485BPOS

As filed with the Securities and Exchange Commission on November 16, 2012

1933 Act File No. 33-37615

1940 Act File No. 811-6174

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C.  20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

POST-EFFECTIVE AMENDMENT NO. 35

AND

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

AMENDMENT NO. 39

MFSÒ INSTITUTIONAL TRUST

(Exact Name of Registrant as Specified in Charter)

500 Boylston, Street, Boston, Massachusetts  02116

(Address of Principal Executive Offices)

Registrant’s Telephone Number, Including Area Code:  (617) 954-5000

Susan S. Newton, Massachusetts Financial Services Company

500 Boylston Street, Boston, Massachusetts  02116

(Name and Address of Agent for Service)

APPROXIMATE DATE OF PROPOSED PUBLIC OFFERING:

It is proposed that this filing will become effective (check appropriate box)

x immediately upon filing pursuant to paragraph (b)

o on [date] pursuant to paragraph (b)

o 60 days after filing pursuant to paragraph (a)(i)

o on [date] pursuant to paragraph (a)(i)

o 75 days after filing pursuant to paragraph (a)(ii)

o on [date] pursuant to paragraph (a)(ii) of rule 485.

If appropriate, check the following box:

o this post-effective amendment designates a new effective date for a previously filed post-effective amendment

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to the Registration Statement to be signed on its behalf by the undersigned, thereto duly authorized, in the City of Boston and The Commonwealth of Massachusetts on the 16th day of November 2012.

MFS® INSTITUTIONAL TRUST
By:	JOHN M. CORCORAN*
Name:	John M. Corcoran
Title:	President

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its Registration Statement has been signed below by the following persons in the capacities indicated on November 16, 2012.

SIGNATURE	TITLE
JOHN M. CORCORAN*	President (Principal Executive Officer)
John M. Corcoran
DAVID L. DILORENZO*	Principal Financial and Accounting Officer
David L. DiLorenzo
ROBERT E. BUTLER*	Trustee
Robert E. Butler
MAUREEN R. GOLDFARB*	Trustee
Maureen R. Goldfarb
DAVID H. GUNNING*	Trustee
David H. Gunning
WILLIAM R. GUTOW*	Trustee
William R. Gutow
MICHAEL HEGARTY*	Trustee
Michael Hegarty

JOHN P. KAVANAUGH*	Trustee
John P. Kavanaugh
ROBERT J. MANNING*	Trustee
Robert J. Manning
J. DALE SHERRATT*	Trustee
J. Dale Sherratt
LAURIE J. THOMSEN*	Trustee
Laurie J. Thomsen
ROBERT W. UEK*	Trustee
Robert W. Uek

*By:	SUSAN S. NEWTON
Name:	Susan S. Newton
	as Attorney-in-fact
Executed by Susan S. Newton on behalf of those indicated pursuant to a Power of Attorney, dated September 11, 2012 (Trustees), and a Power of Attorney, dated September 11, 2012 (Corcoran) (DiLorenzo); filed herewith.

MFS Series Trust I

MFS Series Trust II

MFS Series Trust III

MFS Series Trust IV

MFS Series Trust V

MFS Series Trust VI

MFS Series Trust VII

MFS Series Trust VIII

MFS Series Trust IX

MFS Series Trust X

MFS Series Trust XI

MFS Series Trust XII

MFS Series Trust XIII

MFS Series Trust XIV

MFS Series Trust XV

MFS Series Trust XVI

Massachusetts Investors Growth Stock Fund

Massachusetts Investors Trust

MFS Institutional Trust

MFS Municipal Series Trust

MFS Variable Insurance Trust

MFS Variable Insurance Trust II

MFS California Municipal Fund

MFS Charter Income Trust

MFS Government Markets Income Trust

MFS High Income Municipal Trust

MFS High Yield Municipal Trust

MFS InterMarket Income Trust I

MFS Intermediate High Income Fund

MFS Intermediate Income Trust

MFS Investment Grade Municipal Trust

MFS Multimarket Income Trust

MFS Municipal Income Trust

MFS Special Value Trust

(each a “Registrant”)

POWER OF ATTORNEY

The undersigned, a Trustee of each of the above-mentioned Registrants, hereby severally constitutes and appoints Mark N. Polebaum, Susan S. Newton, Christopher R. Bohane, Thomas H. Connors, Brian E. Langenfeld and Susan A. Pereira, and each of them singly, as true and lawful attorneys, with full power to them and each of them to sign for each of the undersigned, in the names of, and in the capacities indicated below, any

Registration Statement and any and all amendments thereto and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and Exchange Commission for the purpose of registering the Registrant as a management investment company under the Investment Company Act of 1940 and/or the shares issued by the Registrant under the Securities Act of 1933 granting unto my said attorneys, and each of them, acting alone, full power and authority to do and perform each and every act and thing requisite or necessary or desirable to be done in the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorneys or any of them may lawfully do or cause to be done by virtue thereof.

