Label	Element	Value
MFS® Institutional International Equity Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	MFS® Institutional International Equity 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 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 and hold shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy and hold shares of the fund, which are not reflected below.
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 12% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	12.00%
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; 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 (Massachusetts Financial Services Company, the fund's investment adviser) normally invests at least 80% of the fund’s net assets in equity securities. Equity securities include common stocks and other securities that represent an ownership interest (or right to acquire an ownership interest) in a company or other issuer. MFS normally invests the fund’s assets primarily in foreign securities, including emerging market securities. In selecting investments for the fund, MFS is not constrained by any particular investment style. MFS may invest the fund’s assets in the stocks of companies it believes to have above average earnings growth potential compared to other companies (growth companies), in the stocks of companies it believes are undervalued compared to their perceived worth (value companies), or in a combination of growth and value companies. MFS may invest the fund’s assets in securities of companies of any size. MFS normally invests the fund's assets across different industries, sectors, countries, and regions, but MFS may invest a significant percentage of the fund’s assets in issuers in a single industry, sector, country, or region. 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 issuers. Quantitative screening tools that systematically evaluate issuers may also be considered. 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
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	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: 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. Equity Market Risk/Company Risk: Equity markets are volatile and can decline significantly in response to, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions. These conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the equity markets in general. Certain events can have a dramatic adverse effect on equity markets and may lead to periods of high volatility in an equity market or a segment of an equity market. Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions. These factors can make foreign investments, especially those tied economically to emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Emerging Markets Risk: Investments in emerging markets, especially frontier markets, can involve additional and greater risks than the risks associated with investments in developed foreign markets. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, and greater political, social, geopolitical, and economic instability and more susceptible to environmental problems than developed markets. Currency Risk: The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions, and changes in currency exchange rates impact the financial condition of companies or other issuers and may change the value in U.S. dollars of investments denominated in foreign currencies. Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, and other conditions, and the fund's performance will be affected by the conditions in the industries, sectors, countries and regions to which the fund is exposed. 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. 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 costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities.
Risk Lose Money [Text]	rr_RiskLoseMoney	As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.
RIsk Not Insured [Text]	rr_RiskNotInsured	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.
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 a broad measure of market performance. 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 1-877-960-6077.
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 a broad measure of market performance.
Performance Availability Phone [Text]	rr_PerformanceAvailabilityPhone	1-877-960-6077
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	Bar Chart.
Bar Chart Closing [Text Block]	rr_BarChartClosingTextBlock	The total return for the nine-month period ended September 30, 2019, was 17.78%. During the period(s) shown in the bar chart, the highest quarterly return was 21.13% (for the calendar quarter ended September 30, 2009) and the lowest quarterly return was (20.30)% (for the calendar quarter ended September 30, 2011).
Year to Date Return, Label	rr_YearToDateReturnLabel	The total return for the nine-month period ended
Bar Chart, Year to Date Return, Date	rr_BarChartYearToDateReturnDate	Sep. 30, 2019
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	17.78%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Sep. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	21.13%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(20.30%)
Performance Table Heading	rr_PerformanceTableHeading	Performance Table.
Index No Deduction for Fees, Expenses, Taxes [Text]	rr_IndexNoDeductionForFeesExpensesTaxes	(Reflects no deduction for fees, expenses, or taxes)
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.
Performance Table Not Relevant to Tax Deferred	rr_PerformanceTableNotRelevantToTaxDeferred	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 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.
Average Annual Return, Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns (For the Periods Ended December 31, 2018)
MFS® Institutional International Equity Fund \| MSCI EAFE (Europe, Australasia, Far East) Index (net div)
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Index Comparisons (Reflects no deduction for fees, expenses, or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (net div)
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	(13.79%)
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	0.53%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	6.32%
MFS® Institutional International Equity Fund \| MFS Institutional International Equity Fund
Risk/Return:	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.67%
Other Expenses	rr_OtherExpensesOverAssets	0.03%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.70%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	$ 72
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	224
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	390
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	$ 871
Annual Return 2009	rr_AnnualReturn2009	33.12%
Annual Return 2010	rr_AnnualReturn2010	10.97%
Annual Return 2011	rr_AnnualReturn2011	(9.55%)
Annual Return 2012	rr_AnnualReturn2012	22.55%
Annual Return 2013	rr_AnnualReturn2013	18.56%
Annual Return 2014	rr_AnnualReturn2014	(4.21%)
Annual Return 2015	rr_AnnualReturn2015	0.02%
Annual Return 2016	rr_AnnualReturn2016	0.30%
Annual Return 2017	rr_AnnualReturn2017	28.02%
Annual Return 2018	rr_AnnualReturn2018	(10.66%)
Label	rr_AverageAnnualReturnLabel	Returns Before Taxes
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	(10.66%)
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	1.91%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	7.87%
MFS® Institutional International Equity Fund \| MFS Institutional International Equity Fund \| After Taxes on Distributions
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Returns After Taxes on Distributions
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	(10.85%)
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	1.55%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	7.58%
MFS® Institutional International Equity Fund \| MFS Institutional International Equity Fund \| After Taxes on Distributions and Sale of Fund Shares
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Returns After Taxes on Distributions and Sale of Fund Shares
Average Annual Returns, 1 Year	rr_AverageAnnualReturnYear01	(5.91%)
Average Annual Returns, 5 Years	rr_AverageAnnualReturnYear05	1.54%
Average Annual Returns, 10 Years	rr_AverageAnnualReturnYear10	6.52%

Label

Element

Value

MFS® Institutional International Equity Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

MFS® Institutional International Equity 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 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 and hold shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy and hold shares of the fund, which are not reflected below.

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 12% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

12.00%

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; 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 (Massachusetts Financial Services Company, the fund's investment adviser) normally invests at least 80% of the fund’s net assets in equity securities.

Equity securities include common stocks and other securities that represent an ownership interest (or right to acquire an ownership interest) in a company or other issuer.

MFS normally invests the fund’s assets primarily in foreign securities, including emerging market securities.

In selecting investments for the fund, MFS is not constrained by any particular investment style. MFS may invest the fund’s assets in the stocks of companies it believes to have above average earnings growth potential compared to other companies (growth companies), in the stocks of companies it believes are undervalued compared to their perceived worth (value companies), or in a combination of growth and value companies.

MFS may invest the fund’s assets in securities of companies of any size.

MFS normally invests the fund's assets across different industries, sectors, countries, and regions, but MFS may invest a significant percentage of the fund’s assets in issuers in a single industry, sector, country, or region.

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 issuers. Quantitative screening tools that systematically evaluate issuers may also be considered.

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

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

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:

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.

Equity Market Risk/Company Risk: Equity markets are volatile and can decline significantly in response to, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions. These conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the equity markets in general. Certain events can have a dramatic adverse effect on equity markets and may lead to periods of high volatility in an equity market or a segment of an equity market.

Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions. These factors can make foreign investments, especially those tied economically to emerging markets, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market.

Emerging Markets Risk: Investments in emerging markets, especially frontier markets, can involve additional and greater risks than the risks associated with investments in developed foreign markets. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, and greater political, social, geopolitical, and economic instability and more susceptible to environmental problems than developed markets.

Currency Risk: The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, environmental, and other conditions, and changes in currency exchange rates impact the financial condition of companies or other issuers and may change the value in U.S. dollars of investments denominated in foreign currencies.

Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, and other conditions, and the fund's performance will be affected by the conditions in the industries, sectors, countries and regions to which the fund is exposed.

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.

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 costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities.

Risk Lose Money [Text]

rr_RiskLoseMoney

As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund.

RIsk Not Insured [Text]

rr_RiskNotInsured

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.

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 a broad measure of market performance.

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 1-877-960-6077.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

Performance Availability Phone [Text]

rr_PerformanceAvailabilityPhone

1-877-960-6077

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

Bar Chart.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

The total return for the nine-month period ended September 30, 2019, was 17.78%. During the period(s) shown in the bar chart, the highest quarterly return was 21.13% (for the calendar quarter ended September 30, 2009) and the lowest quarterly return was (20.30)% (for the calendar quarter ended September 30, 2011).

Year to Date Return, Label

rr_YearToDateReturnLabel

The total return for the nine-month period ended

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

17.78%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

highest quarterly return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2009

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

21.13%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

lowest quarterly return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2011

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(20.30%)

Performance Table Heading

rr_PerformanceTableHeading

Performance Table.

Index No Deduction for Fees, Expenses, Taxes [Text]

rr_IndexNoDeductionForFeesExpensesTaxes

(Reflects no deduction for fees, expenses, or taxes)

Performance Table Uses Highest Federal Rate

rr_PerformanceTableUsesHighestFederalRate

After-tax returns are calculated using the historical highest individual federal marginal income tax rates and do not reflect the impact of state and local taxes.

Performance Table Not Relevant to Tax Deferred

rr_PerformanceTableNotRelevantToTaxDeferred

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 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.

Average Annual Return, Caption

rr_AverageAnnualReturnCaption

Average Annual Total Returns (For the Periods Ended December 31, 2018)

MFS® Institutional International Equity Fund | MSCI EAFE (Europe, Australasia, Far East) Index (net div)

Risk/Return:

rr_RiskReturnAbstract

Label

rr_AverageAnnualReturnLabel

Index Comparisons (Reflects no deduction for fees, expenses, or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (net div)

Average Annual Returns, 1 Year

rr_AverageAnnualReturnYear01

(13.79%)