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N-2 - USD ($) |  | 3 Months Ended
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Dec. 31, 2022 | Dec. 31, 2022 | Sep. 30, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Mar. 31, 2021
Cover [Abstract] |  |  |  |  |  |  |  |  |  |
Entity Central Index Key |  | 0000832327 |  |  |  |  |  |  |  |
Amendment Flag |  | false |  |  |  |  |  |  |  |
Document Type |  | N-CSR |  |  |  |  |  |  |  |
Entity Registrant Name |  | BlackRock Income Trust, Inc. |  |  |  |  |  |  |  |
Fee Table [Abstract] |  |  |  |  |  |  |  |  |  |
Shareholder Transaction Expenses [Table Text Block] |  |
|  | BKT |
Shareholder Transaction Expenses |  |  |  |
Maximum sales load (as a percentage of offering price) (a) |  |  | 1.00 | %
Offering expenses borne by the Fund (as a percentage of offering price) (a) |  |  | 0.03 | %
|  |  | $0.02 per share for open market purchases of |
Dividend reinvestment plan fees |  |  | common shares | (b)
Dividend reinvestment plan sale transaction fee (b) |  |  | $2.50 |
| (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.’s (the “Reinvestment Plan Agent”) fees for the handling of the reinvestment of dividends will be paid by the Fund. 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] | $ 2.5 |  |  |  |  |  |  |  |
Other Transaction Expenses [Abstract] |  |  |  |  |  |  |  |  |  |
Other Transaction Expenses [Percent] | [1] | 0.03% |  |  |  |  |  |  |  |
Annual Expenses [Table Text Block] |  |
Estimated Annual Expenses (as a percentage of net assets attributable to common shares) |  |  |  |
Investment advisory fees (c),(d) |  |  | 0.65 | %
Other expenses |  |  | 1.00 |
Miscellaneous |  |  | 0.31 |
Interest expense (e) |  |  | 0.69 |
Total annual expenses |  |  | 1.65 |
Fee waiver (d) |  |  | 0.01 |
Total annual Fund operating expenses after fee waiver (d) |  |  | 1.64 |
| (c) | The Fund currently pays the Manager a monthly fee at an annual contractual investment advisory fee rate of 0.65% of the average weekly value of the Fund’s net assets. For purposes of calculating this fee, “net assets” means the total assets of the Fund minus the sum of its accrued liabilities (including the aggregate indebtedness constituting financial leverage). |
| (d) | The Fund 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 the Fund’s assets attributable to investments in any equity and fixed-income mutual funds and exchange-traded funds 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 the Fund 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 the Fund (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 the Fund or a majority of the outstanding voting securities of the Fund), upon 90 days’ written notice by the Fund to the Manager. |
| (e) | The Fund uses leverage in the form of reverse repurchase agreements representing 29.3% of Managed Assets at an annual interest expense to the Fund of 1.7% which is based on current market conditions. The actual amount of interest expenses borne by the Fund will vary over time in accordance with the level of the Fund’s use of reverse repurchase agreements and variations in market interest rates. Interest expense is required to be treated as an expense of the Fund for accounting purposes. |
Management Fees [Percent] | [3],[4] | 0.65% |  |  |  |  |  |  |  |
Other Annual Expenses [Abstract] |  |  |  |  |  |  |  |  |  |
Other Annual Expense 1 [Percent] |  | 0.31% |  |  |  |  |  |  |  |
Other Annual Expense 2 [Percent] | [5] | 0.69% |  |  |  |  |  |  |  |
Other Annual Expenses [Percent] |  | 1.00% |  |  |  |  |  |  |  |
Total Annual Expenses [Percent] |  | 1.65% |  |  |  |  |  |  |  |
Waivers and Reimbursements of Fees [Percent] | [3] | 0.01% |  |  |  |  |  |  |  |
Net Expense over Assets [Percent] | [3] | 1.64% |  |  |  |  |  |  |  |
Expense Example [Table Text Block] |  | The following example illustrates the expenses (including the sales load of $10.00 and offering costs of $0.38) that shareholders would pay on a $1,000 investment in common shares, assuming (i) total net annual expenses of 1.64% of net assets attributable to common shares and (ii) a 5% annual return:
|  | 1 Year |  |  | 3 Years |  |  | 5 Years |  |  | 10 Years |
Total expenses incurred |  | $ | 27 |  |  | $ | 62 |  |  | $ | 99 |  |  | $ | 204 |
Expense Example, Year 01 |  | $ 27 |  |  |  |  |  |  |  |
Expense Example, Years 1 to 3 |  | 62 |  |  |  |  |  |  |  |
Expense Example, Years 1 to 5 |  | 99 |  |  |  |  |  |  |  |
Expense Example, Years 1 to 10 |  | $ 204 |  |  |  |  |  |  |  |
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 BKT’s common shares. |  |  |  |  |  |  |  |
Basis of Transaction Fees, Note [Text Block] | [1] | as a percentage of offering price |  |  |  |  |  |  |  |
Other Transaction Fees, Note [Text Block] |  | Computershare Trust Company, N.A.’s (the “Reinvestment Plan Agent”) fees for the handling of the reinvestment of dividends will be paid by the Fund. 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. |  |  |  |  |  |  |  |
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. The Fund’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] |  | The Fund 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 the Fund’s assets attributable to investments in any equity and fixed-income mutual funds and exchange-traded funds 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 the Fund 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 the Fund (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 the Fund or a majority of the outstanding voting securities of the Fund), upon 90 days’ written notice by the Fund to the Manager. |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Investment Objectives and Practices [Text Block] |  | BlackRock Income Trust, Inc.’s (BKT) (the “Fund”) investment objective is to manage a portfolio of high-quality securities to achieve both preservation of capital and high monthly income. The Fund seeks to achieve its investment objective by investing at least 65% of its assets in mortgage-backed securities. The Fund invests at least 80% of its