| N-2 - USD ($) | 3 Months Ended | 12 Months Ended | |||||||||||||||||
| Jul. 31, 2023 | Jul. 31, 2023 | Apr. 30, 2023 | Jan. 31, 2023 | Oct. 31, 2022 | Jul. 31, 2022 | Apr. 30, 2022 | Jan. 31, 2022 | Oct. 31, 2021 | Jul. 31, 2023 | Jul. 31, 2021 | Jul. 31, 2020 | Jul. 31, 2019 | Jul. 31, 2018 | Jul. 31, 2017 | Jul. 31, 2016 | Jul. 31, 2015 | Jul. 31, 2014 | ||
| Cover [Abstract] | |||||||||||||||||||
| Entity Central Index Key | 0001216583 | ||||||||||||||||||
| Amendment Flag | false | ||||||||||||||||||
| Document Type | N-CSR | ||||||||||||||||||
| Entity Registrant Name | Nuveen Preferred & Income Opportunities Fund | ||||||||||||||||||
| Fee Table [Abstract] | |||||||||||||||||||
| Shareholder Transaction Expenses [Table Text Block] | SUMMARY OF FUND EXPENSES The purpose of the tables and the examples below are to help you understand all fees and expenses that you, as a common shareholder, would bear directly or indirectly. The tables show the expenses of the Fund as a percentage of the average net assets applicable to Common Shares and not as a percentage of total assets or managed assets. Shareholder Transaction Expenses Nuveen Preferred Nuveen Preferred Maximum Sales Charge (as a percentage of offering price) 4.00% (1) 4.00% (1) Dividend Reinvestment Plan Fees (2) $2.50 $2.50 (1) A maximum sales charge of 4.00% applies only to offerings pursuant to a syndicated underwriting. The maximum sales charge for offerings made at‑the‑market is 1.00%. There is no sales charge for offerings pursuant to a private transaction. (2) You will be charged a $2.50 service charge and pay brokerage charges if you direct Computershare Inc. and Computershare Trust Company, N.A., as agent for the common shareholders, to sell your Common Shares held in a dividend reinvestment account. | ||||||||||||||||||
| Sales Load [Percent] | [1] | 4% | |||||||||||||||||
| Dividend Reinvestment and Cash Purchase Fees | [2] | $ 2.50 | |||||||||||||||||
| Other Transaction Expenses [Abstract] | |||||||||||||||||||
| Annual Expenses [Table Text Block] | As a Percentage of Net Assets Annual Expenses Nuveen Preferred Nuveen Preferred Management Fees 1.31% 1.31% Interest and Other Related Expenses (2) 3.07% 3.08% Other Expenses (3) 0.08% 0.06% Total Annual Expenses 4.46% 4.45% (1) Stated as percentages of average net assets attributable to Common Shares for the fiscal year ended July 31, 2023. (2) Interest and Other Related Expenses reflect actual expenses and fees for leverage incurred by a Fund for the fiscal year ended July 31, 2023. The types of leverage used by each Fund during the fiscal year ended July 31, 2023 are described in the Fund Leverage and the Notes to Financial Statements (Note 4 – Portfolio Securities, Note 5 – Derivative Investments, Note 6 – Fund Shares, Note 10 – Borrowings Arrangements and Reverse Repurchase Agreements) sections of this annual report. Actual Interest and Other Related Expenses incurred in the future may be higher or lower. If short-term market interest rates rise in the future, and if a Fund continues to maintain leverage, the cost of which is tied to short-term interest rates, a Fund’s interest expenses on its short-term borrowings can be expected to rise in tandem. A Fund’s use of leverage will increase the amount of management fees paid to the Fund’s adviser and sub‑advisor(s). (3) Other Expenses are based on estimated amounts for the current fiscal year. Expenses attributable to the Fund’s investments, if any, in other investment companies are currently estimated not to exceed 0.01%. | ||||||||||||||||||
| Management Fees [Percent] | [3] | 1.31% | |||||||||||||||||
| Interest Expenses on Borrowings [Percent] | [3],[4] | 3.07% | |||||||||||||||||
| Other Annual Expenses [Abstract] | |||||||||||||||||||
| Other Annual Expenses [Percent] | [3],[5] | 0.08% | |||||||||||||||||
| Total Annual Expenses [Percent] | [3] | 4.46% | |||||||||||||||||
| Expense Example [Table Text Block] | Examples The following examples illustrate the expenses, including the applicable transaction fees (referred to as the “Maximum Sales Charge” in the Shareholder Transaction Expenses table above), if any, that a common shareholder would pay on a $1,000 investment that is held for the time periods provided in the table. Each example assumes that all dividends and other distributions are reinvested in the Fund and that the Fund’s Annual Expenses, as provided above, remain the same. The examples also assume a 5% annual return. Actual expenses may be greater or less than those assumed. Moreover, the Fund’s actual rate of return may be greater or less than the hypothetical 5% return shown in the examples. Example # 1 (At‑the‑Market Transaction) The following example assumes a transaction fee of 1.00%, as a percentage of the offering price. 1 Year 3 Years 5 Years 10 Years Nuveen Preferred & Income Opportunities Fund (JPC) $54 $144 $234 $464 Nuveen Preferred & Income Securities Fund (JPS) $54 $143 $233 $463 Example # 2 (Underwriting Syndicate Transaction) The following example assumes a transaction fee of 4.00%, as a percentage of the offering price. 1 Year 3 Years 5 Years 10 Years Nuveen Preferred & Income Opportunities Fund (JPC) $83 $169 $257 $480 Nuveen Preferred & Income Securities Fund (JPS) $83 $169 $257 $479 Example # 3 (Privately Negotiated Transaction) The following example assumes there is no transaction fee. 1 Year 3 Years 5 Years 10 Years Nuveen Preferred & Income Opportunities Fund (JPC) $45 $135 $226 $458 Nuveen Preferred & Income Securities Fund (JPS) $45 $135 $226 $457 The examples should not be considered a representation of future expenses. Actual expenses may be greater or less than those shown above. | ||||||||||||||||||
| Purpose of Fee Table , Note [Text Block] | The purpose of the tables and the examples below are to help you understand all fees and expenses that you, as a common shareholder, would bear directly or indirectly. The tables show the expenses of the Fund as a percentage of the average net assets applicable to Common Shares and not as a percentage of total assets or managed assets. | ||||||||||||||||||
| Basis of Transaction Fees, Note [Text Block] | as a percentage of offering price | ||||||||||||||||||
| Other Expenses, Note [Text Block] | Other Expenses are based on estimated amounts for the current fiscal year. Expenses attributable to the Fund’s investments, if any, in other investment companies are currently estimated not to exceed 0.01%. | ||||||||||||||||||
| Financial Highlights [Abstract] | |||||||||||||||||||
| Senior Securities [Table Text Block] | The following table sets forth information regarding each Fund’s outstanding senior securities as of the end of each of the Fund’s last ten fiscal years, as applicable. Each Fund’s senior securities during this time period are comprised of borrowings that constitute “senior securities” as defined in the Investment Company Act of 1940, as amended (1940 Act). The information in this table as of and for the fiscal years ended 2023 through 2015 has been audited by KPMG LLP, independent registered public accounting firm. The information with respect to the fiscal years ended prior to 2015, where applicable, has been audited by other auditors. The Funds’ audited financial statements, including the report of KPMG LLP thereon, and accompanying notes thereto, are included in this Annual Report. Nuveen Preferred & Income Opportunities Fund (JPC) Borrowings Outstanding at the End Taxable Fund Preferred (TFP) Shares at Year Ended 7/31: Aggregate Asset Coverage Aggregate Amount Asset Coverage 2023 $ 219,600 $ 5,249 $ 150,000 $ 3,119 2022 $ 423,400 $ 3,088 $ 0 $ 0 2021 $ 462,700 $ 3,223 $ 0 $ 0 2020 $ 400,000 $ 3,280 $ 0 $ 0 2019 $ 455,000 $ 3,303 $ 0 $ 0 2018 $ 437,000 $ 3,403 $ 0 $ 0 2017 $ 540,000 $ 3,079 $ 0 $ 0 2016 $ 404,100 $ 3,526 $ 0 $ 0 2015 $ 404,100 $ 3,506 $ 0 $ 0 2014 $ 402,500 $ 3,572 $ 0 $ 0 (1) Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year and does not include any preferred shares noticed for redemption as noted on the Statement of Assets and Liabilities where applicable. (2) Asset Coverage Per $1,000: Asset coverage per $1,000 is calculated by subtracting the Fund’s liabilities and indebtedness not represented by senior securities from the Fund’s total assets, dividing the result by the aggregate amount of the Fund’s senior securities representing indebtedness then outstanding (if applicable,) plus the aggregate of the involuntary liquidation preference of the outstanding preferred shares, if applicable, and multiplying the result by 1,000. (3) Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year. | ||||||||||||||||||
| Senior Securities, Note [Text Block] | SENIOR SECURITIES The following table sets forth information regarding each Fund’s outstanding senior securities as of the end of each of the Fund’s last ten fiscal years, as applicable. Each Fund’s senior securities during this time period are comprised of borrowings that constitute “senior securities” as defined in the Investment Company Act of 1940, as amended (1940 Act). The information in this table as of and for the fiscal years ended 2023 through 2015 has been audited by KPMG LLP, independent registered public accounting firm. The information with respect to the fiscal years ended prior to 2015, where applicable, has been audited by other auditors. The Funds’ audited financial statements, including the report of KPMG LLP thereon, and accompanying notes thereto, are included in this Annual Report. Nuveen Preferred & Income Opportunities Fund (JPC) Borrowings Outstanding at the End Taxable Fund Preferred (TFP) Shares at Year Ended 7/31: Aggregate Asset Coverage Aggregate Amount Asset Coverage 2023 $ 219,600 $ 5,249 $ 150,000 $ 3,119 2022 $ 423,400 $ 3,088 $ 0 $ 0 2021 $ 462,700 $ 3,223 $ 0 $ 0 2020 $ 400,000 $ 3,280 $ 0 $ 0 2019 $ 455,000 $ 3,303 $ 0 $ 0 2018 $ 437,000 $ 3,403 $ 0 $ 0 2017 $ 540,000 $ 3,079 $ 0 $ 0 2016 $ 404,100 $ 3,526 $ 0 $ 0 2015 $ 404,100 $ 3,506 $ 0 $ 0 2014 $ 402,500 $ 3,572 $ 0 $ 0 (1) Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year and does not include any preferred shares noticed for redemption as noted on the Statement of Assets and Liabilities where applicable. (2) Asset Coverage Per $1,000: Asset coverage per $1,000 is calculated by subtracting the Fund’s liabilities and indebtedness not represented by senior securities from the Fund’s total assets, dividing the result by the aggregate amount of the Fund’s senior securities representing indebtedness then outstanding (if applicable,) plus the aggregate of the involuntary liquidation preference of the outstanding preferred shares, if applicable, and multiplying the result by 1,000. (3) Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year. | ||||||||||||||||||
| Senior Securities Headings, Note [Text Block] | The following table sets forth information regarding each Fund’s outstanding senior securities as of the end of each of the Fund’s last ten fiscal years, as applicable. Each Fund’s senior securities during this time period are comprised of borrowings that constitute “senior securities” as defined in the Investment Company Act of 1940, as amended (1940 Act). The information in this table as of and for the fiscal years ended 2023 through 2015 has been audited by KPMG LLP, independent registered public accounting firm. The information with respect to the fiscal years ended prior to 2015, where applicable, has been audited by other auditors. The Funds’ audited financial statements, including the report of KPMG LLP thereon, and accompanying notes thereto, are included in this Annual Report. | ||||||||||||||||||
