XML 26 R1.htm IDEA: XBRL DOCUMENT
v3.22.2.2

N-2 - USD ($) |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
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 | Apr. 30, 2021 | Apr. 30, 2020 | Apr. 30, 2019 | Apr. 30, 2018 | Apr. 30, 2017 | Apr. 30, 2016 | Apr. 30, 2015 | Apr. 30, 2014 | Apr. 30, 2013
Cover [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Entity Central Index Key |  | 0001137393 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment Flag |  | false |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Document Type |  | N-CSR |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Entity Registrant Name |  | BlackRock Municipal Income Trust |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fee Table [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shareholder Transaction Expenses [Table Text Block] |  |
|  | BFK |
Shareholder Transaction Expenses |  |  |  |
Maximum Sales Load (as a percentage of offering price) (a) |  |  | 1.00 | %
Offering expenses borne by BFK (as a percentage of offering price) (a) |  |  | 0.02 |
Dividend Reinvestment Plan Fees |  |  | $0.02 per share for open market purchases of common shares | (b)
Dividend reinvestment plan sale transaction fee |  |  | $2.50 | (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 BFK. 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 $2.50 sales fee and pay a $0.15 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] | 0.02% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual Expenses [Table Text Block] |  |
Estimated Annual Expenses (as a percentage of net assets attributable to common shares) |  |  |  |
Investment advisory fees (c)(d) |  |  | 0.98 | %
Other expenses |  |  | 1.34 |
Miscellaneous |  |  | 0.08 |
Interest expense (e) |  |  | 1.26 |
Total annual expenses |  |  | 2.32 |
Fee waiver (d) |  |  | — |
Total annual fund operating expenses after fee waiver (d) |  |  | 2.32 |
| (c) | BFK currently pays the Manager a monthly fee at an annual contractual investment management fee rate of 0.60% of its average weekly managed assets. For purposes of calculating these fees, “managed assets” means the total assets of BFK (including any assets attributable to money borrowed for investment purposes) minus the sum of BFK’s accrued liabilities (other than money borrowed for investment purposes). |
| (d) | BFK 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 BFK’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 BFK 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 BFK (upon the vote of a majority of the Trustees who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of BFK (the “Independent Trustees”)) or a majority of the outstanding voting securities of BFK), upon 90 days’ written notice by BFK to the Manager. |
| (e) | The total expense table includes interest expense associated with BFK’s investments in TOBs (also known as “inverse floaters”). Although such interest expense is actually paid by special purpose vehicles in which BFK invests, it is recorded on BFK’s financial statements for accounting purposes. The total expense table also includes, in interest expense, dividends associated with the VMTP Shares, because the VMTP Shares are considered debt of BFK for financial reporting purposes. |
Management Fees [Percent] | [3],[4] | 0.98% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expense 1 [Percent] |  | 0.08% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expense 2 [Percent] | [5] | 1.26% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Percent] |  | 1.34% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total Annual Expenses [Percent] |  | 2.32% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Waivers and Reimbursements of Fees [Percent] | [3] | 0.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Expense over Assets [Percent] | [3] | 2.32% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example [Table Text Block] |  | The following example illustrates BFK’s expenses (including the sales load of $10.00 and offering costs of $0.18) that shareholders would pay on a $1,000 investment in common shares, assuming (i) total net annual expenses of 2.32% of net assets attributable to common shares and (ii) a 5% annual return:
|  | 1 Year |  |  | 3 Years |  |  | 5 Years |  |  | 10 Years |
Total expenses incurred |  | $ | 33 |  |  | $ | 82 |  |  | $ | 133 |  |  | $ | 273 |
Expense Example, Year 01 |  | $ 33 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 3 |  | 82 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 5 |  | 133 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense Example, Years 1 to 10 |  | $ 273 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purpose of Fee Table , Note [Text Block] |  | The following table and example are intended to assist shareholders in understanding the various costs and expenses directly or indirectly associated with investing in BFK’s common shares. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis of Transaction Fees, Note [Text Block] | [1] | 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, BFK’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] |  | BFK 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 BFK’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 BFK 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 BFK (upon the vote of a majority of the Trustees who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of BFK (the “Independent Trustees”)) or a majority of the outstanding voting securities of BFK), upon 90 days’ written notice by BFK to the Manager. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities [Table Text Block] |  | The following table sets forth information regarding BFK’s outstanding senior securities as of the end of each of BFK’s last ten fiscal years, as applicable. BFK’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 |  | $ | 100,175 |  |  | $ | 9,181 | (b) |  | $ | N/A |  |  | $ | 99,657 | (c) |  |  | TOBs |
