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Loomis Sayles Intermediate Municipal Bond Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Loomis Sayles Intermediate Municipal Bond Fund
Objective [Heading] rr_ObjectiveHeading Investment Goal
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
The Fund seeks a high level of federal tax-exempt current income, consistent with the preservation of capital.
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
75 of the Prospectus
, in Appendix A to the
Prospectus and on page 
136 in the section “Reduced Sales Charges” of the Statement of Additional Information (“SAI”).
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 April 30, 2022
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
41% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 41.00%
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock
A 0.75% contingent deferred sales charge (“CDSC”) may apply to certain purchases of Class A shares of $500,000 or more that are redeemed within eighteen months of the
date of purchase.
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
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:
Strategy [Heading] rr_StrategyHeading Investments, Risks and Performance Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
Under normal market conditions, the Fund will invest at least 80% of its net assets (plus borrowings made for investment purposes) in municipal securities
that pay interest exempt from federal income taxes. Municipal securities are debt instruments typically issued by or on behalf of state and local governments,
territories or possessions of the United States, including the District of Columbia, and their political subdivisions, agencies and instrumentalities and may
include general obligation, revenue and private activity bonds and notes. In addition, the Fund may invest up to 20% of its assets in securities that pay
interest subject to federal income taxation. The Fund may invest up to 20% of its assets in debt securities subject to the federal alternative minimum tax. The
Fund’s investments may include securities issued by the U.S. government, its agencies and instrumentalities and corporate debt securities. The Fund will
invest primarily in investment grade fixed-income securities. “Investment grade” securities are those securities that are rated in one of the top four ratings
categories at the time of purchase by at least one of the three major ratings agencies (Moody’s Investors Service, Inc., Fitch Investors Services, Inc. or S&P
Global Ratings), or, if unrated, are determined by Loomis, Sayles & Company, L.P. (“Loomis Sayles” or the “Subadviser”) to be of comparable quality. The
Subadviser considers pre-refunded bonds and municipal securities escrowed to maturity using U.S. Treasury securities or U.S. government agency securities
to be investment grade securities, regardless of rating. The Fund may also invest up to 10% of its assets in securities that are not investment grade
(commonly known as “junk bonds”). Under normal circumstances, the dollar-weighted average effective maturity of the Fund’s portfolio is expected to be
between 3 and 10 years although the Fund may invest in securities of any maturity. 
The portfolio management team seeks to build a portfolio based on a number of factors including sector, duration and maturity distribution, yield, expected
return, credit momentum outlook (sector and security level), credit quality, security structure, issue size and liquidity. Through the use of quantitative and
fundamental analysis, the pool of possible portfolio investments is screened using these factors to arrive at a narrower universe of securities that the
Subadviser believes are suitable for the Fund’s portfolio.


Potential investments are also subject to a portfolio risk assessment that may include the following:
 
•
Determining the ability of creditors to fully repay debt obligations in a timely manner.
 
•
Use of a wide variety of internal and external quantitative and analytical and informational sources to assess likelihood of repayment.
 
•
Monitoring rating agency and third party surveillance sources with an emphasis on core holdings.
 
The Subadviser may sell a security for a variety of reasons, including duration management, yield curve positioning, sector rotation, a change in credit
momentum outlook or if more attractive investment opportunities are identified
.


The Fund may also:
 
•
Invest in when-issued securities and securities issued pursuant to Rule 144A under the Securities Act of 1933 (“Rule 144A securities”).
 
•
Enter into futures transactions for hedging and investment purposes.
 
•
Invest in other investment companies to the extent permitted by the Investment Company Act of 1940.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Risk Narrative [Text Block] rr_RiskNarrativeTextBlock
The principal risks of investing in the Fund are summarized below. The Fund does not represent a complete investment program. You may lose money
by investing in the Fund. 
 
The significance of any specific risk to an investment in the Fund will vary over time, depending on the composition of the Fund’s portfolio, market conditions,
and other factors. You should read all of the risk information presented below carefully, because any one or more of these risks may result in losses to the
Fund.
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.
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. 
 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 monetary policy may affect the level of interest rates. 
Management Risk:
A strategy used by the Fund’s 
portfolio managers may fail to produce the intended result.
 
