| N-2 - USD ($) | 3 Months Ended | 12 Months Ended | ||||||||
| Nov. 30, 2023 | Aug. 31, 2023 | May 31, 2023 | Feb. 28, 2023 | Nov. 30, 2022 | Aug. 31, 2022 | May 31, 2022 | Feb. 28, 2022 | Nov. 30, 2023 | ||
| Cover [Abstract] | ||||||||||
| Entity Central Index Key | 0001074540 | |||||||||
| Amendment Flag | false | |||||||||
| Document Type | N-CSR | |||||||||
| Entity Registrant Name | Eaton Vance Municipal Income Trust | |||||||||
| Fee Table [Abstract] | ||||||||||
| Shareholder Transaction Expenses [Table Text Block] | Common shareholder transaction expenses Sales load paid by you (as a percentage of offering price) — 1 Offering expenses (as a percentage of offering price) None 2 Dividend reinvestment plan fees $5.00 3 1 If common shares are sold to or through underwriters, the Prospectus Supplement will set forth any applicable sales load. 2 Eaton Vance Management (“EVM”) will pay the expenses of the offering (other than the applicable commissions); therefore, offering expenses are not included in the Summary of Fund Expenses. Offering expenses generally include, but are not limited to, the preparation, review and filing with the SEC of the Trust’s registration statement (including its current Prospectus Supplement, the accompanying Prospectus and Statement of Additional Information (“SAI”)), the preparation, review and filing of any associated marketing or similar materials, costs associated with the printing, mailing or other distribution of its current Prospectus Supplement, the accompanying Prospectus, SAI and/or marketing materials, associated filing fees, stock exchange listing fees, and legal and auditing fees associated with the offering. 3 You will be charged a $5.00 service charge and pay brokerage charges if you direct the plan agent to sell your common shares held in a dividend reinvestment account. | |||||||||
| Sales Load [Percent] | [1] | |||||||||
| Dividend Reinvestment and Cash Purchase Fees | [2] | $ 5 | ||||||||
| Other Transaction Expenses [Abstract] | ||||||||||
| Other Transaction Expense 1 [Percent] | [3] | 0% | ||||||||
| Annual Expenses [Table Text Block] | Annual expenses Percentage of net assets 4 Management fee 0.88% 5 Interest expense 1.72% 6 Other expenses 0.10% Total annual Fund operating expenses 2.70% 4 Stated as a percentage of average net assets attributable to common shares for the year ended November 30, 2023. 5 The management fee paid by the Trust to EVM is based on the average daily gross assets of the Trust, including the principal amount of any indebtedness for money borrowed, the amount of any outstanding preferred shares issued by the Trust and, to a limited extent, the amount of floating-rate notes included as a liability in the Trust's Statement of Assets and Liabilities. Accordingly, if the Trust were to increase investment leverage in the future, the management fee will increase as a percentage of net assets. 6 Interest expense relates to the Trust’s liability with respect to floating-rate notes held by third parties in conjunction with investments in residual interest bonds. The Trust records offsetting interest income in an amount at least equal to this expense relating to the municipal obligations underlying such transactions. | |||||||||
| Management Fees [Percent] | [4],[5] | 0.88% | ||||||||
| Interest Expenses on Borrowings [Percent] | [4],[6] | 1.72% | ||||||||
| Other Annual Expenses [Abstract] | ||||||||||
| Other Annual Expenses [Percent] | [4] | 0.10% | ||||||||
| Total Annual Expenses [Percent] | [4] | 2.70% | ||||||||
| Expense Example [Table Text Block] | Example The following Example illustrates the expenses that common shareholders would pay on a $1,000 investment in common shares, assuming (i) total annual expenses of 2.70% of net assets attributable to common shares in years 1 through 10; (ii) a 5% annual return; and (iii) all distributions are reinvested at NAV: 1 Year 3 Years 5 Years 10 Years $27 $84 $143 $303 The above table and example and the assumption in the example of a 5% annual return are required by regulations of the U.S. Securities and Exchange Commission (“SEC”) that are appl ica | |||||||||
| Expense Example, Year 01 | $ 27 | |||||||||
| Expense Example, Years 1 to 3 | 84 | |||||||||
| Expense Example, Years 1 to 5 | 143 | |||||||||
| Expense Example, Years 1 to 10 | $ 303 | |||||||||
| Purpose of Fee Table , Note [Text Block] | The purpose of the table below is to help you understand all fees and expenses that you, as a common shareholder, would bear directly or indirectly. The table reflects leverage attributable to floating-rate notes for the fiscal year ended November 30, 2023 in an amount equal to 31.2% of the Trust’s average gross assets (including floating-rate notes) and shows Trust expenses as a percentage of net assets attributable to common shares. | |||||||||
| Basis of Transaction Fees, Note [Text Block] | as a percentage of offering price | |||||||||
| Other Expenses, Note [Text Block] | Eaton Vance Management (“EVM”) will pay the expenses of the offering (other than the applicable commissions); therefore, offering expenses are not included in the Summary of Fund Expenses. Offering expenses generally include, but are not limited to, the preparation, review and filing with the SEC of the Trust’s registration statement (including its current Prospectus Supplement, the accompanying Prospectus and Statement of Additional Information (“SAI”)), the preparation, review and filing of any associated marketing or similar materials, costs associated with the printing, mailing or other distribution of its current Prospectus Supplement, the accompanying Prospectus, SAI and/or marketing materials, associated filing fees, stock exchange listing fees, and legal and auditing fees associated with the offering. | |||||||||
| Management Fee not based on Net Assets, Note [Text Block] | The management fee paid by the Trust to EVM is based on the average daily gross assets of the Trust, including the principal amount of any indebtedness for money borrowed, the amount of any outstanding preferred shares issued by the Trust and, to a limited extent, the amount of floating-rate notes included as a liability in the Trust's Statement of Assets and Liabilities. Accordingly, if the Trust were to increase investment leverage in the future, the management fee will increase as a percentage of net assets. | |||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Investment Objectives and Practices [Text Block] | Investment Objectives. The Fund’s investment objective is to provide current income exempt from regular federal income tax. Principal Strategies. During normal market conditions, substantially all of the Fund’s total assets (at least 80%) will be invested in debt obligations issued by or on behalf of states, territories and possessions of the United States, and the District of Columbia and their political subdivisions, agencies or instrumentalities, the interest on which is exempt from regular federal income tax (“municipal obligations”). At least 65% of the Fund’s total assets normally will be invested in municipal obligations rated at least investment grade at the time of investment (which are those rated Baa or higher by Moody’s Investors Service, Inc. (“Moody’s”) or BBB or higher by either S&P Global Ratings (“S&P”) or Fitch Ratings (“Fitch”), or, if unrated, determined by Eaton Vance Management (“EVM”) to be of at least investment grade quality. From time to time, the Fund may hold a significant amount of municipal obligations not rated by a nationally recognized statistical rating organization (“Rating Agency”). When the Fund invests in unrated municipal obligations, it may be more dependent on EVM’s research capabilities than when it invests in rated municipal obligations. The Fund may invest up to 35% of its total assets in municipal obligations rated below investment grade by each of Moody’s, S&P and Fitch (but no more than 30% of total assets may be rated lower than B by each of Moody’s, S&P and Fitch) and unrated municipal obligations considered to be of comparable quality by EVM. For purposes of rating restrictions, if an instrument is rated differently by the Rating Agencies, the higher rating is used. The Fund will not purchase securities that are in default at the time of purchase. The Fund may purchase and sell derivative instruments, which derive their value from another instrument, security or index, including financial futures contracts and related options based on various debt securities and securities indices, as well as interest rates swaps and forward rate contracts, to seek to hedge against changes in interest rates, as a substitute for the purchase of securities or for other risk management purposes. The Fund also may invest in residual interest bonds of a trust (the “trust”) that holds municipal securities. The trust will also issue floating-rate notes to third parties that may be senior to the Fund’s residual interest. Except for certain fundamental investment restrictions set forth in the Fund’s registration statement and the 80% requirement set forth above, the investment objective and policies of the Fund may be changed by the Board without shareholder action. The Fund employs leverage to seek opportunities for additional income. Leverage may amplify any increase or decrease in the value of investments held by the Fund. The Fund generally will not use leverage if the investment adviser anticipates that it would result in a lower return to shareholders for any significant amount of time. There can be no assurance that the use of leverage will be successful. | |||||||||