July 31, 2022 |  |  | 270,800 |  |  |  | 247,905 | (d) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2022 |  |  | 270,800 |  |  |  | 302,073 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2021 |  |  | 270,800 |  |  |  | 344,495 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2020 |  |  | 270,800 |  |  |  | 313,740 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2019 |  |  | 270,800 |  |  |  | 334,518 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2018 |  |  | 270,800 |  |  |  | 331,390 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2017 |  |  | 270,800 |  |  |  | 335,616 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2016 |  |  | 270,800 |  |  |  | 351,293 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2015 |  |  | 270,800 |  |  |  | 346,330 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2014 |  |  | 270,800 |  |  |  | 335,811 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2013 |  |  | 270,800 |  |  |  | 354,323 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
| (a) | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of BFK in preference to common shareholders, expressed as a dollar amount per preferred share. VMTP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value. |
| (b) | Calculated by subtracting BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’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 BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VMTP 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 BFK’s total liabilities (not including VMTP Shares) from BFK’s total assets and dividing this by the liquidation value of the VMTP Shares, and by multiplying the results by 100,000. |
Senior Securities, Note [Text Block] |  | Senior Securities The following table sets forth information regarding BFK’s outstanding senior securities as of the end of each of BFK’s last ten fiscal years, as applicable. BFK’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 |  | $ | 100,175 |  |  | $ | 9,181 | (b) |  | $ | N/A |  |  | $ | 99,657 | (c) |  |  | TOBs |
July 31, 2022 |  |  | 270,800 |  |  |  | 247,905 | (d) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2022 |  |  | 270,800 |  |  |  | 302,073 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2021 |  |  | 270,800 |  |  |  | 344,495 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2020 |  |  | 270,800 |  |  |  | 313,740 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2019 |  |  | 270,800 |  |  |  | 334,518 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2018 |  |  | 270,800 |  |  |  | 331,390 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2017 |  |  | 270,800 |  |  |  | 335,616 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2016 |  |  | 270,800 |  |  |  | 351,293 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2015 |  |  | 270,800 |  |  |  | 346,330 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2014 |  |  | 270,800 |  |  |  | 335,811 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
April 30, 2013 |  |  | 270,800 |  |  |  | 354,323 | (e) |  |  | 100,000 |  |  |  | N/A |  |  |  | VMTP Shares |
| (a) | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of BFK in preference to common shareholders, expressed as a dollar amount per preferred share. VMTP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value. |
| (b) | Calculated by subtracting BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’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 BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VMTP 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 BFK’s total liabilities (not including VMTP Shares) from BFK’s total assets and dividing this by the liquidation value of the VMTP Shares, and by multiplying the results by 100,000. |
Common shares of BKN and BFK 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. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Headings, Note [Text Block] |  | The following table sets forth information regarding BFK’s outstanding senior securities as of the end of each of BFK’s last ten fiscal years, as applicable. BFK’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 Municipal Income Trust’s (BFK) (the “Fund”) investment objective is to provide current income exempt from regular U.S. federal income tax. The Fund seeks to achieve its investment objective by investing primarily in municipal bonds that pay interest that is exempt from U.S. federal income taxes (except that the interest may be subject to the U.S. federal alternative minimum tax). The Fund invests, under normal market conditions, at least 80% of its assets in municipal bonds that are investment grade, or if unrated, deemed to be of comparable quality by the investment adviser, 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 current income exempt from federal income taxes. As a matter of fundamental policy, under normal market conditions, the Fund will invest at least 80% of its Managed Assets in investments the income from which is exempt from federal income tax (except that the interest may be subject to the alternative minimum tax). “Managed Assets” means the Fund’s total assets (including any assets attributable