assets in securities that are (i) issued or guaranteed by the U.S. government or one of its agencies or instrumentalities or (ii) rated at the time of investment either AAA by S&P Global Ratings or Aaa by Moody’s Investors Service, Inc. The Fund may invest directly in such securities or synthetically through the use of derivatives . The Fund’s investment objective is to manage a portfolio of high-quality securities to achieve both preservation of capital and high monthly income. The Fund will seek to distribute monthly income that is greater than that obtainable on an annualized basis by investment in United States government securities having the same maturity as the weighted average maturity of the Fund’s investments. The Fund’s portfolio is expected to consist primarily of mortgage-backed securities and, to a lesser extent, asset-backed securities. Mortgage-backed securities are securities that directly or indirectly represent a participation in, or are secured by and payable from, mortgage loans secured by real property. There are three basic types of mortgage-backed securities: (i) those issued or guaranteed by the United States government or one of its agencies or instrumentalities, such as the Government National Mortgage Association (“GNMA”), the Federal National Mortgage Association (“Fannie Mae”) and the Federal Home Loan Mortgage Corporation (“Freddie Mac”); (ii) those issued by private issuers that are collateralized by securities issued or guaranteed by the United States government or one of its agencies or instrumentalities; and (iii) those issued by private issuers and collateralized by securities without a government guarantee but usually with some form of private credit enhancement. The Fund will invest at least 65% of its assets in mortgage-backed securities. The balance of the Fund’s assets generally will be invested in asset-backed securities, which have structural characteristics similar to mortgage-backed securities but have underlying assets that are not mortgage loans or interests in mortgage loans. The Fund may also invest in various derivative mortgage-backed and asset-backed securities, such as collateralized mortgage obligations and asset-backed security residual interests and stripped mortgage-backed securities. The Fund may invest directly in such securities or synthetically through the use of derivatives. In addition, for hedging purposes, the Fund may utilize a portion of its assets for certain options, futures, interest rate swaps and related transactions. For purposes of enhancing liquidity and/or preserving capital, the Fund may invest without limit in securities issued by the United States government and its agencies and instrumentalities, or repurchase agreements collateralized by such securities, certificates of deposit, time deposits or bankers’ acceptances of similar quality. At least 80% of the Fund’s assets will be invested in securities that are (i) issued or guaranteed by the United States government or one of its agencies or instrumentalities or (ii) rated at the time of investment either AAA by S&P Global Ratings (“S&P”) or Aaa by Moody’s Investors Service (“Moody’s”). Securities issued or guaranteed by the United States government or its agencies or instrumentalities are generally considered to be of the same or higher quality than privately issued securities rated AAA or Aaa. No more than 20% of the Fund’s assets will be invested in other securities, all of which will have been determined by BlackRock Advisors, LLC (the “Manager”) or BlackRock International Limited (“BIL” and together with the Manager, the “Advisors”), the Fund’s sub-advisor, to be of comparable credit quality. The yield characteristics of mortgage-backed and asset-backed securities differ from traditional debt securities. Among the major differences are that interest and principal payments are made more frequently, usually monthly, and that principal may be prepaid at any time because the underlying mortgage loans or other assets generally may be prepaid at any time. As a result, if the Fund purchases such a security at a premium, a prepayment rate that is faster than expected will reduce yield to maturity, while a prepayment rate that is slower than expected will have the opposite effect of increasing yield to maturity. Conversely, if the Fund purchases these securities at a discount, faster than expected prepayments will increase, while slower than expected prepayments will reduce, yield to maturity. The Fund may also invest in derivative securities such as stripped mortgage-backed securities or residual interests, which generally are more sensitive to changes in prepayment and interest rates. The Advisors will seek to manage these risks (and potential benefits) by investing in a variety of such securities and through hedging techniques. Prepayments on a pool of mortgage loans are influenced by a variety of economic, geographic, social and other factors, including changes in mortgagors’ housing needs, job transfers, unemployment, mortgagors’ net equity in the mortgaged properties and servicing decisions. Generally, however, prepayments on fixed rate mortgage loans will increase during a period of falling interest rates and decrease during a period of rising interest rates. The same factors apply to prepayments on asset-backed securities but the predominant factor in a particular case may be different than in the case of mortgage-backed securities. Accordingly, amounts available for reinvestment by the Fund are likely to be greater during a period of declining interest rates than during a period of rising interest rates. The Fund’s yield will also be affected by the interest rates on instruments in which the Fund is able to reinvest the proceeds of payments and prepayments. Accelerated prepayments on securities purchased by the Fund at a premium also impose a risk of loss of principal because the premium may not have been fully amortized at the time the principal is repaid in full. Leverage: The Fund may borrow from time to time, at the Advisors’ discretion, for purposes of investment leverage when yields on available investments exceed interest rates and other expenses of related borrowing, or when, in the Advisors’ opinion, unusual market conditions otherwise make it advantageous for the Fund to increase its investment capacity. The Fund may also borrow for emergency purposes, for the payment of dividends or for the clearance of transactions. The Fund may enter into reverse repurchase agreements. |  |  |  |  |  |  |  |