| Risk Factors [Table Text Block] | Principal Risks Market Discount Risk. As with any security, the market value of the common shares may increase or decrease from the amount initially paid for the common shares. The Fund’s common shares have traded both at a premium and at a discount relative to NAV. The shares of closed-end management investment companies frequently trade at a discount from their NAV. This is a risk separate and distinct from the risk that the Fund’s NAV may decrease. Investment and Market Risk. An investment in common shares is subject to investment risk, including the possible loss of the entire principal amount invested. An investment in common shares represents an indirect investment in the securities owned by the Fund, which will generally trade in the over-the-counter (“OTC”) markets. The common shares at any point in time may be worth less than the original investment, even after taking into account any reinvestment of distributions. The value of investments held by the Fund may increase or decrease in response to social, economic, political, financial, public health crises or other disruptive events (whether real, expected or perceived) in t Municipal Obligations Risk. The amount of public information available about municipal obligations is generally less than for corporate equities or bonds, meaning that the investment performance of municipal obligations may be more dependent on the analytical abilities of the investment adviser than stock or corporate bond investments. The secondary market for municipal obligations also tends to be less well-developed and less liquid than many other securities markets, which may limit the Fund’s ability to sell its municipal obligations at attractive prices. The differences between the price at which an obligation can be purchased and the price at which it can be sold may widen during periods of market distress. Less liquid obligations can become more difficult to value and be subject to erratic price movements. The increased presence of nontraditional participants (such as proprietary trading desks of investment banks and hedge funds) or the absence of traditional participants (such as individuals, insurance companies, banks and life insurance companies) in the municipal markets may lead to greater volatility in the markets because non-traditional participants may trade more frequently or in greater volume. Insurance Risk. Municipal obligations may be insured as to their scheduled payment of principal and interest. Although the insurance feature may reduce some financial risks, the premiums for insurance and the higher market price sometimes paid for insured obligations may reduce the current yield on the insured obligation. Insured obligations also may be secured by bank credit agreements or escrow accounts. Changes in the ratings of an insurer may affect the value of an insured obligation, and in some cases may even cause the value of a security to be less than a comparable uninsured obligation. The insurance does not guarantee the market value of the insured obligation or the net asset value of the Fund’s shares. The credit rating of an insured obligation reflects the credit rating of the insurer, based on its claims-paying ability. The obligation of a municipal bond insurance company to pay a claim extends over the life of each insured obligation. Although defaults on insured municipal obligations have been low to date and municipal bond insurers have met their claims, there is no assurance this will continue. A higher than expected default rate could strain the insurer’s loss reserves and adversely affect its ability to pay claims to bondholders. Because a significant portion of insured municipal obligatio ns at in Risks of Municipal Lease Obligations (“MLOs”) and Certificates of Participation. MLOs and certificates of participation involve special risks not normally associated with general obligations or revenue bonds. MLOs are bonds that are secured by lease payments made by the party, typically a state or municipality, leasing the facilities (e.g., schools or office buildings) that were financed by the bond. Interest income from MLOs is generally exempt from local and state taxes in the state of issuance. MLOs, like other municipal debt obligations, are subject to the risk of non-payment. Although MLOs do not constitute general obligations of the issuer for which the issuer’s unlimited taxing power is pledged, the leasing state or municipality may be obligated to appropriate funds from its general tax revenues to make lease payments as long as it utilizes the leased property. Other lease payments may be subject to annual appropriation or may be made only from revenues associated with the facility financed. For example, certain lease obligations contain “non-appropriation” clauses, which provide that the issuer has no obligation to make lease or installment purchase payments in future years unless money is appropriated for such purpose on a yearly basis, which function to render constitutional and statutory requirements for the issuance of debt inapplicable to such obligations. In addition, such leases or contracts may be subject to temporary abatement of payments in the event the governmental issuer is prevented from maintaining occupancy of the leased premises or utilizing the leased equipment. Although “non-appropriation” lease obligations may be secured by the leased property, disposition of the property in the event of foreclosure might prove difficult, time consuming and costly, and may result in a delay in recovering or the failure to fully recover ownership of the assets. A certificate of participation (also referred to as a “participation”) in a municipal lease is an instrument evidencing a pro rata share in a specific pledged revenue stream, usually lease payments by the issuer that are typically subject to annual appropriation. The certificate generally entitles the holder to receive a share, or participation, in the payments from a particular project. Certificates of participation involve the same risks as the underlying municipal leases. In addition, the Fund may be dependent upon the municipal authority issuing the certificate of participation to exercise remedies with respect to an underlying lease. Certificates of participation also entail a risk of default or bankruptcy, both of the issuer of the municipal lease and also the municipal agency issuing the certificate of participation. MLOs and participations therein represent a type of financing that may not have the depth of marketability associated with more conventional securities and, as such, they may be less liquid than conventional securities. Certain MLOs may be deemed illiquid, unless determined by the Adviser, pursuant to guidelines adopted by the Board, to be liquid securities. The Adviser will consider the factors it believes are relevant to the marketability of the obligation, to the extent that information regarding such factor is available to the Adviser and pertinent to the liquidity determination, which may include: (1) the willingness of dealers to bid for the obligation; (2) the number of dealers willing to purchase or sell the obligation and the number of other potential buyers; (3) the frequency of trades and quotes for the obligation; (4) the nature of the marketplace trades, including the time needed to dispose of the obligation, the method of soliciting offers, and the mechanics of transfer; (5) the willingness of the governmental issuer to continue to appropriate funds for the payment of the obligation; (6) how likely or remote an event of non-appropriation may be, which depends in varying degrees on a variety of factors, including those relating to the general creditworthiness of the governmental issuer, its dependence on its continuing access to the credit markets, and the importance to the issuer of the equipment, property or facility covered by the lease or contract; (7) an assessment of the likelihood that the lease may or may not be cancelled; and (8) other factors and information unique to the obligation in determining its liquidity. The ability of issuers of MLOs to make timely lease payments may be adversely impacted in general economic downturns and as relative governmental cost burdens are allocated and reallocated among federal, state and local governmental units. Such non-payment would result in a reduction of income from and value of the obligation. Issuers of MLOs might seek protection under the bankruptcy laws. In the event of bankruptcy of such an issuer, holders of MLOs could experience delays and limitations with respect to the collection of principal and interest on such MLOs and may not, in all circumstances, be able to collect all principal and interest to which it is entitled. To enforce its rights in the event of a default in lease payments, the Fund might take possession of and manage the assets securing the issuer’s obligations on such securities or otherwise incur costs to protect its right, which may increase the Fund’s operating expenses and adversely affect the net asset value of the Fund. When the lease contains a non-appropriation clause, however, the failure to pay would not be a default and the Fund would not have the right to take possession of the assets. Any income derived from the Fund’s ownership or operation of such assets may not be tax-exempt. Interest Rate Risk. In general, the value of debt instruments 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 the time-weighted expected cash flows of a fixed income security, while maturity refers to the amount of time until a fixed income security matures. 