to money borrowed for investment purposes) minus the sum of the Fund’s accrued liabilities (other than money borrowed for investment purposes). The Fund cannot change its investment objectives or the foregoing fundamental policy without the approval of the holders of a majority of the outstanding common shares and the outstanding preferred shares, including the Fund’s variable rate muni term preferred shares (“VMTP Shares”), voting together as a single class, and of the holders of a majority of the outstanding preferred shares, including the VMTP 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’s investment policies provide that, under normal market conditions, the Fund will invest at least 80% of its total assets in investment grade quality municipal obligations issued by or on behalf of states, territories and possessions of the United States and their political subdivisions, agencies or instrumentalities, each of which pays interest that, in the opinion of bond counsel to the issuer, is excludable from gross income for federal income tax purposes (except that the interest may be includable in taxable income for purposes of the federal alternative minimum tax) (“Municipal Bonds”). Investment grade quality means that such bonds are rated, at the time of investment, within the four highest grades (Baa or BBB or better by Moody’s Investor Service Inc. (“Moody’s”), S&P Global Ratings (“S&P”) or Fitch Ratings, Inc. (“Fitch”)) or are unrated but judged to be of comparable quality by the BlackRock Advisors, LLC (the “Manager”). Municipal Bonds rated Baa by Moody’s are investment grade, but Moody’s considers Municipal Bonds rated Baa to have speculative characteristics. Changes in economic conditions or other circumstances are more likely to lead to a weakened capacity for issuers of Municipal Bonds that are rated BBB or Baa (or that have equivalent ratings) to make principal and interest payments than is the case for issues of higher grade Municipal Bonds. In the case of short term notes, the investment grade rating categories are SP-1+ through SP-2 for S&P, MIG-1 through MIG-3 for Moody’s and F-1+ through F-3 for Fitch. In the case of tax exempt commercial paper, the investment grade rating categories are A-1+ through A-3 for S&P, Prime-1 through Prime-3 for Moody’s and F-1+ through F-3 for Fitch. Obligations ranked in the lowest investment grade rating category (BBB, SP-2 and A-3 for S&P; Baa, MIG-3 and Prime-3 for Moody’s and BBB and F-3 for Fitch), while considered “investment grade,” may have certain speculative characteristics. There may be sub-categories or gradations indicating relative standing within the rating categories set forth above. In assessing the quality of Municipal Bonds with respect to the foregoing requirements, the Manager takes into account the nature of any letters of credit or similar credit enhancement to which particular Municipal Bonds are entitled and the creditworthiness of the financial institution that provided such credit enhancement. The Fund may invest up to 20% of its total assets in Municipal Bonds that are rated, at the time of investment, Ba/BB or B by Moody’s, S&P or Fitch or that are unrated but judged to be of comparable quality by the Manager. Bonds of below investment grade quality (Ba/BB or below) are commonly referred to as “junk bonds.” Bonds of below investment grade quality are regarded as having predominantly speculative characteristics with respect to the issuer’s capacity to pay interest and repay principal. Such securities, sometimes referred to as “high yield” or “junk” bonds, are predominantly speculative with respect to the capacity to pay interest and repay principal in accordance with the terms of the security and generally involve a greater volatility of price than securities in higher rating categories. Below investment grade securities and comparable unrated securities involve substantial risk of loss, are considered speculative with respect to the issuer’s ability to pay interest and any required redemption or principal payments and are susceptible to default or decline in market value due to adverse economic and business developments. The foregoing credit quality policies apply only at the time a security is purchased, and the Fund is not required to dispose of a security if a rating agency downgrades its assessment of the credit characteristics of a particular issue. In determining whether to retain or sell a security that a rating agency has downgraded, the Manager may consider such factors as the Manager’s assessment of the credit quality of the issuer of the security, the price at which the security could be sold and the rating, if any, assigned to the security by other rating agencies. Appendix F contains a general description of Moody’s, S&P’s and Fitch’s ratings of municipal bonds. In the event that the Fund disposes of a portfolio security subsequent to its being downgraded, the Fund may experience a greater risk of loss than if such security had been sold prior to such downgrade. The Fund may also invest in securities of other open- or closed-end investment companies that invest primarily in Municipal Bonds of the types in which the Fund may invest directly and in tax-exempt preferred shares that pay dividends that are exempt from regular federal income tax. In addition, the Fund may purchase Municipal Bonds that are additionally secured by insurance, bank credit agreements or escrow accounts. The credit quality of companies which provide these credit enhancements will affect the value of those securities. Although