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Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
SEC Form dei_DocumentType 485BPOS
Period end date dei_DocumentPeriodEndDate Oct. 31, 2020
Registrant Name dei_EntityRegistrantName EATON VANCE SPECIAL INVESTMENT TRUST
Registrant CIK dei_EntityCentralIndexKey 0000031266
Amendment Flag dei_AmendmentFlag false
Class A  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol EARRX
Class C  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol ECRRX
Class I  
Prospectus [Line Items] rr_ProspectusLineItems  
Trading Symbol dei_TradingSymbol EIRRX
Eaton Vance Short Duration Inflation-Protected Income Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk/Return rr_RiskReturnHeading Eaton Vance Short Duration Inflation-Protected Income Fund
Objective rr_ObjectiveHeading Investment Objective
Objective, Primary rr_ObjectivePrimaryTextBlock

The Fund's investment objective is real return.

Expense rr_ExpenseHeading Fees and Expenses of the Fund
Expense Narrative rr_ExpenseNarrativeTextBlock

This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund.  Investors may also pay commissions or other fees to their financial intermediary, which are not reflected below.  You may qualify for a reduced sales charge on purchases of Class A shares if you invest, or agree to invest over a 13-month period, at least $100,000 in Eaton Vance funds. Certain financial intermediaries also may offer variations in Fund sales charges to their customers as described in Appendix A – Financial Intermediary Sales Charge Variations in this Prospectus. More information about these and other discounts is available from your financial intermediary and in Sales Charges beginning on page 29 of this Prospectus and page 22 of the Fund’s Statement of Additional Information.

Shareholder Fees Caption rr_ShareholderFeesCaption Shareholder Fees (fees paid directly from your investment)
Operating Expenses Caption rr_OperatingExpensesCaption Annual Fund Operating Expenses (expenses you pay each year as a percentage of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination February 28, 2022
Portfolio Turnover rr_PortfolioTurnoverHeading Portfolio Turnover
Portfolio Turnover rr_PortfolioTurnoverTextBlock The Fund and the Portfolios in which it invests (see below) pay transaction costs, such as commissions, when they buy and sell securities (or “turn over” the portfolio).  A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account.  These 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  68% of the average value of its portfolio.
Portfolio Turnover, Rate rr_PortfolioTurnoverRate 68.00%
Expense Breakpoint Discounts rr_ExpenseBreakpointDiscounts You may qualify for a reduced sales charge on purchases of Class A shares if you invest, or agree to invest over a 13-month period, at least $100,000 in Eaton Vance funds. Certain financial intermediaries also may offer variations in Fund sales charges to their customers as described in Appendix A – Financial Intermediary Sales Charge Variations in this Prospectus. More information about these and other discounts is available from your financial intermediary and in Sales Charges beginning on page 29 of this Prospectus and page 22 of the Fund’s Statement of Additional Information.
Expense Breakpoint, Minimum Investment Required rr_ExpenseBreakpointMinimumInvestmentRequiredAmount $ 100,000
Expense Example rr_ExpenseExampleHeading Example.
Expense Example Narrative 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 then redeem all of your shares at the end of those periods.  The Example also assumes that your investment has a 5% return each year, that the operating expenses remain the same and that any expense reimbursement arrangement remains in place for the contractual period.  Although your actual costs may be higher or lower, based on these assumptions your costs would be:
Strategy rr_StrategyHeading Principal Investment Strategies
Strategy Narrative rr_StrategyNarrativeTextBlock

Under normal market conditions, the Fund seeks to achieve its investment objective of real return by investing at least 80% of its net assets (plus any borrowings for investment purposes) in “inflation protected” instruments, which include (i) inflation-indexed debt obligations of varying maturities issued by the United States and non-U.S. governments, their agencies or instrumentalities (such as Treasury Inflation Protected Securities, or “TIPS”), corporations and other issuers and (ii) other fixed or floating-rate debt obligations (including junior and senior loans (“loans”)) with respect to which the Fund enters into agreements to swap nominal interest payments for payments based on changes in the U.S. Consumer Price Index (“CPI”) or other measures of inflation (the “80% Policy”). Real return is defined as total return less the estimated cost of inflation (typically measured by the change in an official inflation measure). The Fund will limit its real duration to 3.5 years or less and will maintain a weighted average credit quality of investment grade (rated BBB or higher by Moody’s Investors Service, Inc. (“Moody’s”), or equivalently rated by S&P Global Ratings (“S&P”) or Fitch Ratings (“Fitch”) or, if unrated, determined by the investment adviser to be of comparable quality). The Fund is “non-diversified,” which means it may invest a greater percentage of its assets in the securities of a single issuer than a “diversified” fund.