IN WITNESS WHEREOF, the undersigned have hereunto set their hand on this 11th day of September, 2012.

ROBERT E. BUTLER	Trustee
Robert E. Butler
MAUREEN R. GOLDFARB	Trustee
Maureen R. Goldfarb
DAVID H. GUNNING	Trustee
David H. Gunning
WILLIAM R. GUTOW	Trustee
William R. Gutow
MICHAEL HEGARTY	Trustee
Michael Hegarty
JOHN P. KAVANAUGH	Trustee
John P. Kavanaugh
ROBERT J. MANNING	Trustee
Robert J. Manning
J. DALE SHERRATT	Trustee
J. Dale Sherratt

LAURIE J. THOMSEN	Trustee
Laurie J. Thomsen
ROBERT W. UEK	Trustee
Robert W. Uek

MFS Series Trust I

MFS Series Trust II

MFS Series Trust III

MFS Series Trust IV

MFS Series Trust V

MFS Series Trust VI

MFS Series Trust VII

MFS Series Trust VIII

MFS Series Trust IX

MFS Series Trust X

MFS Series Trust XI

MFS Series Trust XII

MFS Series Trust XIII

MFS Series Trust XIV

MFS Series Trust XV

MFS Series Trust XVI

Massachusetts Investors Growth Stock Fund

Massachusetts Investors Trust

MFS Institutional Trust

MFS Municipal Series Trust

MFS Variable Insurance Trust

MFS Variable Insurance Trust II

MFS California Municipal Fund

MFS Charter Income Trust

MFS Government Markets Income Trust

MFS High Income Municipal Trust

MFS High Yield Municipal Trust

MFS InterMarket Income Trust I

MFS Intermediate High Income Fund

MFS Intermediate Income Trust

MFS Investment Grade Municipal Trust

MFS Multimarket Income Trust

MFS Municipal Income Trust

MFS Special Value Trust

(each a “Registrant”)

POWER OF ATTORNEY

The undersigned, being the President and Principal Executive Officer, and Treasurer and Principal Financial and Accounting Officer, respectively, of each of the above-mentioned Registrants, hereby severally constitutes and appoints Mark N. Polebaum, Susan S. Newton, Christopher R. Bohane, Thomas H. Connors, Brian E. Langenfeld and Susan A. Pereira, and each of them singly, as true and lawful attorneys, with full power to them and each of them to sign for each of the undersigned, in the names of, and in the capacities indicated below, any Registration Statement and any and all amendments thereto and to file the same with all exhibits thereto, and other documents in connection therewith, with the Securities and

Exchange Commission for the purpose of registering the Registrant as a management investment company under the Investment Company Act of 1940 and/or the shares issued by the Registrant under the Securities Act of 1933 granting unto my said attorneys, and each of them, acting alone, full power and authority to do and perform each and every act and thing requisite or necessary or desirable to be done in the premises, as fully to all intents and purposes as he or she might or could do in person, hereby ratifying and confirming all that said attorneys or any of them may lawfully do or cause to be done by virtue thereof.

IN WITNESS WHEREOF, the undersigned have hereunto set their hand on this 11th day of September, 2012.

JOHN M. CORCORAN	President (Principal Executive Officer)
John M. Corcoran
DAVID L. DILORENZO	Principal Financial and Accounting Officer
David L. DiLorenzo

MFS Institutional Trust

MFSÒ INSTITUTIONAL INTERNATIONAL EQUITY FUND

MFSÒ INSTITUTIONAL LARGE CAP VALUE FUND

INDEX TO EXHIBITS

EXHIBIT NO.	DESCRIPTION OF EXHIBIT	PAGE NO.
101	Risk/Return summary of each Fund’s prospectus as an Interactive Data File using eXtensible Business Reporting Language (“XBRL”)

XML

MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund

Summary of Key Information

Investment Objective

The fund's investment objective is to seek capital appreciation.