the insurance feature reduces certain financial risks, the premiums for insurance and the higher market price paid for insured obligations may reduce the Fund’s income. The insurance feature does not guarantee the market value of the insured obligations or the net asset value of the Fund’s common shares. The Fund may purchase insured bonds and may purchase insurance for bonds in its portfolio. The Fund may invest in certain tax exempt securities classified as “private activity bonds” (or industrial development bonds, under pre-1986 law) (in general, bonds that benefit non-governmental entities) that may subject certain investors in the Fund to an alternative minimum tax. The percentage of the Fund’s total assets invested in private activity bonds will vary from time to time. The Fund has not established any limit on the percentage of its portfolio that may be invested in Municipal Bonds subject to the alternative minimum tax provisions of federal tax law, and the Fund expects that a portion of the income it produces will be includable in alternative minimum taxable income. The average maturity of the Fund’s portfolio securities varies from time to time based upon an assessment of economic and market conditions by the Manager. The Fund’s portfolio at any given time may include both long- term and intermediate-term Municipal Bonds. The Fund’s stated expectation is that it will invest in Municipal Bonds that, in the Manager’s opinion, are underrated or undervalued. Underrated Municipal Bonds are those whose ratings do not, in the opinion of the Manager, reflect their true higher creditworthiness. Undervalued Municipal Bonds are bonds that, in the opinion of the Manager, are worth more than the value assigned to them in the marketplace. The Manager may at times believe that bonds associated with a particular municipal market sector (for example, but not limited to electric utilities), or issued by a particular municipal issuer, are undervalued. The Manager may purchase those bonds for the Fund’s portfolio because they represent a market sector or issuer that the Manager considers undervalued, even if the value of those particular bonds appears to be consistent with the value of similar bonds. Municipal Bonds of particular types (for example, but not limited to hospital bonds, industrial revenue bonds or bonds issued by a particular municipal issuer) may be undervalued because there is a temporary excess of supply in that market sector, or because of a general decline in the market price of Municipal Bonds of the market sector for reasons that do not apply to the particular Municipal Bonds that are considered undervalued. The Fund’s investment in underrated or undervalued Municipal Bonds will be based on the Manager’s belief that their yield is higher than that available on bonds bearing equivalent levels of interest rate risk, credit risk and other forms of risk, and that their prices will ultimately rise, relative to the market, to reflect their true value. Any capital appreciation realized by the Fund will generally result in capital gain distributions subject to federal capital gains taxation. The Fund ordinarily does not intend to realize significant investment income not exempt from federal income tax. From time to time, the Fund may realize taxable capital gains. Federal tax legislation has limited the types and volume of bonds the interest on which qualifies for a federal income tax exemption. As a result, this legislation and legislation that may be enacted in the future may affect the availability of Municipal Bonds for investment by the Fund. Leverage: The Fund may utilize leverage to seek to enhance the yield and net asset value of its common shares. However, this objective cannot be achieved in all interest rate environments. The Fund currently leverages its assets through the use of VMTP 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 may purchase and sell futures contracts, enter into various interest rate transactions and may purchase and sell exchange-listed and over-the-counter put and call options on securities, financial indices and futures contracts. The Fund may enter into interest rate swaps and the purchase or sale of interest rate caps and floors. The Fund may enter into credit default swap agreements for hedging purposes or to seek to increase its return. As temporary investments, the Fund may invest in repurchase agreements. The Fund may enter into reverse repurchase agreements with respect to its portfolio investments subject to its investment restrictions. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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. Certain 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. 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. When-Issued and Delayed Delivery Securities and Forward Commitments Risk (BKN and MQT): 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 (BKN, BFK and MQT): 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 (BKN, BFK and MQT): 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 (BKN, BFK and MQT): 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. Leverage Risk: The Fund uses leverage for investment purposes through the issuance of VMTP Shares and investments in TOB Residuals. The Fund may also utilize leverage for investment purposes by entering into reverse repurchase agreements and derivative instruments with leverage embedded in them, 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;
| • |  | lleverage 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.