The Fund may invest up to 10% of its total assets in securities denominated in foreign currencies and may invest beyond this limit in U.S. dollar-denominated securities of foreign issuers. The Fund may invest up to 10% of its total assets in securities and instruments that are economically tied to emerging market countries. The Fund may invest in instruments rated below investment grade (below BBB by Moody’s, or equivalently rated by S&P or Fitch, or, if unrated, determined by the investment adviser to be of comparable quality). The Fund may purchase and sell securities on a when-issued, delayed delivery or forward commitment basis and may invest in repurchase agreements or reverse repurchase agreements. Such instruments may be entered into for purposes of investment leverage. Forward purchases of inflation-indexed debt obligations of varying maturities issued by the United States and non-U.S. governments, their agencies or instrumentalities (such as TIPS), corporations and other issuers will be considered “inflation protected” instruments for purposes of the Fund’s 80% Policy. The Fund may invest in derivative instruments, such as swap agreements, options, forward foreign currency exchange contracts, futures contracts or options on futures. There is no stated limit on the Fund’s use of derivatives. The Fund also may engage in short sales, lend its securities, invest in restricted securities, mortgage-backed securities, commercial mortgage-backed securities or asset-backed securities and is authorized to borrow for investment purposes. The Fund may, without limitation, seek to obtain market exposure to the securities in which it primarily invests by entering into a series of purchase and sale contracts or by using other investment techniques (such as dollar rolls).

The Fund generally invests directly in TIPS and certain other “inflation protected” instruments and may also invest in one or more of the following registered investment companies sponsored by the Eaton Vance organization: Eaton Vance Floating Rate Portfolio and Senior Debt Portfolio (each, a “Portfolio”). Each Portfolio is described in “Further Information about the Portfolios” in the Fund Prospectus. The Fund’s portfolio managers, taking market and other factors into consideration, determine the percentage of the Fund’s assets to be invested directly and in each Portfolio. The Fund also may invest directly in securities or other instruments to gain exposure to sectors of the market the investment adviser believes may not be represented or are underrepresented by the Portfolios, to hedge certain Portfolios and/or to otherwise manage the exposures of the Fund.  The portfolio managers may sell a security when the investment adviser’s valuation target is reached, the fundamentals of the company change or to pursue more attractive investment options.  The portfolio managers may also consider financially material environmental, social and governance factors in evaluating an issuer.  These considerations may be taken into account alongside other fundamental research in the investment selection process.

Strategy Portfolio Concentration rr_StrategyPortfolioConcentration Under normal market conditions, the Fund seeks to achieve its investment objective of real return by investing at least 80% of its net assets (plus any borrowings for investment purposes) in “inflation protected” instruments, which include (i) inflation-indexed debt obligations of varying maturities issued by the United States and non-U.S. governments, their agencies or instrumentalities (such as Treasury Inflation Protected Securities, or “TIPS”), corporations and other issuers and (ii) other fixed or floating-rate debt obligations (including junior and senior loans (“loans”)) with respect to which the Fund enters into agreements to swap nominal interest payments for payments based on changes in the U.S. Consumer Price Index (“CPI”) or other measures of inflation (the “80% Policy”). Real return is defined as total return less the estimated cost of inflation (typically measured by the change in an official inflation measure). The Fund will limit its real duration to 3.5 years or less and will maintain a weighted average credit quality of investment grade (rated BBB or higher by Moody’s Investors Service, Inc. (“Moody’s”), or equivalently rated by S&P Global Ratings (“S&P”) or Fitch Ratings (“Fitch”) or, if unrated, determined by the investment adviser to be of comparable quality). The Fund is “non-diversified,” which means it may invest a greater percentage of its assets in the securities of a single issuer than a “diversified” fund.
Risk rr_RiskHeading Principal Risks
Risk Narrative rr_RiskNarrativeTextBlock

Market Risk.  The value of investments held by the Fund may increase or decrease in response to economic, political, financial, public health crises (such as epidemics or pandemics) or other disruptive events (whether real, expected or perceived) in the U.S. and global markets. The frequency and magnitude of such changes in value cannot be predicted. Certain securities and other investments held by the Fund may experience increased volatility, illiquidity, or other potentially adverse effects in reaction to changing market conditions.  Monetary and/or fiscal actions taken by U.S. or foreign governments to stimulate or stabilize the global economy may not be effective and could lead to high market volatility.  No active trading market may exist for certain investments held by the Fund, which may impair the ability of the Fund to sell or to realize the current valuation of such investments in the event of the need to liquidate such assets.

