to those associated with mortgage-related securities, as well as additional risks associated with the nature of the assets and the servicing of those assets.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Market/Issuer Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Market/Issuer Risk: The market value of the Fund’s investments will move up and down, sometimes rapidly and unpredictably, based upon overall market and economic conditions, as well as a number of reasons that directly relate to the issuers of the Fund’s investments, such as management performance, financial condition and demand for the issuers’ goods and services.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Foreign Securities Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Foreign Securities Risk: Investments in foreign securities may be subject to greater political, economic, environmental, credit/counterparty and information risks. The Fund’s investments in foreign securities also are subject to foreign currency fluctuations and other foreign currency-related risks. Foreign securities may be subject to higher volatility than U.S. securities, varying degrees of regulation and limited liquidity.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Below Investment Grade Fixed-Income Securities Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Below Investment Grade Fixed-Income Securities Risk: The Fund’s investments in below investment grade fixed-income securities, also known as “junk bonds,” may be subject to greater risks than other fixed-income securities, including being subject to greater levels of interest rate risk, credit/counterparty risk (including a greater risk of default) and liquidity risk. The ability of the issuer to make principal and interest payments is predominantly speculative for below investment grade fixed-income securities.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Currency Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Currency Risk: Fluctuations in the exchange rates between different currencies may negatively affect an investment. The Fund may be subject to currency risk because it may invest in securities or other instruments denominated in, or that generate income denominated in, foreign currencies. The Fund may elect not to hedge currency risk, or may hedge such risk imperfectly, which may cause the Fund to incur losses that would not have been incurred had the risk been hedged.  
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Cybersecurity and Technology Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Cybersecurity and Technology Risk: The Fund, its service providers, and other market participants increasingly depend on complex information technology and communications systems, which are subject to a number of different threats and risks that could adversely affect the Fund and its shareholders. Cybersecurity and other operational and technology issues may result in financial losses to the Fund and its shareholders.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Derivatives Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Derivatives Risk: Derivative instruments (such as those in which the Fund may invest, including futures transactions) are subject to changes in the value of the underlying assets or indices on which such instruments are based. There is no guarantee that the use of derivatives will be effective or that suitable
transactions will be available. Even a small investment in derivatives may give rise to leverage risk and can have a significant impact on the Fund’s exposure to securities market values, interest rates or currency exchange rates. It is possible that the Fund’s liquid assets may be insufficient to support its obligations under its derivatives positions. The use of derivatives for other than hedging purposes may be considered a speculative activity, and involves greater risks than are involved in hedging. The use of derivatives may cause the Fund to incur losses greater than those that would have occurred had derivatives not been used. The Fund’s use of derivatives involves other risks, such as credit/counterparty risk relating to the other party to a derivative contract (which is greater for OTC derivatives), the risk of difficulties in pricing and valuation, the risk that changes in the value of a derivative may not correlate as expected with changes in the value of relevant assets, rates or indices, liquidity risk, allocation risk and the risk of losing more than any amounts paid or margin transferred to initiate derivatives positions. There is also the risk that the Fund may be unable to terminate or sell a derivative position at an advantageous time or price. The Fund’s derivative counterparties may experience financial difficulties or otherwise be unwilling or unable to honor their obligations, possibly resulting in losses to the Fund.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Emerging Markets Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Emerging Markets Risk: In addition to the risks of investing in foreign investments generally, emerging markets investments are subject to greater risks arising from political or economic instability, war, nationalization or confiscatory taxation, currency exchange or repatriation restrictions, sanctions by other countries (such as the United States or the European Union), new or inconsistent government treatment of or restrictions on issuers and instruments, and an issuer’s unwillingness or inability to make dividend, principal or interest payments on its securities. Emerging markets companies may be smaller and have shorter operating histories than companies in developed markets.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Large Investor Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Large Investor Risk: Ownership of shares of the Fund may be concentrated in one or a few large investors. Such investors may redeem shares in large quantities or on a frequent basis. Redemptions by a large investor can affect the performance of the Fund, may increase realized capital gains, including short-term capital gains taxable as ordinary income, may accelerate the realization of taxable income to shareholders and may increase transaction costs. These transactions potentially limit the use of any capital loss carryforwards and certain other losses to offset future realized capital gains (if any). Such transactions may also increase the Fund’s expenses.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Leverage Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Leverage Risk: Leverage is the risk associated with securities or investment practices (e.g., borrowing and the use of certain derivatives) that multiply small index, market or asset-price movements into larger changes in value. The use of leverage increases the impact of gains and losses on the Fund’s returns, and may lead to significant losses if investments are not successful.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Liquidity Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Liquidity Risk: Liquidity risk is the risk that the Fund may be unable to find a buyer for its investments when it seeks to sell them or to receive the price it expects. Decreases in the number of financial institutions willing to make markets in the Fund’s investments or in their capacity or willingness to transact may increase the Fund’s exposure to this risk. Events that may lead to increased redemptions, such as market disruptions or increases in interest rates, may also negatively impact the liquidity of the Fund’s investments when it needs to dispose of them. Markets may become illiquid quickly. If the Fund is forced to sell its investments at an unfavorable time and/or under adverse conditions in order to meet redemption requests, such sales could negatively affect the Fund. During times of market turmoil, there may be no buyers or sellers for securities in certain asset classes. Securities acquired in a private placement, such as Rule 144A securities and privately negotiated equity and other investments, are generally subject to significant liquidity risk because they are subject to strict restrictions on resale and there may be no liquid secondary market or ready purchaser for such securities. Derivatives, and particularly OTC derivatives, are generally subject to liquidity risk as well. In other circumstances, liquid investments may become illiquid.Derivatives, and particularly OTC derivatives, are generally subject to liquidity risk as well. Liquidity issues may also make it difficult to value the Fund’s investments. The Fund may invest in liquid investments that become illiquid due to financial distress, or geopolitical events such as sanctions, trading halts or wars.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Management Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
Management Risk: A strategy used by the Fund’s portfolio managers may fail to produce the intended result. 