Risk Factors [Table Text Block] |  |
10. | 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. 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. Infectious Illness Risk: An outbreak of an infectious illness, such as the COVID-19 pandemic, may adversely impact the economies of many nations and the global economy, and may impact individual issuers and capital markets in ways that cannot be foreseen. An infectious illness outbreak may result in, among other things, closed international borders, prolonged quarantines, supply chain disruptions, market volatility or disruptions and other significant economic, social and political impacts. Valuation Risk: The market values of equities, such as common stocks and preferred securities or equity related investments, such as futures and options, may decline due to general market conditions which are not specifically related to a particular company. They may also decline due to factors which affect a particular industry or industries. A Fund may invest in illiquid investments. An illiquid investment is any investment that a Fund reasonably expects cannot be sold or disposed of in current market conditions in seven calendar days or less without the sale or disposition significantly changing the market value of the investment. A Fund may experience difficulty in selling illiquid investments in a timely manner at the price that it believes the investments are worth. Prices may fluctuate widely over short or extended periods in response to company, market or economic news. Markets also tend to move in cycles, with periods of rising and falling prices. This volatility may cause each Fund’s NAV to experience significant increases or decreases over short periods of time. If there is a general decline in the securities and other markets, the NAV of a Fund may lose value, regardless of the individual results of the securities and other instruments in which a Fund invests. The price a Fund could receive upon the sale of any particular portfolio investment may differ from a Fund’s valuation of the investment, particularly for securities that trade in thin or volatile markets or that are valued using a fair valuation technique or a price provided by an independent pricing service. Changes to significant unobservable inputs and assumptions (i.e., publicly traded company multiples, growth rate, time to exit) due to the lack of observable inputs may significantly impact the resulting fair value and therefore a Fund’s results of operations. As a result, the price received upon the sale of an investment may be less than the value ascribed by a Fund, and a Fund could realize a greater than expected loss or lesser than expected gain upon the sale of the investment. A Fund’s ability to value its investments may also be impacted by technological issues and/or errors by pricing services or other third-party service providers. 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. For OTC options purchased, each Fund bears the risk of loss in the amount of the premiums paid plus the positive change in market values net of any collateral held by the Funds should the counterparty fail to perform under the contracts. Options written by the Funds do not typically give rise to counterparty credit risk, as options written generally obligate the Funds, and not the counterparty, to perform. The Funds may be exposed to counterparty credit risk with respect to options written to the extent each Fund deposits collateral with its counterparty to a written option. With exchange-traded options purchased, exchange-traded futures and centrally cleared swaps, 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 and centrally cleared swaps 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 significant portion of their assets in high yield securities. High yield securities that are rated below investment-grade (commonly referred to as “junk bonds”) or are unrated may be deemed speculative, involve greater levels of risk than higher-rated securities of similar maturity and are more likely to default. High yield securities may be issued by less creditworthy issuers, and issuers of high yield securities may be unable to meet their interest or principal payment obligations. High yield securities are subject to extreme price fluctuations, may be less liquid than higher rated fixed-income securities, even under normal economic conditions, and frequently have redemption features. Certain 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. Certain Funds invest a significant portion of their assets in securities backed by commercial or residential mortgage loans or in issuers that hold mortgage and other asset-backed securities. When a Fund concentrates its investments in this manner, it assumes a greater risk of prepayment or payment extension by securities issuers. Changes in economic conditions, including delinquencies and/or defaults on assets underlying these securities, can affect the value, income and/or liquidity of such positions. Investment percentages in these securities are presented in the Schedules of Investments. LIBOR Transition Risk: The United Kingdom’s Financial Conduct Authority announced a phase out of the 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. The order of the below risk factors does not indicate the significance of any particular risk. 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. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because 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. Asecurity 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.