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. Because the Fund is managed toward an income objective, it may hold more longer-duration or maturity obligations and thereby be more exposed to interest rate risk than municipal income funds that are managed with a greater emphasis on total return. The impact of interest rate changes is significantly less for floating rate instruments that have relatively short periodic rate resets (e.g., ninety days or less). In a rising interest rate environment, the durations or maturities 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 were historically based on the London Interbank Offered Rate (“LIBOR”), which was the average offered rate for various maturities of short-term loans between certain major international banks. LIBOR historically was used throughout global banking and financial industries to determine interest rates for a variety of financial instruments (such as debt instruments and derivatives) and borrowing arrangements. Upon a determination by regulators to phase out the use of LIBOR, market participants have been transitioning to the use of alternative reference rates over the past few years. As of June 30, 2023, the administrator of LIBOR ceased publishing LIBOR settings. The impact of the transition away from LIBOR on certain debt securities, derivatives and other financial instruments that utilize LIBOR remains uncertain. The transition away from LIBOR and the use of replacement rates may adversely affect transactions that used LIBOR as Call and Reinvestment Risks. If interest rates fall, it is possible that issuers of callable bonds with high interest coupons will “call” (or prepay) their bonds before their maturity date. If a call were exercised by the issuer during a period of declining interest rates, the Fund would likely replace such called security with a lower yielding security. If that were to happen, it could decrease the Fund’s dividends and possibly could affect the market price of common shares. Similar risks exist when the Fund invests the proceeds from matured or traded municipal obligations at market interest rates that are below the Fund’s current earnings rate. Credit Risk. Investments in municipal obligations and other debt obligations (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 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. Municipal obligations may be insured as to principal and interest payments. If the claims-paying ability or other rating of the insurer is downgraded by a rating agency, the value of such obligations may be negatively affected. Lower Rated Investment Risk. Investments rated below investment grade and comparable unrated investments (sometimes referred to as “junk”) are speculative because of the increased credit risk relative to other fixed income investments. 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. Unrated Securities Risk. The Fund may invest in unrated obligations for which Eaton Vance will make a credit quality determination for purposes of the Fund’s credit quality policy. To the extent that the Fund invests in such unrated obligations, the Fund’s credit quality will be more dependent on Eaton Vance’s credit analysis than if the Fund invested in only rated obligations. Some unrated securities may not have an active trading market or may be difficult to value. 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 also result from borrowings, issuance of preferred shares or participation in residual interest bond transactions. Leverage can increase both the risk and return potential of the Fund. 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 NAV 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 Fund’s portfolio securities. The loss on leveraged investments may substantially exceed the initial investment. Risk of Residual Interest Bonds. The Fund may enter into residual interest bond transactions, which expose the Fund to leverage and greater risk than an investment in a fixed-rate municipal bond. The interest payments that the Fund receives on the residual interest bonds acquired in such transactions vary inversely with short-term interest rates, normally decreasing when short-term rates increase. The value and market for residual interest bonds are volatile and such bonds may have limited liquidity. As required by applicable accounting standards, th e Fun r 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. 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 (or be unable to achieve) the return of collateral or other assets held by the counterparty. The loss on derivative transactions may substantially exceed the initial investment. A derivative investment also involves the risks relating to the reference instrument underlying the investment. Counterparty Risk. Changes in the credit quality of the companies that serve as the Fund’s counterparties with respect to its derivatives positions and liquidity providers for the Fund’s residual interest bonds or other investments supported by another party’s credit will affect the value of those instruments. Certain entities that have served as counterparties in the municipals markets have recently incurred significant financial hardships, including bankruptcy and material loss of credit standing as a result of exposure to investments that have experienced defaults or otherwise suffered extreme credit deterioration. As a result, such hardships have reduced these entities’ capital and called into question their continued ability to perform their obligations. By using derivatives or other instruments that expose the Fund to counterparties, the Fund assumes the risk that its counterparties could experience future financial hardship. The counterparty risk for cleared derivatives is generally lower than for uncleared over-the-counter derivative transactions since generally a clearing organization becomes substituted for each counterparty to a cleared derivative contract and, in effect, guarantees the parties’ performance under the contract as each party to a trade looks only to the clearing organization for performance of financial obligations under the derivative contract. However, there can be no assurance that a clearing organization, or its members, will satisfy its obligations to the Fund. Hedging Risk. The Fund’s use of derivatives or other transactions to reduce risks involves costs and will be subject to Eaton Vance’s ability to predict correctly changes in the relationships of such hedge instruments to the Fund’s portfolio holdings or other factors. No assurance can be given that Eaton Vance’s judgment in this respect will be correct. In addition, no assurance can be given that the Fund will enter into hedging or other transactions at times or under circumstances in which it may be advisable to do so. Hedging transactions have risks, including the imperfect correlation between the value of such instruments and the underlying assets of the Fund, which creates the possibility that the loss on such instruments may be greater than the gain, if any, in the value of the underlying asset in the Fund’s portfolio; the limited availability of such instruments; the loss of principal; the possible default of the other party to the transaction; illiquidity of the derivative investments; and the imperfect correlation between the tax-exempt and taxable markets. Furthermore, the ability to successfully use hedging transactions depends on the Eaton Vance’s ability to predict pertinent market movements, which cannot be assured. Thus, the use of hedging transactions may result in losses greater than if they had not been used, may require the Fund to sell or purchase portfolio investments at inopportune times or for prices other than current market values, may limit the amount of appreciation the Fund can realize on an investment, or may cause the Fund to hold a security that it might otherwise sell. Swaps Risk. Swap agreements are two-party contracts entered into primarily by institutional investors for periods ranging from a few weeks to more than one year. In a standard “swap” transaction, two parties agree to exchange the returns (or differentials in rates of return) earned or realized on a particular predetermined reference instrument or instruments, which can be adjusted for an interest rate factor. The gross returns to be exchanged or “swapped” between the parties are generally calculated with respect to a “notional amount” (i.e., the return on or increase in value of a particular dollar amount invested at a particular interest rate or in a “basket” of securities representing a particular index). Other types of swap agreements may calculate the obligations of the parties to the agreement on a “net basis.” Consequently, a party’s current obligations (or rights) under a swap agreement will generally be equal only to the net amount to be paid or received under the agreement based on the relative values of the positions held by each party to the agreement (the “net amount”). Whether the use of swap agreements will be successful will depend on the Adviser’s ability to predict correctly whether certain types of reference instruments are likely to produce greater returns than other instruments. Swap agreements may be subject to contractual restrictions on transferability and termination and they may have terms of greater than seven days. The Fund’s obligations under a swap agreement will be accrued daily (offset against any amounts owed to the Fund under the swap). Developments in the swaps market, including potential government regulation, could adversely affect the Fund’s ability to terminate existing swap agreements or to realize amounts to be received under such agreements, as well as to participate in swap agreements in the future. If there is a default by the counterparty to a swap, the Fund will have contractual remedies pursuant to the swap agreement, but any recovery may be delayed depending on the circumstances of the default. Private Activity Bonds. Private activity bonds, formerly referred to as industrial development bonds, are issued by or on behalf of public authorities to obtain funds to provide privately operated housing facilities, airport, mass transit or port facilities, sewage disposal, solid waste disposal or hazardous waste treatment or disposal facilities and certain local facilities for water supply, gas or electricity. Other types of private activity bonds, the proceeds of which are used for the construction, equipment, repair or improvement of privately operated industrial or commercial facilities, may constitute municipal obligations, although the current federal tax laws place substantial limitations on the size of such issues. Interest on certain “private activity bonds” issued after August 7, 1986 is exempt from regular federal income tax, but such interest (including a distribution by the Fund derived from such interest) is treated as a tax preference item which could subject the recipient to or increase the recipient’s liability for the AMT. Futures Risk. Although some futures contracts call for making or taking delivery of the