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N-2 - USD ($) |  |  | 3 Months Ended | 12 Months Ended
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Oct. 05, 2022 | Jul. 31, 2022 | Jul. 31, 2022 | Apr. 30, 2022 | Jan. 31, 2022 | Oct. 31, 2021 | Jul. 31, 2021 | Apr. 30, 2021 | Jan. 31, 2021 | Oct. 31, 2020 | Jul. 31, 2020 | Jul. 31, 2022 | Apr. 30, 2022
Cover [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Entity Central Index Key |  | 0000901243 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment Flag |  | false |  |  |  |  |  |  |  |  |  |  |  |  |  |
Document Type |  | N-CSR |  |  |  |  |  |  |  |  |  |  |  |  |  |
Entity Registrant Name |  | BlackRock MuniAssets Fund, Inc. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fee Table [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shareholder Transaction Expenses [Table Text Block] |  |
|  | MUA |
Shareholder Transaction Expenses |  |  |  |
Maximum Sales Load (as a percentage of offering price) (a) |  |  | 1.00% |
Offering expenses borne by MUA (as a percentage of offering price) (a) |  |  | 0.01 |
Dividend Reinvestment Plan Fees |  |  | $0.02 per share for open market purchases of common shares (b) |
| (a) | If the common shares are sold to or through underwriters, the Prospectus Supplement will set forth any applicable sales load and the estimated offering expenses. Fund shareholders will pay all offering expenses involved with an offering.​​​​​​​ |
| (b) | Computershare Trust Company, N.A. (the “Reinvestment Plan Agent”) fees for the handling of the reinvestment of dividends will be paid by MUA. However, shareholders will pay a $0.02 per share fee incurred in connection with open-market purchases, which will be deducted from the value of the dividend. Shareholders will also be charged a $0.02 per share fee if a shareholder directs the Reinvestment Plan Agent to sell the common shares held in a dividend reinvestment account. Per share fees include any applicable brokerage commissions the Reinvestment Plan Agent is required to pay. |
Sales Load [Percent] | [1] | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Dividend Reinvestment and Cash Purchase Fees | [2] | $ 0.02 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Transaction Expenses [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Transaction Expenses [Percent] | [1] | 0.01% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual Expenses [Table Text Block] |  |
Estimated Annual Expenses (as a percentage of net assets attributable to common shares) |  |  |  |
Investment advisory fees |  |  | 0.77% |
Other expenses |  |  | 0.90 |
Miscellaneous |  |  | 0.11 |
Interest expense (c) |  |  | 0.79 |
Total annual expenses |  |  | 1.67 |
Fee waiver (d) |  |  | — |
Total annual fund operating expenses after fee waiver (d) |  |  | 1.67 |
| (c) | Assumes the use of leverage in the form of tender option bond transactions and preferred shares representing 31% of Managed Assets at an annual cost of leverage to MUA of 0.98%, which is based on current market conditions. The actual amount of interest expense borne by MUA will vary over time in accordance with the level of MUA’s use of tender option bond transactions and variations in market interest rates, as well as preferred shares transactions and changes to agreement terms with counterparties. Interest expense is required to be treated as an expense of MUA for accounting purposes. |
| (d) | MUA and the Manager have entered into a fee waiver agreement (the “Fee Waiver Agreement”), pursuant to which the Manager has contractually agreed to waive the investment advisory fees with respect to any portion of MUA’s assets attributable to investments in any equity and fixed-income mutual funds and ETFs managed by the Manager or its affiliates that have a contractual management fee, through June 30, 2024. In addition, pursuant to the Fee Waiver Agreement, the Manager has contractually agreed to waive its investment advisory fees by the amount of investment advisory fees MUA pays to the Manager indirectly through its investment in money market funds managed by the Manager or its affiliates, through June 30, 2024. The Fee Waiver Agreement may be terminated at any time, without the payment of any penalty, only by MUA (upon the vote of a majority of the Directors who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of MUA (the “Independent Directors”)) or a majority of the outstanding voting securities of MUA), upon 90 days’ written notice by MUA to the Manager.​​​​​​​ |
Management Fees [Percent] |  | 0.77% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expense 1 [Percent] |  | 0.11% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expense 2 [Percent] | [3] | 0.79% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Percent] |  | 0.90% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Annual Expenses [Percent] |  | 1.67% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Waivers and Reimbursements of Fees [Percent] | [4] | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Expense over Assets [Percent] | [4] | 1.67% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example [Table Text Block] |  | The following example illustrates MUA’s expenses (including the sales load of $10.00 and offering costs of $0.12) that shareholders would pay on a $1,000 investment in common shares, assuming (i) total net annual expenses of 1.67% of net assets attributable to common shares and (ii) a 5% annual return:
|  | 1 Year |  |  | 3 Years |  |  | 5 Years |  |  | 10 Years |
Total expenses incurred |  | $ | 27 |  |  | $ | 62 |  |  | $ | 100 |  |  | $ | 206 |
Expense Example, Year 01 |  | $ 27 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 3 |  | 62 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 5 |  | 100 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 10 |  | $ 206 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purpose of Fee Table , Note [Text Block] |  | The following t able and example are intended to assist shareholders in understanding the various costs and expenses directly or indirectly associated with investing in MUA’s common shares. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis of Transaction Fees, Note [Text Block] |  | as a percentage of offering price |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Expenses, Note [Text Block] |  | The example should not be considered a representation of future expenses. The example assumes that the estimated “Other expenses” set forth in the Estimated Annual Expenses table are accurate and that all dividends and distributions are reinvested at NAV. Actual expenses may be greater or less than those assumed. Moreover, MUA’s actual rate of return may be greater or less than the hypothetical 5% return shown in the example. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Management Fee not based on Net Assets, Note [Text Block] |  | If the common shares are sold to or through underwriters, the Prospectus Supplement will set forth any applicable sales load and the estimated offering expenses. Fund shareholders will pay all offering expenses involved with an offering. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquired Fund Total Annual Expenses, Note [Text Block] |  | MUA and the Manager have entered into a fee waiver agreement (the “Fee Waiver Agreement”), pursuant to which the Manager has contractually agreed to waive the investment advisory fees with respect to any portion of MUA’s assets attributable to investments in any equity and fixed-income mutual funds and ETFs managed by the Manager or its affiliates that have a contractual management fee, through June 30, 2024. In addition, pursuant to the Fee Waiver Agreement, the Manager has contractually agreed to waive its investment advisory fees by the amount of investment advisory fees MUA pays to the Manager indirectly through its investment in money market funds managed by the Manager or its affiliates, through June 30, 2024. The Fee Waiver Agreement may be terminated at any time, without the payment of any penalty, only by MUA (upon the vote of a majority of the Directors who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of MUA (the “Independent Directors”)) or a majority of the outstanding voting securities of MUA), upon 90 days’ written notice by MUA to the Manager. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities [Table Text Block] |  | The following table sets forth information regarding MUA’s outstanding senior securities as of the end of each of MUA’s last ten fiscal years, as applicable. MUA’s audited financial statements, including Deloitte & Touche LLP’s Report of Independent Registered Public Accounting Firm, and accompanying notes to financial statements, are included in this annual report.
Fiscal Year Ended |  |  | Total Amount Outstanding (000) |  |  |  | Asset Coverage |  |  |  | Liquidating Preference | (a) |  |  | Average Market Value (000) |  |  |  | Type of Senior Security |
July 31, 2022 |  | $ | 42,444 |  |  | $ | 16,471 | (b) |  | $ | N/A |  |  | $ | 37,166 | (c) |  |  | TOBs |
July 31, 2022 |  |  | 175,000 |  |  |  | 321,536 | (d) |  |  | 100,000 |  |  |  | N/A |  |  |  | VRDP Shares |
April 30, 2022 |  |  | 175,000 |  |  |  | 371,729 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VRDP Shares |
| (a) | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of MUA in preference to common shareholders, expressed as a dollar amount per preferred share. VRDP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value. |
| (b) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the amount of TOBs, and by multiplying the results by 1,000. |
| (c) | Represents weighted average daily market value of TOBs. |
| (d) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VRDP Shares, and by multiplying the results by 100,000. Effective July 18, 2022, TOB Trust Certificates are treated as senior securities pursuant to Rule 18f-4 of the 1940 Act. |
| (e) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares) from MUA’s total assets and dividing this by the liquidation value of the VRDP Shares, and by multiplying the results by 100,000. |
Senior Securities, Note [Text Block] |  | Senior Securities The following table sets forth information regarding MUA’s outstanding senior securities as of the end of each of MUA’s last ten fiscal years, as applicable. MUA’s audited financial statements, including Deloitte & Touche LLP’s Report of Independent Registered Public Accounting Firm, and accompanying notes to financial statements, are included in this annual report.