U.S. Government Obligations Risk: Certain securities in which the Fund may invest, including securities issued by certain U.S. Government agencies and U.S. Government sponsored enterprises, are not guaranteed by the U.S. Government or supported by the full faith and credit of the United States. U.S. Government Mortgage-Related Securities Risk (FRA, BKT and BLW): There are a number of important differences among the agencies and instrumentalities of the U.S. Government that issue mortgage-related securities and among the securities that they issue. Mortgage-related securities guaranteed by the Government National Mortgage Association (“GNMA” or “Ginnie Mae”) are guaranteed as to the timely payment of principal and interest by GNMAand such guarantee is backed by the full faith and credit of the United States. GNMA securities also are supported by the right of GNMA to borrow funds from the U.S. Treasury to make payments under its guarantee. Mortgage-related securities issued by Fannie Mae or Freddie Mac are solely the obligations of Fannie Mae or Freddie Mac, as the case may be, and are not backed by or entitled to the full faith and credit of the United States but are supported by the right of the issuer to borrow from the Treasury. Mortgage- and Asset-Backed Securities Risks (FRA, BKT and BLW): Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. Yield and Ratings Risk (BKT): The yields on debt obligations are dependent on a variety of factors, including general market conditions, conditions in the particular market for the obligation, the financial condition of the issuer, the size of the offering, the maturity of the obligation and the ratings of the issue. The ratings of Moody’s, S&P and Fitch, represent their respective opinions as to the quality of the obligations they undertake to rate. Ratings, however, are general and are not absolute standards of quality. Consequently, obligations with the same rating, maturity and interest rate may have different market prices. Subsequent to its purchase by the Fund, a rated security may cease to be rated. The Manager will consider such an event in determining whether the Fund should continue to hold the security. Repurchase Agreements and Purchase and Sale Contracts Risk (BKT): 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. 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.
Leverage Risk: The Fund’s use of leverage may increase or decrease from time to time in its discretion and the Fund may, in the future, determine not to use leverage. The use of leverage creates an opportunity for increased common share net investment income dividends, but also creates risks for the holders of common shares. The Fund cannot assure you that the use of leverage will result in a higher yield on the common shares. Any leveraging strategy the Fund employs may not be successful. Leverage involves risks and special considerations for common shareholders, including:
| • |  | the likelihood of greater volatility of net asset value, market price and dividend rate of the common shares than a comparable portfolio without leverage;
| • |  | the risk that fluctuations in interest rates or dividend rates on any leverage that the Fund must pay will reduce the return to the common shareholders;
| • |  | the effect of leverage in a declining market, which is likely to cause a greater decline in the net asset value of the common shares than if the Fund were not leveraged, which may result in a greater decline in the market price of the common shares;
| • |  | leverage may increase operating costs, which may reduce total return.
Any decline in the net asset value of the Fund’s investments will be borne entirely by the holders of common shares. Therefore, if the market value of the Fund’s portfolio declines, leverage will result in a greater decrease in net asset value to the holders of common shares than if the Fund were not leveraged. This greater net asset value decrease will also tend to cause a greater decline in the market price for the common shares. Reverse Repurchase Agreements Risk: 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. 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. 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 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. |  |  |  |  |  |  |  |
Share Price [Table Text Block] |  | The following tables summarize each Fund’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.