| Senior Securities Highlights Audited, Note [Text Block] | The information in this table as of and for the fiscal years ended 2023 through 2015 has been audited by KPMG LLP, independent registered public accounting firm. The information with respect to the fiscal years ended prior to 2015, where applicable, has been audited by other auditors. The Funds’ audited financial statements, including the report of KPMG LLP thereon, and accompanying notes thereto, are included in this Annual Report. | ||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Investment Objectives and Practices [Text Block] | Investment Objectives The Fund’s primary investment objective is high current income. The Fund’s secondary investment objective is total return. Investment Policies The Fund will invest at least 80% of its Assets (as defined below) in preferred securities and other income producing securities, including hybrid securities such as contingent capital securities and up to 20% in other securities, primarily income-oriented securities such as corporate and taxable municipal debt and common equity. “Assets” mean the net assets of the Fund plus the amount of any borrowings for investment purposes. “Managed Assets” mean the total assets of the Fund, minus the sum of its accrued liabilities (other than Fund liabilities incurred for the express purpose of creating leverage). Total assets for this purpose shall include assets attributable to the Fund’s use of leverage (whether or not those assets are reflected in the Fund’s financial statements for purposes of generally accepted accounting principles), and derivatives will be valued at their market value. Under normal circumstances: • The Fund will invest at least 50% of its Managed Assets in securities rated investment grade (BBB/Baa and above) at the time of investment. Investment grade quality securities are those securities that, at the time of investment, are (i) rated by at least one nationally recognized statistical rating organization (“NRSRO”) within the four highest grades (Baa or BBB or better by Moody’s Investors Service, Inc. (“Moody’s”), Standard & Poor’s Corporation (“S&P”), or Fitch Ratings (“Fitch”)), or are unrated but judged to be of comparable quality. • The Fund will invest more than 25% of its Managed Assets in the securities of companies principally engaged in financial services. • The Fund is not limited in the amount of its investments in non‑U.S. issuers. The Fund may invest up to 10% of its Managed Assets in non‑U.S. dollar‑denominated securities. The Fund may invest up to 5% of its Managed Assets in preferred securities issued by companies located in emerging market countries. The foregoing policies apply only at the time of any new investment. Approving Changes in Investment Policies The Board of Trustees of the Fund may change the policies described above without a shareholder vote. However, with respect to the Fund’s policy of investing at least 80% of its Assets in preferred securities and other income producing securities, such policy may not be changed without 60 days’ prior written notice. Portfolio Contents The Fund invests in preferred securities. The Fund may invest in all types of preferred securities, including both traditional preferred securities and non‑traditional preferred securities. Traditional preferred securities are generally equity securities of the issuer that have priority over the issuer’s common shares as to the payment of dividends (i.e., the issuer cannot pay dividends on its common shares until the dividends on the preferred shares are current) and as to the payout of proceeds of bankruptcy or other liquidation, but are subordinate to an issuer’s senior debt and junior debt as to both types of payments. Additionally, in a bankruptcy or other liquidation, traditional preferred shares are generally subordinate to an issuer’s trade creditors and other general obligations. Traditional preferred securities pay a dividend, typically contingent both upon declaration by the issuer’s board and at times approval by regulators, and on the existence of current earnings (or retained earnings) in sufficient amount to source the payment. Dividend payments can be either cumulative or non‑cumulative and can be passed or deferred without limitation at the option of the issuer. Traditional preferred securities typically have no ordinary right to vote for the board of directors, except in some cases voting rights may arise if the issuer fails to pay the preferred share dividends. Traditional preferred securities may be perpetual, or have a term and typically have a fixed liquidation (or “par”) value. While some preferred securities are issued with a final maturity date, others are perpetual in nature. In certain instances, a final maturity date may be extended and/or the final payment of principal may be deferred at the issuer’s option for a specified time without triggering an event of default for the issuer. No redemption can typically take place unless all cumulative payment obligations to preferred security investors have been met, although issuers may be able to engage in open-market repurchases without regard to any cumulative dividends or interest payable. A portion of the portfolio may include investments in non‑cumulative preferred securities, whereby the issuer does not have an obligation to make up any arrearages to holders of such securities. Should an issuer default on its obligations under such a security, the amount of income earned by the Fund may be adversely affected. Non‑traditional preferred securities include hybrid preferred securities, contingent convertible capital securities and other types of preferred securities that do not have the traditional features described above. Hybrid-preferred securities often behave similarly as investments in traditional preferred securities and are regarded by market investors as being part of the preferred securities market. Hybrid-preferred securities possess varying combinations of features of both debt and preferred shares and as such they may constitute senior debt, junior debt or preferred shares in an issuer’s capital structure. As such, hybrid-preferred securities may not be subordinate to a company’s debt securities (as are traditional preferred shares). Given the various debt and equity characteristics of hybrid-preferred securities, whether a hybrid-preferred security is classified as debt or equity for purposes of reporting the Fund’s portfolio holdings may be based on the portfolio managers’ determination as to whether its debt or preferred features are preponderant, or based on the assessment of an independent data provider. Such determinations may be subjective. Hybrid-preferred securities include trust preferred securities. Trust preferred securities are typically issued by corporations, generally in the form of interest-bearing notes with preferred securities characteristics, or by an affiliated business trust of a corporation, generally in the form of beneficial interests in subordinated debentures or similarly structured securities. The trust preferred securities market consists of both fixed and adjustable coupon rate securities that are either perpetual in nature or have stated maturity dates. Trust preferred securities may defer payment of income without triggering an event of default. These securities may have many characteristics of equity due to their subordinated position in an issuer’s capital structure. Trust preferred securities may be issued by trusts or other special purpose entities. Preferred securities may also include certain forms of debt that have many characteristics of preferred shares, and that are regarded by the investment marketplace to be part of the broader preferred securities market. Among these “preferred securities” are certain exchange-listed debt issues that historically have several attributes, including trading and investment performance characteristics, in common with exchange-listed traditional preferred stock and hybrid-preferred securities. Generally, these types of “preferred securities” are senior debt or junior debt in the capital structure of an issuer. Preferred securities generally pay fixed or adjustable rate dividends or interest to investors and have preference over common stock in the payment of dividends or interest and generally the liquidation of a company’s assets, which means that a company typically must pay dividends or interest on its preferred securities before paying any dividends on its common stock. As a general matter, dividend or interest payments on preferred securities may be cumulative or non‑cumulative. The dividend or interest rates on preferred securities may be fixed or floating, or convert from fixed to floating at a specified future time; the Fund may invest without limit in such floating-rate and fixed‑to‑floating rate preferred securities. Floating-rate and fixed‑to‑floating rate preferred securities may be traditional preferred or hybrid-preferred securities. Floating-rate preferred securities pay a rate of income that resets periodically based on short- and/or longer-term interest rate benchmarks. If the associated interest rate benchmark rises, the income received from the security may increase and therefore the return offered by the floating-rate security may rise as well, making such securities less price sensitive to rising interest rates (or yields). Similarly, a fixed‑to‑floating rate security may be less price sensitive to rising interest rates (or yields), because the period over which the rate of payment is fixed is shorter than the maturity term of the bond, after which period a floating rate of payment applies. On the other hand, preferred securities are junior to most other forms of the company’s debt, including both senior and subordinated debt. Because of their subordinated position in the capital structure of an issuer, the ability to defer dividend or interest payments for extended periods of time without triggering an event of default for the issuer, and certain other features, preferred securities may have, at times, risks similar to equity instruments. The Fund’s portfolio of preferred securities may consist of fixed rate preferred and adjustable rate preferred securities. The preferred securities market continues to evolve. New securities may be developed that may be regarded by market investors as being part of the preferred securities market. Where such securities will fall in the capital structure of the issuer will depend on the structure and characteristics of the new security. For purposes of the Fund’s policy of investing at least 80% of its Assets in preferred securities and other income producing securities, the Fund considers all of the foregoing types of securities that are commonly viewed in the marketplace as preferred securities to be preferred securities, regardless of their classification in the capital structure of the issuer. Preferred securities are typically issued by corporations, generally in the form of interest or dividend bearing instruments, or by an affiliated business trust of a corporation, generally in the form of beneficial interests in subordinated debentures or similarly structured securities. Preferred securities may either trade over‑the‑counter, or trade on an exchange. The preferred securities market is generally divided into the $25 par “retail” and the $1,000 par “institutional” segments. The $25 par segment is typified by securities that are listed on the New York Stock Exchange (“NYSE”), which trade and are quoted with accrued dividend or interest income, and which are often callable. The institutional segment is typified by $1,000 par value securities that are not exchange-listed. The Fund may invest in preferred securities of either segment. The Fund may invest in contingent capital securities. Contingent capital securities (sometimes referred to as “CoCos”) are securities issued primarily by non‑U.S. financial institutions. Specific CoCo structures vary by country of domicile and by each issue. All CoCos have mechanisms that absorb losses or reduces the value of the CoCo due to deterioration of the issuer’s financial condition and status as a going concern. Loss absorption mechanisms, which may include conversion into common equity and principal