Inflation-Linked Investments Risk. Inflation-linked investments are subject to the effects of changes in market interest rates caused by factors other than inflation (real interest rates).  In general, the price of an inflation-linked investment tends to decrease when real interest rates increase and increase when real interest rates decrease. Interest payments on inflation-linked investments may vary widely and will fluctuate as the principal and interest are adjusted for inflation.  Any increase in the principal amount of an inflation-linked investment will be considered taxable ordinary income, even though the Fund will not receive the principal until maturity. There can be no assurance that the inflation index used will

accurately measure the real rate of inflation in the prices of goods and services. The Fund’s investments in inflation-linked investments may lose value in the event that the actual rate of inflation is different from the rate of the inflation index.

Interest Rate Risk. In general, the value of income securities will fluctuate based on changes in interest rates. The value of these securities is likely to increase when interest rates fall and decline when interest rates rise.  Duration measures a fixed-income security’s price sensitivity to changes in the general level of interest rates.  Generally, securities with longer durations or maturities are more sensitive to changes in interest rates than securities with shorter durations or maturities, causing them to be more volatile.  Conversely, fixed-income securities with shorter durations or maturities will be less volatile but may provide lower returns than fixed-income securities with longer durations or maturities.  The Fund may own individual investments that have longer durations than the average duration of the Fund.  The impact of interest rate changes is significantly less for floating-rate investments that have relatively short periodic rate resets (e.g., ninety days or less). In a rising interest rate environment, the duration of income securities that have the ability to be prepaid or called by the issuer may be extended. In a declining interest rate environment, the proceeds from prepaid or maturing instruments may have to be reinvested at a lower interest rate.  Certain instruments held by the Fund pay an interest rate based on the London Interbank Offered Rate (“LIBOR”), which is the average offered rate for various maturities of short-term loans between certain major international banks. It is expected that LIBOR will be phased out beginning at the end of 2021. The phase-out of LIBOR may result in, among other things, increased volatility or illiquidity in markets for instruments based on LIBOR and changes in the value of such instruments.

Credit Risk. Investments in fixed income and other debt obligations, including loans, (referred to below as “debt instruments”) are subject to the risk of non-payment of scheduled principal and interest. Changes in economic conditions or other circumstances may reduce the capacity of the party obligated to make principal and interest payments on such instruments and may lead to defaults. Such non-payments and defaults may reduce the value of Fund shares and income distributions. The value of debt instruments also may decline because of concerns about the issuer’s ability to make principal and interest payments. In addition, the credit ratings of debt instruments may be lowered if the financial condition of the party obligated to make payments with respect to such instruments deteriorates.  In the event of bankruptcy of the issuer of a debt instrument, the Fund could experience delays or limitations with respect to its ability to realize the benefits of any collateral securing the instrument. In order to enforce its rights in the event of a default, bankruptcy or similar situation, the Fund may be required to retain legal or similar counsel, which may increase the Fund’s operating expenses and adversely affect net asset value.

U.S. Government Securities Risk. Although certain U.S. Government-sponsored agencies (such as the Federal Home Loan Mortgage Corporation and the Federal National Mortgage Association) may be chartered or sponsored by acts of Congress, their securities are neither issued nor guaranteed by the U.S. Treasury.  U.S. Treasury securities generally have a lower return than other obligations because of their higher credit quality and market liquidity.  

Lower Rated Investments Risk.  Investments rated below investment grade and comparable unrated investments (sometimes referred to as “junk”) have speculative characteristics because of the credit risk associated with their issuers. Changes in economic conditions or other circumstances typically have a greater effect on the ability of issuers of lower rated investments to make principal and interest payments than they do on issuers of higher rated investments. An economic downturn generally leads to a higher non-payment rate, and a lower rated investment may lose significant value before a default occurs. Lower rated investments typically are subject to greater price volatility and illiquidity than higher rated investments.