underlying reference instrument, generally these obligations are closed out prior to delivery by offsetting purchases or sales of matching futures contracts (same exchange, underlying security or index, and delivery month). Closing a futures contract sale is effected by purchasing a futures contract for the same aggregate amount of the specific type of financial instrument or commodity with the same delivery date. If an offsetting purchase price is less than the original sale price, the Fund realizes a capital gain, or if it is more, the Fund realizes a capital loss. Conversely, if an offsetting sale price is more than the original purchase price, the Fund realizes a capital gain, or if it is less, the Fund realizes a capital loss. The Adviser has claimed an exclusion from the definition of a Commodity Pool Operator (“CPO”) under the Commodity Exchange Act with respect to the Fund and therefore, neither the Adviser nor the Fund are subject to registration or regulation thereunder. Structured Notes Risk. Structured notes are derivative debt instruments, the interest rate or principal of which is determined by an unrelated indicator (for example, a currency, security, commodity or index thereof). Indexed securities may include a multiplier that multiplies the indexed element by a specified factor and, therefore, the value of such securities may be very volatile. The terms of structured notes and indexed securities may provide that in certain circumstances no principal is due at maturity, which may result in a loss of invested capital. Structured notes and indexed securities may be positively or negatively indexed, so that appreciation of the unrelated indicator may produce an increase or a decrease in the interest rate or the value of the structured note or indexed security at maturity may be calculated as a specified multiple of Inflation Risk/Deflation Risk. Inflation risk is the risk that the value of assets or income from investment will be worth less in the future as inflation decreases the value of money. As inflation increases, the real value of the common shares and distributions thereon can decline. In addition, during periods of rising inflation, short-term interest rates and the Fund’s cost of leverage would likely increase, reducing returns to the common shareholders to the extent that such increased cost is not offset by commensurately higher income. Deflation risk is the risk that prices throughout the economy decline over time − the opposite of inflation. Deflation may have an adverse affect on the creditworthiness of issuers and may make issuer defaults more likely, which may result in a decline in the value of the Fund’s investments. Duration and Maturity Risk. Holding long duration and long maturity investments will expose the Fund to certain magnified risks. These risks include interest rate risk and credit risk as discussed above and liquidity risk as discussed below. 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. Current Regulatory Environment Risk. From time to time proposals have been introduced before Congress for the purpose of restricting or eliminating the federal income tax exemption for interest on certain types of municipal obligations, and it can be expected that similar proposals may be introduced in the future. Any proposed or actual changes in such rates or exempt status, therefore, can significantly affect the demand for and supply, liquidity and marketability of municipal obligations. This could in turn affect the Fund’s net asset value and ability to acquire and dispose of municipal obligations at desirable yield and price levels. Legislation may be enacted that could negatively affect the assets of th State Specific Risk. The Fund has no current intention to invest 25% or more of its gross assets in municipal obligations of issuers located in the same state (or U.S. territory), but reserves the flexibility to do so in the future. If the Fund focuses its investments in any one state (or U.S. territory), the Fund may be more susceptible to adverse economic, political or regulatory occurrences affecting a particular state (or territory). Certain municipal bond issuers in Puerto Rico have recently experienced financial difficulties and rating agency downgrades, and two such issuers have defaulted on their payment obligations. Sector and Geographic Risk. Because the Fund may invest a significant portion of its assets in obligations issued in a particular state and/or U.S. territories and in certain types of municipal or other obligations and/or in certain sectors, the value of Fund shares may be affected by events that adversely affect that state, U.S. territory, sector or type of obligation and may fluctuate more than that of a fund that invests more broadly. General obligation bonds issued by municipalities are adversely affected by economic downturns and any resulting decline in tax revenues. Recent Market Conditions. The outbreak of COVID-19 and efforts to contain its spread have resulted in closing borders, enhanced health screenings, changes to healthcare service preparation and delivery, quarantines, cancellations, disruptions to supply chains and customer activity, as well as general concern and uncertainty. The impact of this coronavirus, and the effects of other infectious illness outbreaks, epidemics or pandemics, may be short term or may continue for an extended period of time. Health crises caused by outbreaks of disease, such as the coronavirus outbreak, may exacerbate other pre-existing political, social and economic risks and disrupt normal market conditions and operations. For example, a global pandemic or other widespread health crisis could cause substantial market volatility and exchange trading suspensions and closures. In addition, the increasing interconnectedness of markets around the world may result in many markets being affected by events or conditions in a single country or region or events affecting a single or small number of issuers. The coronavirus outbreak and public and private sector responses thereto have led to large portions of the populations of many countries working from home for indefinite periods of time, temporary or permanent layoffs, disruptions in supply chains, and lack of availability of certain goods. The impact of such responses could adversely affect the information technology and operational systems upon which the Fund and the Fund’s service providers rely, and could otherwise disrupt the ability of the employees of the Fund’s service providers to perform critical tasks relating to the Fund. Any such impact could adversely affect the Fund’s performance, or the performance of the securities in which the Fund invests and may lead to losses on your investment in the Fund. 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 d | |||||||||
| Share Price [Table Text Block] | The Trust’s common shares have traded both at a premium and a dis cou In addition, the Trust’s Board of Trustees has authorized the Trust to repurchase up to 10% of its common shares outstanding as of the last day of the prior calendar year at market prices when shares are trading at a discount to net asset value. The share repurchase program does not obligate the Trust to purchase a specific amount of shares. The results of the share repurchase program are disclosed in the Trust’s annual and semi-annual reports to shareholders. The following table sets forth for each of the periods indicated the high and low closing market prices for the common shares on the New York Stock Exchange, and the corresponding NAV per share and the premium or discount to NAV per share at which the Trust’s common shares were trading as of such date. Market Price ($) NAV per Share on NAV Premium/(Discount) on Fiscal Quarter Ended High Low High Low High Low November 30, 2023 9.60 8.70 10.83 10.24 (11.36) (15.04) August 31, 2023 10.18 9.54 11.33 11.01 (10.15) (13.35) May 31, 2023 10.20 9.70 11.37 11.09 (10.29) (12.53) February 28, 2023 10.82 9.78 11.58 11.18 (6.56) (12.52) November 30, 2022 10.86 9.37 11.56 10.60 (6.06) (11.60) August 31, 2022 12.00 10.43 12.14 11.46 (1.15) (8.99) May 31, 2022 12.79 10.95 13.44 11.56 (4.84) (5.28) February 28, 2022 13.99 12.05 14.18 13.33 (1.34) (9.60) | |||||||||
| Market Discount Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Market Discount Risk. As with any security, the market value of the common shares may increase or decrease from the amount initially paid for the common shares. The Fund’s common shares have traded both at a premium and at a discount relative to NAV. The shares of closed-end management investment companies frequently trade at a discount from their NAV. This is a risk separate and distinct from the risk that the Fund’s NAV may decrease. | |||||||||
| Investment and Market Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Investment and Market Risk. An investment in common shares is subject to investment risk, including the possible loss of the entire principal amount invested. An investment in common shares represents an indirect investment in the securities owned by the Fund, which will generally trade in the over-the-counter (“OTC”) markets. The common shares at any point in time may be worth less than the original investment, even after taking into account any reinvestment of distributions. The value of investments held by the Fund may increase or decrease in response to social, economic, political, financial, public health crises or other disruptive events (whether real, expected or perceived) in t | |||||||||