write-down, are intended for the benefit of the issuer and when triggered will likely negatively impact the value of the CoCo to the detriment of the CoCo investor. Loss absorption mechanisms can be triggered by capital levels or market value metrics of the issuers dropping below a certain predetermined level or at the discretion of the issuer regulator/ supervisory entity. Unlike traditional convertible securities, the conversion is not voluntary and the equity conversion or principal write-down features are tailored to the issuer and its regulatory requirements. Due to increased regulatory requirements for higher capital levels for financial institutions, the issuance of CoCo instruments has increased in the last several years and is expected to continue. The Fund may invest in common stock. Common stock generally represents an equity ownership interest in an issuer. Although common stocks have historically generated higher average total returns than fixed-income securities over the long term, common stocks also have experienced significantly more volatility in those returns and may underperform relative to fixed-income securities during certain periods. An adverse event, such as an unfavorable earnings report, may depress the value of a particular common stock held by the Fund. Also, prices of common stocks are sensitive to general movements in the stock market and a drop in the stock market may depress the price of common stocks to which the Fund has exposure. Common stock prices fluctuate for several reasons including changes in investors’ perceptions of the financial condition of an issuer or the general condition of the relevant stock market, or the occurrence of political or economic events which affect the issuer. In addition, common stock prices may be particularly sensitive to rising interest rates, which increases borrowing costs and the costs of capital. Additional types of equity securities (other than preferred securities) in which the Fund may invest include convertible securities, real estate investment trusts (“REITs”), warrants, rights, depositary receipts (which reference ownership of underlying non‑U.S. securities) and other types of securities with equity characteristics. The Fund’s equity investments also may include securities of other investment companies (including open‑end funds, closed‑end funds and exchange-traded funds (“ETFs”)). The Fund will invest in securities of companies primarily engaged in the financial services industry. A financial services company is one that is primarily involved in banking, mortgage finance, consumer finance, specialized finance, investment banking and brokerage, asset management and custody, corporate lending, insurance, financial instruments or real estate, including business development companies (“BDCs”) and REITs. The Fund may invest in debt securities. The debt securities in which the Fund may invest include corporate debt securities and U.S. government and agency debt securities. Generally, debt securities typically, but not always, possess the following characteristics: a specified maturity or term, at which time the issuer is contractually obligated to pay the associated principal amount of debt to the debtholders; interest payments that are a contractual and enforceable obligation as of the stated payment date, and not contingent either on payment‑by‑payment declaration by the issuer’s board or on the demonstrated existence of company earnings as a source for the payment; and do not entitle the holder to exercise governance of or control over the issuer. In the capital structure of an issuer, debt securities can be senior debt or junior debt. A senior debt security has priority over any other type of security in a company’s capital structure as to the payment of any promised income (typically denoted as interest) from the issuer, and as to payout of the proceeds of the bankruptcy or other liquidation of the company. At times, the issuer will have pledged specific assets or revenues to secure the rights of the holder of the debt security to payments of interest and principal such that the proceeds of the specific assets or revenues must be used to satisfy these debt obligations prior to being applied to any of the issuer’s other obligations in a bankruptcy or other liquidation. In the event that the assets securing the debt security are not sufficient to fully satisfy such obligations in a bankruptcy or other liquidation, the remainder of such obligations will generally have the same priority as an issuer’s trade creditors and other general obligations, but still have priority of payment relative to the issuer’s preferred shares and common shares. Sometimes referred to as subordinated or mezzanine debt, junior debt stands behind the senior debt as to its rights to receive promised income payments (again, typically denoted as interest) from the issuer, and payouts of the proceeds of bankruptcy or other liquidation, but will have priority of payment relative to the issuer’s preferred shares and common shares. The Fund may invest in convertible securities. Convertible securities are hybrid securities that combine the investment characteristics of bonds and common stocks. Convertible securities typically consist of debt securities or preferred securities that may be converted within a specified period of time (typically for the entire life of the security) into a certain amount of common stock or other equity security of the same or a different issuer at a predetermined price. They also include debt securities with warrants or common stock attached and derivatives combining features of debt securities and equity securities. Convertible securities entitle the holder to receive interest paid or accrued on debt securities, or dividends paid or accrued on preferred securities, until the securities mature or are redeemed, converted or exchanged. Before conversion, convertible securities have characteristics similar to nonconvertible income securities in that they ordinarily provide a stable stream of income with generally higher yields than those of common stocks of the same or similar issuers, but lower yields than comparable nonconvertible securities. The value of a convertible security is influenced by changes in interest rates, with investment value generally declining as interest rates increase and increasing as interest rates decline. The credit standing of the issuer and other factors also may have an effect on the convertible security’s investment value. Convertible securities are subordinate in rank to any senior debt obligations of the same issuer and, therefore, an issuer’s convertible securities entail more risk than its debt obligations. The Fund may invest in REITs. REITs are typically publicly traded corporations or trusts that invest in residential or commercial real estate. REITs generally can be divided into the following three types: (i) equity REITs which invest the majority of their assets directly in real property and derive their income primarily from rents and capital gains or real estate appreciation; (ii) mortgage REITs which invest the majority of their assets in real estate mortgage loans and derive their income primarily from interest payments; and (iii) hybrid REITs which combine the characteristics of equity REITs and mortgage REITs. The Fund can invest in common stock, preferred securities, debt securities and convertible securities issued by REITs. The Fund may invest in securities of foreign issuers through the direct investment in securities of such companies and through depositary receipts. For purposes of identifying foreign issuers, the Fund will use Bloomberg classifications, which employ the following factors listed in order of importance: (i) the country in which the company’s management is located, (ii) the country in which the company’s securities are primarily listed, (iii) the country from which the company primarily receives revenue and (iv) the company’s reporting currency. The Fund may purchase depositary receipts such as American Depositary Receipts (“ADRs”), European Depositary Receipts (“EDRs”) and Global Depositary Receipts (“GDRs”). ADRs, EDRs and GDRs are certificates evidencing ownership of shares of foreign issuers and are alternatives to purchasing directly the underlying foreign securities in their national markets and currencies. The Fund may invest in securities of emerging markets issuers. Emerging markets issuers are those (i) whose securities are traded principally on a stock exchange or over‑the‑counter in an emerging market country, (ii) organized under the laws of an emerging market country or (iii) whose principal place of business or principal office(s) is in an emerging market country. Emerging market countries include any country other than Canada, the United States and the countries comprising the MSCI EAFE ® The Fund may invest in zero coupon bonds. A zero coupon bond is a bond that typically does not pay interest for the entire life of the obligation or for an initial period after the issuance of the obligation. The Fund may buy and sell securities on a when-issued or delayed delivery basis, making payment or taking delivery at a later date, normally within 15 to 45 days of the trade date. The Fund may invest in illiquid securities (i.e., securities that are not readily marketable), including, but not limited to, restricted securities (securities the disposition of which is restricted under the federal securities laws), securities that may be resold only pursuant to Rule 144A under the Securities Act of 1933, as amended (the “1933 Act”), and repurchase agreements with maturities in excess of seven days. The Fund may use derivative instruments to seek to hedge some of the risk of the Fund’s investments or its leverage, to enhance return, to serve as a substitute for a position in an underlying asset, to reduce transaction costs, to manage the Fund’s effective interest rate exposure, to maintain full market exposure, to manage cash flows or to preserve capital. Such instruments may include financial futures contracts, swap contracts (including interest rate and credit default swaps), options on equity securities, options on financial futures or other derivative instruments. The Fund may also invest in securities of other open- or closed‑end investment companies (including ETFs) that invest primarily in the types in which the Fund may invest directly, to the extent permitted by the Investment Company Act of 1940, as amended (the “1940 Act”) and the rules and regulations issued thereunder. Use of Leverage The Fund uses leverage to pursue its investment objectives. The Fund may use leverage to the extent permitted by the 1940 Act, including the following forms of leverage: (a) borrowings, including loans from certain financial institutions, and/or the issuance of debt securities; (b) the issuance of preferred shares of beneficial interest (“Preferred Shares”); and (c) engaging in reverse repurchase agreements and economically similar transactions. The Fund also may borrow money for repurchase of its shares or as a temporary measure for extraordinary or emergency purposes, including the payment of dividends and the settlement of securities transactions which otherwise might require untimely dispositions of Fund securities. Temporary Defensive Periods During temporary defensive periods, the Fund may invest up to 100% of its assets in high quality, short-term securities, and in short-, intermediate-, or long-term U.S. Treasury securities. There can be no assurance that such techniques will be successful. Accordingly, during such periods, the Fund may not achieve its investment objectives. | ||||||||||||||||||