Additional Risks of Loans. Loans are traded in a private, unregulated inter-dealer or inter-bank resale market and are generally subject to contractual restrictions that must be satisfied before a loan can be bought or sold. These restrictions may impede the Fund’s ability to buy or sell loans (thus affecting their liquidity) and may negatively impact the transaction price. See also “Market Risk” above. It also may take longer than seven days for transactions in loans to settle. Due to the possibility of an extended loan settlement process, the Fund may hold cash, sell investments or temporarily borrow from banks or other lenders to meet short-term liquidity needs, such as to satisfy redemption requests from Fund shareholders.  The types of covenants included in loan agreements generally vary depending on market conditions, the creditworthiness of the issuer, the nature of the collateral securing the loan and possibly other factors.  Loans with fewer covenants that restrict activities of the borrower may provide the borrower with more flexibility to take actions that may be detrimental to the loan holders and provide fewer investor protections in the event of such actions or if covenants are breached.  The Fund may experience relatively greater realized or unrealized losses or delays and expense in enforcing its rights with respect to loans with fewer restrictive covenants.  Loans to entities located outside of the U.S. may have substantially different lender protections and covenants as compared to loans to U.S. entities and may involve greater risks.  The Fund may have difficulties and incur expense enforcing its rights with respect to non-U.S. loans and such loans could be subject to bankruptcy laws that are materially different than in the U.S.  Loans may be structured such that they are not securities under securities law, and in the event of fraud or misrepresentation by a borrower, lenders may not have the protection of the anti-fraud provisions of the federal securities laws.  Loans are also subject to risks associated with other types of income investments, including credit risk and risks of lower rated investments.

Foreign Investment Risk. Foreign investments can be adversely affected by political, economic and market developments abroad, including the imposition of economic and other sanctions by the United States or another country. There may be less publicly available information about foreign issuers because they may not be subject to reporting practices, requirements or regulations comparable to those to which United States companies are subject.  Foreign markets may be smaller, less liquid and more volatile than the major markets in the United States and, as a result, Fund share values may be more volatile. Trading in foreign markets typically involves higher expense than trading in the United States. The Fund may have difficulties enforcing its legal or contractual rights in a foreign country.

Economic data as reported by sovereign entities may be delayed, inaccurate or fraudulent. In the event of a default by a sovereign entity, there are typically no assets to be seized or cash flows to be attached. Furthermore, the willingness or ability of a sovereign entity to restructure defaulted debt may be limited. Therefore, losses on sovereign defaults may far exceed the losses from the default of a similarly rated U.S. debt issuer.

Emerging Markets Investment Risk.  Investment markets within emerging market countries are typically smaller, less liquid, less developed and more volatile than those in more developed markets like the United States, and may be focused in certain economic sectors.  Emerging market securities often involve greater risks than developed market securities. The information available about an emerging market issuer may be less reliable than for comparable issuers in more developed capital markets.

Currency Risk.  Exchange rates for currencies fluctuate daily.  The value of foreign investments may be affected favorably or unfavorably by changes in currency exchange rates in relation to the U.S. dollar.  Currency markets generally are not as regulated as securities markets and currency transactions are subject to settlement, custodial and other operational risks.

When-Issued and Forward Commitment Risk.  Securities purchased on a when-issued or forward commitment basis are subject to the risk that when delivered they will be worth less than the agreed upon payment price.

Risks of Repurchase Agreements and Reverse Repurchase Agreements. In the event of the insolvency of the counterparty to a repurchase agreement or reverse repurchase agreement, recovery of the repurchase price owed to the Fund or, in the case of a reverse repurchase agreement, the securities sold by the Fund, may be delayed. In a repurchase agreement, such insolvency may result in a loss to the extent that the value of the purchased securities decreases during the delay or that value has otherwise not been maintained at an amount equal to the repurchase price. In a reverse repurchase agreement, the counterparty’s insolvency may result in a loss equal to the amount by which the value of the securities sold by the Fund exceeds the repurchase price payable by the Fund; if the value of the purchased securities increases during such a delay, that loss may also be increased. When the Fund enters into a reverse repurchase agreement, any fluctuations in the market value of either the securities sold to the counterparty or the securities which the Fund purchases with its proceeds from the agreement would affect the value of the Fund’s assets. As a result, such agreements may increase fluctuations in the net asset value of the Fund’s shares. Because reverse repurchase agreements may be considered to be a form of borrowing by the Fund (and a loan from the counterparty), they constitute leverage. If the Fund reinvests the proceeds of a reverse repurchase agreement at a rate lower than the cost of the agreement, entering into the agreement will lower the Fund’s yield.