| Municipal Obligations Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Municipal Obligations Risk. The amount of public information available about municipal obligations is generally less than for corporate equities or bonds, meaning that the investment performance of municipal obligations may be more dependent on the analytical abilities of the investment adviser than stock or corporate bond investments. The secondary market for municipal obligations also tends to be less well-developed and less liquid than many other securities markets, which may limit the Fund’s ability to sell its municipal obligations at attractive prices. The differences between the price at which an obligation can be purchased and the price at which it can be sold may widen during periods of market distress. Less liquid obligations can become more difficult to value and be subject to erratic price movements. The increased presence of nontraditional participants (such as proprietary trading desks of investment banks and hedge funds) or the absence of traditional participants (such as individuals, insurance companies, banks and life insurance companies) in the municipal markets may lead to greater volatility in the markets because non-traditional participants may trade more frequently or in greater volume. | |||||||||
| Insurance Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Insurance Risk. Municipal obligations may be insured as to their scheduled payment of principal and interest. Although the insurance feature may reduce some financial risks, the premiums for insurance and the higher market price sometimes paid for insured obligations may reduce the current yield on the insured obligation. Insured obligations also may be secured by bank credit agreements or escrow accounts. Changes in the ratings of an insurer may affect the value of an insured obligation, and in some cases may even cause the value of a security to be less than a comparable uninsured obligation. The insurance does not guarantee the market value of the insured obligation or the net asset value of the Fund’s shares. The credit rating of an insured obligation reflects the credit rating of the insurer, based on its claims-paying ability. The obligation of a municipal bond insurance company to pay a claim extends over the life of each insured obligation. Although defaults on insured municipal obligations have been low to date and municipal bond insurers have met their claims, there is no assurance this will continue. A higher than expected default rate could strain the insurer’s loss reserves and adversely affect its ability to pay claims to bondholders. Because a significant portion of insured municipal obligatio ns at in | |||||||||
| Risks of Municipal Lease Obligations (MLOs) and Certificates of Participation [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Risks of Municipal Lease Obligations (“MLOs”) and Certificates of Participation. MLOs and certificates of participation involve special risks not normally associated with general obligations or revenue bonds. MLOs are bonds that are secured by lease payments made by the party, typically a state or municipality, leasing the facilities (e.g., schools or office buildings) that were financed by the bond. Interest income from MLOs is generally exempt from local and state taxes in the state of issuance. MLOs, like other municipal debt obligations, are subject to the risk of non-payment. Although MLOs do not constitute general obligations of the issuer for which the issuer’s unlimited taxing power is pledged, the leasing state or municipality may be obligated to appropriate funds from its general tax revenues to make lease payments as long as it utilizes the leased property. Other lease payments may be subject to annual appropriation or may be made only from revenues associated with the facility financed. For example, certain lease obligations contain “non-appropriation” clauses, which provide that the issuer has no obligation to make lease or installment purchase payments in future years unless money is appropriated for such purpose on a yearly basis, which function to render constitutional and statutory requirements for the issuance of debt inapplicable to such obligations. In addition, such leases or contracts may be subject to temporary abatement of payments in the event the governmental issuer is prevented from maintaining occupancy of the leased premises or utilizing the leased equipment. Although “non-appropriation” lease obligations may be secured by the leased property, disposition of the property in the event of foreclosure might prove difficult, time consuming and costly, and may result in a delay in recovering or the failure to fully recover ownership of the assets. A certificate of participation (also referred to as a “participation”) in a municipal lease is an instrument evidencing a pro rata share in a specific pledged revenue stream, usually lease payments by the issuer that are typically subject to annual appropriation. The certificate generally entitles the holder to receive a share, or participation, in the payments from a particular project. Certificates of participation involve the same risks as the underlying municipal leases. In addition, the Fund may be dependent upon the municipal authority issuing the certificate of participation to exercise remedies with respect to an underlying lease. Certificates of participation also entail a risk of default or bankruptcy, both of the issuer of the municipal lease and also the municipal agency issuing the certificate of participation. MLOs and participations therein represent a type of financing that may not have the depth of marketability associated with more conventional securities and, as such, they may be less liquid than conventional securities. Certain MLOs may be deemed illiquid, unless determined by the Adviser, pursuant to guidelines adopted by the Board, to be liquid securities. The Adviser will consider the factors it believes are relevant to the marketability of the obligation, to the extent that information regarding such factor is available to the Adviser and pertinent to the liquidity determination, which may include: (1) the willingness of dealers to bid for the obligation; (2) the number of dealers willing to purchase or sell the obligation and the number of other potential buyers; (3) the frequency of trades and quotes for the obligation; (4) the nature of the marketplace trades, including the time needed to dispose of the obligation, the method of soliciting offers, and the mechanics of transfer; (5) the willingness of the governmental issuer to continue to appropriate funds for the payment of the obligation; (6) how likely or remote an event of non-appropriation may be, which depends in varying degrees on a variety of factors, including those relating to the general creditworthiness of the governmental issuer, its dependence on its continuing access to the credit markets, and the importance to the issuer of the equipment, property or facility covered by the lease or contract; (7) an assessment of the likelihood that the lease may or may not be cancelled; and (8) other factors and information unique to the obligation in determining its liquidity. The ability of issuers of MLOs to make timely lease payments may be adversely impacted in general economic downturns and as relative governmental cost burdens are allocated and reallocated among federal, state and local governmental units. Such non-payment would result in a reduction of income from and value of the obligation. Issuers of MLOs might seek protection under the bankruptcy laws. In the event of bankruptcy of such an issuer, holders of MLOs could experience delays and limitations with respect to the collection of principal and interest on such MLOs and may not, in all circumstances, be able to collect all principal and interest to which it is entitled. To enforce its rights in the event of a default in lease payments, the Fund might take possession of and manage the assets securing the issuer’s obligations on such securities or otherwise incur costs to protect its right, which may increase the Fund’s operating expenses and adversely affect the net asset value of the Fund. When the lease contains a non-appropriation clause, however, the failure to pay would not be a default and the Fund would not have the right to take possession of the assets. Any income derived from the Fund’s ownership or operation of such assets may not be tax-exempt. | |||||||||
| Call and Reinvestment Risks [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Call and Reinvestment Risks. If interest rates fall, it is possible that issuers of callable bonds with high interest coupons will “call” (or prepay) their bonds before their maturity date. If a call were exercised by the issuer during a period of declining interest rates, the Fund would likely replace such called security with a lower yielding security. If that were to happen, it could decrease the Fund’s dividends and possibly could affect the market price of common shares. Similar risks exist when the Fund invests the proceeds from matured or traded municipal obligations at market interest rates that are below the Fund’s current earnings rate. | |||||||||
| Credits Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Credit Risk. Investments in municipal obligations and other debt obligations (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 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. Municipal obligations may be insured as to principal and interest payments. If the claims-paying ability or other rating of the insurer is downgraded by a rating agency, the value of such obligations may be negatively affected. | |||||||||
| Lower Rated Investment Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Lower Rated Investment Risk. Investments rated below investment grade and comparable unrated investments (sometimes referred to as “junk”) are speculative because of the increased credit risk relative to other fixed income investments. 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. | |||||||||
| Unrated Securities Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Unrated Securities Risk. The Fund may invest in unrated obligations for which Eaton Vance will make a credit quality determination for purposes of the Fund’s credit quality policy. To the extent that the Fund invests in such unrated obligations, the Fund’s credit quality will be more dependent on Eaton Vance’s credit analysis than if the Fund invested in only rated obligations. Some unrated securities may not have an active trading market or may be difficult to value. | |||||||||