|  | NYSE Market Price Per Common Share |  |  | NAV per Common Share on Date of Market Price |  |  | Premium/ (Discount) on Date of Market Price |  |  |  |
BKT — During Quarter Ended |  | High |  |  | Low |  |  | High |  |  | Low |  |  | High |  |  | Low |  |  | Trading Volume |
December 31, 2022 |  |  | $ 12.82 |  |  |  | $ 11.63 |  |  |  | $ 13.54 |  |  |  | $ 12.70 |  |  |  | (5.32 | )% |  |  | (8.43 | )% |  |  | 4,348,585 |
September 30, 2022 |  |  | 14.43 |  |  |  | 12.06 |  |  |  | 14.82 |  |  |  | 13.08 |  |  |  | (2.63 | ) |  |  | (7.80 | ) |  |  | 7,412,087 |
June 30, 2022 |  |  | 15.36 |  |  |  | 12.93 |  |  |  | 15.51 |  |  |  | 14.43 |  |  |  | (0.97 | ) |  |  | (10.40 | ) |  |  | 18,907,464 |
March 31, 2022 |  |  | 16.92 |  |  |  | 14.85 |  |  |  | 16.89 |  |  |  | 15.66 |  |  |  | 0.18 |  |  |  | (5.17 | ) |  |  | 21,937,873 |
December 31, 2021 |  |  | 18.90 |  |  |  | 16.95 |  |  |  | 17.58 |  |  |  | 16.95 |  |  |  | 7.51 |  |  |  | — |  |  |  | 16,500,738 |
September 30, 2021 |  |  | 19.50 |  |  |  | 17.88 |  |  |  | 17.88 |  |  |  | 17.67 |  |  |  | 9.06 |  |  |  | 1.19 |  |  |  | 13,508,607 |
June 30, 2021 |  |  | 19.23 |  |  |  | 18.27 |  |  |  | 18.18 |  |  |  | 18.39 |  |  |  | 5.78 |  |  |  | (0.65 | ) |  |  | 16,030,457 |
March 31, 2021 |  |  | 18.51 |  |  |  | 18.00 |  |  |  | 18.54 |  |  |  | 18.57 |  |  |  | (0.16 | ) |  |  | (3.07 | ) |  |  | 14,365,814 |
As of December 31, 2022, BKT’s market price, NAV per Common Share, and premium/(discount) to NAV per Common Share were $12.34, $13.10, and (5.80)%, respectively. |  |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |  |  |  |
Capital Stock [Table Text Block] |  |
11. | CAPITAL SHARE TRANSACTIONS
DSU is authorized to issue 400 million shares, all of which were initially classified as Common Shares. FRA and BKT are authorized to issue 200 million shares, all of which were initially classified as Common Shares. BLW is authorized to issue an unlimited number of shares, all of which were initially classified as Common Shares. The par value for DSU’s, FRA’s, BKT’s and BLW’s shares is $0.10, $0.10, $0.010 and $0.001, respectively. The Board for DSU, FRA and BLW are each authorized, however, to reclassify any unissued Common Shares to Preferred Shares without the approval of Common Shareholders. The Board authorized a 1-for-3 reverse stock split for BKT, effective after the close of trading on October 17, 2022, for the shareholders of record on October 17, 2022. The impact of the reverse stock split was a decrease in the number of shares outstanding by a factor of three, while increasing the NAV per share by a factor of three, resulting in no effect on the net assets of BKT. The financial statements for BKT have been adjusted to reflect the reverse stock split. Common Shares For the periods shown, shares issued and outstanding increased by the following amounts as a result of dividend reinvestment:
|  | Year Ended |
Fund Name |  | 12/31/22 |  |  | 12/31/21 |
DSU |  |  | 3,721 |  |  |  | 14,784 |
BKT (a) |  |  | 3,887 |  |  |  | 34,687 |
BLW |  |  | — |  |  |  | 49,148 |
| (a) | Share transactions reflect 1-for-3 reverse stock split effective after the close of trading on October 17, 2022. |
The Funds participate in an open market share repurchase program (the “Repurchase Program”). From December 1, 2021 through November 30, 2022, each Fund may repurchase up to 5% of its outstanding common shares under the Repurchase Program, based on common shares outstanding as of the close of business on November 30, 2021, subject to certain conditions. From December 1, 2022 through November 30, 2023, each Fund may repurchase up to 5% of its outstanding common shares under the Repurchase Program, based on common shares outstanding as of the close of business on November 30, 2022, subject to certain conditions. The Repurchase Program has an accretive effect as shares are purchased at a discount to the Fund’s NAV. There is no assurance that the Funds will purchase shares in any particular amounts. For the year ended December 31, 2022, the Funds did not repurchase any shares. For the year ended December 31, 2021, shares issued and outstanding decreased by 102,939 as a result of share repurchase for FRA. DSU filed a registration statement with the SEC seeking the ability to issue additional Common Shares through a Shelf Offering, which was declared effective on December 28, 2022. DSU may not sell any Common Shares in the Shelf Offering until a definitive prospectus relating to the Shelf Offering has been filed with the SEC. Under the Shelf Offering, DSU subject to market conditions, may raise additional equity capital from time to time in varying amounts and utilizing various offering methods at a net price at or above DSU’s NAV per Common Share (calculated within 48 hours of pricing). See Additional Information - Shelf Offering Program for additional information. BKT and BLW have filed a prospectus with the SEC allowing them to issue an additional 8,333,333 and 10,000,000 Common Shares, respectively, through an equity Shelf Offering. Under the Shelf Offering, BKT and BLW, subject to market conditions, may raise additional equity capital from time to time in varying amounts and utilizing various offering methods at a net price at or above each Fund’s NAV per Common Share (calculated within 48 hours of pricing). As of period end, 8,333,333 and 10,000,000 Common Shares, respectively, remain available for issuance under the Shelf Offering. For the year ended December 31, 2022, Common Shares issued and outstanding under the Shelf Offering remained constant for BKT and BLW. See Additional Information - Shelf Offering Program for additional information. Initial costs incurred by DSU, BKT and BLW in connection with their Shelf Offerings are recorded as “Deferred offering costs” in the Statements of Assets and Liabilities. As shares are sold, a portion of the costs attributable to the shares sold will be charged against paid-in-capital. Any remaining deferred charges at the end of the Shelf Offering period will be charged to expense. |  |  |  |  |  |  |  |
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. |  |  |  |  |  |  |  |
Infectious Illness Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Infectious Illness Risk: An outbreak of an infectious illness, such as the COVID-19 pandemic, may adversely impact the economies of many nations and the global economy, and may impact individual issuers and capital markets in ways that cannot be foreseen. An infectious illness outbreak may result in, among other things, closed international borders, prolonged quarantines, supply chain disruptions, market volatility or disruptions and other significant economic, social and political impacts. |  |  |  |  |  |  |  |