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 (BFK) : 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 (BFK): 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 may utilize leverage to seek to enhance the yield and net asset value of its common shares. However, this objective cannot be achieved in all interest rate environments. The Fund currently leverages its assets through the use of VMTP 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 may purchase and sell futures contracts, enter into various interest rate transactions and may purchase and sell exchange-listed and over-the-counter put and call options on securities, financial indices and futures contracts. The Fund may enter into interest rate swaps and the purchase or sale of interest rate caps and floors. The Fund may enter into credit default swap agreements for hedging purposes or to seek to increase its return. As temporary investments, the Fund may invest in repurchase agreements. The Fund may enter into reverse repurchase agreements with respect to its portfolio investments subject to its investment restrictions. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Price [Table Text Block] |  | Share Price Data The following table summarizes BFK’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, BFK 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 |  | $ | 12.91 |  |  | $ | 10.37 |  |  | $ | 12.33 |  |  | $ | 11.45 |  |  |  | 4.70 | % |  |  | (9.43 | )% |  |  | 11,716,243 |
April 30, 2022 |  |  | 13.97 |  |  |  | 11.50 |  |  |  | 14.07 |  |  |  | 12.34 |  |  |  | (0.71 | ) |  |  | (6.81 | ) |  |  | 8,454,787 |
January 31, 2022 |  |  | 15.54 |  |  |  | 13.82 |  |  |  | 14.67 |  |  |  | 13.94 |  |  |  | 5.93 |  |  |  | (0.86 | ) |  |  | 4,396,715 |
October 31, 2021 |  |  | 15.82 |  |  |  | 14.49 |  |  |  | 14.94 |  |  |  | 14.42 |  |  |  | 5.89 |  |  |  | 0.49 |  |  |  | 3,844,602 |
July 31, 2021 |  |  | 15.71 |  |  |  | 14.86 |  |  |  | 15.12 |  |  |  | 14.72 |  |  |  | 3.90 |  |  |  | 0.95 |  |  |  | 4,000,173 |
April 30, 2021 |  |  | 15.40 |  |  |  | 14.39 |  |  |  | 15.07 |  |  |  | 14.43 |  |  |  | 2.19 |  |  |  | (0.28 | ) |  |  | 4,550,065 |
January 31, 2021 |  |  | 15.47 |  |  |  | 13.72 |  |  |  | 14.75 |  |  |  | 14.10 |  |  |  | 4.88 |  |  |  | (2.70 | ) |  |  | 3,829,337 |
October 31, 2020 |  |  | 14.95 |  |  |  | 13.57 |  |  |  | 14.64 |  |  |  | 14.08 |  |  |  | 2.12 |  |  |  | (3.62 | ) |  |  | 3,865,889 |
July 31, 2020 |  |  | 14.69 |  |  |  | 12.08 |  |  |  | 14.51 |  |  |  | 13.01 |  |  |  | 1.24 |  |  |  | (7.15 | ) |  |  | 4,759,351 |
As of July 31, 2022, BFK’s market price, NAV per Common Share, and premium/(discount) to NAV per Common Share are $11.25, $12.18, and (7.64)% respectively. Common shares of BFK 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. Certain 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. 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. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Leverage Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Leverage Risk: The Fund uses leverage for investment purposes through the issuance of VMTP Shares and investments in TOB Residuals. The Fund may also utilize leverage for investment purposes by entering into reverse repurchase agreements and derivative instruments with leverage embedded in them, 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;
| • |  | lleverage 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.
