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Municipal High Income Fund Blended Index MFS Municipal High Income Fund Blended Index Document Type Document Type Document Effective Date Document Effective Date Document Period End Date Document Period End Date Entity Central Index Key Entity Central Index Key Document Creation Date Document Creation Date Amendment Flag Amendment Flag EntityInvCompanyType EntityInvCompanyType Registrant Name Registrant Name Prospectus Date Prospectus Date Risk/Return [Heading] Risk/Return [Heading] Trading Symbol Trading Symbol Objective [Heading] Objective [Heading] Objective, Primary [Text Block] Objective, Primary [Text Block] Expense [Heading] Expense [Heading] Expense Narrative [Text Block] Expense Narrative [Text Block] Expense Breakpoint Discounts [Text] Expense Breakpoint Discounts [Text] Expense Breakpoint, Minimum Investment Required [Amount] Expense Breakpoint, Minimum Investment Required [Amount] Shareholder Fees Caption [Text] Shareholder Fees Caption [Text] Operating Expenses Caption [Text] Operating Expenses Caption [Text] Expense Example [Heading] Expense Example [Heading] Expense Example Narrative [Text Block] Expense Example Narrative [Text Block] Expense Example by, Year, Caption [Text] Expense Example by, Year, Caption [Text] Expense Example, with Redemption, 1 Year 1 Year Expense Example, with Redemption, 3 Years 3 Years Expense Example, with Redemption, 5 Years 5 Years Expense Example, with Redemption, 10 Years 10 Years Expense Example, No Redemption, 1 Year 1 Year Expense Example, No Redemption, 3 Years 3 Years Expense Example, No Redemption, 5 Years 5 Years Expense Example, No Redemption, 10 Years 10 Years Portfolio Turnover [Heading] Portfolio Turnover [Heading] Portfolio Turnover [Text Block] Portfolio Turnover [Text Block] Portfolio Turnover, Rate Portfolio Turnover, Rate Strategy [Heading] Strategy [Heading] Strategy Narrative [Text Block] Strategy Narrative [Text Block] Strategy Portfolio Concentration [Text] Strategy Portfolio Concentration [Text] Risk [Heading] Risk [Heading] Risk [Text Block] Risk [Text Block] Bar Chart and Performance Table [Heading] Bar Chart and Performance Table [Heading] Performance Narrative [Text Block] Performance Narrative [Text Block] Performance Information Illustrates Variability of Returns [Text] Performance Information Illustrates Variability of Returns [Text] Performance Past Does Not Indicate Future [Text] Performance Past Does Not Indicate Future [Text] Performance Availability Website Address [Text] Performance Availability Website Address [Text] Performance Availability Phone [Text] Performance Availability Phone [Text] Bar Chart [Heading] Bar Chart [Heading] Bar Chart Narrative [Text Block] Bar Chart Narrative [Text Block] Bar Chart Does Not Reflect Sales Loads [Text] Bar Chart Does Not Reflect Sales Loads [Text] Annual Return 2015 Annual Return 2015 Annual Return 2016 Annual Return 2016 Annual Return 2017 Annual Return 2017 Annual Return 2018 Annual Return 2018 Annual Return 2019 Annual Return 2019 Annual Return 2020 Annual Return 2020 Annual Return 2021 Annual Return 2021 Annual Return 2022 Annual Return 2022 Annual Return 2023 Annual Return 2023 Annual Return 2024 Annual Return 2024 Bar Chart Footnotes [Text Block] Bar Chart Footnotes [Text Block] Bar Chart Closing [Text Block] Bar Chart Closing [Text Block] Bar Chart, Year to Date Return Bar Chart, Year to Date Return Bar Chart, Year to Date Return, Date Bar Chart, Year to Date Return, Date Year to Date Return, Label Year to Date Return, Label Highest Quarterly Return, Label Label Highest Quarterly Return Highest Quarterly Return Highest Quarterly Return, Date Highest Quarterly Return, Date Lowest Quarterly Return, Label Label Lowest Quarterly Return Lowest Quarterly Return Lowest Quarterly Return, Date Lowest Quarterly Return, Date Performance One Year or Less [Text] Performance One Year or Less [Text] Performance Table Heading Performance Table Heading Average Annual Return, Caption Average Annual Return, Caption Performance Table Closing [Text Block] Performance Table Closing [Text Block] Performance Table Market Index Changed Performance Table Market Index Changed Performance Additional Market Index [Text] Performance Additional Market Index [Text] Performance Table Uses Highest Federal Rate Performance Table Uses Highest Federal Rate Performance Table Not Relevant to Tax Deferred Performance Table Not Relevant to Tax Deferred Performance Table One Class of after Tax Shown [Text] Performance Table One Class of after Tax Shown [Text] Label Label 1 Year 1 Year 5 Years 5 Years 10 Years 10 Years Index No Deduction for Fees, Expenses, Taxes [Text] Index No Deduction for Fees, Expenses, Taxes [Text] Expenses Deferred Charges [Text Block] Expenses Deferred Charges [Text Block] Fee Waiver or Reimbursement over Assets, Date of Termination Fee Waiver or Reimbursement over Assets, Date of Termination Document [Axis] Document [Axis] Series [Axis] Series [Axis] Prospectus Prospectus Prospectus: Prospectus: Risk/Return: Risk/Return: Shareholder Fees [Table] Shareholder Fees [Table] Annual Fund Operating Expenses [Table] Annual Fund Operating Expenses [Table] Expense Example, With Redemption [Table] Expense Example, With Redemption [Table] Expense Example, No Redemption [Table] Expense Example, No Redemption [Table] Bar Chart [Table] Bar Chart [Table] Performance [Table] Performance [Table] Market Index Performance [Table] Market Index Performance [Table] Performance Measure [Axis] Performance Measure [Axis] Share Class [Axis] Share Class [Axis] Before Taxes Before Taxes After Taxes on Distributions After Taxes on Distributions After Taxes on Distributions and Sales After Taxes on Distributions and Sales Risk [Axis] Risk [Axis] All Risks [Member] All Risks [Member] Risk Lose Money [Member] Risk Lose Money [Member] Risk Not Insured [Member] Risk Not Insured [Member] Risk Nondiversified Status [Member] Risk Nondiversified Status [Member] Risk Not Insured Depository Institution [Member] Risk Not Insured Depository Institution [Member] Risk Money Market Fund Price Fluctuates [Member] Risk Money Market Fund Price Fluctuates [Member] Risk Money Market Fund May Impose Fees or Suspend Sales [Member] Risk Money Market Fund May Impose Fees or Suspend Sales [Member] Risk Money Market Fund Sponsor May Not Provide Support [Member] Risk Money Market Fund Sponsor May Not Provide Support [Member] Risk Money Market Fund May Not Preserve Dollar [Member] Risk Money Market Fund May Not Preserve Dollar [Member] Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) Maximum Deferred Sales Charge (Load) (as a percentage of original purchase price or redemption proceeds, whichever is less) Management Fee Distribution and/or Service (12b-1) Fees Other Expenses Acquired (Underlying) Fund Fees and Expenses Total Annual Fund Operating Expenses Fee Reductions and/or Expense Reimbursements Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements Risks [Table Text Block] Risks [Table Text Block]