Mortgage- and Asset-Backed Securities Risk.  Mortgage- and asset-backed securities represent interests in “pools” of commercial or residential mortgages or other assets, including consumer loans or receivables.  Movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain types of mortgage- and asset-backed securities.  Although certain mortgage- and asset-backed securities are guaranteed as to timely payment of interest and principal by a government entity, the market price for such securities is not guaranteed and will fluctuate.  The purchase of mortgage- and asset-backed securities issued by non-government entities may entail greater risk than such securities that are issued or guaranteed by a government entity.  Mortgage- and asset-backed securities issued by non-government entities may offer higher yields than those issued by government entities, but may also be subject to greater volatility than government issues and can also be subject to greater credit risk and the risk of default on the underlying mortgages or other assets.  Investments in mortgage- and asset-backed securities are subject to both extension risk, where borrowers pay off their debt obligations more slowly in times of rising interest rates, and prepayment risk, where borrowers pay off their debt obligations sooner than expected in times of declining interest rates.

Leverage Risk.  Certain Fund transactions may give rise to leverage.  Leverage can result from a non-cash exposure to an underlying reference instrument.  Leverage can increase both the risk and return potential of the Fund.  The Fund is required to segregate liquid assets or otherwise cover the Fund’s obligation created by a transaction that may give rise to leverage. The use of leverage may cause the Fund to liquidate portfolio positions when it may not be advantageous to do so to satisfy its obligations or to meet segregation requirements. Leverage may cause the Fund’s share price to be more volatile than if it had not been leveraged, as certain types of leverage may exaggerate the effect of any increase or decrease in the value of the Fund’s portfolio securities. The loss on leveraged investments may substantially exceed the initial investment.

Derivatives Risk. The Fund’s exposure to derivatives involves risks different from, or possibly greater than, the risks associated with investing directly in securities and other investments. The use of derivatives can lead to losses because of adverse movements in the price or value of the security, instrument, index, currency, commodity, economic indicator or event underlying a derivative (“reference instrument”), due to failure of a counterparty or due to tax or regulatory constraints. Derivatives may create leverage in the Fund, which represents a non-cash exposure to the underlying reference instrument.  Leverage can increase both the risk and return potential of the Fund.  Derivatives risk may be more significant when derivatives are used to enhance return or as a substitute for a cash investment position, rather than solely to hedge the risk of a position held by the Fund. Use of derivatives involves the exercise of specialized skill and judgment, and a transaction may be unsuccessful in whole or in part because of market behavior or unexpected events. Changes in the value of a derivative (including one used for hedging) may not correlate perfectly with the underlying reference instrument. Derivative instruments traded in over-the-counter markets may be difficult to value, may be illiquid, and may be subject to wide swings in valuation caused by changes in the value of the underlying reference instrument. If a derivative’s counterparty is unable to honor its commitments, the value of Fund shares may decline and the Fund could experience delays in the return of collateral or other assets held by the counterparty. The loss on derivative transactions may substantially exceed the initial investment, particularly when there is no stated limit on the Fund’s use of derivatives.  A derivative investment also involves the risks relating to the reference instrument underlying the investment.

Short Sale Risk. The Fund will incur a loss as a result of a short sale if the price of the security sold short increases in value between the date of the short sale and the date on which the Fund purchases the security to replace the borrowed security. Short sale risks include, among others, the potential loss of more money than the actual cost of the investment, and the risk that the third party to the short sale may fail to honor its contract terms, causing a loss to the Fund.

Borrowing Risk. Borrowing cash to increase investments (sometimes referred to as “leverage”) may exaggerate the effect on the Fund’s net asset value of any increase or decrease in the value of the security purchased with the borrowing. There can be no assurance that the use of borrowings will be successful. In connection with its borrowings, the Fund will be required to maintain specified asset coverage with respect to such borrowings by applicable federal securities laws and the terms of its credit facility with the lender. The Fund may be required to dispose of portfolio investments on unfavorable terms if market fluctuations or other factors cause the required asset coverage to be less than the prescribed amount. Borrowings involve additional expense to the Fund.

Restricted Securities Risk.  Unless registered for sale to the public under applicable federal securities law, restricted securities can be sold only in private transactions to qualified purchasers pursuant to an exemption from registration. The sale price realized from a private transaction could be less than the Fund’s purchase price for the restricted security. It may be difficult to identify a qualified purchaser for a restricted security held by the Fund and such security could be deemed illiquid. It may also be more difficult to value such securities.

Liquidity Risk.  The Fund is exposed to liquidity risk when trading volume, lack of a market maker or trading partner, large position size, market conditions, or legal restrictions impair its ability to sell particular investments or to sell them at advantageous market prices.  Consequently, the Fund may have to accept a lower price to sell an investment or continue to hold it or keep the position open, sell other investments to raise cash or abandon an investment opportunity, any of which could have a negative effect on the Fund’s performance. These effects may be exacerbated during times of financial or political stress.