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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Investment Companies and ETFs Risk (BFK) : 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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Preferred Securities Risk (BFK): 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. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
WhenIssued and Delayed Delivery Securities and Forward Commitments Risk [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | When-Issued and Delayed Delivery Securities and Forward Commitments Risk (BKN and MQT): 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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Defensive Investing Risk (BKN, BFK and MQT): 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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Repurchase Agreements and Purchase and Sale Contracts Risk (BKN, BFK and MQT): 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 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Reverse Repurchase Agreements Risk (BKN, BFK and MQT): 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. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [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 |  |  |  | $ 10.37 | $ 11.5 | $ 13.82 | $ 14.49 | $ 14.86 | $ 14.39 | $ 13.72 | $ 13.57 | $ 12.08 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Highest Price or Bid |  |  |  | 12.91 | 13.97 | 15.54 | 15.82 | 15.71 | 15.4 | 15.47 | 14.95 | 14.69 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lowest Price or Bid, NAV |  |  |  | 11.45 | 12.34 | 13.94 | 14.42 | 14.72 | 14.43 | 14.1 | 14.08 | 13.01 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Highest Price or Bid, NAV |  |  |  | $ 12.33 | $ 14.07 | $ 14.67 | $ 14.94 | $ 15.12 | $ 15.07 | $ 14.75 | $ 14.64 | $ 14.51 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Highest Price or Bid, Premium (Discount) to NAV [Percent] |  |  |  | 4.70% | (0.71%) | 5.93% | 5.89% | 3.90% | 2.19% | 4.88% | 2.12% | 1.24% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Lowest Price or Bid, Premium (Discount) to NAV [Percent] |  |  |  | (9.43%) | (6.81%) | (0.86%) | 0.49% | 0.95% | (0.28%) | (2.70%) | (3.62%) | (7.15%) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Latest Share Price |  |  | $ 11.25 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Latest Premium (Discount) to NAV [Percent] |  |  | (7.64%) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Latest NAV |  |  | $ 12.18 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding Security, Held [Shares] |  |  | 45,041,077 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tobs [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,175 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Coverage per Unit | [6] |  |  |  |  |  |  |  |  |  |  |  | $ 9,181 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Involuntary Liquidating Preference per Unit | [7] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Average Market Value per Unit | [8] |  |  |  |  |  |  |  |  |  |  |  | $ 99,657 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vmtp Shares [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Financial Highlights [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Senior Securities Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |  | $ 270,800 |
Senior Securities Coverage per Unit |  |  |  |  |  |  |  |  |  |  |  |  | $ 247,905 | [9] | $ 302,073 | [10] | $ 344,495 | [10] | $ 313,740 | [10] | $ 334,518 | [10] | $ 331,390 | [10] | $ 335,616 | [10] | $ 351,293 | [10] | $ 346,330 | [10] | $ 335,811 | [10] | $ 354,323 | [10]
Senior Securities Involuntary Liquidating Preference per Unit | [7] |  |  |  |  |  |  |  |  |  |  |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 100,000 |  | 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 VMTP Fund is required to redeem its VMTP Shares on the term redemption date, unless earlier redeemed or repurchased or unless extended. There is no assurance that a term will be extended further or that any VMTP Shares will be replaced with any other preferred shares or other form of leverage upon the redemption or repurchase of the VMTP Shares. Six months prior to the term redemption date, a VMTP Fund is required to begin to segregate liquid assets with its custodian to fund the redemption. In addition, a VMTP Fund is required to redeem certain of its outstanding VMTP Shares if it fails to comply with certain asset coverage, basic maintenance amount or leverage requirements. Subject to certain conditions, VMTP Shares may be redeemed, in whole or in part, at any time at the option of the VMTP Fund. With respect to each Fund, the redemption price per VMTP Share is equal to the liquidation preference per share plus any outstanding unpaid dividends and applicable redemption premium. If each Fund redeems the VMTP Shares prior to the term redemption date and the VMTP Shares have long-term ratings above A1/A+ or its equivalent by the ratings agencies then rating the VMTP Shares, then such redemption may be subject to a prescribed redemption premium (up to 2% of the liquidation preference) payable to the holder of the VMTP Shares based on the time remaining until the term redemption date, subject to certain exceptions for redemptions that are required to comply with minimum asset coverage requirements. Dividends: Dividends on the VMTP Shares are declared daily and payable monthly at a variable rate set weekly at a fixed rate spread to the Securities Industry and Financial Markets Association (“SIFMA”) Municipal Swap Index or to a percentage of the one-month LIBOR rate, as set forth in the VMTP Shares governing instrument. The fixed spread is determined based on the long-term preferred share rating assigned to the VMTP Shares by the ratings agencies then rating the VMTP Shares. The dividend rate on VMTP Shares is subject to a step-up spread if the VMTP Fund fails to comply with certain provisions, including, among other things, the timely payment of dividends, redemptions or gross-up payments, and complying with certain asset coverage and leverage requirements. |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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, Held [Shares] |  |  | 2,708 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[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 BFK. 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 $2.50 sales fee and pay a $0.15 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] | BFK 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 BFK’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 BFK 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 BFK (upon the vote of a majority of the Trustees who are not “interested persons” (as defined in the Investment Company Act of 1940, as amended (the “Investment Company Act”), of BFK (the “Independent Trustees”)) or a majority of the outstanding voting securities of BFK), upon 90 days’ written notice by BFK to the Manager.