Valuation Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Valuation Risk: The market values of equities, such as common stocks and preferred securities or equity related investments, such as futures and options, may decline due to general market conditions which are not specifically related to a particular company. They may also decline due to factors which affect a particular industry or industries. A Fund may invest in illiquid investments. An illiquid investment is any investment that a Fund reasonably expects cannot be sold or disposed of in current market conditions in seven calendar days or less without the sale or disposition significantly changing the market value of the investment. A Fund may experience difficulty in selling illiquid investments in a timely manner at the price that it believes the investments are worth. Prices may fluctuate widely over short or extended periods in response to company, market or economic news. Markets also tend to move in cycles, with periods of rising and falling prices. This volatility may cause each Fund’s NAV to experience significant increases or decreases over short periods of time. If there is a general decline in the securities and other markets, the NAV of a Fund may lose value, regardless of the individual results of the securities and other instruments in which a Fund invests. The price a Fund could receive upon the sale of any particular portfolio investment may differ from a Fund’s valuation of the investment, particularly for securities that trade in thin or volatile markets or that are valued using a fair valuation technique or a price provided by an independent pricing service. Changes to significant unobservable inputs and assumptions (i.e., publicly traded company multiples, growth rate, time to exit) due to the lack of observable inputs may significantly impact the resulting fair value and therefore a Fund’s results of operations. As a result, the price received upon the sale of an investment may be less than the value ascribed by a Fund, and a Fund could realize a greater than expected loss or lesser than expected gain upon the sale of the investment. A Fund’s ability to value its investments may also be impacted by technological issues and/or errors by pricing services or other third-party service providers. |  |  |  |  |  |  |  |
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. For OTC options purchased, each Fund bears the risk of loss in the amount of the premiums paid plus the positive change in market values net of any collateral held by the Funds should the counterparty fail to perform under the contracts. Options written by the Funds do not typically give rise to counterparty credit risk, as options written generally obligate the Funds, and not the counterparty, to perform. The Funds may be exposed to counterparty credit risk with respect to options written to the extent each Fund deposits collateral with its counterparty to a written option. With exchange-traded options purchased, exchange-traded futures and centrally cleared swaps, 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 and centrally cleared swaps 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 significant portion of their assets in high yield securities. High yield securities that are rated below investment-grade (commonly referred to as “junk bonds”) or are unrated may be deemed speculative, involve greater levels of risk than higher-rated securities of similar maturity and are more likely to default. High yield securities may be issued by less creditworthy issuers, and issuers of high yield securities may be unable to meet their interest or principal payment obligations. High yield securities are subject to extreme price fluctuations, may be less liquid than higher rated fixed-income securities, even under normal economic conditions, and frequently have redemption features. Certain 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. Certain Funds invest a significant portion of their assets in securities backed by commercial or residential mortgage loans or in issuers that hold mortgage and other asset-backed securities. When a Fund concentrates its investments in this manner, it assumes a greater risk of prepayment or payment extension by securities issuers. Changes in economic conditions, including delinquencies and/or defaults on assets underlying these securities, can affect the value, income and/or liquidity of such positions. Investment percentages in these securities are presented in the Schedules of Investments. |  |  |  |  |  |  |  |
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 LIBOR. Although many LIBOR rates ceased to be published or no longer are representative of the underlying market they seek to measure after December 31, 2021, a selection of widely used USD LIBOR rates will continue to be published through June 2023 in order to assist with the transition. The Funds may be exposed to financial instruments tied to LIBOR to determine payment obligations, financing terms, hedging strategies or investment value. The transition process away from LIBOR might lead to increased volatility and illiquidity in markets for, and reduce the effectiveness of new hedges placed against instruments whose terms currently include LIBOR. The ultimate effect of the LIBOR transition process on the Funds is uncertain. |  |  |  |  |  |  |  |
Investment and Market Discount Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Investment and Market Discount Risk: An investment in the Fund’s common shares is subject to investment risk, including the possible loss of the entire amount that you invest. As with any stock, the price of the Fund’s common shares will fluctuate with market conditions and other factors. If shares are sold, the price received may be more or less than the original investment. Common shares are designed for long-term investors and the Fund should not be treated as a trading vehicle. Shares of closed-end management investment companies frequently trade at a discount from their net asset value. This risk is separate and distinct from the risk that the Fund’s net asset value could decrease as a result of its investment activities. At any point in time an investment in the Fund’s common shares may be worth less than the original amount invested, even after taking into account distributions paid by the Fund. During periods in which the Fund may use leverage, the Fund’s investment, market discount and certain other risks will be magnified. |  |  |  |  |  |  |  |
Debt Securities Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Debt Securities Risk: Debt securities, such as bonds, involve interest rate risk, credit risk, extension risk, and prepayment risk, among other things.
| • |  | Interest Rate Risk — The market value of bonds and other fixed-income securities changes in response to interest rate changes and other factors. Interest rate risk is the risk that prices of bonds and other fixed-income securities will increase as interest rates fall and decrease as interest rates rise.