Issuer Diversification Risk. The Fund is “non-diversified,” which means it may invest a greater percentage of its assets in the securities of a single issuer than a fund that is “diversified.” Non-diversified funds may focus their investments in a small number of issuers, making them more susceptible to risks affecting such issuers than a more diversified fund might be.

Securities Lending Risk. Securities lending involves a possible delay in recovery of the loaned securities or a possible loss of rights in the collateral if the borrower fails financially.  The Fund could also lose money if the value of the collateral decreases.

Risks Associated with Active Management.  The success of the Fund’s investment strategy depends on portfolio management’s successful application of analytical skills and investment judgment.  Active management involves subjective decisions.

General Fund Investing Risks. The Fund is not a complete investment program and there is no guarantee that the Fund will achieve its investment objective. It is possible to lose money by investing in the Fund. The Fund is not suited for short-term trading, and investors in the Fund should be able to tolerate potentially sharp declines in value over time.  Purchase and redemption activities by Fund shareholders may impact the management of the Fund and its ability to achieve its investment objective(s).  In addition, the redemption by one or more large shareholders or groups of shareholders of their holdings in the Fund could have an adverse impact on the remaining shareholders in the Fund.  The Fund relies on various service providers, including the investment adviser, in its operations and is susceptible to operational, information security and related events (such as public health crises, cyber or hacking attacks) that may

affect the service providers or the services that they provide to the Fund.  An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Lose Money rr_RiskLoseMoney The Fund is not a complete investment program and there is no guarantee that the Fund will achieve its investment objective. It is possible to lose money by investing in the Fund.
Risk Not Insured Depository Institution rr_RiskNotInsuredDepositoryInstitution An investment in the Fund is not a deposit in a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Risk Nondiversified Status rr_RiskNondiversifiedStatus Issuer Diversification Risk. The Fund is “non-diversified,” which means it may invest a greater percentage of its assets in the securities of a single issuer than a fund that is “diversified.” Non-diversified funds may focus their investments in a small number of issuers, making them more susceptible to risks affecting such issuers than a more diversified fund might be.
Bar Chart and Performance Table rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative rr_PerformanceNarrativeTextBlock The following bar chart and table provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and how the Fund’s average annual total returns over time compare with those of a broad-based securities market index.   The returns in the bar chart are for Class A shares and do not reflect a sales charge. If the sales charge was reflected, the returns would be lower. Past performance (both before and after taxes) is not necessarily an indication of how the Fund will perform in the future.  The Fund’s performance reflects the effects of expense reductions.  Absent these reductions, performance would have been lower.  Updated Fund performance information can be obtained by visiting www.eatonvance.com.
Performance Information Illustrates Variability of Returns rr_PerformanceInformationIllustratesVariabilityOfReturns The following bar chart and table provide some indication of the risks of investing in the Fund by showing changes in the Fund’s performance from year to year and how the Fund’s average annual total returns over time compare with those of a broad-based securities market index.
Performance Availability Website Address rr_PerformanceAvailabilityWebSiteAddress www.eatonvance.com
Performance Past Does Not Indicate Future rr_PerformancePastDoesNotIndicateFuture Past performance (both before and after taxes) is not necessarily an indication of how the Fund will perform in the future.
Bar Chart Does Not Reflect Sales Loads rr_BarChartDoesNotReflectSalesLoads The returns in the bar chart are for Class A shares and do not reflect a sales charge. If the sales charge was reflected, the returns would be lower.
Annual Return Caption rr_AnnualReturnCaption Calendar year-by-year total return
Bar Chart Closing rr_BarChartClosingTextBlock

For the ten years ended December 31, 2020, the highest quarterly total return for Class A was 6.13% for the quarter ended June 30, 2020, and the lowest quarterly return was -6.83% for the quarter ended March 31, 2020.  
Performance Table Does Reflect Sales Loads rr_PerformanceTableDoesReflectSalesLoads These returns reflect the maximum sales charge for Class A (2.25%) and any applicable contingent deferred sales charge (“CDSC”) for Class C.
Performance Table Uses Highest Federal Rate rr_PerformanceTableUsesHighestFederalRate After-tax returns are calculated using the highest historical individual federal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on a shareholder’s tax situation and the actual characterization of distributions, and may differ from those shown.
Performance Table Not Relevant to Tax Deferred rr_PerformanceTableNotRelevantToTaxDeferred After-tax returns are not relevant to shareholders who hold shares in tax-deferred accounts or to shares held by non-taxable entities.
Performance Table One Class of after Tax Shown rr_PerformanceTableOneClassOfAfterTaxShown After-tax returns for other Classes of shares will vary from the after-tax returns presented for Class A shares.
Performance Table Explanation after Tax Higher rr_PerformanceTableExplanationAfterTaxHigher Return After Taxes on Distributions for a period may be the same as Return Before Taxes for that period because no taxable distributions were made during that period. Also, Return After Taxes on Distributions and Sale of Fund Shares for a period may be greater than or equal to Return Before Taxes and/or Return After Taxes on Distributions for the same period because of losses realized on the sale of Fund shares.
Performance Table Closing rr_PerformanceTableClosingTextBlock