Fiscal Year Ended |  |  | Total Amount Outstanding (000) |  |  |  | Asset Coverage |  |  |  | Liquidating Preference | (a) |  |  | Average Market Value (000) |  |  |  | Type of Senior Security |
July 31, 2022 |  | $ | 42,444 |  |  | $ | 16,471 | (b) |  | $ | N/A |  |  | $ | 37,166 | (c) |  |  | TOBs |
July 31, 2022 |  |  | 175,000 |  |  |  | 321,536 | (d) |  |  | 100,000 |  |  |  | N/A |  |  |  | VRDP Shares |
April 30, 2022 |  |  | 175,000 |  |  |  | 371,729 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VRDP Shares |
| (a) | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of MUA in preference to common shareholders, expressed as a dollar amount per preferred share. VRDP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value. |
| (b) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the amount of TOBs, and by multiplying the results by 1,000. |
| (c) | Represents weighted average daily market value of TOBs. |
| (d) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VRDP Shares, and by multiplying the results by 100,000. Effective July 18, 2022, TOB Trust Certificates are treated as senior securities pursuant to Rule 18f-4 of the 1940 Act. |
| (e) | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares) from MUA’s total assets and dividing this by the liquidation value of the VRDP Shares, and by multiplying the results by 100,000. |
Senior Securities Headings, Note [Text Block] |  | The following table sets forth information regarding MUA’s outstanding senior securities as of the end of each of MUA’s last ten fiscal years, as applicable. MUA’s audited financial statements, including Deloitte & Touche LLP’s Report of Independent Registered Public Accounting Firm, and accompanying notes to financial statements, are included in this annual report. |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Objectives and Practices [Text Block] |  | BlackRock MuniAssets Fund, Inc.’s (MUA) (the “Fund”) investment objective is to provide high current income exempt from U.S. federal income taxes by investing primarily in a portfolio of medium- to lower-grade or unrated municipal obligations, the interest on which, in the opinion of bond counsel to the issuer, is exempt from U.S. federal income taxes. The Fund seeks to achieve its investment objective by investing, under normal market conditions, at least 80% of its assets in municipal bonds exempt from U.S. federal income taxes (except that the interest may be subject to the federal alternative minimum tax). The Fund invests at least 65% of its assets in municipal bonds that are rated in the medium to lower rating categories by nationally recognized rating services (for example, Baa or lower by Moody’s Investors Service, Inc. (“Moody’s”) or BBB or lower by S&P Global Ratings, or securities that are unrated but are deemed by the investment adviser to be of comparable quality at the time of investment. The Fund may invest directly in such securities or synthetically through the use of derivatives. The Fund’s investment objective is to provide high current income exempt from Federal income taxes by investing primarily in a portfolio of medium to lower grade or unrated municipal obligations the interest on which, in the opinion of bond counsel to the issuer, is exempt from Federal income taxes. The Fund seeks to achieve its investment objective by investing at least 80% of its assets, except during temporary defensive periods, in a portfolio of obligations issued by or on behalf of states, territories and possessions of the United States and their political subdivisions, agencies or instrumentalities paying interest which, in the opinion of bond counsel to the issuer, is exempt from Federal income taxes (“Municipal Bonds”). The Fund at all times, except during temporary defensive periods, will maintain at least 65% of its assets in Municipal Bonds which are rated in any one of the medium and lower rating categories of a nationally recognized statistical rating organization or are unrated. These ratings are currently Baa (Moody’s Investor Service Inc. (“Moody’s”)) or BBB (S&P Global Ratings (“S&P”) and Fitch Ratings, Inc. (“Fitch”)) or lower. These are fundamental policies of the Fund and, therefore, may not be changed without the approval of a majority of the Fund’s outstanding common stock and the outstanding preferred stock, including the Fund’s outstanding Variable Rate Demand Preferred Shares (the “VRDP Shares”), voting together as a single class, and of the holders of a majority of the outstanding preferred stock, including the VRDP Shares, voting as a separate class. A majority of the outstanding means (1) 67% or more of the shares present at a meeting, if the holders of more than 50% of the outstanding shares are present or represented by proxy, or (2) more than 50% of the outstanding shares, whichever is less. The Fund may invest directly in such securities or synthetically through the use of derivatives. The Fund is not intended as, and you should not construe it to be, a complete investment program. There can be no assurance that the Fund’s investment objective will be achieved or that the Fund’s investment program will be successful. The Fund has the authority to invest as much as 35% of its total assets in Municipal Bonds in the higher rating categories of nationally recognized statistical rating organizations (ratings of A or higher by Moody’s, S&P or Fitch or comparable unrated securities). In addition, the Fund reserves the right to temporarily invest more than 20% of its total assets in short-term municipal securities, or short-term taxable money market securities (including commercial paper, certificates of deposit and repurchase agreements) for defensive purposes when, in the opinion of BlackRock Advisors, LLC (the “Manager”), prevailing market or financial conditions warrant. The Fund does not invest more than 25% of its total assets (taken at market value) in Municipal Bonds whose issuers are located in the same state. “Total assets” of the Fund means the Fund’s net assets plus the amount of any borrowings for investment purposes. Ordinarily, the Fund does not intend to realize significant interest income that is subject to Federal income taxes. However, the Fund may invest all or a portion of its assets in certain tax-exempt securities classified as “private activity bonds” (“PABs”) (in general, bonds that benefit non-governmental entities) that may subject certain investors in the Fund to a Federal alternative minimum tax. The Fund may invest in securities not issued by or on behalf of a state or territory or by an agency or instrumentality thereof, if the Fund receives an opinion of counsel to the issuer that such securities pay interest that is excludable from gross income for federal income tax purposes (“Non-Municipal Tax-Exempt Securities”), which could include trust certificates, partnership interests or other instruments evidencing interest in one or more long-term Municipal Bonds. Non-Municipal Tax-Exempt Securities also may include securities issued by other investment companies that invest in Municipal Bonds, to the extent such investments are permitted by the Fund’s investment restrictions and applicable law. The Fund ordinarily does not intend to realize significant investment income not exempt from federal income taxes. From time to time, the Fund may realize taxable capital gains. Investments in lower rated Municipal Bonds generally provide a higher yield and are less affected by interest rate fluctuations than higher rated tax-exempt securities of similar maturity but are subject to greater overall market risk and are also subject to a greater degree of risk with r espect to the ability of the issuer to meet its principal and interest obligations. The Fund seeks to reduce risk through investing in multiple issuers, credit analysis and monitoring of current developments regarding the obligor and trends in both the economy and financial markets. The Manager will use various means to research the stability and/or potential for improvement of various municipal issuers in connection with the proposed purchase of their securities by the Fund. Evaluation of each Municipal Bond may include the analysis of financial performance, debt structure, economic factors and the administrative structure of the issuer. Additionally, the priority of liens and the overall structure of the particular issue may be factors that will determine suitability for purchase. Further investigation may be performed and may include, among other things, discussions with project management, corporate officers and industry experts as well as site inspections, area analysis, and project and financial projection analysis. All purchases and sales also may be subject to the review of market data, economic projections and the performance of the financial markets. Certain economic indicators also may be monitored. Additionally, the Manager will vary the average maturity of the Fund’s portfolio securities based upon its assessment of economic and market conditions.​​​​​​​​​​​​​​ Leverage: The Fund currently leverages its assets through the use of VRDP Shares and residual interest municipal tender option bonds (“TOB Residuals”), which are derivative interests in municipal bonds. The TOB Residuals in which the Fund will invest pay interest or income that, in the opinion of counsel to the issuer of such TOB Residuals, is exempt from regular U.S. federal income tax. The Fund currently does not intend to borrow money or issue debt securities. Although it has no present intention to do so, the Fund reserves the right to borrow money from banks or other financial institutions, or issue debt securities, in the future if it believes that market conditions would be conducive to the successful implementation of a leveraging strategy through borrowing money or issuing debt securities. Any such leveraging will not be fully achieved until the proceeds resulting from the use of leverage have been invested in accordance with the Fund’s investment objective and policies. The Fund may enter into derivative transactions that have economic leverage embedded in them. The Fund may also borrow money as a temporary measure for extraordinary or emergency purposes, including the payment of dividends and the settlement of securities transactions which otherwise might require untimely dispositions of Fund securities. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk Factors [Table Text Block] |  |
9. | PRINCIPAL RISKS
In the normal course of business, the Funds invest in securities or other instruments and may enter into certain transactions, and such activities subject each Fund to various risks, including among others, fluctuations in the market (market risk) or failure of an issuer to meet all of its obligations. The value of securities or other instruments may also be affected by various factors, including, without limitation: (i) the general economy; (ii) the overall market as well as local, regional or global political and/or social instability; (iii) regulation, taxation or international tax treaties between various countries; or (iv) currency, interest rate and price fluctuations. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues, recessions, or other events could have a significant impact on the Funds and their investments. The Funds may hold a significant amount of bonds subject to calls by the issuers at defined dates and prices. When bonds are called by issuers and the Funds reinvest the proceeds received, such investments may be in securities with lower yields than the bonds originally held, and correspondingly, could adversely impact the yield and total return performance of a Fund. A Fund structures and “sponsors” the TOB Trusts in which it holds TOB Residuals and has certain duties and responsibilities, which may give rise to certain additional risks including, but not limited to, compliance, securities law and operational risks. As short-term interest rates rise, the Funds’ investments in the TOB Trusts may adversely affect the Funds’ net investment income and dividends to Common Shareholders. Also, fluctuations in the market value of municipal bonds deposited into the TOB Trust may adversely affect the Funds’ NAVs per share. The U.S. Securities and Exchange Commission (“SEC”) and various federal banking and housing agencies have adopted credit risk retention rules for securitizations (the “Risk Retention Rules”). The Risk Retention Rules would require the sponsor of a TOB Trust to retain at least 5% of the credit risk of the underlying assets supporting the TOB Trust’s municipal bonds. The Risk Retention Rules may adversely affect the Funds’ ability to engage in TOB Trust transactions or increase the costs of such transactions in certain circumstances. TOB Trusts constitute an important component of the municipal bond market. Any modifications or changes to rules governing TOB Trusts may adversely impact the municipal market and the Funds, including through reduced demand for and liquidity of municipal bonds and increased financing costs for municipal issuers. The ultimate impact of any potential modifications on the TOB Trust market and the overall municipal market is not yet certain. Each Fund may invest without limitation in illiquid or less liquid investments or investments in which no secondary market is readily available or which are otherwise illiquid, including private placement securities. A Fund may not be able to readily dispose of such investments at prices that approximate those at which a Fund could sell such investments if they were more widely traded and, as a result of such illiquidity, a Fund may have to sell other investments or engage in borrowing transactions if necessary to raise funds to meet its obligations. Limited liquidity can also affect the market price of investments, thereby adversely affecting a Fund’s NAV and ability to make dividend distributions. Privately issued debt securities are often of below investment grade quality, frequently are unrated and present many of the same risks as investing in below investment grade public debt securities. Market Risk: Each Fund may be exposed to prepayment risk, which is the risk that borrowers may exercise their option to prepay principal earlier than scheduled during periods of declining interest rates, which would force each Fund to reinvest in lower yielding securities. Each Fund may also be exposed to reinvestment risk, which is the risk that income from each Fund’s portfolio will decline if each Fund invests the proceeds from matured, traded or called fixed-income securities at market interest rates that are below each Fund portfolio’s current earnings rate. Municipal securities are subject to the risk that litigation, legislation or other political events, local business or economic conditions, credit rating downgrades, or the bankruptcy of the issuer could have a significant effect on an issuer’s ability to make payments of principal and/or interest or otherwise affect the value of such securities. Municipal securities can be significantly affected by political or economic changes, including changes made in the law after issuance of the securities, as well as uncertainties in the municipal market related to, taxation, legislative changes or the rights of municipal security holders, including in connection with an issuer insolvency. Municipal securities backed by current or anticipated revenues from a specific project or specific assets can be negatively affected by the discontinuance of the tax benefits supporting the project or assets or the inability to collect revenues for the project or from the assets. Municipal securities may be less liquid than taxable bonds, and there may be less publicly available information on the financial condition of municipal security issuers than for issuers of other securities. An outbreak of respiratory disease caused by a novel coronavirus has developed into a global pandemic and has resulted in closing