The Fund may be subject to a greater risk of rising interest rates due to the recent period of historically low interest rates. For example, if interest rates increase by 1%, assuming a current portfolio duration of ten years, and all other factors being equal, the value of the Fund’s investments would be expected to decrease by 10%. (Duration is a measure of the price sensitivity of a debt security or portfolio of debt securities to relative changes in interest rates.) The magnitude of these fluctuations in the market price of bonds and other fixed-income securities is generally greater for those securities with longer maturities. Fluctuations in the market price of the Fund’s investments will not affect interest income derived from instruments already owned by the Fund, but will be reflected in the Fund’s net asset value. The Fund may lose money if short-term or long-term interest rates rise sharply in a manner not anticipated by Fund management. To the extent the Fund invests in debt securities that may be prepaid at the option of the obligor (such as mortgage-backed securities), the sensitivity of such securities to changes in interest rates may increase (to the detriment of the Fund) when interest rates rise. Moreover, because 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. Asecurity 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.
U.S. Government Obligations Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | U.S. Government Obligations Risk: Certain securities in which the Fund may invest, including securities issued by certain U.S. Government agencies and U.S. Government sponsored enterprises, are not guaranteed by the U.S. Government or supported by the full faith and credit of the United States. |  |  |  |  |  |  |  |
U.S. Government Mortgage-Related Securities Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | U.S. Government Mortgage-Related Securities Risk (FRA, BKT and BLW): There are a number of important differences among the agencies and instrumentalities of the U.S. Government that issue mortgage-related securities and among the securities that they issue. Mortgage-related securities guaranteed by the Government National Mortgage Association (“GNMA” or “Ginnie Mae”) are guaranteed as to the timely payment of principal and interest by GNMAand such guarantee is backed by the full faith and credit of the United States. GNMA securities also are supported by the right of GNMA to borrow funds from the U.S. Treasury to make payments under its guarantee. Mortgage-related securities issued by Fannie Mae or Freddie Mac are solely the obligations of Fannie Mae or Freddie Mac, as the case may be, and are not backed by or entitled to the full faith and credit of the United States but are supported by the right of the issuer to borrow from the Treasury. |  |  |  |  |  |  |  |
Mortgage- and Asset-Backed Securities Risks [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Mortgage- and Asset-Backed Securities Risks (FRA, BKT and BLW): Mortgage- and asset-backed securities represent interests in “pools” of mortgages or other assets, including consumer loans or receivables held in trust. Mortgage- and asset-backed securities are subject to credit, interest rate, prepayment and extension risks. These securities also are subject to risk of default on the underlying mortgage or asset, particularly during periods of economic downturn. Small movements in interest rates (both increases and decreases) may quickly and significantly reduce the value of certain mortgage-backed securities. |  |  |  |  |  |  |  |
Yield and Ratings Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Yield and Ratings Risk (BKT): The yields on debt obligations are dependent on a variety of factors, including general market conditions, conditions in the particular market for the obligation, the financial condition of the issuer, the size of the offering, the maturity of the obligation and the ratings of the issue. The ratings of Moody’s, S&P and Fitch, represent their respective opinions as to the quality of the obligations they undertake to rate. Ratings, however, are general and are not absolute standards of quality. Consequently, obligations with the same rating, maturity and interest rate may have different market prices. Subsequent to its purchase by the Fund, a rated security may cease to be rated. The Manager will consider such an event in determining whether the Fund should continue to hold the security. |  |  |  |  |  |  |  |
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 (BKT): 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. |  |  |  |  |  |  |  |
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.
Leverage Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Leverage Risk: The Fund’s use of leverage may increase or decrease from time to time in its discretion and the Fund may, in the future, determine not to use leverage. The use of leverage creates an opportunity for increased common share net investment income dividends, but also creates risks for the holders of common shares. The Fund cannot assure you that the use of leverage will result in a higher yield on the common shares. Any leveraging strategy the Fund employs may not be successful. Leverage involves risks and special considerations for common shareholders, including:
| • |  | the likelihood of greater volatility of net asset value, market price and dividend rate of the common shares than a comparable portfolio without leverage;
| • |  | the risk that fluctuations in interest rates or dividend rates on any leverage that the Fund must pay will reduce the return to the common shareholders;
| • |  | the effect of leverage in a declining market, which is likely to cause a greater decline in the net asset value of the common shares than if the Fund were not leveraged, which may result in a greater decline in the market price of the common shares;
| • |  | leverage may increase operating costs, which may reduce total return.