Fees and Expenses

This table describes the fees and expenses that you may pay when you buy and
hold shares of the fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Annual Fund Operating Expenses		MFS Institutional Large Cap Value Fund
Management Fee		0.55%
Distribution and/or Service (12b-1) Fees		none
Other Expenses		0.12%
Total Annual Fund Operating Expenses		0.67%
Fee Reductions and/or Expense Reimbursements	[1]	(0.02%)
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements		0.65%

Example

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.

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Expense Example (USD $)	Expense Example, with Redemption, 1 Year	Expense Example, with Redemption, 3 Years	Expense Example, with Redemption, 5 Years	Expense Example, with Redemption, 10 Years
MFS Institutional Large Cap Value Fund	66	212	371	833

Portfolio Turnover

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 18% of the average value of its portfolio.

Principal Investment Strategies

MFS (Massachusetts Financial Services Company, the fund's investment adviser)
normally invests at least 80% of the fund's net assets in issuers with large
market capitalizations. MFS generally defines large market capitalization
issuers as issuers with market capitalizations of at least $5 billion at the
time of purchase.

MFS normally invests the fund's assets primarily in U.S. equity securities.
Equity securities include common stocks, preferred stocks, securities
convertible into stocks, and depositary receipts for those securities.

MFS focuses on investing the fund's assets in the stocks of companies it
believes are undervalued compared to their perceived worth (value companies).

MFS uses a bottom-up investment approach to buying and selling investments for
the fund. Investments are selected primarily based on fundamental analysis of
individual issuers. Quantitative models that systematically evaluate issuers may
also be considered.

Principal Risks

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

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline significantly
in response to issuer, market, economic, industry, political, regulatory, geopolitical,
and other conditions, as well as to investor perceptions of these conditions. The
price of an equity security can decrease significantly in response to these conditions,
and these conditions can affect a single issuer or type of security, issuers within a
broad market sector, industry or geographic region, or the market in general.

Value Company Risk:  The stocks of value companies can continue to be undervalued for
long periods of time and not realize their expected value and can be more volatile than
the market in general.

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.

Liquidity Risk:  It may not be possible to sell certain investments, types of investments,
and/or segments of the market at any particular time or at an acceptable price.

Performance Information

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.

Bar Chart.

The total return for the nine-month period ended September 30, 2012, was 15.52%.
During the period(s) shown in the bar chart, the highest quarterly return was
16.86% (for the calendar quarter ended June 30, 2009) and the lowest quarterly
return was (19.05)% (for the calendar quarter ended December 31, 2008).

Performance Table.

Average Annual Total Returns (for the Periods Ended December 31, 2011)

Average Annual Total Returns	Label	1 Year	5 Years	10 Years
MFS Institutional Large Cap Value Fund	Fund Shares Returns Before Taxes	(0.08%)	(0.29%)	4.79%
MFS Institutional Large Cap Value Fund After Taxes on Distributions	Fund's Shares Returns After Taxes on Distributions	(0.77%)	(1.45%)	3.54%
MFS Institutional Large Cap Value Fund After Taxes on Distributions and Sales	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares	0.83%	(0.51%)	3.74%
MFS Institutional Large Cap Value Fund Russell 1000 Value Index	Index Comparison (Reflects no deduction for fees, expenses or taxes) Russell 1000 Value Index	0.39%	(2.64%)	3.89%

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-deferred arrangements, such as 401(k) plans or
individual retirement accounts.

[1] Massachusetts Financial Services Company has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.65% of the fund's average daily net assets annually. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least October 31, 2013.

XML

MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund

Summary of Key Information

Investment Objective

The fund's investment objective is to seek capital appreciation.

Fees and Expenses

This table describes the fees and expenses that you may pay when you buy and
hold shares of the fund.

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):

Annual Fund Operating Expenses		MFS Institutional International Equity Fund
Management Fee		0.71%
Distribution and/or Service (12b-1) Fees		none
Other Expenses		0.07%
Total Annual Fund Operating Expenses		0.78%
Fee Reductions and/or Expense Reimbursements	[1]	(0.03%)
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements		0.75%

Example

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.

Although your actual costs will likely be higher or lower, under these assumptions your costs would be:

Expense Example (USD $)	Expense Example, with Redemption, 1 Year	Expense Example, with Redemption, 3 Years	Expense Example, with Redemption, 5 Years	Expense Example, with Redemption, 10 Years
MFS Institutional International Equity Fund	77	246	430	963

Portfolio Turnover

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 22% of the average value of its portfolio.

Principal Investment Strategies

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, preferred stocks, securities convertible
into stocks, and depositary receipts for those securities.

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

MFS may invest a large percentage of the fund's assets in issuers in a single
country, a small number of countries, or a particular geographic region.

In selecting investments for the fund, MFS is not constrained to 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 companies of any size.

MFS uses a bottom-up investment approach to buying and selling investments for the
fund. Investments are selected primarily based on fundamental analysis of individual
issuers. Quantitative models that systematically evaluate issuers may also be
considered.

Principal Risks

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

The principal risks of investing in the fund are:

Stock Market/Company Risk:  Stock markets are volatile and can decline
significantly in response to issuer, market, economic, industry, political,
regulatory, geopolitical, and other conditions, as well as to investor
perceptions of these conditions. The price of an equity security can decrease
significantly in response to these conditions, and these conditions can affect
a single issuer or type of security, issuers within a broad market sector,
industry or geographic region, or the market in general.

Foreign and Emerging Markets Risk:  Exposure to foreign markets, especially
emerging markets, through issuers or currencies can involve additional risks
relating to market, economic, political, regulatory, geopolitical, or other
conditions. These factors can make foreign investments, especially those in
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 can have less developed markets, greater custody
and operational risk, and less developed legal, regulatory, and accounting
systems, and greater political, social, and economic instability than developed
markets.

Currency Risk:  The value of foreign currencies relative to the U.S. dollar
fluctuates in response to market, economic, political, regulatory, geopolitical
or other conditions, and a decline in the value of a foreign currency versus the
U.S. dollar reduces the value in U.S. dollars of investments denominated in that
foreign currency.

Geographic Concentration Risk:  The fund's performance could be closely tied to
the market, currency, economic, political, regulatory, geopolitical, or other
conditions in the countries or regions in which the fund invests and could be
more volatile than the performance of more geographically-diversified funds.

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.

Liquidity Risk:  It may not be possible to sell certain investments, types of
investments, and/or segments of the market at any particular time or at an
acceptable price.

Performance Information

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.

Bar Chart.

The total return for the nine-month period ended September 30, 2012, was 12.94%.
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).

Performance Table.

Average Annual Total Returns (for the Periods Ended December 31, 2011)

Average Annual Total Returns	Label	1 Year	5 Years	10 Years
MFS Institutional International Equity Fund	Fund Shares Returns Before Taxes	(9.55%)	(1.06%)	7.14%
MFS Institutional International Equity Fund After Taxes on Distributions	Fund's Shares Returns After Taxes on Distributions	(9.75%)	(1.48%)	6.47%
MFS Institutional International Equity Fund After Taxes on Distributions and Sales	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares	(5.83%)	(0.82%)	6.19%
MFS Institutional International Equity Fund MSCI EAFE (Europe, Australasia, Far East) Index (gross div)	Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (gross div)	(11.73%)	(4.26%)	5.12%

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-deferred arrangements, such as 401(k)
plans or individual retirement accounts.