These returns reflect the maximum sales charge for Class A (2.25%) and any applicable contingent deferred sales charge (“CDSC”) for Class C. The Fund commenced operations on April 1, 2010. ICE® BofA® indices are not for redistribution or other uses; provided “as is”, without warranties, and with no liability.  Eaton Vance has prepared this report and ICE Data Indices, LLC does not endorse it, or guarantee, review, or endorse Eaton Vance’s products.  BofA® is a licensed registered trademark of Bank of America Corporation in the United States and other countries.  Investors cannot invest directly in an Index.  

After-tax returns are calculated using the highest historical individual federal income tax rates and do not reflect the impact of state and local taxes. Actual after-tax returns depend on a shareholder’s tax situation and the actual characterization of distributions, and may differ from those shown.   After-tax returns are not relevant to shareholders who hold shares in tax-deferred accounts or to shares held by non-taxable entities.   After-tax returns for other Classes of shares will vary from the after-tax returns presented for Class A shares.    Return After Taxes on Distributions for a period may be the same as Return Before Taxes for that period because no taxable distributions were made during that period. Also, Return After Taxes on Distributions and Sale of Fund Shares for a period may be greater than or equal to Return Before Taxes and/or Return After Taxes on Distributions for the same period because of losses realized on the sale of Fund shares.