| Leverage Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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 also result from borrowings, issuance of preferred shares or participation in residual interest bond transactions. Leverage can increase both the risk and return potential of the Fund. 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 NAV 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 Fund’s portfolio securities. The loss on leveraged investments may substantially exceed the initial investment. | |||||||||
| Risk of Residual Interest Bonds [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Risk of Residual Interest Bonds. The Fund may enter into residual interest bond transactions, which expose the Fund to leverage and greater risk than an investment in a fixed-rate municipal bond. The interest payments that the Fund receives on the residual interest bonds acquired in such transactions vary inversely with short-term interest rates, normally decreasing when short-term rates increase. The value and market for residual interest bonds are volatile and such bonds may have limited liquidity. As required by applicable accounting standards, th e Fun r | |||||||||
| Restricted Securities Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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. | |||||||||
| Derivatives Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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 (or be unable to achieve) the return of collateral or other assets held by the counterparty. The loss on derivative transactions may substantially exceed the initial investment. A derivative investment also involves the risks relating to the reference instrument underlying the investment. | |||||||||
| Counterparty Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Counterparty Risk. Changes in the credit quality of the companies that serve as the Fund’s counterparties with respect to its derivatives positions and liquidity providers for the Fund’s residual interest bonds or other investments supported by another party’s credit will affect the value of those instruments. Certain entities that have served as counterparties in the municipals markets have recently incurred significant financial hardships, including bankruptcy and material loss of credit standing as a result of exposure to investments that have experienced defaults or otherwise suffered extreme credit deterioration. As a result, such hardships have reduced these entities’ capital and called into question their continued ability to perform their obligations. By using derivatives or other instruments that expose the Fund to counterparties, the Fund assumes the risk that its counterparties could experience future financial hardship. The counterparty risk for cleared derivatives is generally lower than for uncleared over-the-counter derivative transactions since generally a clearing organization becomes substituted for each counterparty to a cleared derivative contract and, in effect, guarantees the parties’ performance under the contract as each party to a trade looks only to the clearing organization for performance of financial obligations under the derivative contract. However, there can be no assurance that a clearing organization, or its members, will satisfy its obligations to the Fund. | |||||||||
| Hedging Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Hedging Risk. The Fund’s use of derivatives or other transactions to reduce risks involves costs and will be subject to Eaton Vance’s ability to predict correctly changes in the relationships of such hedge instruments to the Fund’s portfolio holdings or other factors. No assurance can be given that Eaton Vance’s judgment in this respect will be correct. In addition, no assurance can be given that the Fund will enter into hedging or other transactions at times or under circumstances in which it may be advisable to do so. Hedging transactions have risks, including the imperfect correlation between the value of such instruments and the underlying assets of the Fund, which creates the possibility that the loss on such instruments may be greater than the gain, if any, in the value of the underlying asset in the Fund’s portfolio; the limited availability of such instruments; the loss of principal; the possible default of the other party to the transaction; illiquidity of the derivative investments; and the imperfect correlation between the tax-exempt and taxable markets. Furthermore, the ability to successfully use hedging transactions depends on the Eaton Vance’s ability to predict pertinent market movements, which cannot be assured. Thus, the use of hedging transactions may result in losses greater than if they had not been used, may require the Fund to sell or purchase portfolio investments at inopportune times or for prices other than current market values, may limit the amount of appreciation the Fund can realize on an investment, or may cause the Fund to hold a security that it might otherwise sell. | |||||||||
| Swaps Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Swaps Risk. Swap agreements are two-party contracts entered into primarily by institutional investors for periods ranging from a few weeks to more than one year. In a standard “swap” transaction, two parties agree to exchange the returns (or differentials in rates of return) earned or realized on a particular predetermined reference instrument or instruments, which can be adjusted for an interest rate factor. The gross returns to be exchanged or “swapped” between the parties are generally calculated with respect to a “notional amount” (i.e., the return on or increase in value of a particular dollar amount invested at a particular interest rate or in a “basket” of securities representing a particular index). Other types of swap agreements may calculate the obligations of the parties to the agreement on a “net basis.” Consequently, a party’s current obligations (or rights) under a swap agreement will generally be equal only to the net amount to be paid or received under the agreement based on the relative values of the positions held by each party to the agreement (the “net amount”). Whether the use of swap agreements will be successful will depend on the Adviser’s ability to predict correctly whether certain types of reference instruments are likely to produce greater returns than other instruments. Swap agreements may be subject to contractual restrictions on transferability and termination and they may have terms of greater than seven days. The Fund’s obligations under a swap agreement will be accrued daily (offset against any amounts owed to the Fund under the swap). Developments in the swaps market, including potential government regulation, could adversely affect the Fund’s ability to terminate existing swap agreements or to realize amounts to be received under such agreements, as well as to participate in swap agreements in the future. If there is a default by the counterparty to a swap, the Fund will have contractual remedies pursuant to the swap agreement, but any recovery may be delayed depending on the circumstances of the default. | |||||||||
| Private Activity Bonds [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Private Activity Bonds. Private activity bonds, formerly referred to as industrial development bonds, are issued by or on behalf of public authorities to obtain funds to provide privately operated housing facilities, airport, mass transit or port facilities, sewage disposal, solid waste disposal or hazardous waste treatment or disposal facilities and certain local facilities for water supply, gas or electricity. Other types of private activity bonds, the proceeds of which are used for the construction, equipment, repair or improvement of privately operated industrial or commercial facilities, may constitute municipal obligations, although the current federal tax laws place substantial limitations on the size of such issues. Interest on certain “private activity bonds” issued after August 7, 1986 is exempt from regular federal income tax, but such interest (including a distribution by the Fund derived from such interest) is treated as a tax preference item which could subject the recipient to or increase the recipient’s liability for the AMT. | |||||||||
| Futures Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Futures Risk. Although some futures contracts call for making or taking delivery of the underlying reference instrument, generally these obligations are closed out prior to delivery by offsetting purchases or sales of matching futures contracts (same exchange, underlying security or index, and delivery month). Closing a futures contract sale is effected by purchasing a futures contract for the same aggregate amount of the specific type of financial instrument or commodity with the same delivery date. If an offsetting purchase price is less than the original sale price, the Fund realizes a capital gain, or if it is more, the Fund realizes a capital loss. Conversely, if an offsetting sale price is more than the original purchase price, the Fund realizes a capital gain, or if it is less, the Fund realizes a capital loss. The Adviser has claimed an exclusion from the definition of a Commodity Pool Operator (“CPO”) under the Commodity Exchange Act with respect to the Fund and therefore, neither the Adviser nor the Fund are subject to registration or regulation thereunder. | |||||||||
| Structured Notes Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Structured Notes Risk. Structured notes are derivative debt instruments, the interest rate or principal of which is determined by an unrelated indicator (for example, a currency, security, commodity or index thereof). Indexed securities may include a multiplier that multiplies the indexed element by a specified factor and, therefore, the value of such securities may be very volatile. The terms of structured notes and indexed securities may provide that in certain circumstances no principal is due at maturity, which may result in a loss of invested capital. Structured notes and indexed securities may be positively or negatively indexed, so that appreciation of the unrelated indicator may produce an increase or a decrease in the interest rate or the value of the structured note or indexed security at maturity may be calculated as a specified multiple of | |||||||||