| Risk Factors [Table Text Block] | PRINCIPAL RISKS OF THE FUNDS The factors that are most likely to have a material effect on a particular Fund’s portfolio as a whole are called “principal risks.” Each Fund is subject to the principal risks indicated below, whether through direct investment or derivative positions. Each Fund may be subject to additional risks other than those identified and described below because the types of investments made by a Fund can change over time. Risk Nuveen Nuveen Preferred Nuveen Preferred Nuveen Nuveen Variable Portfolio Level Risks Asset-Backed Securities (“ABS”) Risk - - - - X Below Investment Grade Risk X X X X X Call Risk X X X X X Collateralized Mortgage-Backed Securities (“CMBS”) and Mortgage-Backed Securities (“MBS”) Risk - - - - X Common Stock Risk X X X X - Concentration and Financial Services Sector Risk X X X X X Contingent Capital Securities (“CoCos”) Risk X X X X X Convertible Securities Risk X X X X X Credit Risk X X X X X Credit Spread Risk X X X X X Debt Securities Risk X X X X X Defaulted and Distressed Investments Risk - - - X - Deflation Risk X X X X X Derivatives Risk X X X X X Duration Risk X X X X X Emerging Markets Risk X X X X X Equity Securities Risk - - - - X Financial Futures and Options Transactions Risk X X X X X Floating-Rate and Fixed‑to‑Floating Rate Securities Risk X X X X - Foreign Currency Risk X - X - - Hedging Risk X X X X X Illiquid Investments Risk X X X X X Income Risk X X X X X Inflation Risk X X X X X Inflation Correlation Risk X X X X X Interest Rate Risk X X X X X Inverse Floating Rate Securities Risk - - - - X Loan Risk - - - - X Municipal Securities Market Liquidity Risk - X - X X Municipal Securities Market Risk - X - X X Non‑U.S. Securities Risk X X X X X Other Investment Companies Risk X X X X X Preferred and Hybrid Preferred Securities Risk X X X X X Risk Nuveen Nuveen Preferred Nuveen Preferred Nuveen Nuveen Variable Portfolio Level Risks Reinvestment Risk X X X X X Senior Loan Risk - - - - X Sovereign Government and Supranational Debt Risk - - - - X Swap Transactions Risk X X X X X Unrated Securities Risk X X X X X U.S. Government Securities Risk X X X X X Valuation Risk X X X X X Warrants and Equity Securities Risk X X X X - When-Issued and Delayed-Delivery Transactions X X X X X Zero Coupon Bonds Risk X X X X - Risk Fund Level and Other Risks Anti-Takeover Provisions X X X X X Borrowing Risk X X X X X Counterparty Risk X X X X X Cybersecurity Risk X X X X X Fund Tax Risk X X X X X Global Economic Risk X X X X X Investment and Market Risk X X X X X Legislation and Regulatory Risk X X X X X Leverage Risk X X X X X Limited Term Risk - X - - - Market Discount from Net Asset Value X X X X X Recent Market Conditions X X X X X Reverse Repurchase Agreement Risk X X X X X Tax Risk X X X X X Portfolio Level Risks: Below Investment Grade Risk. If a below investment grade investment goes into default, or its issuer enters bankruptcy, it might be difficult to sell that investment in a timely manner at a reasonable price. Call Risk. Common Stock Risk. Concentration and Financial Services Sector Risk. • financial services companies may suffer a setback if regulators change the rules under which they operate, which may increase costs for or limit the ability to offer new services or products and make it difficult to pass increased costs on to consumers; • unstable interest rates can have a disproportionate effect on the financial services sector; • financial services companies whose securities the Fund may purchase may themselves have concentrated portfolios, such as a high level of loans to real estate developers, which makes them vulnerable to economic conditions that affect that sector; and • financial services companies have been affected by increased competition, which could adversely affect the profitability or viability of such companies. The profitability of many types of financial services companies may be adversely affected in certain market cycles, including periods of rising interest rates, which may restrict the availability and increase the cost of capital, and declining economic conditions, which may cause credit losses due to financial difficulties of borrowers. Because many types of financial services companies are especially vulnerable to these economic cycles, the Fund’s investments in these companies may lose significant value during such periods. Contingent Capital Securities (“CoCos”) Risk. CoCos may be subject to an automatic write-down (i.e., the automatic write-down of the principal amount or value of the securities, potentially to zero, and the cancellation of the securities) under certain circumstances, which could result in the Fund losing a portion or all of its investment in such securities. In addition, the Fund may not have any rights with respect to repayment of the principal amount of the securities that has not become due or the payment of interest or dividends on such securities for any period from (and including) the interest or dividend payment date falling immediately prior to the occurrence of such automatic write-down. An automatic write-down could also result in a reduced income rate if the dividend or interest payment is based on the security’s par value. Coupon payments on CoCos may be discretionary and may be cancelled by the issuer for any reason or may be subject to approval by the issuer’s regulator and may be suspended in the event there are insufficient distributable reserves. In certain scenarios, investors in CoCos may suffer a loss of capital ahead of equity holders or when equity holders do not. There is no guarantee that the Fund will receive a return of principal on CoCos. Any indication that an automatic write-down or conversion event may occur can be expected to have a material adverse effect on the market price of CoCos. The prices of CoCos may be volatile. Additionally, the trading behavior of a given issuer’s CoCo may be strongly impacted by the trading behavior of other issuers’ CoCos, such that negative information from an unrelated CoCo may cause a decline in value of one or more CoCos held by a fund. Accordingly, the trading behavior of CoCos may not follow the trading behavior of other similarly structured securities. CoCos are issued primarily by financial institutions. Therefore, CoCos present substantially increased risks at times of financial turmoil, which could affect financial institutions more than companies in other sectors and industries. Convertible Securities Risk. Convertible securities have characteristics of both equity and debt securities and, as a result, are exposed to certain additional risks that are typically associated with debt, including but not limited to Interest Rate Risk, Credit Risk, Below Investment Grade Risk and Unrated Securities Risk. The value of a convertible security is influenced by both the yield of non‑convertible securities of comparable issuers and by the value of the underlying common stock. Convertible securities generally offer lower interest or dividend yields than non‑convertible securities of similar credit quality. The market values of convertible securities tend to decline as interest rates increase and, conversely, to increase as interest rates decline. However, the convertible security’s market value tends to reflect the market price of the common stock of the issuing company when that stock price is greater than the convertible security’s “conversion price.” The conversion price is defined as the predetermined price at which the convertible security could be exchanged for the associated common stock. As the market price of the underlying common stock declines, the price of the convertible security tends to be influenced more by the yield of the convertible security. Thus, the convertible security may not decline in price to the same extent as the underlying common stock. Convertible securities fall below debt obligations of the same issuer in order of preference or priority in the event of a liquidation and are typically unrated or rated lower than such debt obligations. Credit Risk. Credit Spread Risk. Debt Securities Risk. Deflation Risk. Derivatives Risk. It is possible that regulatory or other developments in the derivatives market, including changes in government regulation could adversely impact the Fund’s ability to invest in certain derivatives or successfully use derivative instruments. Duration Risk. Emerging Markets Risk. Financial Futures and Options Transactions Risk. If the Fund engages in futures transactions or in the writing of options on futures, it will be required to maintain initial margin and maintenance margin and may be required to make daily variation margin payments in accordance with applicable rules of the exchanges and the Commodity Futures Trading Commission (“CFTC”). If the Fund purchases a financial futures contract or a call option or writes a put option in order to hedge the anticipated purchase of securities, and if the Fund fails to complete the anticipated purchase transaction, the Fund may have a loss or a gain on the futures or options transaction that will not be offset by price movements in the securities that were the subject of the anticipatory hedge. There can be no assurance that a liquid market will exist at a time when the Fund seeks to close out a derivatives or futures or a futures option position, and the Fund would remain obligated to meet margin requirements until the position is closed. Floating-Rate and Fixed‑to‑Floating‑Rate Securities Risk. Foreign Currency Risk. Hedging Risk. Illiquid Investments Risk. Income Risk Inflation Risk. Inflation Correlation Risk. Interest Rate Risk. vice versa . As interest rates decline, issuers of securities may prepay principal earlier than scheduled, forcing the Fund to reinvest in lower-yielding securities and potentially reducing the Fund’s income. As interest rates increase, slower than expected principal payments may extend the average life of securities, potentially locking in a below-market interest rate and reducing the Fund’s value. In typical market interest rate environments, the prices of longer-term securities generally fluctuate more than prices of shorter-term securities as interest rates change. Non‑U.S. Securities Risk. Other Investment Companies Risk. pro rata portion of the other investment companies’ expenses, including advisory fees. These expenses are in addition to the direct expenses of the Fund’s own operations. As a result, the cost of investing in investment company shares may exceed the costs of investing directly in its underlying investments. In addition, securities of other investment companies may be leveraged. As a result, the Fund may be indirectly exposed to leverage through an investment in such securities and therefore magnify the Fund’s leverage risk. With respect to ETF’s, an ETF that is based on a specific index may not be able to replicate and maintain exactly the composition and relative weighting of securities in the index. The value of an ETF based on a specific index is subject to change as the values of its respective component assets fluctuate according to market volatility. ETFs typically rely on a limited pool of authorized participants to create and redeem shares, and an active trading market for ETF shares may not develop or be maintained. The market value of shares of ETFs and closed‑end funds may differ from their NAV. Preferred and Hybrid Preferred Securities Risk. • Limited Voting Rights Risk. Generally, preferred security holders (such as the Fund) have no voting rights with respect to the issuing company unless preferred dividends have been in arrears for a specified number of periods, at which time the preferred security holders may elect a number of directors to the issuer’s board. Generally, once all the arrearages have been paid, the preferred security holders no longer have voting rights. In the case of certain preferred securities issued by trusts or special purpose entities, holders generally have no voting rights except if a declaration of default occurs and is continuing. In such an event, preferred security holders generally would have the right to appoint and authorize a trustee to enforce the