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Prospectus Summary | MFS Municipal Intermediate Fund  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading MFS® Municipal Intermediate Fund Summary of Key Information
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock

The fund’s investment objective is to seek total return with an emphasis on income exempt from federal income tax, but also considering capital appreciation.

Expense [Heading] rr_ExpenseHeading Fees and Expenses
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below.

You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 11 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus.

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):
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 when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 25% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 25.00%
Expense Breakpoint Discounts [Text] rr_ExpenseBreakpointDiscounts You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds.
Expense Breakpoint, Minimum Investment Required [Amount] rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
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 you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same.

Expense Example by, Year, Caption [Text] rr_ExpenseExampleByYearCaption Although your actual costs will likely be higher or lower, under these assumptions your costs would be:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock

MFS normally invests at least 80% of the fund’s net assets in municipal instruments, the interest on which is exempt from federal income tax. Interest from the fund’s investments may be subject to the federal alternative minimum tax.

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments.

MFS primarily invests the fund’s assets in investment grade quality debt instruments, but may also invest in below investment grade quality debt instruments.

The fund’s dollar-weighted average effective maturity will normally range between three and ten years.

MFS may invest 25% or more of the fund’s total assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water, or sewers.

MFS may invest a significant percentage of the fund's assets in issuers in a single state, territory, or possession, or a small number of states, territories, or possessions.

While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, inverse floating rate instruments, and swaps.

MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.

For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes.
Risk [Heading] rr_RiskHeading Principal Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance Information
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock

The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.

The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.
Performance Availability Phone [Text] rr_PerformanceAvailabilityPhone 1-800-225-2606
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress mfs.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 Class A Bar Chart.
Bar Chart Narrative [Text Block] rr_BarChartNarrativeTextBlock The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown.
Bar Chart Does Not Reflect Sales Loads [Text] rr_BarChartDoesNotReflectSalesLoads The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown.
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock

The total return for the six-month period ended June 30, 2024, was 1.02%. During the period(s) shown in the bar chart, the highest quarterly return was 6.82% (for the calendar quarter ended December 31, 2023) and the lowest quarterly return was (5.08)% (for the calendar quarter ended March 31, 2022).
Performance Table Heading rr_PerformanceTableHeading Performance Table.
Performance Table Market Index Changed rr_PerformanceTableMarketIndexChanged Effective May 1, 2024, the Bloomberg Municipal Bond Index replaced the Bloomberg Municipal 1-15 Year Index as a broad measure of market performance in accordance with new regulatory disclosure requirements. The fund continue to use the Bloomberg Municipal 1-15 Year Index as an additional benchmark that MFS believes more closely reflects the market segments in which the fund invests.
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 Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) 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 of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown.
Performance Table Closing [Text Block] rr_PerformanceTableClosingTextBlock

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. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts. The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown.
Average Annual Return, Caption rr_AverageAnnualReturnCaption Average Annual Total Returns (For the Periods Ended December 31, 2023)
Prospectus Summary | MFS Municipal Intermediate Fund | Principal Risks  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Principal Risks

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.

The principal risks of investing in the fund are:
Prospectus Summary | MFS Municipal Intermediate Fund | Risk Lose Money [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.
Prospectus Summary | MFS Municipal Intermediate Fund | Risk Not Insured [Member]  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.
Prospectus Summary | MFS Municipal Intermediate Fund | Investment Selection Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results.

Prospectus Summary | MFS Municipal Intermediate Fund | Debt Market Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market.

Prospectus Summary | MFS Municipal Intermediate Fund | Interest Rate Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest.
Prospectus Summary | MFS Municipal Intermediate Fund | Credit Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions.

The credit quality of, and the ability to pay principal and interest when due by, an issuer of a municipal instrument depends on the credit quality of the entity supporting the municipal instrument, how essential any services supported by the municipal instrument are, the sufficiency of any revenues or taxes that support the municipal instrument, and/or the willingness or ability of the appropriate government entity to approve any appropriations necessary to support the municipal instrument. In addition, the price of a municipal instrument also depends on its credit quality and ability to meet the credit support obligations of any insurer or other entity providing credit support to a municipal instrument.