borders, quarantines, disruptions to supply chains and customer activity, as well as general concern and uncertainty. The impact of this pandemic, and other global health crises that may arise in the future, could affect the economies of many nations, individual companies and the market in general in ways that cannot necessarily be foreseen at the present time. This pandemic may result in substantial market volatility and may adversely impact the prices and liquidity of a fund’s investments. Although vaccines have been developed and approved for use by various governments, the duration of this pandemic and its effects cannot be determined with certainty. Counterparty Credit Risk: The Funds may be exposed to counterparty credit risk, or the risk that an entity may fail to or be unable to perform on its commitments related to unsettled or open transactions, including making timely interest and/or principal payments or otherwise honoring its obligations. The Funds manage counterparty credit risk by entering into transactions only with counterparties that the Manager believes have the financial resources to honor their obligations and by monitoring the financial stability of those counterparties. Financial assets, which potentially expose the Funds to market, issuer and counterparty credit risks, consist principally of financial instruments and receivables due from counterparties. The extent of the Funds’ exposure to market, issuer and counterparty credit risks with respect to these financial assets is approximately their value recorded in the Statements of Assets and Liabilities, less any collateral held by the Funds. A derivative contract may suffer a mark-to-market loss if the value of the contract decreases due to an unfavorable change in the market rates or values of the underlying instrument. Losses can also occur if the counterparty does not perform under the contract. With exchange-traded futures, there is less counterparty credit risk to the Funds since the exchange or clearinghouse, as counterparty to such instruments, guarantees against a possible default. The clearinghouse stands between the buyer and the seller of the contract; therefore, credit risk is limited to failure of the clearinghouse. While offset rights may exist under applicable law, a Fund does not have a contractual right of offset against a clearing broker or clearinghouse in the event of a default (including the bankruptcy or insolvency). Additionally, credit risk exists in exchange-traded futures with respect to initial and variation margin that is held in a clearing broker’s customer accounts. While clearing brokers are required to segregate customer margin from their own assets, in the event that a clearing broker becomes insolvent or goes into bankruptcy and at that time there is a shortfall in the aggregate amount of margin held by the clearing broker for all its clients, typically the shortfall would be allocated on a pro rata basis across all the clearing broker’s customers, potentially resulting in losses to the Funds. Concentration Risk: A diversified portfolio, where this is appropriate and consistent with a fund’s objectives, minimizes the risk that a price change of a particular investment will have a material impact on the NAV of a fund. The investment concentrations within each Fund’s portfolio are disclosed in its Schedule of Investments. The Funds invest a substantial amount of their assets in issuers located in a single state or limited number of states. When a Fund concentrates its investments in this manner, it assumes the risk that economic, regulatory, political or social conditions affecting that state or group of states could have a significant impact on the fund and could affect the income from, or the value or liquidity of, the fund’s portfolio. Investment percentages in specific states or U.S. territories are presented in the Schedules of Investments. Certain Funds invest a significant portion of their assets in securities within a single or limited number of market sectors. When a fund concentrates its investments in this manner, it assumes the risk that economic, regulatory, political and social conditions affecting such sectors may have a significant impact on the Fund and could affect the income from, or the value or liquidity of, the Fund’s portfolio. Investment percentages in specific sectors are presented in the Schedules of Investments. Certain Funds invest a significant portion of their assets in high yield securities. High yield securities that are rated below investment-grade (commonly referred to as “junk bonds”) or are unrated may be deemed speculative, involve greater levels of risk than higher-rated securities of similar maturity and are more likely to default. High yield securities may be issued by less creditworthy issuers, and issuers of high yield securities may be unable to meet their interest or principal payment obligations. High yield securities are subject to extreme price fluctuations, may be less liquid than higher rated fixed-income securities, even under normal economic conditions, and frequently have redemption features. The Funds invest a significant portion of their assets in fixed-income securities and/or use derivatives tied to the fixed-income markets. Changes in market interest rates or economic conditions may affect the value and/or liquidity of such investments. Interest rate risk is the risk that prices of bonds and other fixed-income securities will decrease as interest rates rise and increase as interest rates fall. The Funds may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. The Federal Reserve has recently begun to raise the federal funds rate as part of its efforts to address inflation. There is a risk that interest rates will continue to rise, which will likely drive down the prices of bonds and other fixed-income securities, and could negatively impact the Funds’ performance. LIBOR Transition Risk: The United Kingdom’s Financial Conduct Authority announced a phase out of the London Interbank Offered Rate (“LIBOR”). Although many LIBOR rates ceased to be published or no longer are representative of the underlying market they seek to measure after December 31, 2021, a selection of widely used USD LIBOR rates will continue to be published through June 2023 in order to assist with the transition. The Funds may be exposed to financial instruments tied to LIBOR to determine payment obligations, financing terms, hedging strategies or investment value. The transition process away from LIBOR might lead to increased volatility and illiquidity in markets for, and reduce the effectiveness of new hedges placed against instruments whose terms currently include LIBOR. The ultimate effect of the LIBOR transition process on the Funds is uncertain. Risk Factors This section contains a discussion of the general risks of investing in each Fund. The net asset value and market price of, and dividends paid on, the common shares will fluctuate with and be affected by, among other things, the risks more fully described below. As with any fund, there can be no guarantee that a Fund will meet its investment objective or that the Fund’s performance will be positive for any period of time. Each risk noted below is applicable to each Fund unless the specific Fund or Funds are noted in a parenthetical. Investment and Market Discount Risk: An investment in the Fund’s common shares is subject to investment risk, including the possible loss of the entire amount that you invest. As with any stock, the price of the Fund’s common shares will fluctuate with market conditions and other factors. If shares are sold, the price received may be more or less than the original investment. Common shares are designed for long-term investors and the Fund should not be treated as a trading vehicle. Shares of closed-end management investment companies frequently trade at a discount from their net asset value. This risk is separate and distinct from the risk that the Fund’s net asset value could decrease as a result of its investment activities. At any point in time an investment in the Fund’s common shares may be worth less than the original amount invested, even after taking into account distributions paid by the Fund. During periods in which the Fund may use leverage, the Fund’s investment, market discount and certain other risks will be magnified. Debt Securities Risk: Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
| • |  | Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise.
The Fund may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. Rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
| • |  | Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
| • |  | Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
| • |  | Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.
Municipal Securities Risks: Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:
| • |  | General Obligation Bonds Risks — Timely payments depend on the issuer’s credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
| • |  | Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
| • |  | Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its full faith, credit and taxing power for repayment.
| • |  | Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
| • |  | Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
| • |  | Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
| • |  | Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers’ bond counsel and, in the case of derivative securities, sponsors’ counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities
Taxability Risk: The Fund intends to minimize the payment of taxable income to shareholders by investing in tax-exempt or municipal securities in reliance at the time of purchase on an opinion of bond counsel to the issuer that the interest paid on those securities will be excludable from gross income for U.S. federal income tax purposes. Such securities, however, may be determined to pay, or have paid, taxable income subsequent to the Fund’s acquisition of the securities. In that event, the treatment of dividends previously paid or to be paid by the Fund as “exempt interest dividends” could be adversely affected, subjecting the Fund’s shareholders to increased U.S. federal income tax liabilities. Alternatively, the Fund might enter into an agreement with the IRS to pay an agreed upon amount in lieu of the IRS adjusting individual shareholders’ income tax liabilities. If the Fund agrees to enter into such an agreement, the Fund’s yield could be adversely affected. Further, shareholders at the time the Fund enters into such an agreement that were not shareholders when the dividends in question were paid would bear some cost for a benefit they did not receive. Federal tax legislation may limit the types and volume of bonds the interest on which qualifies for a federal income tax-exemption. As a result, current legislation and legislation that may be enacted in the future may affect the availability of municipal securities for investment by the Fund. In addition, future laws, regulations, rulings or court decisions may cause interest on municipal securities to be subject, directly or indirectly, to U.S. federal income taxation or interest on state municipal securities to be subject to state or local income taxation, or the value of state municipal securities to be subject to state or local intangible personal property tax, or may otherwise prevent the Fund from realizing the full current benefit of the tax-exempt status of such securities. Any such change could also affect the market price of such securities, and thus the value of an investment in the Fund. Insurance Risk: Insurance guarantees that interest payments on a municipal security will be made on time and that the principal will be repaid when the security matures. However, insurance does not protect against losses caused by declines in a municipal security’s value. The Fund cannot be certain that any insurance company will make the payments it guarantees. If a municipal security’s insurer fails to fulfill its obligations or loses its credit rating, the value of the security could drop Junk Bonds Risk: Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that are considered speculative and may cause income and principal losses for the Fund. Variable Rate Demand Obligations Risk (MUI, MYD and MQY): Variable rate demand obligations are floating rate securities that combine an interest in a long term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money. When-Issued and Delayed Delivery Securities and Forward Commitments Risk (BTA): When-issued and delayed delivery securities and forward commitments involve the risk that the security the Fund buys will lose value prior to its delivery. There also is the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Fund may lose both the investment opportunity for the assets it set aside to pay for the security and any gain in the security’s price. Defensive Investing Risk (BTA, MYD and MQY): For defensive purposes, the Fund may, as part of its proprietary volatility control process, allocate assets into cash or short-term fixed-income securities without limitation. In doing so, the Fund may succeed in avoiding losses but may otherwise fail to achieve its investment objective. Further, the value of short-term fixed-income securities may be affected by changing interest rates and by changes in credit ratings of the investments. If the Fund holds cash uninvested it will be subject to the credit risk of the depositary institution holding the cash. Repurchase Agreements and Purchase and Sale Contracts Risk (MUA and MYD): If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. Reverse Repurchase Agreements Risk (BTA and MYD): Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences for the Fund. In addition, reverse repurchase agreements involve the risk that the interest income earned in the investment of the proceeds will be less than the interest expense. Sector Risk (MUI): Sector risk is the risk that the Fund’s concentration in the securities of companies in a specific market sector or industry will cause the Fund to be more exposed to the price movements of companies in and developments affecting that sector than a more broadly diversified fund. To the extent that the Fund concentrates its investments in a particular sector, there is the risk that the Fund will perform poorly during a downturn in that sector. Leverage Risk: The Fund uses leverage for investment purposes through the issuance of VRDP Shares and investments in TOB Residuals. The Fund may also utilize leverage for investment purposes by entering into derivative instruments with leverage embedded in them and reverse repurchase agreements, as applicable. The Fund’s use of leverage may increase or decrease from time to time in its discretion and the Fund may, in the future, determine not to use leverage. The use of leverage creates an opportunity for increased common share net investment income dividends, but also creates risks for the holders of common shares. The Fund cannot assure you that the use of leverage will result in a higher yield on the common shares. Any leveraging strategy the Fund employs may not be successful. Leverage involves risks and special considerations for common shareholders, including:
| • |  | the likelihood of greater volatility of net asset value, market price and dividend rate of the common shares than a comparable portfolio without leverage;
| • |  | the risk that fluctuations in interest rates or dividend rates on any leverage that the Fund must pay will reduce the return to the common shareholders;
| • |  | the effect of leverage in a declining market, which is likely to cause a greater decline in the net asset value of the common shares than if the Fund were not leveraged, which may result in a greater decline in the market price of the common shares;
| • |  | leverage may increase operating costs, which may reduce total return.
Any decline in the net asset value of the Fund’s investments will be borne entirely by the holders of common shares. Therefore, if the market value of the Fund’s portfolio declines, leverage will result in a greater decrease in net asset value to the holders of common shares than if the Fund were not leveraged. This greater net asset value decrease will also tend to cause a greater decline in the market price for the common shares. Derivatives Risk: The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
| • |  | Leverage Risk — The Fund’s use of derivatives can magnify the Fund’s gains and losses. Relatively small market movements may result in large changes in the value of a derivatives position and can result in losses that greatly exceed the amount originally invested.
| • |  | Market Risk — Some derivatives are more sensitive to interest rate changes and market price fluctuations than other securities. The Fund could also suffer losses related to its derivatives positions as a result of unanticipated market movements, which losses are potentially unlimited. Finally, the Manager may not be able to predict correctly the direction of securities prices, interest rates and other economic factors, which could cause the Fund’s derivatives positions to lose value.
| • |  | Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will be unable or unwilling to fulfill its contractual obligation, and the related risks of having concentrated exposure to such a counterparty.
| • |  | Illiquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
| • |  | Operational Risk — The use of derivatives includes the risk of potential operational issues, including documentation issues, settlement issues, systems failures, inadequate controls and human error.
| • |  | Legal Risk — The risk of insufficient documentation, insufficient capacity or authority of counterparty, or legality or enforceability of a contract
| • |  | Volatility and Correlation Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
| • |  | Valuation Risk — Valuation for derivatives may not be readily available in the market. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
| • |  | Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
| • |  | Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
| • |  | Regulatory Risk — Derivative contracts are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (the “Dodd-Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, with respect to uncleared swaps, swap dealers are required to collect variation margin from the Fund and may be required by applicable regulations to collect initial margin from the Fund. Both initial and variation margin may be comprised of cash and/or securities, subject to applicable regulatory haircuts. Shares of investment companies (other than certain money market funds) may not be posted as collateral under applicable regulations. In addition, regulations adopted by global prudential regulators that are now in effect require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.
Indexed and Inverse Securities Risk (BTA, MYD and MQY): Indexed and inverse securities provide a potential return based on a particular index of value or interest rates. The Fund’s return on these securities will be subject to risk with respect to the value of the particular index. These securities are subject to leverage risk and correlation risk. Certain indexed and inverse securities have greater sensitivity to changes in interest rates or index levels than other securities, and the Fund’s investment in such instruments may decline significantly in value if interest rates or index levels move in a way Fund management does not anticipate. Tender Option Bonds Risk: The Fund’s participation in tender option bond transactions may reduce the Fund’s returns and/or increase volatility. Investments in tender option bond transactions expose the Fund to counterparty risk and leverage risk. An investment in a tender option bond transaction typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on TOB Residuals will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals generally will underperform the market for fixed rate municipal securities in a rising interest rate environment. The Fund may invest special purpose trusts formed for the purpose of holding municipal bonds contributed by one or more funds (“TOB Trusts”) on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis, it could suffer losses in excess of the value of its TOB Residuals.​​​​​​​ Illiquid Investments Risk: The Fund may invest without limitation in illiquid or less liquid investments or investments in which no secondary market is readily available or which are otherwise illiquid, including private placement securities. The Fund may not be able to readily dispose of such investments at prices that approximate those at which the Fund could sell such investments if they were more widely traded and, as a result of such illiquidity, the Fund may have to sell other investments or engage in borrowing transactions if necessary to raise cash to meet its obligations. Limited liquidity can also affect the market price of investments, thereby adversely affecting the Fund’s net asset value and ability to make dividend distributions. The financial markets in general, and certain segments of the mortgage-related securities markets in particular, have in recent years experienced periods of extreme secondary market supply and demand imbalance, resulting in a loss of liquidity during which market prices were suddenly and substantially below traditional measures of intrinsic value. During such periods, some investments could be sold only at arbitrary prices and with substantial losses. Periods of such market dislocation may occur again at any time. Privately issued debt securities are often of below investment grade quality, frequently are unrated and present many of the same risks as investing in below investment grade public debt securities. Investment Companies and ETFs Risk (BTA): Subject to the limitations set forth in the Investment Company Act of 1940, as amended, and the rules thereunder, the Fund may acquire shares in other investment companies and in exchange-traded funds (“ETFs”), some of which may be affiliated investment companies. The market value of the shares of other investment companies and ETFs may differ from their net asset value. As an investor in investment companies and ETFs, the Fund would bear its ratable share of that entity’s expenses, including its investment advisory and administration fees, while continuing to pay its own advisory and administration fees and other expenses (to the extent not offset by the Manager through waivers). As a result, shareholders will be absorbing duplicate levels of fees with respect to investments in other investment companies and ETFs (to the extent not offset by the Manager through waivers). The securities of other investment companies and ETFs in which the Fund may invest may be leveraged. As a result, the Fund may be indirectly exposed to leverage through an investment in such securities. An investment in securities of other investment companies and ETFs that use leverage may expose the Fund to higher volatility in the market value of such securities and the possibility that the Fund’s long-term returns on such securities (and, indirectly, the long-term returns of shares of the Fund) will be diminished. As with other investments, investments in other investment companies, including ETFs, are subject to market and selection risk. To the extent the Fund is held by an affiliated fund, the ability of the Fund itself to hold other investment companies may be limited. Preferred Securities Risk (BTA): Preferred securities may pay fixed or adjustable rates of return. Preferred securities are subject to issuer-specific and market risks applicable generally to equity securities. In addition, a company’s preferred securities generally pay dividends only after the company makes required payments to holders of its bonds and other debt. For this reason, the value of preferred securities will usually react more strongly than bonds and other debt to actual or perceived changes in the company’s financial condition or prospects. Preferred securities of smaller companies may be more vulnerable to adverse developments than preferred securities of larger companies. Market Risk and Selection Risk: Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. The value of a security or other asset may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security or other asset, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues like pandemics or epidemics, recessions, or other events could have a significant impact on the Fund and its investments. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. An outbreak of an infectious coronavirus (COVID-19) that was first detected in December 2019 developed into a global pandemic that has resulted in numerous disruptions in the market and has had significant economic impact leaving general concern and uncertainty. Although vaccines have been developed and approved for use by various governments, the duration of the pandemic and its effects cannot be predicted with certainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Effects of Leverage [Text Block] |  | Leverage: The Fund currently leverages its assets through the use of VRDP Shares and residual interest municipal tender option bonds (“TOB Residuals”), which are derivative interests in municipal bonds. The TOB Residuals in which the Fund will invest pay interest or income that, in the opinion of counsel to the issuer of such TOB Residuals, is exempt from regular U.S. federal income tax. The Fund currently does not intend to borrow money or issue debt securities. Although it has no present intention to do so, the Fund reserves the right to borrow money from banks or other financial institutions, or issue debt securities, in the future if it believes that market conditions would be conducive to the successful implementation of a leveraging strategy through borrowing money or issuing debt securities. Any such leveraging will not be fully achieved until the proceeds resulting from the use of leverage have been invested in accordance with the Fund’s investment objective and policies. The Fund may enter into derivative transactions that have economic leverage embedded in them. The Fund may also borrow money as a temporary measure for extraordinary or emergency purposes, including the payment of dividends and the settlement of securities transactions which otherwise might require untimely dispositions of Fund securities. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Price [Table Text Block] |  | The following table summarizes MUA’s highest and lowest daily closing market prices on the NYSE per common share, the NAV per common share, and the premium to or discount from NAV, on the date of each of the high and low market prices. The trading volume indicates the number of common shares traded on the NYSE during the respective quarters. Effective July 31, 2022, MUA changed its fiscal year end from April 30 to July 31.
|  | NYSE Market Price Per Common Share |  |  | NAV per Common Share on Date of Market Price |  |  | Premium/ (Discount) on Date of Market Price |  |  |  |
During Quarter Ended |  | High |  |  | Low |  |  | High |  |  | Low |  |  | High |  |  | Low |  |  | Trading Volume |
July 31, 2022 |  | $ | 12.57 |  |  | $ | 10.83 |  |  | $ | 12.21 |  |  | $ | 11.65 |  |  |  | 2.95 | % |  |  | (7.04 | )% |  |  | 6,751,266 |
April 30, 2022 |  |  | 14.44 |  |  |  | 11.70 |  |  |  | 14.36 |  |  |  | 12.75 |  |  |  | 0.56 |  |  |  | (8.24 | ) |  |  | 4,880,887 |
January 31, 2022 |  |  | 16.43 |  |  |  | 13.96 |  |  |  | 15.02 |  |  |  | 14.57 |  |  |  | 9.39 |  |  |  | (4.19 | ) |  |  | 3,487,411 |
October 31, 2021 |  |  | 16.77 |  |  |  | 14.76 |  |  |  | 15.08 |  |  |  | 14.83 |  |  |  | 11.21 |  |  |  | (0.47 | ) |  |  | 2,704,969 |
July 31, 2021 |  |  | 17.46 |  |  |  | 15.25 |  |  |  | 15.04 |  |  |  | 14.78 |  |  |  | 16.09 |  |  |  | 3.18 |  |  |  | 2,765,542 |
April 30, 2021 |  |  | 15.91 |  |  |  | 14.23 |  |  |  | 14.84 |  |  |  | 14.53 |  |  |  | 7.21 |  |  |  | (2.06 | ) |  |  | 3,335,653 |
January 31, 2021 |  |  | 15.28 |  |  |  | 13.55 |  |  |  | 14.66 |  |  |  | 13.92 |  |  |  | 4.23 |  |  |  | (2.66 | ) |  |  | 3,179,354 |
October 31, 2020 |  |  | 14.74 |  |  |  | 13.49 |  |  |  | 14.01 |  |  |  | 13.92 |  |  |  | 5.21 |  |  |  | (3.09 | ) |  |  | 2,844,590 |
July 31, 2020 |  |  | 14.27 |  |  |  | 12.58 |  |  |  | 14.00 |  |  |  | 12.91 |  |  |  | 1.93 |  |  |  | (2.56 | ) |  |  | 3,317,978 |
As of July 31, 2022, MUA’s market price, NAV per Common Share, and premium/(discount) to NAV per Common Share are $12.55, $12.53, and 0.16% respectively. Common shares of MUA have historically traded at both a premium and discount to NAV. Shares of closed-end funds frequently trade at a discount to their NAV. Because of this possibility and the recognition that any such discount may not be in the interest of shareholders, the Board might consider from time to time engaging in open-market repurchases, managed distribution plans, or other programs intended to reduce the discount. We cannot guarantee or assure, however, that the Board will decide to engage in any of these actions. Nor is there any guarantee or assurance that such actions, if undertaken, would result in the shares trading at a price equal or close to the NAV. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Market Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Market Risk: Each Fund may be exposed to prepayment risk, which is the risk that borrowers may exercise their option to prepay principal earlier than scheduled during periods of declining interest rates, which would force each Fund to reinvest in lower yielding securities. Each Fund may also be exposed to reinvestment risk, which is the risk that income from each Fund’s portfolio will decline if each Fund invests the proceeds from matured, traded or called fixed-income securities at market interest rates that are below each Fund portfolio’s current earnings rate. Municipal securities are subject to the risk that litigation, legislation or other political events, local business or economic conditions, credit rating downgrades, or the bankruptcy of the issuer could have a significant effect on an issuer’s ability to make payments of principal and/or interest or otherwise affect the value of such securities. Municipal securities can be significantly affected by political or economic changes, including changes made in the law after issuance of the securities, as well as uncertainties in the municipal market related to, taxation, legislative changes or the rights of municipal security holders, including in connection with an issuer insolvency. Municipal securities backed by current or anticipated revenues from a specific project or specific assets can be negatively affected by the discontinuance of the tax benefits supporting the project or assets or the inability to collect revenues for the project or from the assets. Municipal securities may be less liquid than taxable bonds, and there may be less publicly available information on the financial condition of municipal security issuers than for issuers of other securities. An outbreak of respiratory disease caused by a novel coronavirus has developed into a global pandemic and has resulted in closing borders, quarantines, disruptions to supply chains and customer activity, as well as general concern and uncertainty. The impact of this pandemic, and other global health crises that may arise in the future, could affect the economies of many nations, individual companies and the market in general in ways that cannot necessarily be foreseen at the present time. This pandemic may result in substantial market volatility and may adversely impact the prices and liquidity of a fund’s investments. Although vaccines have been developed and approved for use by various governments, the duration of this pandemic and its effects cannot be determined with certainty. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Counterparty Credit Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Counterparty Credit Risk: The Funds may be exposed to counterparty credit risk, or the risk that an entity may fail to or be unable to perform on its commitments related to unsettled or open transactions, including making timely interest and/or principal payments or otherwise honoring its obligations. The Funds manage counterparty credit risk by entering into transactions only with counterparties that the Manager believes have the financial resources to honor their obligations and by monitoring the financial stability of those counterparties. Financial assets, which potentially expose the Funds to market, issuer and counterparty credit risks, consist principally of financial instruments and receivables due from counterparties. The extent of the Funds’ exposure to market, issuer and counterparty credit risks with respect to these financial assets is approximately their value recorded in the Statements of Assets and Liabilities, less any collateral held by the Funds. A derivative contract may suffer a mark-to-market loss if the value of the contract decreases due to an unfavorable change in the market rates or values of the underlying instrument. Losses can also occur if the counterparty does not perform under the contract. With exchange-traded futures, there is less counterparty credit risk to the Funds since the exchange or clearinghouse, as counterparty to such instruments, guarantees against a possible default. The clearinghouse stands between the buyer and the seller of the contract; therefore, credit risk is limited to failure of the clearinghouse. While offset rights may exist under applicable law, a Fund does not have a contractual right of offset against a clearing broker or clearinghouse in the event of a default (including the bankruptcy or insolvency). Additionally, credit risk exists in exchange-traded futures with respect to initial and variation margin that is held in a clearing broker’s customer accounts. While clearing brokers are required to segregate customer margin from their own assets, in the event that a clearing broker becomes insolvent or goes into bankruptcy and at that time there is a shortfall in the aggregate amount of margin held by the clearing broker for all its clients, typically the shortfall would be allocated on a pro rata basis across all the clearing broker’s customers, potentially resulting in losses to the Funds. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Concentration Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Concentration Risk: A diversified portfolio, where this is appropriate and consistent with a fund’s objectives, minimizes the risk that a price change of a particular investment will have a material impact on the NAV of a fund. The investment concentrations within each Fund’s portfolio are disclosed in its Schedule of Investments. The Funds invest a substantial amount of their assets in issuers located in a single state or limited number of states. When a Fund concentrates its investments in this manner, it assumes the risk that economic, regulatory, political or social conditions affecting that state or group of states could have a significant impact on the fund and could affect the income from, or the value or liquidity of, the fund’s portfolio. Investment percentages in specific states or U.S. territories are presented in the Schedules of Investments. Certain Funds invest a significant portion of their assets in securities within a single or limited number of market sectors. When a fund concentrates its investments in this manner, it assumes the risk that economic, regulatory, political and social conditions affecting such sectors may have a significant impact on the Fund and could affect the income from, or the value or liquidity of, the Fund’s portfolio. Investment percentages in specific sectors are presented in the Schedules of Investments. Certain Funds invest a significant portion of their assets in high yield securities. High yield securities that are rated below investment-grade (commonly referred to as “junk bonds”) or are unrated may be deemed speculative, involve greater levels of risk than higher-rated securities of similar maturity and are more likely to default. High yield securities may be issued by less creditworthy issuers, and issuers of high yield securities may be unable to meet their interest or principal payment obligations. High yield securities are subject to extreme price fluctuations, may be less liquid than higher rated fixed-income securities, even under normal economic conditions, and frequently have redemption features. The Funds invest a significant portion of their assets in fixed-income securities and/or use derivatives tied to the fixed-income markets. Changes in market interest rates or economic conditions may affect the value and/or liquidity of such investments. Interest rate risk is the risk that prices of bonds and other fixed-income securities will decrease as interest rates rise and increase as interest rates fall. The Funds may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. The Federal Reserve has recently begun to raise the federal funds rate as part of its efforts to address inflation. There is a risk that interest rates will continue to rise, which will likely drive down the prices of bonds and other fixed-income securities, and could negatively impact the Funds’ performance. |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR Transition Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | LIBOR Transition Risk: The United Kingdom’s Financial Conduct Authority announced a phase out of the London Interbank Offered Rate (“LIBOR”). Although many LIBOR rates ceased to be published or no longer are representative of the underlying market they seek to measure after December 31, 2021, a selection of widely used USD LIBOR rates will continue to be published through June 2023 in order to assist with the transition. The Funds may be exposed to financial instruments tied to LIBOR to determine payment obligations, financing terms, hedging strategies or investment value. The transition process away from LIBOR might lead to increased volatility and illiquidity in markets for, and reduce the effectiveness of new hedges placed against instruments whose terms currently include LIBOR. The ultimate effect of the LIBOR transition process on the Funds is uncertain. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment and Market Discount Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Investment and Market Discount Risk: An investment in the Fund’s common shares is subject to investment risk, including the possible loss of the entire amount that you invest. As with any stock, the price of the Fund’s common shares will fluctuate with market conditions and other factors. If shares are sold, the price received may be more or less than the original investment. Common shares are designed for long-term investors and the Fund should not be treated as a trading vehicle. Shares of closed-end management investment companies frequently trade at a discount from their net asset value. This risk is separate and distinct from the risk that the Fund’s net asset value could decrease as a result of its investment activities. At any point in time an investment in the Fund’s common shares may be worth less than the original amount invested, even after taking into account distributions paid by the Fund. During periods in which the Fund may use leverage, the Fund’s investment, market discount and certain other risks will be magnified. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Securities Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Debt Securities Risk: Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
| • |  | Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise.
The Fund may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. Rates on certain floating rate debt securities typically reset only periodically, changes in prevailing interest rates (and particularly sudden and significant changes) can be expected to cause some fluctuations in the net asset value of the Fund to the extent that it invests in floating rate debt securities. These basic principles of bond prices also apply to U.S. Government securities. A security backed by the “full faith and credit” of the U.S. Government is guaranteed only as to its stated interest rate and face value at maturity, not its current market price. Just like other fixed-income securities, government-guaranteed securities will fluctuate in value when interest rates change. A general rise in interest rates has the potential to cause investors to move out of fixed-income securities on a large scale, which may increase redemptions from funds that hold large amounts of fixed-income securities. Heavy redemptions could cause the Fund to sell assets at inopportune times or at a loss or depressed value and could hurt the Fund’s performance.
| • |  | Credit Risk — Credit risk refers to the possibility that the issuer of a debt security (i.e., the borrower) will not be able to make payments of interest and principal when due. Changes in an issuer’s credit rating or the market’s perception of an issuer’s creditworthiness may also affect the value of the Fund’s investment in that issuer. The degree of credit risk depends on both the financial condition of the issuer and the terms of the obligation.
| • |  | Extension Risk — When interest rates rise, certain obligations will be paid off by the obligor more slowly than anticipated, causing the value of these obligations to fall.
| • |  | Prepayment Risk — When interest rates fall, certain obligations will be paid off by the obligor more quickly than originally anticipated, and the Fund may have to invest the proceeds in securities with lower yields.
Municipal Securities Risks [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Municipal Securities Risks: Municipal securities risks include the ability of the issuer to repay the obligation, the relative lack of information about certain issuers of municipal securities, and the possibility of future legislative changes which could affect the market for and value of municipal securities. These risks include:
| • |  | General Obligation Bonds Risks — Timely payments depend on the issuer’s credit quality, ability to raise tax revenues and ability to maintain an adequate tax base.
| • |  | Revenue Bonds Risks — These payments depend on the money earned by the particular facility or class of facilities, or the amount of revenues derived from another source.
| • |  | Private Activity Bonds Risks — Municipalities and other public authorities issue private activity bonds to finance development of industrial facilities for use by a private enterprise. The private enterprise pays the principal and interest on the bond, and the issuer does not pledge its full faith, credit and taxing power for repayment.
| • |  | Moral Obligation Bonds Risks — Moral obligation bonds are generally issued by special purpose public authorities of a state or municipality. If the issuer is unable to meet its obligations, repayment of these bonds becomes a moral commitment, but not a legal obligation, of the state or municipality.
| • |  | Municipal Notes Risks — Municipal notes are shorter term municipal debt obligations. If there is a shortfall in the anticipated proceeds, the notes may not be fully repaid and the Fund may lose money.
| • |  | Municipal Lease Obligations Risks — In a municipal lease obligation, the issuer agrees to make payments when due on the lease obligation. Although the issuer does not pledge its unlimited taxing power for payment of the lease obligation, the lease obligation is secured by the leased property.
| • |  | Tax-Exempt Status Risk — The Fund and its investment manager will rely on the opinion of issuers’ bond counsel and, in the case of derivative securities, sponsors’ counsel, on the tax-exempt status of interest on municipal bonds and payments under derivative securities. Neither the Fund nor its investment manager will independently review the bases for those tax opinions, which may ultimately be determined to be incorrect and subject the Fund and its shareholders to substantial tax liabilities
Taxability Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Taxability Risk: The Fund intends to minimize the payment of taxable income to shareholders by investing in tax-exempt or municipal securities in reliance at the time of purchase on an opinion of bond counsel to the issuer that the interest paid on those securities will be excludable from gross income for U.S. federal income tax purposes. Such securities, however, may be determined to pay, or have paid, taxable income subsequent to the Fund’s acquisition of the securities. In that event, the treatment of dividends previously paid or to be paid by the Fund as “exempt interest dividends” could be adversely affected, subjecting the Fund’s shareholders to increased U.S. federal income tax liabilities. Alternatively, the Fund might enter into an agreement with the IRS to pay an agreed upon amount in lieu of the IRS adjusting individual shareholders’ income tax liabilities. If the Fund agrees to enter into such an agreement, the Fund’s yield could be adversely affected. Further, shareholders at the time the Fund enters into such an agreement that were not shareholders when the dividends in question were paid would bear some cost for a benefit they did not receive. Federal tax legislation may limit the types and volume of bonds the interest on which qualifies for a federal income tax-exemption. As a result, current legislation and legislation that may be enacted in the future may affect the availability of municipal securities for investment by the Fund. In addition, future laws, regulations, rulings or court decisions may cause interest on municipal securities to be subject, directly or indirectly, to U.S. federal income taxation or interest on state municipal securities to be subject to state or local income taxation, or the value of state municipal securities to be subject to state or local intangible personal property tax, or may otherwise prevent the Fund from realizing the full current benefit of the tax-exempt status of such securities. Any such change could also affect the market price of such securities, and thus the value of an investment in the Fund. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Insurance Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Insurance Risk: Insurance guarantees that interest payments on a municipal security will be made on time and that the principal will be repaid when the security matures. However, insurance does not protect against losses caused by declines in a municipal security’s value. The Fund cannot be certain that any insurance company will make the payments it guarantees. If a municipal security’s insurer fails to fulfill its obligations or loses its credit rating, the value of the security could drop |  |  |  |  |  |  |  |  |  |  |  |  |  |
Junk Bonds Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Junk Bonds Risk: Although junk bonds generally pay higher rates of interest than investment grade bonds, junk bonds are high risk investments that are considered speculative and may cause income and principal losses for the Fund. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Variable Rate Demand Obligations Risk MUI, MYD and MQY [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Variable Rate Demand Obligations Risk (MUI, MYD and MQY): Variable rate demand obligations are floating rate securities that combine an interest in a long term municipal bond with a right to demand payment before maturity from a bank or other financial institution. If the bank or financial institution is unable to pay, the Fund may lose money. |  |  |  |  |  |  |  |  |  |  |  |  |  |
When Issued and Delayed Delivery Securities and Forward Commitments Risk (BTA) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | When-Issued and Delayed Delivery Securities and Forward Commitments Risk (BTA): When-issued and delayed delivery securities and forward commitments involve the risk that the security the Fund buys will lose value prior to its delivery. There also is the risk that the security will not be issued or that the other party to the transaction will not meet its obligation. If this occurs, the Fund may lose both the investment opportunity for the assets it set aside to pay for the security and any gain in the security’s price. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Defensive Investing Risk (BTA, MYD and MQY) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Defensive Investing Risk (BTA, MYD and MQY): For defensive purposes, the Fund may, as part of its proprietary volatility control process, allocate assets into cash or short-term fixed-income securities without limitation. In doing so, the Fund may succeed in avoiding losses but may otherwise fail to achieve its investment objective. Further, the value of short-term fixed-income securities may be affected by changing interest rates and by changes in credit ratings of the investments. If the Fund holds cash uninvested it will be subject to the credit risk of the depositary institution holding the cash. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repurchase Agreements and Purchase and Sale Contracts Risk (MUA and MYD [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Repurchase Agreements and Purchase and Sale Contracts Risk (MUA and MYD): If the other party to a repurchase agreement or purchase and sale contract defaults on its obligation under the agreement, the Fund may suffer delays and incur costs or lose money in exercising its rights under the agreement. If the seller fails to repurchase the security in either situation and the market value of the security declines, the Fund may lose money. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reverse Repurchase Agreements Risk (BTA and MYD) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Reverse Repurchase Agreements Risk (BTA and MYD): Reverse repurchase agreements involve the sale of securities held by the Fund with an agreement to repurchase the securities at an agreed-upon price, date and interest payment. Reverse repurchase agreements involve the risk that the other party may fail to return the securities in a timely manner or at all. The Fund could lose money if it is unable to recover the securities and the value of the collateral held by the Fund, including the value of the investments made with cash collateral, is less than the value of the securities. These events could also trigger adverse tax consequences for the Fund. In addition, reverse repurchase agreements involve the risk that the interest income earned in the investment of the proceeds will be less than the interest expense. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Sector Risk (MUI) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Sector Risk (MUI): Sector risk is the risk that the Fund’s concentration in the securities of companies in a specific market sector or industry will cause the Fund to be more exposed to the price movements of companies in and developments affecting that sector than a more broadly diversified fund. To the extent that the Fund concentrates its investments in a particular sector, there is the risk that the Fund will perform poorly during a downturn in that sector. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leverage Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Leverage Risk: The Fund uses leverage for investment purposes through the issuance of VRDP Shares and investments in TOB Residuals. The Fund may also utilize leverage for investment purposes by entering into derivative instruments with leverage embedded in them and reverse repurchase agreements, as applicable. The Fund’s use of leverage may increase or decrease from time to time in its discretion and the Fund may, in the future, determine not to use leverage. The use of leverage creates an opportunity for increased common share net investment income dividends, but also creates risks for the holders of common shares. The Fund cannot assure you that the use of leverage will result in a higher yield on the common shares. Any leveraging strategy the Fund employs may not be successful. Leverage involves risks and special considerations for common shareholders, including:
| • |  | the likelihood of greater volatility of net asset value, market price and dividend rate of the common shares than a comparable portfolio without leverage;
| • |  | the risk that fluctuations in interest rates or dividend rates on any leverage that the Fund must pay will reduce the return to the common shareholders;
| • |  | the effect of leverage in a declining market, which is likely to cause a greater decline in the net asset value of the common shares than if the Fund were not leveraged, which may result in a greater decline in the market price of the common shares;
| • |  | leverage may increase operating costs, which may reduce total return.
Any decline in the net asset value of the Fund’s investments will be borne entirely by the holders of common shares. Therefore, if the market value of the Fund’s portfolio declines, leverage will result in a greater decrease in net asset value to the holders of common shares than if the Fund were not leveraged. This greater net asset value decrease will also tend to cause a greater decline in the market price for the common shares. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivatives Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Derivatives Risk: The Fund’s use of derivatives may increase its costs, reduce the Fund’s returns and/or increase volatility. Derivatives involve significant risks, including:
| • |  | Leverage Risk — The Fund’s use of derivatives can magnify the Fund’s gains and losses. Relatively small market movements may result in large changes in the value of a derivatives position and can result in losses that greatly exceed the amount originally invested.
| • |  | Market Risk — Some derivatives are more sensitive to interest rate changes and market price fluctuations than other securities. The Fund could also suffer losses related to its derivatives positions as a result of unanticipated market movements, which losses are potentially unlimited. Finally, the Manager may not be able to predict correctly the direction of securities prices, interest rates and other economic factors, which could cause the Fund’s derivatives positions to lose value.
| • |  | Counterparty Risk — Derivatives are also subject to counterparty risk, which is the risk that the other party in the transaction will be unable or unwilling to fulfill its contractual obligation, and the related risks of having concentrated exposure to such a counterparty.
| • |  | Illiquidity Risk — The possible lack of a liquid secondary market for derivatives and the resulting inability of the Fund to sell or otherwise close a derivatives position could expose the Fund to losses and could make derivatives more difficult for the Fund to value accurately.
| • |  | Operational Risk — The use of derivatives includes the risk of potential operational issues, including documentation issues, settlement issues, systems failures, inadequate controls and human error.
| • |  | Legal Risk — The risk of insufficient documentation, insufficient capacity or authority of counterparty, or legality or enforceability of a contract
| • |  | Volatility and Correlation Risk — Volatility is defined as the characteristic of a security, an index or a market to fluctuate significantly in price within a short time period. A risk of the Fund’s use of derivatives is that the fluctuations in their values may not correlate with the overall securities markets.
| • |  | Valuation Risk — Valuation for derivatives may not be readily available in the market. Valuation may be more difficult in times of market turmoil since many investors and market makers may be reluctant to purchase complex instruments or quote prices for them.
| • |  | Hedging Risk — Hedges are sometimes subject to imperfect matching between the derivative and the underlying security, and there can be no assurance that the Fund’s hedging transactions will be effective. The use of hedging may result in certain adverse tax consequences.
| • |  | Tax Risk — Certain aspects of the tax treatment of derivative instruments, including swap agreements and commodity-linked derivative instruments, are currently unclear and may be affected by changes in legislation, regulations or other legally binding authority. Such treatment may be less favorable than that given to a direct investment in an underlying asset and may adversely affect the timing, character and amount of income the Fund realizes from its investments.
| • |  | Regulatory Risk — Derivative contracts are subject to regulation under the Dodd-Frank Wall Street Reform and Consumer Protection Act (the “Dodd-Frank Act”) in the United States and under comparable regimes in Europe, Asia and other non-U.S. jurisdictions. Under the Dodd-Frank Act, with respect to uncleared swaps, swap dealers are required to collect variation margin from the Fund and may be required by applicable regulations to collect initial margin from the Fund. Both initial and variation margin may be comprised of cash and/or securities, subject to applicable regulatory haircuts. Shares of investment companies (other than certain money market funds) may not be posted as collateral under applicable regulations. In addition, regulations adopted by global prudential regulators that are now in effect require certain bank-regulated counterparties and certain of their affiliates to include in certain financial contracts, including many derivatives contracts, terms that delay or restrict the rights of counterparties, such as the Fund, to terminate such contracts, foreclose upon collateral, exercise other default rights or restrict transfers of credit support in the event that the counterparty and/or its affiliates are subject to certain types of resolution or insolvency proceedings. The implementation of these requirements with respect to derivatives, as well as regulations under the Dodd-Frank Act regarding clearing, mandatory trading and margining of other derivatives, may increase the costs and risks to the Fund of trading in these instruments and, as a result, may affect returns to investors in the Fund.
Indexed and Inverse Securities Risk (BTA, MYD and MQY) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Indexed and Inverse Securities Risk (BTA, MYD and MQY): Indexed and inverse securities provide a potential return based on a particular index of value or interest rates. The Fund’s return on these securities will be subject to risk with respect to the value of the particular index. These securities are subject to leverage risk and correlation risk. Certain indexed and inverse securities have greater sensitivity to changes in interest rates or index levels than other securities, and the Fund’s investment in such instruments may decline significantly in value if interest rates or index levels move in a way Fund management does not anticipate. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tender Option Bonds Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Tender Option Bonds Risk: The Fund’s participation in tender option bond transactions may reduce the Fund’s returns and/or increase volatility. Investments in tender option bond transactions expose the Fund to counterparty risk and leverage risk. An investment in a tender option bond transaction typically will involve greater risk than an investment in a municipal fixed rate security, including the risk of loss of principal. Distributions on TOB Residuals will bear an inverse relationship to short-term municipal security interest rates. Distributions on TOB Residuals paid to the Fund will be reduced or, in the extreme, eliminated as short-term municipal interest rates rise and will increase when short-term municipal interest rates fall. TOB Residuals generally will underperform the market for fixed rate municipal securities in a rising interest rate environment. The Fund may invest special purpose trusts formed for the purpose of holding municipal bonds contributed by one or more funds (“TOB Trusts”) on either a non-recourse or recourse basis. If the Fund invests in a TOB Trust on a recourse basis, it could suffer losses in excess of the value of its TOB Residuals.​​​​​​​ |  |  |  |  |  |  |  |  |  |  |  |  |  |
Illiquid Investments Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Illiquid Investments Risk: The Fund may invest without limitation in illiquid or less liquid investments or investments in which no secondary market is readily available or which are otherwise illiquid, including private placement securities. The Fund may not be able to readily dispose of such investments at prices that approximate those at which the Fund could sell such investments if they were more widely traded and, as a result of such illiquidity, the Fund may have to sell other investments or engage in borrowing transactions if necessary to raise cash to meet its obligations. Limited liquidity can also affect the market price of investments, thereby adversely affecting the Fund’s net asset value and ability to make dividend distributions. The financial markets in general, and certain segments of the mortgage-related securities markets in particular, have in recent years experienced periods of extreme secondary market supply and demand imbalance, resulting in a loss of liquidity during which market prices were suddenly and substantially below traditional measures of intrinsic value. During such periods, some investments could be sold only at arbitrary prices and with substantial losses. Periods of such market dislocation may occur again at any time. Privately issued debt securities are often of below investment grade quality, frequently are unrated and present many of the same risks as investing in below investment grade public debt securities. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Investment Companies and ETFs Risk (BTA) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Investment Companies and ETFs Risk (BTA): Subject to the limitations set forth in the Investment Company Act of 1940, as amended, and the rules thereunder, the Fund may acquire shares in other investment companies and in exchange-traded funds (“ETFs”), some of which may be affiliated investment companies. The market value of the shares of other investment companies and ETFs may differ from their net asset value. As an investor in investment companies and ETFs, the Fund would bear its ratable share of that entity’s expenses, including its investment advisory and administration fees, while continuing to pay its own advisory and administration fees and other expenses (to the extent not offset by the Manager through waivers). As a result, shareholders will be absorbing duplicate levels of fees with respect to investments in other investment companies and ETFs (to the extent not offset by the Manager through waivers). The securities of other investment companies and ETFs in which the Fund may invest may be leveraged. As a result, the Fund may be indirectly exposed to leverage through an investment in such securities. An investment in securities of other investment companies and ETFs that use leverage may expose the Fund to higher volatility in the market value of such securities and the possibility that the Fund’s long-term returns on such securities (and, indirectly, the long-term returns of shares of the Fund) will be diminished. As with other investments, investments in other investment companies, including ETFs, are subject to market and selection risk. To the extent the Fund is held by an affiliated fund, the ability of the Fund itself to hold other investment companies may be limited. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Securities Risk (BTA) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Preferred Securities Risk (BTA): Preferred securities may pay fixed or adjustable rates of return. Preferred securities are subject to issuer-specific and market risks applicable generally to equity securities. In addition, a company’s preferred securities generally pay dividends only after the company makes required payments to holders of its bonds and other debt. For this reason, the value of preferred securities will usually react more strongly than bonds and other debt to actual or perceived changes in the company’s financial condition or prospects. Preferred securities of smaller companies may be more vulnerable to adverse developments than preferred securities of larger companies. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Market Risk and Selection Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Market Risk and Selection Risk: Market risk is the risk that one or more markets in which the Fund invests will go down in value, including the possibility that the markets will go down sharply and unpredictably. The value of a security or other asset may decline due to changes in general market conditions, economic trends or events that are not specifically related to the issuer of the security or other asset, or factors that affect a particular issuer or issuers, exchange, country, group of countries, region, market, industry, group of industries, sector or asset class. Local, regional or global events such as war, acts of terrorism, the spread of infectious illness or other public health issues like pandemics or epidemics, recessions, or other events could have a significant impact on the Fund and its investments. Selection risk is the risk that the securities selected by Fund management will underperform the markets, the relevant indices or the securities selected by other funds with similar investment objectives and investment strategies. This means you may lose money. An outbreak of an infectious coronavirus (COVID-19) that was first detected in December 2019 developed into a global pandemic that has resulted in numerous disruptions in the market and has had significant economic impact leaving general concern and uncertainty. Although vaccines have been developed and approved for use by various governments, the duration of the pandemic and its effects cannot be predicted with certainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis of Transaction Fees, Note [Text Block] |  | as a percentage of net assets attributable to common shares |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lowest Price or Bid |  |  |  | $ 10.83 | $ 11.7 | $ 13.96 | $ 14.76 | $ 15.25 | $ 14.23 | $ 13.55 | $ 13.49 | $ 12.58 |  |  |  |