trust’s or special purpose entity’s rights as a creditor under the agreement with its operating company. • Special Redemption Rights Risk. In certain circumstances, an issuer of preferred securities may redeem the securities at par prior to their stated maturity date. For instance, for certain types of preferred securities, a redemption may be triggered by a change in federal income tax or securities laws or regulatory or major corporate action. A redemption by the issuer may negatively impact the return of the security held by the Fund. • Payment Deferral and Omission Risk. Generally, preferred securities may be subject to provisions that allow an issuer, under certain conditions, to skip (“non‑cumulative” preferred securities) or defer (“cumulative” preferred securities) distributions for a stated period without any adverse consequences to the issuer. Non‑cumulative preferred securities can defer distributions indefinitely. Cumulative preferred securities typically contain provisions that allow an issuer, at its discretion, to defer distribution payments for up to 10 years. If the Fund owns a preferred security that is deferring its distribution, the Fund may be required to report income for tax purposes although it has not yet received such income. In addition, recent changes in bank regulations may increase the likelihood for issuers to defer or omit distributions. • Credit and Subordination Risk. Credit risk is the risk that a security in the Fund’s portfolio will decline in price or the issuer of the security will fail to make dividend, interest or principal payments when due because the issuer experiences a decline in its financial status. Preferred securities are generally subordinated to bonds and other debt instruments in a company’s capital structure in terms of having priority to corporate income, claims to corporate assets and liquidation payments, and therefore will be subject to greater credit risk than more senior debt instruments. • Floating Rate and Fixed‑to‑Floating Rate Securities Risk. The market value of floating rate securities is a reflection of discounted expected cash flows based on expectations for future interest rate resets. The market value of such securities may fall in a declining interest rate environment and may also fall in a rising interest rate environment if there is a lag between the rise in interest rates and the reset. This risk may also be present with respect to fixed‑to‑floating rate securities in which the Fund may invest. A secondary risk associated with declining interest rates is the risk that income earned by the Fund on floating rate and fixed‑to‑floating rate securities may decline due to lower coupon payments on floating-rate securities. • Liquidity Risk. Certain preferred securities may be substantially less liquid than many other securities, such as U.S. Government securities or common stock. Illiquid securities involve the risk that the securities will not be able to be sold at the time desired by the Fund or at prices approximating the value at which the Fund is carrying the securities on its books. • Regulatory Risk. Issuers of preferred securities may be in industries that are heavily regulated and that may receive government funding. The value of preferred securities issued by these companies may be affected by changes in government policy, such as increased regulation, ownership restrictions, deregulation or reduced government funding. • New Types of Securities Risk. From time to time, preferred securities, including hybrid-preferred securities, have been, and may in the future be, offered having features other than those described herein. The Fund reserves the right to invest in these securities if the Sub‑Advisers believe that doing so would be consistent with the Fund’s investment objective and policies. Since the market for these instruments would be new, the Fund may have difficulty disposing of them at a suitable price and time. In addition to limited liquidity, these instruments may present other risks, such as high price volatility. Reinvestment Risk. Swap Transactions Risk. Unrated Securities Risk. which means the Fund might have difficulty selling them promptly at an acceptable price. To the extent that the Fund invests in unrated securities, the Fund’s ability to achieve its investment objectives will be more dependent on the investment adviser’s credit analysis than would be the case when the Fund invests in rated securities. U.S. Government Securities Risk. Valuation Risk. Warrants and Equity Securities Risk. When-Issued and Delayed-Delivery Transactions Risk. Zero Coupon Bonds Risk. Fund Level and Other Risks: Anti-Takeover Provisions. Borrowing Risk. Counterparty Risk. Cybersecurity Risk. Fund Tax Risk. Global Economic Risk. The Fund does not know and cannot predict how long the securities markets may be affected by these events and the effects of these and similar events in the future on the U.S. economy and securities markets. The Fund may be adversely affected by abrogation of international agreements and national laws which have created the market instruments in which the Fund may invest, failure of the designated national and international authorities to enforce compliance with the same laws and agreements, failure of local, national and international organizations to carry out the duties prescribed to them under the relevant agreements, revisions of these laws and agreements which dilute their effectiveness or conflicting interpretation of provisions of the same laws and agreements. Investment and Market Risk. Legislation and Regulatory Risk. Leverage Risk. The Fund will pay (and common shareholders will bear) any costs and expenses relating to the Fund’s use of leverage, which will result in a reduction in the Fund’s NAV. The investment adviser may, based on its assessment of market conditions and composition of the Fund’s holdings, increase or decrease the amount of leverage. Such changes may impact the Fund’s distributions and the price of the common shares in the secondary market. The Fund may seek to refinance its leverage over time, in the ordinary course, as current forms of leverage mature or it is otherwise desirable to refinance; however, the form that such leverage will take cannot be predicted at this time. If the Fund is unable to replace existing leverage on comparable terms, its costs of leverage will increase. Accordingly, there is no assurance that the use of leverage may result in a higher yield or return to common shareholders. The amount of fees paid to the investment adviser and the sub‑advisor for investment advisory services will be higher if the Fund uses leverage because the fees will be calculated based on the Fund’s Managed Assets - this may create an incentive for the investment adviser and the sub‑advisor to leverage the Fund or increase the Fund’s leverage. Market Discount from Net Asset Value. Recent Market Conditions. Ukraine has experienced ongoing military conflict, most recently in February 2022 when Russia invaded Ukraine; this conflict may expand and military attacks could occur elsewhere in Europe. Europe has also been struggling with mass migration from the Middle East and Africa. The ultimate effects of these events and other socio-political or geographical issues are not known but could profoundly affect global economies and markets. The ongoing trade war between China and the United States, including the imposition of tariffs by each country on the other country’s products, has created a tense political environment. These actions may trigger a significant reduction in international trade, the oversupply of certain manufactured goods, substantial price reductions of goods and possible failure of individual companies and/or large segments of China’s export industry, which could have a negative impact on the Fund’s performance. U.S. companies that source material and goods from China and those that make large amounts of sales in China would be particularly vulnerable to an escalation of trade tensions. Uncertainty regarding the outcome of the trade tensions and the potential for a trade war could cause the U.S. dollar to decline against safe haven currencies, such as the Japanese yen and the euro. Events such as these and their consequences are difficult to predict and it is unclear whether further tariffs may be imposed or other escalating actions may be taken in the future. Recently the U.S. Federal Reserve (the “Fed”) has sharply raised interest rates and has signaled an intention to continue to do so or maintain higher interest rates until current inflation levels re‑align with the Fed’s long-term inflation target. Changing interest rate environments impact the various sectors of the economy in different ways. For example, in March 2023, the Federal Deposit Insurance Corporation (““FDIC””) was appointed receiver for each of Silicon Valley Bank and Signature Bank, the second- and third-largest bank failures in U.S. history, which failures may be attributable, in part, to rising interest rates. Bank failures may have a destabilizing impact on the broader banking industry or markets generally. The impact of these developments in the near- and long-term is unknown and could have additional adverse effects on economies, financial markets and asset valuations around the world. Reverse Repurchase Agreement Risk. | ||||||||||||||||||
| Effects of Leverage [Text Block] | EFFECTS OF LEVERAGE The following table is furnished in response to requirements of the SEC. It is designed to illustrate the effects of leverage through the use of senior securities, as that term is defined under Section 18 of the 1940 Act, as well as certain other forms of leverage, such as reverse repurchase agreements, on common share total return, assuming investment portfolio total returns (consisting of income and changes in the value of investments held in the Fund’s portfolio) of ‑10%, ‑5%, 0%, 5% and 10%. The table below reflects each Fund’s (i) continued use of leverage as of July 31, 2023 as a percentage of Managed Assets (including assets attributable to such leverage), (ii) the estimated annual effective interest expense rate payable by the Funds on such instruments (based on actual leverage costs incurred during the fiscal year ended July 31, 2023) as set forth in the table, and (iii) the annual return that the Fund’s portfolio must experience (net of expenses) in order to cover such costs of leverage based on such estimated annual effective interest expense rate. The information below does not reflect any Fund’s use of certain other forms of economic leverage achieved through the use of certain derivative instruments. The numbers are merely estimates, used for illustration. The costs of leverage may vary frequently and may be significantly higher or lower than the estimated rate. The assumed investment portfolio returns in the table below are hypothetical figures and are not necessarily indicative of the investment portfolio returns experienced or expected to be experienced by the Funds. Your actual returns may be greater or less than those appearing below. Nuveen Nuveen Preferred Nuveen Preferred Nuveen Nuveen Variable Estimated Leverage as a Percentage of Managed Assets (Including Assets Attributable to Leverage) 37.59 % 36.93 % 35.47 % 34.29 % 36.49 % Estimated Annual Effective Leverage Expense Rate Payable by Fund on Leverage 5.01 % 5.00 % 5.06 % 4.92 % 4.90 % Annual Return Fund Portfolio Must Experience (net of expenses) to Cover Estimated Annual Effective Interest Expense Rate on Leverage 1.88 % 1.85 % 1.80 % 1.69 % 1.79 % Common Share Total Return for (10.00)% Assumed Portfolio Total Return (19.04 )% (18.78 )% (18.28 )% (17.79 )% (18.56 )% Common Share Total Return for (5.00)% Assumed Portfolio Total Return (11.03 )% (10.86 )% (10.53 )% (10.18 )% (10.69 )% Common Share Total Return for 0.00% Assumed Portfolio Total Return (3.02 )% (2.93 )% (2.78 )% (2.57 )% (2.82 )% Common Share Total Return for 5.00% Assumed Portfolio Total Return 4.99 % 5.00 % 4.97 % 5.04 % 5.06 % Common Share Total Return for 10.00% Assumed Portfolio Total Return 13.00 % 12.93 % 12.71 % 12.65 % 12.93 % Common Share total return is composed of two elements — the distributions paid by the Fund to holders of common shares (the amount of which is largely determined by the net investment income of the Fund after paying dividend payments on any preferred shares issued by the Fund and expenses on any forms of leverage outstanding) and gains or losses on the value of the securities and other instruments the Fund owns. As required by SEC rules, the table assumes that the Funds are more likely to suffer capital losses than to enjoy capital appreciation. For example, to assume a total return of 0%, the Fund must assume that the income it receives on its investments is entirely offset by losses in the value of those investments. This table reflects hypothetical performance of the Fund’s portfolio and not the actual performance of the Fund’s common shares, the value of which is determined by market forces and other factors. Should the Fund elect to add additional leverage to its portfolio, any benefits of such additional leverage cannot be fully achieved until the proceeds resulting from the use of such leverage have been received by the Fund and invested in accordance with the Fund’s investment objectives and policies. As noted above, the Fund’s willingness to use additional leverage, and the extent to which leverage is used at any time, will depend on many factors. | ||||||||||||||||||