Below investment grade quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Below investment grade quality debt instruments are regarded as having predominantly speculative characteristics. Below investment grade quality debt

instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments.
Prospectus Summary | MFS Municipal Intermediate Fund | Municipal Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Municipal Risk: The price of a municipal instrument can be volatile and significantly affected by adverse tax changes or court rulings, legislative or political changes, changes in specific or general market and economic conditions and developments, and the financial condition of municipal issuers and insurers. Because many municipal instruments are issued to finance similar projects, conditions in certain industries can significantly affect the fund and the overall municipal market. Municipal instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could affect the market values and marketability of many or all municipal obligations of issuers in a state, U.S. territory, or possession.

Prospectus Summary | MFS Municipal Intermediate Fund | Focus Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Focus Risk: The fund’s performance will be closely tied to the issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions in the states, territories, and possessions of the United States in which the fund's assets are invested. If MFS invests a significant percentage of the fund's assets in a single state, territory, or possession, or a small number of states, territories, or possessions, these conditions will have a significant impact on the fund's performance and the fund's performance may be more volatile than the performance of more geographically-diversified funds.
Prospectus Summary | MFS Municipal Intermediate Fund | Prepayment/Extension Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended.

Prospectus Summary | MFS Municipal Intermediate Fund | Derivatives Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage.
Prospectus Summary | MFS Municipal Intermediate Fund | Leveraging Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses.

Prospectus Summary | MFS Municipal Intermediate Fund | Counterparty and Third Party Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction.
Prospectus Summary | MFS Municipal Intermediate Fund | Liquidity Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs.

Prospectus Summary | MFS Municipal Intermediate Fund | Large Shareholder Risk  
Risk/Return: rr_RiskReturnAbstract  
Risk [Text Block] rr_RiskTextBlock

Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would.
Prospectus Summary | MFS Municipal Intermediate Fund | Bloomberg Municipal Bond Index  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Reflects no deduction for fees, expenses, or taxes
Label rr_AverageAnnualReturnLabel Bloomberg Municipal Bond Index
1 Year rr_AverageAnnualReturnYear01 6.40%
Since Inception rr_AverageAnnualReturnSinceInception (0.67%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | Bloomberg Municipal 1-15 Year Index  
Risk/Return: rr_RiskReturnAbstract  
Index No Deduction for Fees, Expenses, Taxes [Text] rr_IndexNoDeductionForFeesExpensesTaxes Reflects no deduction for fees, expenses, or taxes
Label rr_AverageAnnualReturnLabel Bloomberg Municipal 1-15 Year Index
1 Year rr_AverageAnnualReturnYear01 5.26%
Since Inception rr_AverageAnnualReturnSinceInception (0.18%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | A  
Risk/Return: rr_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MIUAX
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 4.25%
Maximum Deferred Sales Charge (Load) (as a percentage of original purchase price or redemption proceeds, whichever is less) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00% [1]
Management Fee rr_ManagementFeesOverAssets 0.35%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.25%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.85%
Fee Reductions and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.15%) [2]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements rr_NetExpensesOverAssets 0.70%
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination July 31, 2025
Expenses Deferred Charges [Text Block] rr_ExpensesDeferredChargesTextBlock This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase.
1 Year rr_ExpenseExampleYear01 $ 493
3 Years rr_ExpenseExampleYear03 670
5 Years rr_ExpenseExampleYear05 862
10 Years rr_ExpenseExampleYear10 $ 1,416
Annual Return 2022 rr_AnnualReturn2022 (7.41%)
Annual Return 2023 rr_AnnualReturn2023 5.85%
Year to Date Return, Label rr_YearToDateReturnLabel The total return for the six-month period ended
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Jun. 30, 2024
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 1.02%
Label rr_HighestQuarterlyReturnLabel highest quarterly return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Dec. 31, 2023
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 6.82%
Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2022
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (5.08%)
Label rr_AverageAnnualReturnLabel Returns Before Taxes
1 Year rr_AverageAnnualReturnYear01 1.35%
Since Inception rr_AverageAnnualReturnSinceInception (2.22%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | A | After Taxes on Distributions  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Returns After Taxes on Distributions
1 Year rr_AverageAnnualReturnYear01 1.31%
Since Inception rr_AverageAnnualReturnSinceInception (2.30%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | A | After Taxes on Distributions and Sales  
Risk/Return: rr_RiskReturnAbstract  
Label rr_AverageAnnualReturnLabel Returns After Taxes on Distributions and Sale of Fund Shares
1 Year rr_AverageAnnualReturnYear01 1.98%
Since Inception rr_AverageAnnualReturnSinceInception (1.25%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | C  
Risk/Return: rr_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MIUCX
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, whichever is less) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00%
Management Fee rr_ManagementFeesOverAssets 0.35%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses rr_OtherExpensesOverAssets 0.25%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.60%
Fee Reductions and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.15%) [2]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements rr_NetExpensesOverAssets 1.45%
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination July 31, 2025
1 Year rr_ExpenseExampleYear01 $ 248 [3]
3 Years rr_ExpenseExampleYear03 490 [3]
5 Years rr_ExpenseExampleYear05 857 [3]
10 Years rr_ExpenseExampleYear10 1,686 [3]
1 Year rr_ExpenseExampleNoRedemptionYear01 148 [3]
3 Years rr_ExpenseExampleNoRedemptionYear03 490 [3]
5 Years rr_ExpenseExampleNoRedemptionYear05 857 [3]
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,686 [3]
Label rr_AverageAnnualReturnLabel Returns Before Taxes
1 Year rr_AverageAnnualReturnYear01 4.07%
Since Inception rr_AverageAnnualReturnSinceInception (1.36%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | I  
Risk/Return: rr_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MIUIX
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, whichever is less) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management Fee rr_ManagementFeesOverAssets 0.35%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.25%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.60%
Fee Reductions and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.15%) [2]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements rr_NetExpensesOverAssets 0.45%
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination July 31, 2025
1 Year rr_ExpenseExampleYear01 $ 46
3 Years rr_ExpenseExampleYear03 177
5 Years rr_ExpenseExampleYear05 320
10 Years rr_ExpenseExampleYear10 $ 736
Label rr_AverageAnnualReturnLabel Returns Before Taxes
1 Year rr_AverageAnnualReturnYear01 6.12%
Since Inception rr_AverageAnnualReturnSinceInception (0.38%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
Prospectus Summary | MFS Municipal Intermediate Fund | R6  
Risk/Return: rr_RiskReturnAbstract  
Trading Symbol dei_TradingSymbol MIURX
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, whichever is less) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management Fee rr_ManagementFeesOverAssets 0.35%
Distribution and/or Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.18%
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.53%
Fee Reductions and/or Expense Reimbursements rr_FeeWaiverOrReimbursementOverAssets (0.15%) [2]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements rr_NetExpensesOverAssets 0.38%
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination July 31, 2025
1 Year rr_ExpenseExampleYear01 $ 39
3 Years rr_ExpenseExampleYear03 155
5 Years rr_ExpenseExampleYear05 281
10 Years rr_ExpenseExampleYear10 $ 650
Label rr_AverageAnnualReturnLabel Returns Before Taxes
1 Year rr_AverageAnnualReturnYear01 6.18%
Since Inception rr_AverageAnnualReturnSinceInception (0.33%)
Inception Date rr_AverageAnnualReturnInceptionDate May 18, 2021
[1] This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase.
[2] Massachusetts Financial Services Company (MFS) has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least July 31, 2025. MFS has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, certain tax reclaim recovery expenses (including contingency fees and closing agreement expenses), and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.70% of the class' average daily net assets annually for Class A shares, 1.45% of the class' average daily net assets annually for Class C shares, 0.45% of the class' average daily net assets annually for Class I shares, and 0.38% of the class' average daily net assets annually for Class R6 shares. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least July 31, 2025.
[3] Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years.