[4] | BFK currently pays the Manager a monthly fee at an annual contractual investment management fee rate of 0.60% of its average weekly managed assets. For purposes of calculating these fees, “managed assets” means the total assets of BFK (including any assets attributable to money borrowed for investment purposes) minus the sum of BFK’s accrued liabilities (other than money borrowed for investment purposes).
[5] | The total expense table includes interest expense associated with BFK’s investments in TOBs (also known as “inverse floaters”). Although such interest expense is actually paid by special purpose vehicles in which BFK invests, it is recorded on BFK’s financial statements for accounting purposes. The total expense table also includes, in interest expense, dividends associated with the VMTP Shares, because the VMTP Shares are considered debt of BFK for financial reporting purposes.
[6] | Calculated by subtracting BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’s total assets and dividing this by the amount of TOBs, and by multiplying the results by 1,000.
[7] | Represents the amount to which a holder of preferred shares would be entitled upon the liquidation of BFK in preference to common shareholders, expressed as a dollar amount per preferred share. VMTP Shares are considered debt of the issuer; therefore, the liquidation preference approximates fair value.
[8] | Represents weighted average daily market value of TOBs.
[9] | Calculated by subtracting BFK’s total liabilities (not including VMTP Shares and TOBs) from BFK’s total assets and dividing this by the sum of the amount of TOBs and liquidation value of the VMTP 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.
[10] | Calculated by subtracting BFK’s total liabilities (not including VMTP Shares) from BFK’s total assets and dividing this by the liquidation value of the VMTP Shares, and by multiplying the results by 100,000.

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 10 -Subparagraph i + Details
Name: | cef_AcquiredFundTotalAnnualExpensesNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 6 + Details
Name: | cef_AnnualExpensesTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 4 + Details
Name: | cef_BasisOfTransactionFeesNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 + Details
Name: | cef_CapitalStockLongTermDebtAndOtherSecuritiesAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 + Details
Name: | cef_DividendReinvestmentAndCashPurchaseFees
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 3 -Paragraph b + Details
Name: | cef_EffectsOfLeverageTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 + Details
Name: | cef_ExpenseExampleTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 11 + Details
Name: | cef_ExpenseExampleYear01
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 11 + Details
Name: | cef_ExpenseExampleYears1to10
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 11 + Details
Name: | cef_ExpenseExampleYears1to3
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 11 + Details
Name: | cef_ExpenseExampleYears1to5
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 + Details
Name: | cef_FeeTableAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 + Details
Name: | cef_FinancialHighlightsAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 + Details
Name: | cef_GeneralDescriptionOfRegistrantAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b + Details
Name: | cef_HighestPriceOrBid
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b -Subparagraph Instruction 4 + Details
Name: | cef_HighestPriceOrBidNav
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b -Subparagraph Instructions 4, 5 + Details
Name: | cef_HighestPriceOrBidPremiumDiscountToNavPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 2 -Paragraph b, d + Details
Name: | cef_InvestmentObjectivesAndPracticesTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph c + Details
Name: | cef_LatestNav
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph c + Details
Name: | cef_LatestPremiumDiscountToNavPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph c + Details
Name: | cef_LatestSharePrice
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b + Details
Name: | cef_LowestPriceOrBid
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b -Subparagraph Instruction 4 + Details
Name: | cef_LowestPriceOrBidNav
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b -Subparagraph Instructions 4, 5 + Details
Name: | cef_LowestPriceOrBidPremiumDiscountToNavPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 7 -Subparagraph b + Details
Name: | cef_ManagementFeeNotBasedOnNetAssetsNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 7 -Subparagraph a + Details
Name: | cef_ManagementFeesPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_NetExpenseOverAssetsPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_OtherAnnualExpense1Percent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_OtherAnnualExpense2Percent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_OtherAnnualExpensesAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_OtherAnnualExpensesPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 6 + Details
Name: | cef_OtherExpensesNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 5 + Details
Name: | cef_OtherTransactionExpensesAbstract
Namespace Prefix: | cef_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 5 + Details
Name: | cef_OtherTransactionExpensesPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 5 -Paragraph 3 + Details
Name: | cef_OutstandingSecurityHeldShares
Namespace Prefix: | cef_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph b -Subparagraph 1 + Details