| Annual Interest Rate [Percent] | 37.59% | ||||||||||||||||||
| Annual Interest Rate, Current [Percent] | 5.01% | ||||||||||||||||||
| Annual Coverage Return Rate [Percent] | 1.88% | ||||||||||||||||||
| Effects of Leverage [Table Text Block] | The numbers are merely estimates, used for illustration. The costs of leverage may vary frequently and may be significantly higher or lower than the estimated rate. The assumed investment portfolio returns in the table below are hypothetical figures and are not necessarily indicative of the investment portfolio returns experienced or expected to be experienced by the Funds. Your actual returns may be greater or less than those appearing below. Nuveen Nuveen Preferred Nuveen Preferred Nuveen Nuveen Variable Estimated Leverage as a Percentage of Managed Assets (Including Assets Attributable to Leverage) 37.59 % 36.93 % 35.47 % 34.29 % 36.49 % Estimated Annual Effective Leverage Expense Rate Payable by Fund on Leverage 5.01 % 5.00 % 5.06 % 4.92 % 4.90 % Annual Return Fund Portfolio Must Experience (net of expenses) to Cover Estimated Annual Effective Interest Expense Rate on Leverage 1.88 % 1.85 % 1.80 % 1.69 % 1.79 % Common Share Total Return for (10.00)% Assumed Portfolio Total Return (19.04 )% (18.78 )% (18.28 )% (17.79 )% (18.56 )% Common Share Total Return for (5.00)% Assumed Portfolio Total Return (11.03 )% (10.86 )% (10.53 )% (10.18 )% (10.69 )% Common Share Total Return for 0.00% Assumed Portfolio Total Return (3.02 )% (2.93 )% (2.78 )% (2.57 )% (2.82 )% Common Share Total Return for 5.00% Assumed Portfolio Total Return 4.99 % 5.00 % 4.97 % 5.04 % 5.06 % Common Share Total Return for 10.00% Assumed Portfolio Total Return 13.00 % 12.93 % 12.71 % 12.65 % 12.93 % | ||||||||||||||||||
| Return at Minus Ten [Percent] | (19.04%) | ||||||||||||||||||
| Return at Minus Five [Percent] | (11.03%) | ||||||||||||||||||
| Return at Zero [Percent] | (3.02%) | ||||||||||||||||||
| Return at Plus Five [Percent] | 4.99% | ||||||||||||||||||
| Return at Plus Ten [Percent] | 13% | ||||||||||||||||||
| Effects of Leverage, Purpose [Text Block] | The following table is furnished in response to requirements of the SEC. It is designed to illustrate the effects of leverage through the use of senior securities, as that term is defined under Section 18 of the 1940 Act, as well as certain other forms of leverage, such as reverse repurchase agreements, on common share total return, assuming investment portfolio total returns (consisting of income and changes in the value of investments held in the Fund’s portfolio) of ‑10%, ‑5%, 0%, 5% and 10%. The table below reflects each Fund’s (i) continued use of leverage as of July 31, 2023 as a percentage of Managed Assets (including assets attributable to such leverage), (ii) the estimated annual effective interest expense rate payable by the Funds on such instruments (based on actual leverage costs incurred during the fiscal year ended July 31, 2023) as set forth in the table, and (iii) the annual return that the Fund’s portfolio must experience (net of expenses) in order to cover such costs of leverage based on such estimated annual effective interest expense rate. The information below does not reflect any Fund’s use of certain other forms of economic leverage achieved through the use of certain derivative instruments. | ||||||||||||||||||
| Share Price [Table Text Block] | The following table shows for the periods indicated: (i) the high and low sales prices for the Common Shares reported as of the end of the day on the NYSE, (ii) the high and low net asset value (NAV) of the Common Shares, and (iii) the high and low of the premium/(discount) to NAV (expressed as a percentage) of shares of the Common Shares. Market Price NAV Premium/(Discount) Fiscal Quarter End High Low High Low High Low July 2023 $6.60 $5.98 $7.45 $6.86 (9.37)% (13.08)% April 2023 $8.03 $6.06 $8.47 $6.62 (4.40)% (12.02)% January 2023 $7.96 $7.12 $8.38 $7.67 (2.77)% (9.64)% October 2022 $8.41 $6.70 $8.52 $7.56 (0.36)% (11.84)% July 2022 $8.29 $7.35 $8.70 $7.99 (2.50)% (9.25)% April 2022 $9.31 $7.99 $9.53 $8.70 (2.31)% (8.91)% January 2022 $9.99 $8.99 $9.90 $9.48 1.33% (6.05)% October 2021 $10.06 $9.84 $9.98 $9.81 1.41% (0.50)% The following table shows, as of July 31, 2023 the Fund’s: (i) NAV per Common Share, (ii) market price, (iii) percentage of premium/(discount) to NAV per Common Share and, (iv) net assets attributable to Common Shares. July 31, 2023 Nuveen Preferred Nuveen Preferred NAV per Common Share $ 7.45 $ 7.48 Market Price $ 6.60 $ 6.56 Percentage of Premium/(Discount) to NAV per Common Share (11.41)% (12.30)% Net Assets Attributable to Common Shares $ 783,007,340 $ 1,539,324,694 Shares of closed‑end investment companies, including those of the Funds, may frequently trade at prices lower than NAV, the Fund’s Board of Trustees (Board) has currently determined that, at least annually, it will consider action that might be taken to reduce or eliminate any material discount from NAV in respect of Common Shares, which may include the repurchase of such shares in the open market or in private transactions, the making of a tender offer for such shares at NAV, or the conversion of the Fund to an open‑end investment company. The Funds cannot assure you that their Board will decide to take any of these actions, or that share repurchases or tender offers will actually reduce market discount. | ||||||||||||||||||
| Below Investment Grade Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Below Investment Grade Risk. If a below investment grade investment goes into default, or its issuer enters bankruptcy, it might be difficult to sell that investment in a timely manner at a reasonable price. | ||||||||||||||||||
| Call Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Call Risk. | ||||||||||||||||||
| Common Stock Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Common Stock Risk. | ||||||||||||||||||
| Concentration and Financial Services Sector Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Concentration and Financial Services Sector Risk. • financial services companies may suffer a setback if regulators change the rules under which they operate, which may increase costs for or limit the ability to offer new services or products and make it difficult to pass increased costs on to consumers; • unstable interest rates can have a disproportionate effect on the financial services sector; • financial services companies whose securities the Fund may purchase may themselves have concentrated portfolios, such as a high level of loans to real estate developers, which makes them vulnerable to economic conditions that affect that sector; and • financial services companies have been affected by increased competition, which could adversely affect the profitability or viability of such companies. The profitability of many types of financial services companies may be adversely affected in certain market cycles, including periods of rising interest rates, which may restrict the availability and increase the cost of capital, and declining economic conditions, which may cause credit losses due to financial difficulties of borrowers. Because many types of financial services companies are especially vulnerable to these economic cycles, the Fund’s investments in these companies may lose significant value during such periods. | ||||||||||||||||||
| Contingent Capital Securities CoCos Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Contingent Capital Securities (“CoCos”) Risk. CoCos may be subject to an automatic write-down (i.e., the automatic write-down of the principal amount or value of the securities, potentially to zero, and the cancellation of the securities) under certain circumstances, which could result in the Fund losing a portion or all of its investment in such securities. In addition, the Fund may not have any rights with respect to repayment of the principal amount of the securities that has not become due or the payment of interest or dividends on such securities for any period from (and including) the interest or dividend payment date falling immediately prior to the occurrence of such automatic write-down. An automatic write-down could also result in a reduced income rate if the dividend or interest payment is based on the security’s par value. Coupon payments on CoCos may be discretionary and may be cancelled by the issuer for any reason or may be subject to approval by the issuer’s regulator and may be suspended in the event there are insufficient distributable reserves. In certain scenarios, investors in CoCos may suffer a loss of capital ahead of equity holders or when equity holders do not. There is no guarantee that the Fund will receive a return of principal on CoCos. Any indication that an automatic write-down or conversion event may occur can be expected to have a material adverse effect on the market price of CoCos. The prices of CoCos may be volatile. Additionally, the trading behavior of a given issuer’s CoCo may be strongly impacted by the trading behavior of other issuers’ CoCos, such that negative information from an unrelated CoCo may cause a decline in value of one or more CoCos held by a fund. Accordingly, the trading behavior of CoCos may not follow the trading behavior of other similarly structured securities. CoCos are issued primarily by financial institutions. Therefore, CoCos present substantially increased risks at times of financial turmoil, which could affect financial institutions more than companies in other sectors and industries. | ||||||||||||||||||