Highest Price or Bid |  |  |  | 12.57 | 14.44 | 16.43 | 16.77 | 17.46 | 15.91 | 15.28 | 14.74 | 14.27 |  |  |  |
Lowest Price or Bid, NAV |  |  |  | 11.65 | 12.75 | 14.57 | 14.83 | 14.78 | 14.53 | 13.92 | 13.92 | 12.91 |  |  |  |
Highest Price or Bid, NAV |  |  |  | $ 12.21 | $ 14.36 | $ 15.02 | $ 15.08 | $ 15.04 | $ 14.84 | $ 14.66 | $ 14.01 | $ 14 |  |  |  |
Highest Price or Bid, Premium (Discount) to NAV [Percent] |  |  |  | 2.95% | 0.56% | 9.39% | 11.21% | 16.09% | 7.21% | 4.23% | 5.21% | 1.93% |  |  |  |
Lowest Price or Bid, Premium (Discount) to NAV [Percent] |  |  |  | (7.04%) | (8.24%) | (4.19%) | (0.47%) | 3.18% | (2.06%) | (2.66%) | (3.09%) | (2.56%) |  |  |  |
Latest Share Price |  |  | $ 12.55 |  |  |  |  |  |  |  |  |  |  |  |  |
Latest Premium (Discount) to NAV [Percent] |  |  | 0.16% |  |  |  |  |  |  |  |  |  |  |  |  |
Latest NAV |  |  | $ 12.53 |  |  |  |  |  |  |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Security, Authorized [Shares] |  |  | 199,998,250 |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Security, Held [Shares] |  |  | 38,440,728 |  |  |  |  |  |  |  |  |  |  |  |  |
TOBs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 42,444,000 |  |  |
Senior Securities Coverage per Unit | [5] |  |  |  |  |  |  |  |  |  |  |  | $ 16,471 |  |  |
Senior Securities Involuntary Liquidating Preference per Unit | [6] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Average Market Value per Unit | [7] |  |  |  |  |  |  |  |  |  |  |  | $ 37,166,000 |  |  |
VRDP Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 175,000,000 |  | $ 175,000,000 |
Senior Securities Coverage per Unit |  |  |  |  |  |  |  |  |  |  |  |  | $ 321,536 | [8] | $ 371,729 | [9]
Senior Securities Involuntary Liquidating Preference per Unit | [6] |  |  |  |  |  |  |  |  |  |  |  | 100,000 |  | 100,000 |
Senior Securities Average Market Value per Unit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Security Title [Text Block] |  | Preferred Shares A Fund’s Preferred Shares rank prior to its Common Shares as to the payment of dividends by the Fund and distribution of assets upon dissolution or liquidation of the Fund.​​​​​​​ The 1940 Act prohibits the declaration of any dividend on Common Shares or the repurchase of Common Shares if the Fund fails to maintain asset coverage of at least 200% of the liquidation preference of the Fund’s outstanding Preferred Shares. In addition, pursuant to the Preferred Shares’ governing instruments, a Fund is restricted from declaring and paying dividends on classes of shares ranking junior to or on parity with its Preferred Shares or repurchasing such shares if the Fund fails to declare and pay dividends on the Preferred Shares, redeem any Preferred Shares required to be redeemed under the Preferred Shares’ governing instruments or comply with the basic maintenance amount requirement of the ratings agencies rating the Preferred Shares.​​​​​​​​​​​​​​ Holders of Preferred Shares have voting rights equal to the voting rights of holders of Common Shares (one vote per share) and vote together with holders of Common Shares (one vote per share) as a single class on certain matters. Holders of Preferred Shares, voting as a separate class, are also entitled to (i) elect two members of the Board, (ii) elect the full Board if dividends on the Preferred Shares are not paid for a period of two years and (iii) a separate class vote to amend the Preferred Share governing documents. In addition, the 1940 Act requires the approval of the holders of a majority of any outstanding Preferred Shares, voting as a separate class, to (a) adopt any plan of reorganization that would adversely affect the Preferred Shares, (b) change a Fund’s sub-classification as a closed-end investment company or change its fundamental investment restrictions or (c) change its business so as to cease to be an investment company.​​​​​​​​​​​​​​ Redemption Terms: A VRDP Fund is required to redeem its VRDP Shares on the maturity date, unless earlier redeemed or repurchased. Six months prior to the maturity date, a VRDP Fund is required to begin to segregate liquid assets with the Fund’s custodian to fund the redemption. In addition, a VRDP Fund is required to redeem certain of its outstanding VRDP Shares if it fails to comply with certain asset coverage, basic maintenance amount or leverage requirements. Subject to certain conditions, the VRDP Shares may also be redeemed, in whole or in part, at any time at the option of a VRDP Fund. The redemption price per VRDP Share is equal to the liquidation preference per share plus any outstanding unpaid dividends. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Security Dividends [Text Block] |  | A Fund’s Preferred Shares rank prior to its Common Shares as to the payment of dividends by the Fund and distribution of assets upon dissolution or liquidation of the Fund.​​​​​​​ The 1940 Act prohibits the declaration of any dividend on Common Shares or the repurchase of Common Shares if the Fund fails to maintain asset coverage of at least 200% of the liquidation preference of the Fund’s outstanding Preferred Shares. In addition, pursuant to the Preferred Shares’ governing instruments, a Fund is restricted from declaring and paying dividends on classes of shares ranking junior to or on parity with its Preferred Shares or repurchasing such shares if the Fund fails to declare and pay dividends on the Preferred Shares, redeem any Preferred Shares required to be redeemed under the Preferred Shares’ governing instruments or comply with the basic maintenance amount requirement of the ratings agencies rating the Preferred Shares.​​​​​​​​​​​​​​ |  |  |  |  |  |  |  |  |  |  |  |  |  |
Security Voting Rights [Text Block] |  | Holders of Preferred Shares have voting rights equal to the voting rights of holders of Common Shares (one vote per share) and vote together with holders of Common Shares (one vote per share) as a single class on certain matters. Holders of Preferred Shares, voting as a separate class, are also entitled to (i) elect two members of the Board, (ii) elect the full Board if dividends on the Preferred Shares are not paid for a period of two years and (iii) a separate class vote to amend the Preferred Share governing documents. In addition, the 1940 Act requires the approval of the holders of a majority of any outstanding Preferred Shares, voting as a separate class, to (a) adopt any plan of reorganization that would adversely affect the Preferred Shares, (b) change a Fund’s sub-classification as a closed-end investment company or change its fundamental investment restrictions or (c) change its business so as to cease to be an investment company.​​​​​​​​​​​​​​ |  |  |  |  |  |  |  |  |  |  |  |  |  |
Security Liquidation Rights [Text Block] |  | A Fund’s Preferred Shares rank prior to its Common Shares as to the payment of dividends by the Fund and distribution of assets upon dissolution or liquidation of the Fund. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred Stock Restrictions, Arrearage [Text Block] |  | The 1940 Act prohibits the declaration of any dividend on Common Shares or the repurchase of Common Shares if the Fund fails to maintain asset coverage of at least 200% of the liquidation preference of the Fund’s outstanding Preferred Shares. In addition, pursuant to the Preferred Shares’ governing instruments, a Fund is restricted from declaring and paying dividends on classes of shares ranking junior to or on parity with its Preferred Shares or repurchasing such shares if the Fund fails to declare and pay dividends on the Preferred Shares, redeem any Preferred Shares required to be redeemed under the Preferred Shares’ governing instruments or comply with the basic maintenance amount requirement of the ratings agencies rating the Preferred Shares. |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rights Limited by Other Securities [Text Block] |  | A Fund’s Preferred Shares rank prior to its Common Shares as to the payment of dividends by the Fund and distribution of assets upon dissolution or liquidation of the Fund.​​​​​​​ Holders of Preferred Shares have voting rights equal to the voting rights of holders of Common Shares (one vote per share) and vote together with holders of Common Shares (one vote per share) as a single class on certain matters. Holders of Preferred Shares, voting as a separate class, are also entitled to (i) elect two members of the Board, (ii) elect the full Board if dividends on the Preferred Shares are not paid for a period of two years and (iii) a separate class vote to amend the Preferred Share governing documents. In addition, the 1940 Act requires the approval of the holders of a majority of any outstanding Preferred Shares, voting as a separate class, to (a) adopt any plan of reorganization that would adversely affect the Preferred Shares, (b) change a Fund’s sub-classification as a closed-end investment company or change its fundamental investment restrictions or (c) change its business so as to cease to be an investment company.​​​​​​​​​​​​​​ |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Security, Authorized [Shares] |  |  | 1,750 |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Security, Held [Shares] |  |  | 1,750 |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | If the common shares are sold to or through underwriters, the Prospectus Supplement will set forth any applicable sales load and the estimated offering expenses. Fund shareholders will pay all offering expenses involved with an offering.
[2] | Computershare Trust Company, N.A. (the “Reinvestment Plan Agent”) fees for the handling of the reinvestment of dividends will be paid by MUA. However, shareholders will pay a $0.02 per share fee incurred in connection with open-market purchases, which will be deducted from the value of the dividend. Shareholders will also be charged a $0.02 per share fee if a shareholder directs the Reinvestment Plan Agent to sell the common shares held in a dividend reinvestment account. Per share fees include any applicable brokerage commissions the Reinvestment Plan Agent is required to pay.
[3] | Assumes the use of leverage in the form of tender option bond transactions and preferred shares representing 31% of Managed Assets at an annual cost of leverage to MUA of 0.98%, which is based on current market conditions. The actual amount of interest expense borne by MUA will vary over time in accordance with the level of MUA’s use of tender option bond transactions and variations in market interest rates, as well as preferred shares transactions and changes to agreement terms with counterparties. Interest expense is required to be treated as an expense of MUA for accounting purposes.
[4] | MUA and the Manager have entered into a fee waiver agreement (the “Fee Waiver Agreement”), pursuant to which the Manager has contractually agreed to waive the investment advisory fees with respect to any portion of MUA’s assets attributable to investments in any equity and fixed-income mutual funds and ETFs managed by the Manager or its affiliates that have a contractual management fee, through June 30, 2024. In addition, pursuant to the Fee Waiver Agreement, the Manager has contractually agreed to waive its investment advisory fees by the amount of investment advisory fees MUA pays to the Manager indirectly through its investment in money market funds managed by the Manager or its affiliates, through June 30, 2024. The Fee Waiver Agreement may be terminated at any time, without the payment of any penalty, only by MUA (upon the vote of a majority of the Directors who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of MUA (the “Independent Directors”)) or a majority of the outstanding voting securities of MUA), upon 90 days’ written notice by MUA to the Manager.
[5] | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the amount of TOBs, and by multiplying the results by 1,000.
[6] | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of MUA in preference to common shareholders, expressed as a dollar amount per preferred share. VRDP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value.
[7] | Represents weighted average daily market value of TOBs.
[8] | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares and TOBs) from MUA’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VRDP Shares, and by multiplying the results by 100,000. Effective July 18, 2022, TOB Trust Certificates are treated as senior securities pursuant to Rule 18f-4 of the 1940 Act.
[9] | Calculated by subtracting MUA’s total liabilities (not including VRDP Shares) from MUA’s total assets and dividing this by the liquidation value of the VRDP Shares, and by multiplying the results by 100,000.

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