[1] Massachusetts Financial Services Company has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.75% of the fund's average daily net assets annually. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least October 31, 2013.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Oct. 28, 2012
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	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.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2013-10-31
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 22% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	22.00%
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, preferred stocks, securities convertible into stocks, and depositary receipts for those securities. MFS normally invests the fund's assets primarily in foreign securities, including emerging market securities. MFS may invest a large percentage of the fund's assets in issuers in a single country, a small number of countries, or a particular geographic region. In selecting investments for the fund, MFS is not constrained to 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 companies of any size. MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.
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: Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general. Foreign and Emerging Markets Risk:  Exposure to foreign markets, especially emerging markets, through issuers or currencies can involve additional risks relating to market, economic, political, regulatory, geopolitical, or other conditions. These factors can make foreign investments, especially those in 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 can have less developed markets, greater custody and operational risk, and less developed legal, regulatory, and accounting systems, and greater political, social, and economic instability than developed markets. Currency Risk:  The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, political, regulatory, geopolitical or other conditions, and a decline in the value of a foreign currency versus the U.S. dollar reduces the value in U.S. dollars of investments denominated in that foreign currency. Geographic Concentration Risk:  The fund's performance could be closely tied to the market, currency, economic, political, regulatory, geopolitical, or other conditions in the countries or regions in which the fund invests and could be more volatile than the performance of more geographically-diversified funds. 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. Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.
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 Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	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, 2012, was 12.94%. 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).
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-deferred 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-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns (for the Periods Ended December 31, 2011)
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund | MSCI EAFE (Europe, Australasia, Far East) Index (gross div)
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Index Comparison (Reflects no deduction for fees, expenses or taxes) MSCI EAFE (Europe, Australasia, Far East) Index (gross div)
1 Year	rr_AverageAnnualReturnYear01	(11.73%)
5 Years	rr_AverageAnnualReturnYear05	(4.26%)
10 Years	rr_AverageAnnualReturnYear10	5.12%
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund | MFS Institutional International Equity Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.71%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.07%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.78%
Fee Reductions and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.03%)	[1]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.75%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	77
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	246
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	430
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	963
Annual Return 2002	rr_AnnualReturn2002	(6.57%)
Annual Return 2003	rr_AnnualReturn2003	31.02%
Annual Return 2004	rr_AnnualReturn2004	18.86%
Annual Return 2005	rr_AnnualReturn2005	12.92%
Annual Return 2006	rr_AnnualReturn2006	27.94%
Annual Return 2007	rr_AnnualReturn2007	9.11%
Annual Return 2008	rr_AnnualReturn2008	(34.95%)
Annual Return 2009	rr_AnnualReturn2009	33.12%
Annual Return 2010	rr_AnnualReturn2010	10.97%
Annual Return 2011	rr_AnnualReturn2011	(9.55%)
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, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	12.94%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	the 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	the lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Sep. 30, 2011
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(20.30%)
Label	rr_AverageAnnualReturnLabel	Fund Shares Returns Before Taxes
1 Year	rr_AverageAnnualReturnYear01	(9.55%)
5 Years	rr_AverageAnnualReturnYear05	(1.06%)
10 Years	rr_AverageAnnualReturnYear10	7.14%
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund | MFS Institutional International Equity Fund | After Taxes on Distributions
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Fund's Shares Returns After Taxes on Distributions
1 Year	rr_AverageAnnualReturnYear01	(9.75%)
5 Years	rr_AverageAnnualReturnYear05	(1.48%)
10 Years	rr_AverageAnnualReturnYear10	6.47%
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund | MFS Institutional International Equity Fund | After Taxes on Distributions and Sales
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares
1 Year	rr_AverageAnnualReturnYear01	(5.83%)
5 Years	rr_AverageAnnualReturnYear05	(0.82%)
10 Years	rr_AverageAnnualReturnYear10	6.19%

[1] Massachusetts Financial Services Company has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.75% of the fund's average daily net assets annually. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least October 31, 2013.

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Jun. 30, 2012
Registrant Name	dei_EntityRegistrantName	MFS INSTITUTIONAL TRUST
Central Index Key	dei_EntityCentralIndexKey	0000867969
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Oct. 25, 2012
Document Effective Date	dei_DocumentEffectiveDate	Oct. 28, 2012
MFS Institutional International Equity Fund (Prospectus Summary) | MFS Institutional International Equity Fund | MFS Institutional International Equity Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	MIEIX
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund | MFS Institutional Large Cap Value Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Trading Symbol	dei_TradingSymbol	ILVAX