Name: | cef_PreferredStockRestrictionsArrearageTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 1 + Details
Name: | cef_PurposeOfFeeTableNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph d + Details
Name: | cef_RightsLimitedByOtherSecuritiesTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 3 -Paragraph a + Details
Name: | cef_RiskFactorsTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | cef_RiskTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 + Details
Name: | cef_SalesLoadPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph a -Subparagraph 1 + Details
Name: | cef_SecurityDividendsTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph a -Subparagraph 3 + Details
Name: | cef_SecurityLiquidationRightsTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph a + Details
Name: | cef_SecurityTitleTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 10 -Subsection 1 -Paragraph a -Subparagraph 2 + Details
Name: | cef_SecurityVotingRightsTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph 2 + Details
Name: | cef_SeniorSecuritiesAmount
Namespace Prefix: | cef_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph 5 + Details
Name: | cef_SeniorSecuritiesAverageMarketValuePerUnit
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph 3 -Subparagraph Instruction 2 + Details
Name: | cef_SeniorSecuritiesCoveragePerUnit
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph Instruction 5 + Details
Name: | cef_SeniorSecuritiesHeadingsNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph 4 -Subparagraph Instruction 3 + Details
Name: | cef_SeniorSecuritiesInvoluntaryLiquidatingPreferencePerUnit
Namespace Prefix: | cef_
Data Type: | dtr-types:perShareItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 -Paragraph Instruction 1 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 1 -Paragraph Instruction 2 + Details
Name: | cef_SeniorSecuritiesNoteTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 4 -Subsection 3 + Details
Name: | cef_SeniorSecuritiesTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 8 -Subsection 5 -Paragraph b -Subparagraph 4 + Details
Name: | cef_SharePriceTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 + Details
Name: | cef_ShareholderTransactionExpensesTableTextBlock
Namespace Prefix: | cef_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 8 + Details
Name: | cef_TotalAnnualExpensesPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Form N-2 -Section Item 3 -Subsection 1 -Paragraph Instruction 9 + Details
Name: | cef_WaiversAndReimbursementsOfFeesPercent
Namespace Prefix: | cef_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Boolean flag that is true when the XBRL content amends previously-filed or accepted submission. + References No definition available. + Details
Name: | dei_AmendmentFlag
Namespace Prefix: | dei_
Data Type: | xbrli:booleanItemType
Balance Type: | na
Period Type: | duration

X
- Definition Cover page. + References No definition available. + Details
Name: | dei_CoverAbstract
Namespace Prefix: | dei_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'. + References No definition available. + Details
Name: | dei_DocumentType
Namespace Prefix: | dei_
Data Type: | dei:submissionTypeItemType
Balance Type: | na
Period Type: | duration

X
- Definition A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2 + Details
Name: | dei_EntityCentralIndexKey
Namespace Prefix: | dei_
Data Type: | dei:centralIndexKeyItemType
Balance Type: | na
Period Type: | duration

X
- Definition The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Exchange Act -Number 240 -Section 12 -Subsection b-2 + Details
Name: | dei_EntityRegistrantName
Namespace Prefix: | dei_
Data Type: | xbrli:normalizedStringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | cef_RiskAxis=cik0001137393_MarketRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_CounterpartyCreditRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_ConcentrationRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_LiborTransitionRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_DebtSecuritiesRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_MunicipalSecuritiesRisksMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_TaxabilityRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_InsuranceRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_JunkBondsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_LeverageRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_DerivativesRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_TenderOptionBondsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_IlliquidInvestmentsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_InvestmentCompaniesAndEtfsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_PreferredSecuritiesRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_MarketRiskAndSelectionRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_WhenissuedAndDelayedDeliverySecuritiesAndForwardCommitmentsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_DefensiveInvestingRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_RepurchaseAgreementsAndPurchaseAndSaleContractsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_RiskAxis=cik0001137393_ReverseRepurchaseAgreementsRiskMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_SecurityAxis=cik0001137393_CommonStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_SecurityAxis=cik0001137393_TobsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_SecurityAxis=cik0001137393_VmtpSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | cef_SecurityAxis=cik0001137393_PreferredSharesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |