v3.26.1
Investment Strategy - Prospectus Summary
Mar. 31, 2026
MFS Alabama Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Alabama. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Alabama.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Alabama.
MFS Arkansas Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Arkansas. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Arkansas.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Arkansas.
MFS California Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of California. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of California.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of California.
MFS Georgia Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Georgia. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Georgia.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Georgia.
MFS Maryland Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Maryland. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Maryland.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Maryland.
MFS Massachusetts Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Massachusetts. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Massachusetts.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Massachusetts.
MFS Mississippi Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Mississippi. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Mississippi.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

The fund is a non-diversified fund. This means that MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Mississippi.
MFS Municipal Income Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax.
MFS Municipal Income Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax.
MFS Municipal Intermediate Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

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

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

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] MFS normally invests at least 80% of the fund’s net assets in municipal instruments, the interest on which is exempt from U.S. federal income tax.
MFS New York Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of New York. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of New York.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of New York.
MFS North Carolina Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of North Carolina. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of North Carolina.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of North Carolina.
MFS Pennsylvania Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Pennsylvania. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Pennsylvania.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Pennsylvania.
MFS South Carolina Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of South Carolina. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of South Carolina.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of South Carolina.
MFS Virginia Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Virginia. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of Virginia.

MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of Virginia.
MFS West Virginia Municipal Bond Fund  
Prospectus [Line Items]  
Strategy [Heading] Principal Investment Strategies
Strategy Narrative [Text Block]

MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.

The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of West Virginia. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax. MFS may also invest the fund's assets in taxable instruments, including municipal instruments of other states.

MFS invests a significant percentage of the fund’s assets in municipal instruments of West Virginia.

MFS may invest 25% or more of the fund’s assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water or sewers. Municipal instruments whose interest is exempt from U.S. federal and state personal income tax include instruments issued by U.S. territories and possessions (such as Puerto Rico, Guam, and the U.S. Virgin Islands) and their political subdivisions and public corporations.

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

The fund is a non-diversified fund. This means that MFS may invest a significant percentage of the fund’s assets in a single issuer or a small number of issuers.

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.

Summary of Definition of Rule 35d-1 Term in Fund Name [Text Block] MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds.
Rule 35d-1 Eighty Percent Investment Policy [Text Block] The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax and personal income tax, if any, of West Virginia.