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | REITs Risk [Member]  
Prospectus: rr_ProspectusTable  
Risk [Text Block] rr_RiskTextBlock
REITs Risk: Investments in the real estate industry, including REITs, are particularly sensitive to economic downturns and are sensitive to factors such as changes in real estate values, property taxes and tax laws, interest rates, cash flow of underlying real estate assets, occupancy rates, government regulations affecting zoning, land use and rents and the management skill and creditworthiness of the issuer. Companies in the real estate industry also may be subject to liabilities under environmental and hazardous waste laws. In addition, the value of a REIT is affected by changes in the value of the properties owned by the REIT or mortgage loans held by the REIT. REITs are also subject to default and prepayment risk. Many REITs are highly leveraged, increasing their risk. The Fund will indirectly bear its proportionate share of expenses, including management fees, paid by each REIT in which it invests in addition to the expenses of the Fund.
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Bloomberg U.S. Aggregate Bond Index  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 5.53% [1]
Past 5 Years rr_AverageAnnualReturnYear05 1.10% [1]
Past 10 Years rr_AverageAnnualReturnYear10 1.81% [1]
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Bloomberg U.S. Intermediate Government/Credit Bond Index  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 5.24%
Past 5 Years rr_AverageAnnualReturnYear05 1.59%
Past 10 Years rr_AverageAnnualReturnYear10 1.72%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class A  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 4.25%
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none [2]
Redemption Fee (as a percentage of Amount Redeemed) rr_RedemptionFeeOverRedemption none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.21%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.71%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.06% [3],[4]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.65%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 489
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 637
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 798
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,265
Past 1 Year rr_AverageAnnualReturnYear01 0.90%
Past 5 Years rr_AverageAnnualReturnYear05 0.90%
Past 10 Years rr_AverageAnnualReturnYear10 1.35%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class C  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther 1.00%
Redemption Fee (as a percentage of Amount Redeemed) rr_RedemptionFeeOverRedemption none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.21%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.46%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.06% [3],[4]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 1.40%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 243
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 456
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 792
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 1,537
Expense Example, No Redemption, 1 Year rr_ExpenseExampleNoRedemptionYear01 143
Expense Example, No Redemption, 3 Years rr_ExpenseExampleNoRedemptionYear03 456
Expense Example, No Redemption, 5 Years rr_ExpenseExampleNoRedemptionYear05 792
Expense Example, No Redemption, 10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,537
Past 1 Year rr_AverageAnnualReturnYear01 3.66%
Past 5 Years rr_AverageAnnualReturnYear05 1.03%
Past 10 Years rr_AverageAnnualReturnYear10 1.18%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class N  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Redemption Fee (as a percentage of Amount Redeemed) rr_RedemptionFeeOverRedemption none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.14%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.39%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.04% [3],[4]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.35%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 36
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 121
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 215
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 489
Past 1 Year rr_AverageAnnualReturnYear01 5.81%
Since Inception rr_AverageAnnualReturnSinceInception 1.95%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class N | Bloomberg U.S. Aggregate Bond Index  
Prospectus: rr_ProspectusTable  
Since Inception rr_AverageAnnualReturnSinceInception 0.90% [1]
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class N | Bloomberg U.S. Intermediate Government/Credit Bond Index  
Prospectus: rr_ProspectusTable  
Since Inception rr_AverageAnnualReturnSinceInception 1.48%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class T  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 2.50%