XML

Label	Element	Value
Risk Return [Abstract]	rr_RiskReturnAbstract
ProspectusDate	rr_ProspectusDate	Oct. 28, 2012
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	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.
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment):
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	2013-10-31
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 18% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	18.00%
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 issuers with large market capitalizations. MFS generally defines large market capitalization issuers as issuers with market capitalizations of at least $5 billion at the time of purchase. MFS normally invests the fund's assets primarily in U.S. equity securities. Equity securities include common stocks, preferred stocks, securities convertible into stocks, and depositary receipts for those securities. MFS focuses on investing the fund's assets in the stocks of companies it believes are undervalued compared to their perceived worth (value companies). MFS uses a bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual issuers. Quantitative models that systematically evaluate issuers may also be considered.
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: Stock Market/Company Risk:  Stock markets are volatile and can decline significantly in response to issuer, market, economic, industry, political, regulatory, geopolitical, and other conditions, as well as to investor perceptions of these conditions. The price of an equity security can decrease significantly in response to these conditions, and these conditions can affect a single issuer or type of security, issuers within a broad market sector, industry or geographic region, or the market in general. Value Company Risk:  The stocks of value companies can continue to be undervalued for long periods of time and not realize their expected value and can be more volatile than the market in general. 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. Liquidity Risk:  It may not be possible to sell certain investments, types of investments, and/or segments of the market at any particular time or at an acceptable price.
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 Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	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, 2012, was 15.52%. During the period(s) shown in the bar chart, the highest quarterly return was 16.86% (for the calendar quarter ended June 30, 2009) and the lowest quarterly return was (19.05)% (for the calendar quarter ended December 31, 2008).
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-deferred 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-deferred arrangements, such as 401(k) plans or individual retirement accounts.
Caption	rr_AverageAnnualReturnCaption	Average Annual Total Returns (for the Periods Ended December 31, 2011)
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund | Russell 1000 Value Index
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Index Comparison (Reflects no deduction for fees, expenses or taxes) Russell 1000 Value Index
1 Year	rr_AverageAnnualReturnYear01	0.39%
5 Years	rr_AverageAnnualReturnYear05	(2.64%)
10 Years	rr_AverageAnnualReturnYear10	3.89%
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund | MFS Institutional Large Cap Value Fund
Risk Return [Abstract]	rr_RiskReturnAbstract
Management Fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) Fees	rr_DistributionAndService12b1FeesOverAssets	none
Other Expenses	rr_OtherExpensesOverAssets	0.12%
Total Annual Fund Operating Expenses	rr_ExpensesOverAssets	0.67%
Fee Reductions and/or Expense Reimbursements	rr_FeeWaiverOrReimbursementOverAssets	(0.02%)	[1]
Total Annual Fund Operating Expenses After Fee Reductions and/or Expense Reimbursements	rr_NetExpensesOverAssets	0.65%
Expense Example, with Redemption, 1 Year	rr_ExpenseExampleYear01	66
Expense Example, with Redemption, 3 Years	rr_ExpenseExampleYear03	212
Expense Example, with Redemption, 5 Years	rr_ExpenseExampleYear05	371
Expense Example, with Redemption, 10 Years	rr_ExpenseExampleYear10	833
Annual Return 2002	rr_AnnualReturn2002	(14.27%)
Annual Return 2003	rr_AnnualReturn2003	25.63%
Annual Return 2004	rr_AnnualReturn2004	15.98%
Annual Return 2005	rr_AnnualReturn2005	6.84%
Annual Return 2006	rr_AnnualReturn2006	21.38%
Annual Return 2007	rr_AnnualReturn2007	8.28%
Annual Return 2008	rr_AnnualReturn2008	(32.34%)
Annual Return 2009	rr_AnnualReturn2009	20.99%
Annual Return 2010	rr_AnnualReturn2010	11.30%
Annual Return 2011	rr_AnnualReturn2011	(0.08%)
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, 2012
Bar Chart, Year to Date Return	rr_BarChartYearToDateReturn	15.52%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	the highest quarterly return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2009
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	16.86%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	the lowest quarterly return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Dec. 31, 2008
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(19.05%)
Label	rr_AverageAnnualReturnLabel	Fund Shares Returns Before Taxes
1 Year	rr_AverageAnnualReturnYear01	(0.08%)
5 Years	rr_AverageAnnualReturnYear05	(0.29%)
10 Years	rr_AverageAnnualReturnYear10	4.79%
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund | MFS Institutional Large Cap Value Fund | After Taxes on Distributions
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Fund's Shares Returns After Taxes on Distributions
1 Year	rr_AverageAnnualReturnYear01	(0.77%)
5 Years	rr_AverageAnnualReturnYear05	(1.45%)
10 Years	rr_AverageAnnualReturnYear10	3.54%
MFS Institutional Large Cap Value Fund (Prospectus Summary) | MFS Institutional Large Cap Value Fund | MFS Institutional Large Cap Value Fund | After Taxes on Distributions and Sales
Risk Return [Abstract]	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Fund's Shares Returns After Taxes on Distributions and Sale of Fund Shares
1 Year	rr_AverageAnnualReturnYear01	0.83%
5 Years	rr_AverageAnnualReturnYear05	(0.51%)
10 Years	rr_AverageAnnualReturnYear10	3.74%

[1] Massachusetts Financial Services Company has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.65% of the fund's average daily net assets annually. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least October 31, 2013.