| Convertible Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Convertible Securities Risk. Convertible securities have characteristics of both equity and debt securities and, as a result, are exposed to certain additional risks that are typically associated with debt, including but not limited to Interest Rate Risk, Credit Risk, Below Investment Grade Risk and Unrated Securities Risk. The value of a convertible security is influenced by both the yield of non‑convertible securities of comparable issuers and by the value of the underlying common stock. Convertible securities generally offer lower interest or dividend yields than non‑convertible securities of similar credit quality. The market values of convertible securities tend to decline as interest rates increase and, conversely, to increase as interest rates decline. However, the convertible security’s market value tends to reflect the market price of the common stock of the issuing company when that stock price is greater than the convertible security’s “conversion price.” The conversion price is defined as the predetermined price at which the convertible security could be exchanged for the associated common stock. As the market price of the underlying common stock declines, the price of the convertible security tends to be influenced more by the yield of the convertible security. Thus, the convertible security may not decline in price to the same extent as the underlying common stock. Convertible securities fall below debt obligations of the same issuer in order of preference or priority in the event of a liquidation and are typically unrated or rated lower than such debt obligations. | ||||||||||||||||||
| Credit Spread Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Credit Spread Risk. | ||||||||||||||||||
| Debt Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Debt Securities Risk. | ||||||||||||||||||
| Deflation Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Deflation Risk. | ||||||||||||||||||
| Derivatives Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Derivatives Risk. It is possible that regulatory or other developments in the derivatives market, including changes in government regulation could adversely impact the Fund’s ability to invest in certain derivatives or successfully use derivative instruments. | ||||||||||||||||||
| Duration Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Duration Risk. | ||||||||||||||||||
| Emerging Markets Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Emerging Markets Risk. | ||||||||||||||||||
| Financial Futures and Options Transactions Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Financial Futures and Options Transactions Risk. If the Fund engages in futures transactions or in the writing of options on futures, it will be required to maintain initial margin and maintenance margin and may be required to make daily variation margin payments in accordance with applicable rules of the exchanges and the Commodity Futures Trading Commission (“CFTC”). If the Fund purchases a financial futures contract or a call option or writes a put option in order to hedge the anticipated purchase of securities, and if the Fund fails to complete the anticipated purchase transaction, the Fund may have a loss or a gain on the futures or options transaction that will not be offset by price movements in the securities that were the subject of the anticipatory hedge. There can be no assurance that a liquid market will exist at a time when the Fund seeks to close out a derivatives or futures or a futures option position, and the Fund would remain obligated to meet margin requirements until the position is closed. | ||||||||||||||||||
| Floating Rate and Fixed to Floating Rate Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Floating-Rate and Fixed‑to‑Floating‑Rate Securities Risk. | ||||||||||||||||||
| Foreign Currency Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Foreign Currency Risk. | ||||||||||||||||||
| Hedging Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Hedging Risk. | ||||||||||||||||||
| Illiquid Investments Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Illiquid Investments Risk. | ||||||||||||||||||
| Income Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Income Risk | ||||||||||||||||||
| Inflation Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Inflation Risk. | ||||||||||||||||||
| Inflation Correlation Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Inflation Correlation Risk. | ||||||||||||||||||
| Non U S Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Non‑U.S. Securities Risk. | ||||||||||||||||||
| Other Investment Companies Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Other Investment Companies Risk. pro rata portion of the other investment companies’ expenses, including advisory fees. These expenses are in addition to the direct expenses of the Fund’s own operations. As a result, the cost of investing in investment company shares may exceed the costs of investing directly in its underlying investments. In addition, securities of other investment companies may be leveraged. As a result, the Fund may be indirectly exposed to leverage through an investment in such securities and therefore magnify the Fund’s leverage risk. With respect to ETF’s, an ETF that is based on a specific index may not be able to replicate and maintain exactly the composition and relative weighting of securities in the index. The value of an ETF based on a specific index is subject to change as the values of its respective component assets fluctuate according to market volatility. ETFs typically rely on a limited pool of authorized participants to create and redeem shares, and an active trading market for ETF shares may not develop or be maintained. The market value of shares of ETFs and closed‑end funds may differ from their NAV. | ||||||||||||||||||
| Preferred and Hybrid Preferred Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Preferred and Hybrid Preferred Securities Risk. • Limited Voting Rights Risk. Generally, preferred security holders (such as the Fund) have no voting rights with respect to the issuing company unless preferred dividends have been in arrears for a specified number of periods, at which time the preferred security holders may elect a number of directors to the issuer’s board. Generally, once all the arrearages have been paid, the preferred security holders no longer have voting rights. In the case of certain preferred securities issued by trusts or special purpose entities, holders generally have no voting rights except if a declaration of default occurs and is continuing. In such an event, preferred security holders generally would have the right to appoint and authorize a trustee to enforce the trust’s or special purpose entity’s rights as a creditor under the agreement with its operating company. • Special Redemption Rights Risk. In certain circumstances, an issuer of preferred securities may redeem the securities at par prior to their stated maturity date. For instance, for certain types of preferred securities, a redemption may be triggered by a change in federal income tax or securities laws or regulatory or major corporate action. A redemption by the issuer may negatively impact the return of the security held by the Fund. • Payment Deferral and Omission Risk. Generally, preferred securities may be subject to provisions that allow an issuer, under certain conditions, to skip (“non‑cumulative” preferred securities) or defer (“cumulative” preferred securities) distributions for a stated period without any adverse consequences to the issuer. Non‑cumulative preferred securities can defer distributions indefinitely. Cumulative preferred securities typically contain provisions that allow an issuer, at its discretion, to defer distribution payments for up to 10 years. If the Fund owns a preferred security that is deferring its distribution, the Fund may be required to report income for tax purposes although it has not yet received such income. In addition, recent changes in bank regulations may increase the likelihood for issuers to defer or omit distributions. • Credit and Subordination Risk. Credit risk is the risk that a security in the Fund’s portfolio will decline in price or the issuer of the security will fail to make dividend, interest or principal payments when due because the issuer experiences a decline in its financial status. Preferred securities are generally subordinated to bonds and other debt instruments in a company’s capital structure in terms of having priority to corporate income, claims to corporate assets and liquidation payments, and therefore will be subject to greater credit risk than more senior debt instruments. • Floating Rate and Fixed‑to‑Floating Rate Securities Risk. The market value of floating rate securities is a reflection of discounted expected cash flows based on expectations for future interest rate resets. The market value of such securities may fall in a declining interest rate environment and may also fall in a rising interest rate environment if there is a lag between the rise in interest rates and the reset. This risk may also be present with respect to fixed‑to‑floating rate securities in which the Fund may invest. A secondary risk associated with declining interest rates is the risk that income earned by the Fund on floating rate and fixed‑to‑floating rate securities may decline due to lower coupon payments on floating-rate securities. • Liquidity Risk. Certain preferred securities may be substantially less liquid than many other securities, such as U.S. Government securities or common stock. Illiquid securities involve the risk that the securities will not be able to be sold at the time desired by the Fund or at prices approximating the value at which the Fund is carrying the securities on its books. • Regulatory Risk. Issuers of preferred securities may be in industries that are heavily regulated and that may receive government funding. The value of preferred securities issued by these companies may be affected by changes in government policy, such as increased regulation, ownership restrictions, deregulation or reduced government funding. • New Types of Securities Risk. From time to time, preferred securities, including hybrid-preferred securities, have been, and may in the future be, offered having features other than those described herein. The Fund reserves the right to invest in these securities if the Sub‑Advisers believe that doing so would be consistent with the Fund’s investment objective and policies. Since the market for these instruments would be new, the Fund may have difficulty disposing of them at a suitable price and time. In addition to limited liquidity, these instruments may present other risks, such as high price volatility. | ||||||||||||||||||
| Reinvestment Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Reinvestment Risk. | ||||||||||||||||||
| Swap Transactions Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Swap Transactions Risk. | ||||||||||||||||||
| Unrated Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Unrated Securities Risk. which means the Fund might have difficulty selling them promptly at an acceptable price. To the extent that the Fund invests in unrated securities, the Fund’s ability to achieve its investment objectives will be more dependent on the investment adviser’s credit analysis than would be the case when the Fund invests in rated securities. | ||||||||||||||||||
| U S Government Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | U.S. Government Securities Risk. | ||||||||||||||||||
| Valuation Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Valuation Risk. | ||||||||||||||||||
| Warrants and Equity Securities Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Warrants and Equity Securities Risk. | ||||||||||||||||||
| When Issued and Delayed Delivery Transactions [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | When-Issued and Delayed-Delivery Transactions Risk. | ||||||||||||||||||
| Zero Coupon Bonds Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Zero Coupon Bonds Risk. | ||||||||||||||||||
| Anti Takeover Provisions [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Anti-Takeover Provisions. | ||||||||||||||||||
| Borrowing Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Borrowing Risk. | ||||||||||||||||||
| Counterparty Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Counterparty Risk. | ||||||||||||||||||
| Cybersecurity Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Cybersecurity Risk. | ||||||||||||||||||
| Fund Tax Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Fund Tax Risk. | ||||||||||||||||||
| Global Economic Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Global Economic Risk. The Fund does not know and cannot predict how long the securities markets may be affected by these events and the effects of these and similar events in the future on the U.S. economy and securities markets. The Fund may be adversely affected by abrogation of international agreements and national laws which have created the market instruments in which the Fund may invest, failure of the designated national and international authorities to enforce compliance with the same laws and agreements, failure of local, national and international organizations to carry out the duties prescribed to them under the relevant agreements, revisions of these laws and agreements which dilute their effectiveness or conflicting interpretation of provisions of the same laws and agreements. | ||||||||||||||||||
| Investment and Market Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Investment and Market Risk. | ||||||||||||||||||
| Legislation and Regulatory Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Legislation and Regulatory Risk. | ||||||||||||||||||
| Leverage Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Leverage Risk. The Fund will pay (and common shareholders will bear) any costs and expenses relating to the Fund’s use of leverage, which will result in a reduction in the Fund’s NAV. The investment adviser may, based on its assessment of market conditions and composition of the Fund’s holdings, increase or decrease the amount of leverage. Such changes may impact the Fund’s distributions and the price of the common shares in the secondary market. The Fund may seek to refinance its leverage over time, in the ordinary course, as current forms of leverage mature or it is otherwise desirable to refinance; however, the form that such leverage will take cannot be predicted at this time. If the Fund is unable to replace existing leverage on comparable terms, its costs of leverage will increase. Accordingly, there is no assurance that the use of leverage may result in a higher yield or return to common shareholders. The amount of fees paid to the investment adviser and the sub‑advisor for investment advisory services will be higher if the Fund uses leverage because the fees will be calculated based on the Fund’s Managed Assets - this may create an incentive for the investment adviser and the sub‑advisor to leverage the Fund or increase the Fund’s leverage. | ||||||||||||||||||
| Market Discount from Net Asset Value [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Market Discount from Net Asset Value. | ||||||||||||||||||
| Recent Market Conditions [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Recent Market Conditions. Ukraine has experienced ongoing military conflict, most recently in February 2022 when Russia invaded Ukraine; this conflict may expand and military attacks could occur elsewhere in Europe. Europe has also been struggling with mass migration from the Middle East and Africa. The ultimate effects of these events and other socio-political or geographical issues are not known but could profoundly affect global economies and markets. The ongoing trade war between China and the United States, including the imposition of tariffs by each country on the other country’s products, has created a tense political environment. These actions may trigger a significant reduction in international trade, the oversupply of certain manufactured goods, substantial price reductions of goods and possible failure of individual companies and/or large segments of China’s export industry, which could have a negative impact on the Fund’s performance. U.S. companies that source material and goods from China and those that make large amounts of sales in China would be particularly vulnerable to an escalation of trade tensions. Uncertainty regarding the outcome of the trade tensions and the potential for a trade war could cause the U.S. dollar to decline against safe haven currencies, such as the Japanese yen and the euro. Events such as these and their consequences are difficult to predict and it is unclear whether further tariffs may be imposed or other escalating actions may be taken in the future. Recently the U.S. Federal Reserve (the “Fed”) has sharply raised interest rates and has signaled an intention to continue to do so or maintain higher interest rates until current inflation levels re‑align with the Fed’s long-term inflation target. Changing interest rate environments impact the various sectors of the economy in different ways. For example, in March 2023, the Federal Deposit Insurance Corporation (““FDIC””) was appointed receiver for each of Silicon Valley Bank and Signature Bank, the second- and third-largest bank failures in U.S. history, which failures may be attributable, in part, to rising interest rates. Bank failures may have a destabilizing impact on the broader banking industry or markets generally. The impact of these developments in the near- and long-term is unknown and could have additional adverse effects on economies, financial markets and asset valuations around the world. | ||||||||||||||||||
| Reverse Repurchase Agreement Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Reverse Repurchase Agreement Risk. | ||||||||||||||||||
| Credit Risks [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Credit Risk. | ||||||||||||||||||
| Interest Rate Risk [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Risk [Text Block] | Interest Rate Risk. vice versa . As interest rates decline, issuers of securities may prepay principal earlier than scheduled, forcing the Fund to reinvest in lower-yielding securities and potentially reducing the Fund’s income. As interest rates increase, slower than expected principal payments may extend the average life of securities, potentially locking in a below-market interest rate and reducing the Fund’s value. In typical market interest rate environments, the prices of longer-term securities generally fluctuate more than prices of shorter-term securities as interest rates change. | ||||||||||||||||||
| At The Market Transaction [Member] | |||||||||||||||||||
| Other Annual Expenses [Abstract] | |||||||||||||||||||
| Expense Example, Year 01 | $ 54 | ||||||||||||||||||
| Expense Example, Years 1 to 3 | 144 | ||||||||||||||||||
| Expense Example, Years 1 to 5 | 234 | ||||||||||||||||||
| Expense Example, Years 1 to 10 | 464 | ||||||||||||||||||
| Underwriting Syndicate Transaction [Member] | |||||||||||||||||||
| Other Annual Expenses [Abstract] | |||||||||||||||||||
| Expense Example, Year 01 | 83 | ||||||||||||||||||
| Expense Example, Years 1 to 3 | 169 | ||||||||||||||||||
| Expense Example, Years 1 to 5 | 257 | ||||||||||||||||||
| Expense Example, Years 1 to 10 | 480 | ||||||||||||||||||
| Privately Negotiated Transaction [Member] | |||||||||||||||||||
| Other Annual Expenses [Abstract] | |||||||||||||||||||
| Expense Example, Year 01 | 45 | ||||||||||||||||||
| Expense Example, Years 1 to 3 | 135 | ||||||||||||||||||
| Expense Example, Years 1 to 5 | 226 | ||||||||||||||||||
| Expense Example, Years 1 to 10 | $ 458 | ||||||||||||||||||
| Common Shares [Member] | |||||||||||||||||||
| General Description of Registrant [Abstract] | |||||||||||||||||||
| Lowest Price or Bid | $ 5.98 | $ 6.06 | $ 7.12 | $ 6.70 | $ 7.35 | $ 7.99 | $ 8.99 | $ 9.84 | |||||||||||
| Highest Price or Bid | 6.60 | 8.03 | 7.96 | 8.41 | 8.29 | 9.31 | 9.99 | 10.06 | |||||||||||
| Lowest Price or Bid, NAV | 6.86 | 6.62 | 7.67 | 7.56 | 7.99 | 8.70 | 9.48 | 9.81 | |||||||||||
| Highest Price or Bid, NAV | $ 7.45 | $ 8.47 | $ 8.38 | $ 8.52 | $ 8.70 | $ 9.53 | $ 9.90 | $ 9.98 | |||||||||||
| Highest Price or Bid, Premium (Discount) to NAV [Percent] | (9.37%) | (4.40%) | (2.77%) | (0.36%) | (2.50%) | (2.31%) | 1.33% | 1.41% | |||||||||||
| Lowest Price or Bid, Premium (Discount) to NAV [Percent] | (13.08%) | (12.02%) | (9.64%) | (11.84%) | (9.25%) | (8.91%) | (6.05%) | (0.50%) | |||||||||||
| Share Price | $ 7.45 | $ 7.45 | $ 7.45 | ||||||||||||||||
| NAV Per Share | $ 6.60 | $ 6.60 | $ 6.60 | ||||||||||||||||
| Latest Premium (Discount) to NAV [Percent] | (11.41%) | ||||||||||||||||||
| Capital Stock, Long-Term Debt, and Other Securities [Abstract] | |||||||||||||||||||
| Outstanding Security, Title [Text Block] | Common shares | ||||||||||||||||||
| Outstanding Security, Held [Shares] | 105,069,232 | ||||||||||||||||||
| Borrowings Outstanding [Member] | |||||||||||||||||||
| Financial Highlights [Abstract] | |||||||||||||||||||
| Senior Securities Amount | [6] | $ 219,600,000 | $ 219,600,000 | $ 423,400,000 | $ 219,600,000 | $ 462,700,000 | $ 400,000,000 | $ 455,000,000 | $ 437,000,000 | $ 540,000,000 | $ 404,100,000 | $ 404,100,000 | $ 402,500,000 | ||||||
| Senior Securities Coverage per Unit | [7] | $ 5,249,000 | $ 5,249,000 | $ 3,088,000 | $ 5,249,000 | $ 3,223,000 | $ 3,280,000 | $ 3,303,000 | $ 3,403,000 | $ 3,079,000 | $ 3,526,000 | $ 3,506,000 | $ 3,572,000 | ||||||
| Taxable Fund Preferred TFP Shares [Member] | |||||||||||||||||||
| Financial Highlights [Abstract] | |||||||||||||||||||
| Senior Securities Amount | [8] | $ 150,000,000 | $ 150,000,000 | $ 0 | $ 150,000,000 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | ||||||
| Senior Securities Coverage per Unit | [7] | $ 3,119,000 | $ 3,119,000 | $ 0 | $ 3,119,000 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | ||||||
| [1]A maximum sales charge of 4.00% applies only to offerings pursuant to a syndicated underwriting. The maximum sales charge for offerings made at‑the‑market is 1.00%. There is no sales charge for offerings pursuant to a private transaction.[2]You will be charged a $2.50 service charge and pay brokerage charges if you direct Computershare Inc. and Computershare Trust Company, N.A., as agent for the common shareholders, to sell your Common Shares held in a dividend reinvestment account.[3]Stated as percentages of average net assets attributable to Common Shares for the fiscal year ended July 31, 2023.[4]Interest and Other Related Expenses reflect actual expenses and fees for leverage incurred by a Fund for the fiscal year ended July 31, 2023. The types of leverage used by each Fund during the fiscal year ended July 31, 2023 are described in the Fund Leverage and the Notes to Financial Statements (Note 4 – Portfolio Securities, Note 5 – Derivative Investments, Note 6 – Fund Shares, Note 10 – Borrowings Arrangements and Reverse Repurchase Agreements) sections of this annual report. Actual Interest and Other Related Expenses incurred in the future may be higher or lower. If short-term market interest rates rise in the future, and if a Fund continues to maintain leverage, the cost of which is tied to short-term interest rates, a Fund’s interest expenses on its short-term borrowings can be expected to rise in tandem. A Fund’s use of leverage will increase the amount of management fees paid to the Fund’s adviser and sub‑advisor(s).[5]Other Expenses are based on estimated amounts for the current fiscal year. Expenses attributable to the Fund’s investments, if any, in other investment companies are currently estimated not to exceed 0.01%.[6]Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year and does not include any preferred shares noticed for redemption as noted on the Statement of Assets and Liabilities where applicable.[7]Asset Coverage Per $1,000: Asset coverage per $1,000 is calculated by subtracting the Fund’s liabilities and indebtedness not represented by senior securities from the Fund’s total assets, dividing the result by the aggregate amount of the Fund’s senior securities representing indebtedness then outstanding (if applicable,) plus the aggregate of the involuntary liquidation preference of the outstanding preferred shares, if applicable, and multiplying the result by 1,000.[8]Aggregate Amount Outstanding: Aggregate amount outstanding represents the liquidation preference as of the end of the relevant fiscal year. | |||||||||||||||||||