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. 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. Securities acquired in a private
placement, such as Rule 144A securities, 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 over-the-counter (“OTC”) derivatives, are generally
subject to liquidity risk as well. Liquidity issues may also make it difficult to value the Fund’s investments.
Investments in Other Investment Companies Risk:
The Fund will indirectly bear the management, service and other fees of any other investment
companies, including exchange-traded funds, in which it invests in addition to its own expenses. 
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.
Credit/Counterparty Risk:
Credit/counterparty risk is the risk that the issuer or guarantor of a fixed-income security, or the counterparty to a derivatives 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. For centrally cleared derivatives, such as futures, the primary credit/counterparty risk is the
creditworthiness of the Fund’s clearing broker and the central clearing house itself.
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.
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 markets 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, such as
futures transactions, involves other risks, such as the credit risk relating to the other party to a derivative contract
(which is greater for over-the-counter traded 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 the initial
margin (if any) required to initiate derivatives positions. There is also the risk that the Fund may be unable to terminate or sell a derivatives 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. 
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.
Municipal Securities Risk:
Municipal bonds are investments issued by states, cities, public authorities or political subdivisions to raise money for public
purposes, including general obligation bonds and revenue obligations. Municipal securities are subject to information risk, liquidity risk, credit risk and the
risks that economic, political, fiscal or regulatory events, legislative changes and the enforceability of rights of municipal bond holders could adversely affect
the values of municipal bonds. Municipal obligations may be susceptible to downgrades or defaults during recessions or similar periods of economic stress
and insolvent municipalities may file for bankruptcy, which could significantly affect the rights of creditors and the value of the municipal securities. In
addition, if the municipal securities held by the Fund fail to meet certain legal requirements allowing interest distributed from such securities to be tax-
exempt, the interest received and distributed to shareholders by the Fund may be taxable
.
Risk Lose Money [Text] rr_RiskLoseMoney You may lose money by investing in the Fund. 
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 and life-of-fund periods (as applicable) compare to those of
a broad
measure of market performance.
Class C shares will automatically convert 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.
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 and life-of-fund periods (as applicable) compare to those of a broad measure of market performance.
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.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
Highest Quarterly Return:

First Quarter 2019,
2.66%


Lowest Quarterly Return:

Fourth Quarter 2016,
-3.76%
Performance Table Heading rr_PerformanceTableHeading Average Annual Total Returns (for the periods ended December 31, 2020)
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.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock
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, r
estated
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 Intermediate Municipal Bond Fund | Class A  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 3.00%
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, as applicable) rr_MaximumDeferredSalesChargeOverOther none [1]
Redemption fees rr_RedemptionFee none
Management fees rr_ManagementFeesOverAssets 0.40%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.78% [2]
Total annual fund operating expenses rr_ExpensesOverAssets 1.43%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.72% [3]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.71%
1 year rr_ExpenseExampleYear01 $ 370
3 years rr_ExpenseExampleYear03 670
5 years rr_ExpenseExampleYear05 992
10 years rr_ExpenseExampleYear10 $ 1,901
Past 1 Year rr_AverageAnnualReturnYear01 0.38%
Past 5 Years rr_AverageAnnualReturnYear05 2.21%
Life of Class N rr_AverageAnnualReturnSinceInception 2.06%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Class C  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, as applicable) rr_MaximumDeferredSalesChargeOverOther 1.00%
Redemption fees rr_RedemptionFee none
Management fees rr_ManagementFeesOverAssets 0.40%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other expenses rr_OtherExpensesOverAssets 0.78% [2]
Total annual fund operating expenses rr_ExpensesOverAssets 2.18%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.72% [3]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 1.46%
1 year rr_ExpenseExampleYear01 $ 249
3 years rr_ExpenseExampleYear03 613
5 years rr_ExpenseExampleYear05 1,104
10 years rr_ExpenseExampleYear10 2,266
1 year rr_ExpenseExampleNoRedemptionYear01 149
3 years rr_ExpenseExampleNoRedemptionYear03 613
5 years rr_ExpenseExampleNoRedemptionYear05 1,104
10 years rr_ExpenseExampleNoRedemptionYear10 $ 2,266
Past 1 Year rr_AverageAnnualReturnYear01 1.71%
Past 5 Years rr_AverageAnnualReturnYear05 2.06%
Life of Class N rr_AverageAnnualReturnSinceInception 1.70%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Class T  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 2.50%
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, as applicable) rr_MaximumDeferredSalesChargeOverOther none
Redemption fees rr_RedemptionFee none
Management fees rr_ManagementFeesOverAssets 0.40%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other expenses rr_OtherExpensesOverAssets 0.78% [2],[4]
Total annual fund operating expenses rr_ExpensesOverAssets 1.43%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.72% [3]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.71%
1 year rr_ExpenseExampleYear01 $ 321
3 years rr_ExpenseExampleYear03 622
5 years rr_ExpenseExampleYear05 945
10 years rr_ExpenseExampleYear10 $ 1,860
Past 1 Year rr_AverageAnnualReturnYear01 0.85%
Past 5 Years rr_AverageAnnualReturnYear05 2.31%
Life of Class N rr_AverageAnnualReturnSinceInception 2.14%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Class Y  
Risk/Return: rr_RiskReturnAbstract  
Maximum sales charge (load) imposed on purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum deferred sales charge (load) (as a percentage of original purchase price or redemption proceeds, as applicable) rr_MaximumDeferredSalesChargeOverOther none
Redemption fees rr_RedemptionFee none
Management fees rr_ManagementFeesOverAssets 0.40%
Distribution and/or service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets none
Other expenses rr_OtherExpensesOverAssets 0.77% [2]
Total annual fund operating expenses rr_ExpensesOverAssets 1.17%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.71% [3]
Total annual fund operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.46%
1 year rr_ExpenseExampleYear01 $ 47
3 years rr_ExpenseExampleYear03 301
5 years rr_ExpenseExampleYear05 575
10 years rr_ExpenseExampleYear10 $ 1,357
2013 rr_AnnualReturn2013 (2.31%)
2014 rr_AnnualReturn2014 6.36%
2015 rr_AnnualReturn2015 2.63%
2016 rr_AnnualReturn2016 (0.55%)
2017 rr_AnnualReturn2017 5.13%
2018 rr_AnnualReturn2018 0.58%
2019 rr_AnnualReturn2019 6.80%
2020 rr_AnnualReturn2020 3.63%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarterly Return:
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Mar. 31, 2019
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 2.66%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarterly Return:
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Dec. 31, 2016
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (3.76%)
Past 1 Year rr_AverageAnnualReturnYear01 3.63%
Past 5 Years rr_AverageAnnualReturnYear05 3.08%
Life of Class N rr_AverageAnnualReturnSinceInception 2.74%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Return After Taxes on Distributions | Class Y  
Risk/Return: rr_RiskReturnAbstract  
Past 1 Year rr_AverageAnnualReturnYear01 3.63%
Past 5 Years rr_AverageAnnualReturnYear05 3.08%
Life of Class N rr_AverageAnnualReturnSinceInception 2.74%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Return After Taxes on Distributions and Sale of Fund Shares | Class Y  
Risk/Return: rr_RiskReturnAbstract  
Past 1 Year rr_AverageAnnualReturnYear01 2.99%
Past 5 Years rr_AverageAnnualReturnYear05 2.87%
Life of Class N rr_AverageAnnualReturnSinceInception 2.55%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
Loomis Sayles Intermediate Municipal Bond Fund | Bloomberg Barclays Municipal Bond Index  
Risk/Return: rr_RiskReturnAbstract  
Past 1 Year rr_AverageAnnualReturnYear01 5.21%
Past 5 Years rr_AverageAnnualReturnYear05 3.91%
Life of Class N rr_AverageAnnualReturnSinceInception 3.61%
Inception Date rr_AverageAnnualReturnInceptionDate Dec. 31, 2012
[1] A 0.75% contingent deferred sales charge (“CDSC”) may apply to certain purchases of Class A shares of $500,000 or more that are redeemed within eighteen months of the date of purchase.
[2] The expense information shown in the table above includes acquired fund fees and expenses of less than 0.01%; the ratios differ from the expense information disclosed in the Fund’s financial highlights table because the financial highlights table reflects the operating expenses of the Fund and does not include acquired fund fees and expenses.
[3] The Fund’s investment adviser has given a binding contractual undertaking to the Fund to limit the amount of the Fund’s total annual fund operating expenses to 0.70%, 1.45%, 0.70% and 0.45% of the Fund’s average daily net assets for Class A, Class C, Class T and Class 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 April 30, 2022 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 the applicable expense limitations for Class A, Class C, Class T and Class Y shares. 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] Other expenses for Class T shares are estimated for the current fiscal year.