Any decline in the net asset value of the Fund’s investments will be borne entirely by the holders of common shares. Therefore, if the market value of the Fund’s portfolio declines, leverage will result in a greater decrease in net asset value to the holders of common shares than if the Fund were not leveraged. This greater net asset value decrease will also tend to cause a greater decline in the market price for the common shares. |  |  |  |  |  |  |  |
Reverse Repurchase Agreements Risk [Member] |  |  |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Risk [Text Block] |  | Reverse Repurchase Agreements Risk: 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. |  |  |  |  |  |  |  |
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. |  |  |  |  |  |  |  |
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 developed into a global pandemic that has resulted in numerous disruptions in the market and has had significant economic impact leaving general concern and uncertainty. Although vaccines have been developed and approved for use by various governments, the duration of the pandemic and its effects cannot be predicted with certainty. The impact of this coronavirus, and other epidemics and pandemics that may arise in the future, could affect the economies of many nations, individual companies and the market in general ways that cannot necessarily be foreseen at the present time. |  |  |  |  |  |  |  |
Common Shares [Member] |  |  |  |  |  |  |  |  |  |
Other Annual Expenses [Abstract] |  |  |  |  |  |  |  |  |  |
Basis of Transaction Fees, Note [Text Block] |  | as a percentage of net assets attributable to common shares |  |  |  |  |  |  |  |
General Description of Registrant [Abstract] |  |  |  |  |  |  |  |  |  |
Lowest Price or Bid |  |  | $ 11.63 | $ 12.06 | $ 12.93 | $ 14.85 | $ 16.95 | $ 17.88 | $ 18.27 | $ 18
Highest Price or Bid |  |  | 12.82 | 14.43 | 15.36 | 16.92 | 18.9 | 19.5 | 19.23 | 18.51
Lowest Price or Bid, NAV |  |  | 12.7 | 13.08 | 14.43 | 15.66 | 16.95 | 17.67 | 18.39 | 18.57
Highest Price or Bid, NAV |  |  | $ 13.54 | $ 14.82 | $ 15.51 | $ 16.89 | $ 17.58 | $ 17.88 | $ 18.18 | $ 18.54
Highest Price or Bid, Premium (Discount) to NAV [Percent] |  |  | (5.32%) | (2.63%) | (0.97%) | 0.18% | 7.51% | 9.06% | 5.78% | (0.16%)
Lowest Price or Bid, Premium (Discount) to NAV [Percent] |  |  | (8.43%) | (7.80%) | (10.40%) | (5.17%) |  | 1.19% | (0.65%) | (3.07%)
Latest Share Price |  | $ 12.34 |  |  |  |  |  |  |  |
Latest Premium (Discount) to NAV [Percent] |  | (5.80%) |  |  |  |  |  |  |  |
Latest NAV |  | $ 13.1 |  |  |  |  |  |  |  |
Capital Stock, Long-Term Debt, and Other Securities [Abstract] |  |  |  |  |  |  |  |  |  |
Outstanding Securities [Table Text Block] |  | DSU is authorized to issue 400 million shares, all of which were initially classified as Common Shares. FRA and BKT are authorized to issue 200 million shares, all of which were initially classified as Common Shares. |  |  |  |  |  |  |  |
Outstanding Security, Authorized [Shares] |  | 200,000,000 |  |  |  |  |  |  |  |
Outstanding Security, Held [Shares] |  | 21,304,278 |  |  |  |  |  |  |  |
[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.’s (the “Reinvestment Plan Agent”) fees for the handling of the reinvestment of dividends will be paid by the Fund. 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] | The Fund 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 the Fund’s assets attributable to investments in any equity and fixed-income mutual funds and exchange-traded funds 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 the Fund 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 the Fund (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 the Fund or a majority of the outstanding voting securities of the Fund), upon 90 days’ written notice by the Fund to the Manager.
[4] | The Fund currently pays the Manager a monthly fee at an annual contractual investment advisory fee rate of 0.65% of the average weekly value of the Fund’s net assets. For purposes of calculating this fee, “net assets” means the total assets of the Fund minus the sum of its accrued liabilities (including the aggregate indebtedness constituting financial leverage).
[5] | The Fund uses leverage in the form of reverse repurchase agreements representing 29.3% of Managed Assets at an annual interest expense to the Fund of 1.7% which is based on current market conditions. The actual amount of interest expenses borne by the Fund will vary over time in accordance with the level of the Fund’s use of reverse repurchase agreements and variations in market interest rates. Interest expense is required to be treated as an expense of the Fund for accounting purposes.

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