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Redemption Fee (as a percentage of Amount Redeemed) rr_RedemptionFeeOverRedemption none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.21% [5]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.71%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.06% [3],[4]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.65%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 315
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 465
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 629
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 1,105
Past 1 Year rr_AverageAnnualReturnYear01 2.70%
Past 5 Years rr_AverageAnnualReturnYear05 1.25%
Past 10 Years rr_AverageAnnualReturnYear10 1.54%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class Y  
Prospectus: rr_ProspectusTable  
Maximum Sales Charge Imposed on Purchases (as a percentage of Offering Price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (as a percentage) rr_MaximumDeferredSalesChargeOverOther none
Redemption Fee (as a percentage of Amount Redeemed) rr_RedemptionFeeOverRedemption none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets 0.21%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.46%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.06% [3],[4]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.40%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 41
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 142
Expense Example, with Redemption, 5 Years rr_ExpenseExampleYear05 252
Expense Example, with Redemption, 10 Years rr_ExpenseExampleYear10 $ 573
Annual Return 2014 rr_AnnualReturn2014 3.87%
Annual Return 2015 rr_AnnualReturn2015 0.76%
Annual Return 2016 rr_AnnualReturn2016 2.74%
Annual Return 2017 rr_AnnualReturn2017 2.66%
Annual Return 2018 rr_AnnualReturn2018 0.52%
Annual Return 2019 rr_AnnualReturn2019 7.64%
Annual Return 2020 rr_AnnualReturn2020 7.74%
Annual Return 2021 rr_AnnualReturn2021 (1.38%)
Annual Return 2022 rr_AnnualReturn2022 (8.55%)
Annual Return 2023 rr_AnnualReturn2023 5.75%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarterly Return:
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 5.03%
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarterly Return:
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (4.85%)
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2022
Past 1 Year rr_AverageAnnualReturnYear01 5.75%
Past 5 Years rr_AverageAnnualReturnYear05 2.04%
Past 10 Years rr_AverageAnnualReturnYear10 2.07%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class Y | Return After Taxes on Distributions  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 4.13%
Past 5 Years rr_AverageAnnualReturnYear05 0.78%
Past 10 Years rr_AverageAnnualReturnYear10 0.87%
(Loomis Sayles Funds I) | (Loomis Sayles Intermediate Duration Bond Fund) | Class Y | Return After Taxes on Distributions and Sale of Fund Shares  
Prospectus: rr_ProspectusTable  
Past 1 Year rr_AverageAnnualReturnYear01 3.37%
Past 5 Years rr_AverageAnnualReturnYear05 1.05%
Past 10 Years rr_AverageAnnualReturnYear10 1.07%
[1]
1 Effective February 1, 2024, the Fund’s primary broad-based performance index changed to the Bloomberg U.S. Aggregate Bond Index. The Bloomberg U.S. Aggregate Bond Index is a broad-based securities market index that represents the overall market applicable to the Fund. The Fund will retain the Bloomberg U.S. Intermediate Government/Credit Bond Index as its additional benchmark for performance comparison.
[2]
* A 1.00% contingent deferred sales charge (“CDSC”) may apply to certain purchases of Class A shares of $1,000,000 or more that are redeemed within eighteen months of the date of purchase.
[3]
2 Loomis, Sayles & Company, L.P. (“Loomis Sayles” or the “Adviser”) has given a binding contractual undertaking to the Fund to limit the amount of the Fund’s total annual fund operating expenses to 0.65%, 1.40%, 0.35%, 0.65% and 0.40% of the Fund’s average daily net assets for Class A, C, N, T and Y shares, respectively, exclusive of brokerage expenses, interest expense, taxes, acquired fund fees and expenses, organizational and extraordinary expenses, such as litigation and indemnification expenses. This undertaking is in effect through January 31, 2025 and may be terminated before then only with the consent of the Fund’s Board of Trustees. The Adviser will be permitted to recover, on a class-by-class basis, management fees waived and/or expenses reimbursed to the extent that expenses in later periods fall below both (1) the class’ applicable expense limitation at the time such amounts were waived/reimbursed and (2) the class’ current applicable expense limitation. The Fund will not be obligated to repay any such waived/reimbursed fees and expenses more than one year after the end of the fiscal year in which the fees or expenses were waived/reimbursed.
[4]
3 Natixis Advisors, LLC (“Natixis Advisors”) has given a binding contractual undertaking to the Fund to reimburse any and all transfer agency expenses for Class N shares. This undertaking is in effect through January 31, 2025 and may be terminated before then only with the consent of the Fund’s Board of Trustees.
[5]
1 Other expenses for Class T shares are estimated for the current fiscal year.