Average Annual Return, Caption rr_AverageAnnualReturnCaption Average Annual Total Return as of December 31, 2020
Eaton Vance Short Duration Inflation-Protected Income Fund | ICE BofAML 1-5 Year U.S. Inflation-Linked Treasury Index  
Prospectus [Line Items] rr_ProspectusLineItems  
One Year rr_AverageAnnualReturnYear01 5.69%
Five Years rr_AverageAnnualReturnYear05 3.00%
Ten Years rr_AverageAnnualReturnSinceInception 1.92%
Eaton Vance Short Duration Inflation-Protected Income Fund | Class A  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice 2.25%
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of net asset value at purchase or redemption) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management Fees rr_ManagementFeesOverAssets 0.32%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 0.25%
Other Expenses rr_OtherExpensesOverAssets 0.15%
Acquired Fund Fees and Expenses(1) rr_AcquiredFundFeesAndExpensesOverAssets 0.29% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.01%
Expense Reimbursement(2) rr_FeeWaiverOrReimbursementOverAssets (0.11%) [2]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.90%
1 Year rr_ExpenseExampleYear01 $ 315
3 Years rr_ExpenseExampleYear03 529
5 Years rr_ExpenseExampleYear05 760
10 Years rr_ExpenseExampleYear10 1,424
1 Year rr_ExpenseExampleNoRedemptionYear01 315
3 Years rr_ExpenseExampleNoRedemptionYear03 529
5 Years rr_ExpenseExampleNoRedemptionYear05 760
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,424
2011 rr_AnnualReturn2011 4.19%
2012 rr_AnnualReturn2012 3.92%
2013 rr_AnnualReturn2013 (0.17%)
2014 rr_AnnualReturn2014 (2.01%)
2015 rr_AnnualReturn2015 (1.10%)
2016 rr_AnnualReturn2016 5.80%
2017 rr_AnnualReturn2017 1.80%
2018 rr_AnnualReturn2018 (0.49%)
2019 rr_AnnualReturn2019 5.28%
2020 rr_AnnualReturn2020 5.04%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel highest quarterly total return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2020
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 6.13%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel lowest quarterly return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2020
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn (6.83%)
One Year rr_AverageAnnualReturnYear01 2.63%
Five Years rr_AverageAnnualReturnYear05 2.98%
Ten Years rr_AverageAnnualReturnSinceInception 1.96%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 01, 2010
Eaton Vance Short Duration Inflation-Protected Income Fund | Class A | After Taxes on Distributions  
Prospectus [Line Items] rr_ProspectusLineItems  
One Year rr_AverageAnnualReturnYear01 1.76%
Five Years rr_AverageAnnualReturnYear05 2.04%
Ten Years rr_AverageAnnualReturnSinceInception 1.05%
Eaton Vance Short Duration Inflation-Protected Income Fund | Class A | After Taxes on Distributions and Sales  
Prospectus [Line Items] rr_ProspectusLineItems  
One Year rr_AverageAnnualReturnYear01 1.53%
Five Years rr_AverageAnnualReturnYear05 1.89%
Ten Years rr_AverageAnnualReturnSinceInception 1.10%
Eaton Vance Short Duration Inflation-Protected Income Fund | Class C  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of net asset value at purchase or redemption) rr_MaximumDeferredSalesChargeOverOfferingPrice 1.00%
Management Fees rr_ManagementFeesOverAssets 0.32%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets 1.00%
Other Expenses rr_OtherExpensesOverAssets 0.15%
Acquired Fund Fees and Expenses(1) rr_AcquiredFundFeesAndExpensesOverAssets 0.29% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 1.76%
Expense Reimbursement(2) rr_FeeWaiverOrReimbursementOverAssets (0.11%) [2]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 1.65%
1 Year rr_ExpenseExampleYear01 $ 268
3 Years rr_ExpenseExampleYear03 543
5 Years rr_ExpenseExampleYear05 944
10 Years rr_ExpenseExampleYear10 1,866
1 Year rr_ExpenseExampleNoRedemptionYear01 168
3 Years rr_ExpenseExampleNoRedemptionYear03 543
5 Years rr_ExpenseExampleNoRedemptionYear05 944
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 1,866
One Year rr_AverageAnnualReturnYear01 3.21%
Five Years rr_AverageAnnualReturnYear05 2.68%
Ten Years rr_AverageAnnualReturnSinceInception 1.43%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 01, 2010
Eaton Vance Short Duration Inflation-Protected Income Fund | Class I  
Prospectus [Line Items] rr_ProspectusLineItems  
Maximum Sales Charge (Load) Imposed on Purchases (as a percentage of offering price) rr_MaximumSalesChargeImposedOnPurchasesOverOfferingPrice none
Maximum Deferred Sales Charge (Load) (as a percentage of the lower of net asset value at purchase or redemption) rr_MaximumDeferredSalesChargeOverOfferingPrice none
Management Fees rr_ManagementFeesOverAssets 0.32%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses rr_OtherExpensesOverAssets 0.15%
Acquired Fund Fees and Expenses(1) rr_AcquiredFundFeesAndExpensesOverAssets 0.29% [1]
Total Annual Fund Operating Expenses rr_ExpensesOverAssets 0.76%
Expense Reimbursement(2) rr_FeeWaiverOrReimbursementOverAssets (0.11%) [2]
Total Annual Fund Operating Expenses After Expense Reimbursement rr_NetExpensesOverAssets 0.65%
1 Year rr_ExpenseExampleYear01 $ 66
3 Years rr_ExpenseExampleYear03 232
5 Years rr_ExpenseExampleYear05 412
10 Years rr_ExpenseExampleYear10 932
1 Year rr_ExpenseExampleNoRedemptionYear01 66
3 Years rr_ExpenseExampleNoRedemptionYear03 232
5 Years rr_ExpenseExampleNoRedemptionYear05 412
10 Years rr_ExpenseExampleNoRedemptionYear10 $ 932
One Year rr_AverageAnnualReturnYear01 5.24%
Five Years rr_AverageAnnualReturnYear05 3.70%
Ten Years rr_AverageAnnualReturnSinceInception 2.44%
Inception Date rr_AverageAnnualReturnInceptionDate Apr. 01, 2010
[1] Reflects the Fund’s allocable share of the advisory fee and other expenses of the Portfolios in which it invests. Acquired Fund Fees and Expenses include borrowing costs of 0.15% incurred by the Portfolios in which the Fund invests.
[2] The investment adviser and administrator have agreed to reimburse the Fund’s expenses to the extent that Total Annual Fund Operating Expenses exceed 0.75% for Class A shares, 1.50% for Class C shares and 0.50% for Class I shares.  This expense reimbursement will continue through February 28, 2022.  Any amendment to or termination of this reimbursement would require approval of the Board of Trustees.  The expense reimbursement relates to ordinary operating expenses only and does not include expenses such as: brokerage commissions, acquired fund fees and expenses of unaffiliated funds, borrowing costs (including borrowing costs of any acquired funds), taxes or litigation expenses.  Amounts reimbursed may be recouped by the investment adviser and administrator during the same fiscal year to the extent actual expenses are less than the contractual expense cap during such year.