| Inflation RiskDeflation Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Inflation Risk/Deflation Risk. Inflation risk is the risk that the value of assets or income from investment will be worth less in the future as inflation decreases the value of money. As inflation increases, the real value of the common shares and distributions thereon can decline. In addition, during periods of rising inflation, short-term interest rates and the Fund’s cost of leverage would likely increase, reducing returns to the common shareholders to the extent that such increased cost is not offset by commensurately higher income. Deflation risk is the risk that prices throughout the economy decline over time − the opposite of inflation. Deflation may have an adverse affect on the creditworthiness of issuers and may make issuer defaults more likely, which may result in a decline in the value of the Fund’s investments. | |||||||||
| Duration and Maturity Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Duration and Maturity Risk. Holding long duration and long maturity investments will expose the Fund to certain magnified risks. These risks include interest rate risk and credit risk as discussed above and liquidity risk as discussed below. | |||||||||
| Liquidity Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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. | |||||||||
| Current Regulatory Environment Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Current Regulatory Environment Risk. From time to time proposals have been introduced before Congress for the purpose of restricting or eliminating the federal income tax exemption for interest on certain types of municipal obligations, and it can be expected that similar proposals may be introduced in the future. Any proposed or actual changes in such rates or exempt status, therefore, can significantly affect the demand for and supply, liquidity and marketability of municipal obligations. This could in turn affect the Fund’s net asset value and ability to acquire and dispose of municipal obligations at desirable yield and price levels. Legislation may be enacted that could negatively affect the assets of th | |||||||||
| State Specific Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | State Specific Risk. The Fund has no current intention to invest 25% or more of its gross assets in municipal obligations of issuers located in the same state (or U.S. territory), but reserves the flexibility to do so in the future. If the Fund focuses its investments in any one state (or U.S. territory), the Fund may be more susceptible to adverse economic, political or regulatory occurrences affecting a particular state (or territory). Certain municipal bond issuers in Puerto Rico have recently experienced financial difficulties and rating agency downgrades, and two such issuers have defaulted on their payment obligations. | |||||||||
| Sector and Geographic Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Sector and Geographic Risk. Because the Fund may invest a significant portion of its assets in obligations issued in a particular state and/or U.S. territories and in certain types of municipal or other obligations and/or in certain sectors, the value of Fund shares may be affected by events that adversely affect that state, U.S. territory, sector or type of obligation and may fluctuate more than that of a fund that invests more broadly. General obligation bonds issued by municipalities are adversely affected by economic downturns and any resulting decline in tax revenues. | |||||||||
| Recent Market Conditions [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Recent Market Conditions. The outbreak of COVID-19 and efforts to contain its spread have resulted in closing borders, enhanced health screenings, changes to healthcare service preparation and delivery, quarantines, cancellations, disruptions to supply chains and customer activity, as well as general concern and uncertainty. The impact of this coronavirus, and the effects of other infectious illness outbreaks, epidemics or pandemics, may be short term or may continue for an extended period of time. Health crises caused by outbreaks of disease, such as the coronavirus outbreak, may exacerbate other pre-existing political, social and economic risks and disrupt normal market conditions and operations. For example, a global pandemic or other widespread health crisis could cause substantial market volatility and exchange trading suspensions and closures. In addition, the increasing interconnectedness of markets around the world may result in many markets being affected by events or conditions in a single country or region or events affecting a single or small number of issuers. The coronavirus outbreak and public and private sector responses thereto have led to large portions of the populations of many countries working from home for indefinite periods of time, temporary or permanent layoffs, disruptions in supply chains, and lack of availability of certain goods. The impact of such responses could adversely affect the information technology and operational systems upon which the Fund and the Fund’s service providers rely, and could otherwise disrupt the ability of the employees of the Fund’s service providers to perform critical tasks relating to the Fund. Any such impact could adversely affect the Fund’s performance, or the performance of the securities in which the Fund invests and may lead to losses on your investment in the Fund. | |||||||||
| Risks Associated with Active Management [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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 and there is no guarantee that such decisions will produce the desired results or expected returns. | |||||||||
| Tax Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Tax Risk. Income from tax-exempt municipal obligations could be declared taxable because of changes in tax laws, adverse interpretations by the relevant taxing authority or the non-compliant conduct of the issuer of an obligation. | |||||||||
| TaxSensitive Investing Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Tax-Sensitive Investing Risk. The Fund may hold a security in order to achieve more favorable tax-treatment or to sell a security in order to create tax losses. The Fund’s utilization of various tax-management techniques may be curtailed or eliminated by tax legislation, regulation or interpretations. The Fund may not be able to minimize taxable distributions to shareholders and a portion of the Fund’s distributions may be taxable. | |||||||||
| Cybersecurity Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Cybersecurity Risk. With the increased use of technologies by Fund service providers to conduct business, such as the Internet, the Fund is susceptible to operational, information security and related risks. In general, cyber incidents can result from deliberate attacks or unintentional events. Cybersecurity failures by or breaches of the Fund’s investment adviser or administrator and other service providers (including, but not limited to, the custodian or transfer agent), and the issuers of securities in which the Fund invests, may disrupt and otherwise adversely affect their business operations. This may result in financial losses to the Fund, impede Fund trading, interfere with the Fund’s ability to calculate its net asset value, interfere with Fund shareholders’ ability to transact business or cause violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, reimbursement or other compensation costs, or additional compliance costs. | |||||||||
| WhenIssued and DelayedDelivery Transactions Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | When-Issued and Delayed-Delivery Transactions Risk. Securities may be purchased on a “forward commitment,” “when-issued” or “delayed delivery” basis (meaning securities are purchased or sold with payment and delivery taking place in the future) in order to secure what is considered to be an advantageous price and yield at the time of entering into the transaction. When the Fund agrees to purchase such securities, it assumes the risk of any decline in value of the security from the date of the agreement to purchase. The Fund does not earn interest on the securities it has committed to purchase until they are paid for and delivered on the settlement date. From the time of entering into the transaction until delivery and payment is made at a later date, the securities that are the subject of the transaction are subject to market fluctuations. In forward commitment, when-issued or delayed delivery transactions, if the seller or buyer, as the case may be, fails to consummate the transaction the counterparty may miss the opportunity of obtaining a price or yield considered to be advantageous. However, no payment or delivery is made until payment is received or delivery is made from the other party to the transaction. Such transactions may be considered a form of leverage. | |||||||||
| Other Investment Companies and ETF Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Other Investment Companies and ETF Risk . The Fund may, subject to the limitations of the 1940 Act, invest in the securities of other investment companies. Such securities may be leveraged. As a result, the Fund may be indirectly exposed to leverage through an investment in such securities. Utilization of leverage is a speculative investment technique and involves certain risks. The Fund, as a holder of the securities of other investment companies, will bear its pro rata portion of the other investment companies’ expenses, including advisory fees. These expenses are in addition to the direct expenses of the Fund’s own operations. The Fund may invest in the securities of ETFs, to the extent permitted by law. ETFs are often designed to provide investment results that generally correspond to the price and yield performance of the component securities (or commodities) of the benchmark index. ETFs are listed on an exchange and trade in the secondary market on a per-share basis. The values of ETFs are subject to change as the values of their respective component securities (or commodities) fluctuate according to market volatility. Pooled investment vehicles, including ETFs, are subject to the risks of investing in the underlying securities or other instruments that they own. The market for common shares of ETFs, which are generally traded on an exchange and may be traded at a premium or discount to net asset value, is affected by the demand for those securities, regardless of the value of such ETF’s underlying securities. Additionally, natural or environmental disasters, widespread disease or other public health issues, war, acts of terrorism or other events could result in increased premiums or discounts to an ETF’s net asset value. Investments in ETFs may not exactly match the performance of a direct investment in the respective indices to which they are intended to correspond due to the temporary unavailability of certain index securities in the secondary market or other extraordinary circumstances, such as discrepancies with respect to the weighting of securities. Typically, the ETF bears its own operational expenses, which are deducted from its assets. To the extent that the Fund invests in ETFs, the Fund must bear these expenses in addition to the expenses of its own operation. | |||||||||
| Geopolitical Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Geopolitical Risk. The increasing interconnectivity between global economies and financial markets increases the likelihood that events or conditions in one region or financial market may adversely impact issuers in a different country, region or financial market. Securities in a Fund’s portfolio may underperform due to inflation (or expectations for inflation), interest rates, global demand for particular products or resources, natural disasters, health emergencies (such as epidemics and pandemics), terrorism, regulatory events and governmental or quasi-governmental actions. The occurrence of global events similar to those in recent years, such as terrorist attacks around the world, natural disasters, health emergencies, social and political discord, war or debt crises and downgrades, among others, may result in market volatility and may have long term effects on both the U.S. and global financial markets. Other financial, economic and other global market and social developments or disruptions may result in similar adverse circumstances, and it is difficult to predict when similar events affecting the U.S. or global financial markets may occur, the effects that such events may have and the duration of those effects (which may last for extended periods). Such global events may negatively impact broad segments of businesses and populations, cause a significant negative impact on the performance of the Fund’s investments, adversely affect and increase the volatility of the Fund’s share price and/or exacerbate preexisting political , | |||||||||
| Market Disruption [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Market Disruption. Global instability, war, geopolitical tensions and terrorist attacks in the United States and around the world have previously resulted, and may in the future result in market volatility and may have long-term effects on the United States and worldwide financial markets and may cause further economic uncertainties in the United States and worldwide. The Fund cannot predict the effects of significant future events on the global economy and securities markets. A similar disruption of the financial markets could impact interest rates, auctions, secondary trading, ratings, credit risk, inflation and other factors relating to the common shares. | |||||||||
| AntiTakeover Provisions [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Anti-Takeover Provisions. The Fund’s Agreement and Declaration of Fund (the “Declaration of Fund”) and Amended and Restated By-Laws include provisions that could have the effect of limiting the ability of other persons or entities to acquire control of the Fund or to change the composition of its Board. For example, pursuant to the Fund's Declaration of Fund, the Board is divided into three classes of Trustees with each class serving for a three-year term and certain types of transactions require the favorable vote of holders of at least 75% of the outstanding shares of the Fund. | |||||||||
| General Fund Investing Risks [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | 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. 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. | |||||||||
| Interest Rate Risk [Member] | ||||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Risk [Text Block] | Interest Rate Risk. In general, the value of debt instruments 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 the time-weighted expected cash flows of a fixed income security, while maturity refers to the amount of time until a fixed income security matures. 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. Because the Fund is managed toward an income objective, it may hold more longer-duration or maturity obligations and thereby be more exposed to interest rate risk than municipal income funds that are managed with a greater emphasis on total return. The impact of interest rate changes is significantly less for floating rate instruments that have relatively short periodic rate resets (e.g., ninety days or less). In a rising interest rate environment, the durations or maturities 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 were historically based on the London Interbank Offered Rate (“LIBOR”), which was the average offered rate for various maturities of short-term loans between certain major international banks. LIBOR historically was used throughout global banking and financial industries to determine interest rates for a variety of financial instruments (such as debt instruments and derivatives) and borrowing arrangements. Upon a determination by regulators to phase out the use of LIBOR, market participants have been transitioning to the use of alternative reference rates over the past few years. As of June 30, 2023, the administrator of LIBOR ceased publishing LIBOR settings. The impact of the transition away from LIBOR on certain debt securities, derivatives and other financial instruments that utilize LIBOR remains uncertain. The transition away from LIBOR and the use of replacement rates may adversely affect transactions that used LIBOR as | |||||||||
| Common Shares [Member] | ||||||||||
| Other Annual Expenses [Abstract] | ||||||||||
| Basis of Transaction Fees, Note [Text Block] | Percentage of net assets | |||||||||
| General Description of Registrant [Abstract] | ||||||||||
| Lowest Price or Bid | $ 8.7 | $ 9.54 | $ 9.7 | $ 9.78 | $ 9.37 | $ 10.43 | $ 10.95 | $ 12.05 | ||
| Highest Price or Bid | 9.6 | 10.18 | 10.2 | 10.82 | 10.86 | 12 | 12.79 | 13.99 | ||
| Lowest Price or Bid, NAV | 10.24 | 11.01 | 11.09 | 11.18 | 10.6 | 11.46 | 11.56 | 13.33 | ||
| Highest Price or Bid, NAV | $ 10.83 | $ 11.33 | $ 11.37 | $ 11.58 | $ 11.56 | $ 12.14 | $ 13.44 | $ 14.18 | ||
| Highest Price or Bid, Premium (Discount) to NAV [Percent] | (11.36%) | (10.15%) | (10.29%) | (6.56%) | (6.06%) | (1.15%) | (4.84%) | (1.34%) | ||
| Lowest Price or Bid, Premium (Discount) to NAV [Percent] | (15.04%) | (13.35%) | (12.53%) | (12.52%) | (11.60%) | (8.99%) | (5.28%) | (9.60%) | ||
| Capital Stock, Long-Term Debt, and Other Securities [Abstract] | ||||||||||
| Security Title [Text Block] | Common Shares | |||||||||
| Outstanding Security, Held [Shares] | 39,667,163 | |||||||||
| Offering Expenses [Member] | ||||||||||
| Other Annual Expenses [Abstract] | ||||||||||
| Basis of Transaction Fees, Note [Text Block] | as a percentage of offering price | |||||||||
| [1]If common shares are sold to or through underwriters, the Prospectus Supplement will set forth any applicable sales load.[2]You will be charged a $5.00 service charge and pay brokerage charges if you direct the plan agent to sell your common shares held in a dividend reinvestment account.[3]Eaton Vance Management (“EVM”) will pay the expenses of the offering (other than the applicable commissions); therefore, offering expenses are not included in the Summary of Fund Expenses. Offering expenses generally include, but are not limited to, the preparation, review and filing with the SEC of the Trust’s registration statement (including its current Prospectus Supplement, the accompanying Prospectus and Statement of Additional Information (“SAI”)), the preparation, review and filing of any associated marketing or similar materials, costs associated with the printing, mailing or other distribution of its current Prospectus Supplement, the accompanying Prospectus, SAI and/or marketing materials, associated filing fees, stock exchange listing fees, and legal and auditing fees associated with the offering.[4]Stated as a percentage of average net assets attributable to common shares for the year ended November 30, 2023.[5]The management fee paid by the Trust to EVM is based on the average daily gross assets of the Trust, including the principal amount of any indebtedness for money borrowed, the amount of any outstanding preferred shares issued by the Trust and, to a limited extent, the amount of floating-rate notes included as a liability in the Trust's Statement of Assets and Liabilities. Accordingly, if the Trust were to increase investment leverage in the future, the management fee will increase as a percentage of net assets.[6]Interest expense relates to the Trust’s liability with respect to floating-rate notes held by third parties in conjunction with investments in residual interest bonds. The Trust records offsetting interest income in an amount at least equal to this expense relating to the municipal obligations underlying such transactions. | ||||||||||
