Risk is inherent in all investing. Investing in any investment company security involves risk, including the risk that you may receive little or no return on your investment or even that you may lose part or all of your investment. Please refer to the section of the Fund’s most recent annual report on Form N-CSR entitled “Shareholder Update—Current Investment Objectives, Investment Policies and Principal Risks of the Funds—Principal Risks of the Funds,” as such principal risks may be supplemented from time to time, which is incorporated by reference herein, for a discussion of the principal risks you should consider before making an investment in the Fund. Any additional risks applicable to a particular offering of Securities will be set forth in the related prospectus supplement.

Share Price [Table Text Block]

TRADING AND NET ASSET VALUE INFORMATION

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 corresponding NAV per share, and (iii) the premium/(discount) to NAV per share at which the Common Shares were trading as of such date. The Fund’s Common Shares have historically traded both at premiums and discounts in relation to the Fund’s NAV per share. The Fund cannot predict whether its Common Shares will trade at a premium or discount to NAV in the future. The Board of Trustees 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 Fund cannot assure you that its Board of Trustees will decide to take any of these actions, or that share repurchases or tender offers will actually reduce market discount.


	Closing Market Price per Common Share				NAV per Common Share on Date of Market Price				Premium/(Discount) on Date of Market Price
Fiscal Quarter Ended	High		Low		High		Low		High			Low
November 2024	$	14.06	$	12.85	$	13.37	$	13.17		5.16	%		(2.43	)%
August 2024	$	13.40	$	11.48	$	13.26	$	12.90		1.06	%		(11.01	)%
May 2024	$	12.23	$	11.40	$	13.26	$	12.89		(7.77	)%		(11.56	)%
February 2024	$	11.65	$	11.24	$	13.45	$	13.12		(13.38	)%		(14.33	)%
November 2023	$	11.57	$	9.80	$	12.50	$	11.69		(7.44	)%		(16.17	)%
August 2023	$	11.60	$	10.90	$	13.20	$	12.68		(12.12	)%		(14.04	)%
May 2023	$	11.74	$	10.99	$	13.23	$	12.89		(11.26	)%		(14.74	)%
February 2023	$	12.90	$	11.71	$	13.31	$	13.00		(3.08	)%		(9.92	)%
November 2022	$	13.30	$	10.83	$	13.31	$	12.14		(0.08	)%		(10.79	)%

The net asset value per Common Share, the market price, and percentage of premium/(discount) to net asset value per Common Share on January 6, 2025, $12.88, $12.74 and (1.09)%, respectively. As of January 6, 2025, the Fund had 47,561,542 Common Shares outstanding and net assets applicable to Common Shares of $612,546,986.

Highest Price or Bid

14.06

13.40

12.23

11.65

11.57

11.60

11.74

12.90

13.30

Highest Price or Bid, NAV

$ 13.37

$ 13.26

$ 13.45

$ 12.50

$ 13.20

$ 13.23

$ 13.31

Lowest Price or Bid, Premium (Discount) to NAV [Percent]

(2.43%)

(11.01%)

(11.56%)

(14.33%)

(16.17%)

(14.04%)

(14.74%)

(9.92%)

(10.79%)

Capital Stock [Table Text Block]

DESCRIPTION OF SHARES

Common Shares

The Declaration of Trust authorizes the issuance of an unlimited number of Common Shares. The Common Shares have a par value of $0.01 per share and, subject to the rights of holders of any Preferred Shares, have equal rights to the payment of dividends and the distribution of assets upon liquidation. The Common Shares when issued, are fully paid and, subject to matters discussed in “Certain Provisions in the Declaration of Trust and By-Laws,” non-assessable, and have no preemptive or conversion rights or rights to cumulative voting. A copy of the Declaration of Trust is filed with the SEC as an exhibit to the Fund’s registration statement of which this Prospectus is a part.

Each whole Common Share has one vote with respect to matters submitted for a vote by the Fund’s Common Shareholders and on which the shareholder is entitled to vote, and each fractional share shall be entitled to a proportional fractional vote consistent with the requirements of the 1940 Act and the rules promulgated thereunder, and will vote together as a single class. Whenever the Fund incurs borrowings and/or Preferred Shares are outstanding, Common Shareholders will not be entitled to receive any cash distributions from the Fund unless all interest on such borrowings has been paid and all accumulated dividends on Preferred Shares have been paid, unless asset coverage (as defined in the 1940 Act) with respect to any borrowings would be at least 300% after giving effect to the distributions and asset coverage (as defined in the 1940 Act) with respect to Preferred Shares would be at least 200% after giving effect to the distributions. See “—Preferred Shares” below.

The Common Shares are listed on the NYSE and trade under the ticker symbol “NKX.” The Fund intends to hold annual meetings of shareholders so long as the Common Shares are listed on a national securities exchange and such meetings are required as a condition to such listing. The Fund does not issue share certificates.

Unlike open-end funds, closed-end funds like the Fund do not provide daily redemptions. Rather, if a shareholder determines to buy additional Common Shares or sell shares already held, the shareholder may conveniently do so by trading on the exchange through a broker or otherwise. Common shares of closed-end investment companies may frequently trade on an exchange at prices lower than NAV. Common shares of closed-end investment companies like the Fund have during some periods traded at prices higher than NAV and have during other periods traded at prices lower than NAV.

Because the market value of the Common Shares may be influenced by such factors as distribution levels (which are in turn affected by expenses), call protection, dividend stability, portfolio credit quality, NAV, relative demand for and supply of such shares in the market, general market and economic conditions, and other factors beyond the control of the Fund, the Fund cannot assure you that Common Shares will trade at a price equal to or higher than NAV in the future. The Common Shares are designed primarily for long-term investors, and investors in the Common Shares should not view the Fund as a vehicle for trading purposes. See “Repurchase of Fund Shares; Conversion to Open-End Fund.”

Preferred Shares

The Fund’s Declaration of Trust authorizes the issuance of an unlimited number of Preferred Shares in one or more classes or series, with rights as determined by the Board of Trustees, by action of the Board of Trustees without the approval of the Common Shareholders. As of January 6, 2025, there were 2,567 VRDP Shares and 1,404 MFP Shares outstanding. The MFP and VRDP Shares have various rights that were approved by the Board of Trustees without the approval of Common Shareholders, which are specified in the Fund’s statement establishing and fixing the rights and preferences with respect to such Shares (each, a “Statement”). As of January 6, 2025, there were 3,971 Preferred Shares outstanding. The discussion below generally describes the rights of the holders of Preferred Shares, including rights generally applicable to the holders of the Fund’s outstanding MFP and VRDP Shares, although the terms of any Preferred Shares that may be issued by the Fund may be the same as, or different from, the terms described below, subject to the applicable Statement, applicable law and the Declaration of Trust.

Under the 1940 Act, the Fund is not permitted to issue “senior securities” that are Preferred Shares if, immediately after the issuance of Preferred Shares, the asset coverage ratio would be less than 200%. See “Use of Leverage.” Additionally, the Fund will generally not be permitted to purchase any of its Common Shares or declare dividends (except a dividend payable in Common Shares) or other distributions on its Common Shares unless, at the time of such purchase or declaration, the asset coverage ratio with respect to such Preferred Shares, after taking into account such purchase or distribution, is at least 200%. Preferred Shares issued by the Fund have priority over the Common Shares.

For so long as any Preferred Shares are outstanding, the Fund will not: (1) declare or pay any dividend or other distribution (other than a dividend or distribution paid in Common Shares) in respect of the Common Shares, (2) call for redemption, redeem, purchase or otherwise acquire for consideration any Common Shares, or (3) pay any proceeds of the liquidation of the Fund in respect of the Common Shares, unless, in each case, (A) immediately thereafter, the Fund shall be in compliance with the 200% asset coverage limitations set forth under the 1940 Act after deducting the amount of such dividend or other distribution or redemption or purchase price or liquidation proceeds and (B) all cumulative dividends and other distributions of shares of all series of Preferred Shares of the Fund due on or prior to the date of the applicable dividend, distribution, redemption, purchase or acquisition shall have been declared and paid.

Distribution Preference

The Fund’s Preferred Shares have complete priority over the Common Shares as to distribution of assets.

Liquidation Preference

In the event of any voluntary or involuntary liquidation, dissolution or winding up of the affairs of the Fund, holders of Preferred Shares would be entitled to receive a preferential liquidating distribution (expected to equal the original purchase price per share plus accumulated and unpaid dividends thereon, whether or not earned or declared) before any distribution of assets is made to Common Shareholders. After payment of the full amount of the liquidating distribution to which they are entitled, holders of Preferred Shares will not be entitled to any further participation in any distribution of assets by the Fund. A consolidation or merger of the Fund with or into another entity or a sale of all or substantially all of the assets of the Fund shall not be deemed to be a liquidation, dissolution or winding up of the Fund.

Voting Rights

In connection with any issuance of Preferred Shares, the Fund must comply with Section 18(i) of the 1940 Act, which requires, among other things, that Preferred Shares be voting shares and have equal voting rights with Common Shares. Except with respect to certain matters affecting only the holders of the Preferred Shares and except as discussed further below, holders of Preferred Shares vote together with Common Shareholders as a single class on matters submitted to Fund shareholders.

In connection with the election of the Fund’s trustees, holders of Preferred Shares, voting as a separate class, are entitled to elect two of the Fund’s trustees, and the remaining trustees are elected by Common Shareholders and holders of Preferred Shares, voting together as a single class. In addition, if at any time dividends on the Fund’s outstanding Preferred Shares are unpaid in an amount equal to two full years’ dividends thereon, the holders of all outstanding Preferred Shares, voting as a separate class, would be entitled to elect a majority of the Fund’s trustees until all dividends in arrears have been paid or declared and set apart for payment.

The Statement with respect to each series of the Fund’s Preferred Shares sets forth certain voting and consent rights of the holders of such Shares, including with respect to certain actions that would affect the preferences, rights, or powers of such class or series or the authorization or issuance of any class or series ranking prior to the Preferred Shares. Except as may otherwise be required by law, the Fund’s Declaration of Trust requires that (1) the affirmative vote of the holders of at least two-thirds of the Fund’s Preferred Shares outstanding at the time, voting as a separate class, would be required to approve any conversion of the Fund from a closed-end to an open-end investment company and (2) the affirmative vote of the holders of at least two-thirds of the outstanding Preferred Shares, voting as a separate class, would be required to approve any plan of reorganization (as such term is used in the 1940 Act) adversely affecting such shares; provided however, that such separate class vote would be a majority vote if the action in question has previously been approved, adopted or authorized by the affirmative vote of two-thirds of the total number of trustees fixed in accordance with the Declaration of Trust or the By-laws. The affirmative vote of the holders of a majority of the outstanding Preferred Shares, voting as a separate class, would be required to approve any action not described in the preceding sentence requiring a vote of security holders under Section 13(a) of the 1940 Act including, among other things, changes in the Fund’s investment objectives or changes in the investment restrictions described as fundamental policies under “Investment Restrictions” in the SAI. The class or series vote of holders of Preferred Shares described above would in each case be in addition to any separate vote of the requisite percentage of Common Shares and Preferred Shares necessary to authorize the action in question.

The foregoing voting provisions would not apply with respect to the Fund’s Preferred Shares if, at or prior to the time when a vote was required, such shares have been (1) redeemed or (2) called for redemption and sufficient funds would have been deposited in trust to effect such redemption.

Redemption, Purchase and Sale of Preferred Shares

The terms of the Preferred Shares may provide that they are redeemable by the Fund at certain times, in whole or in part, at the liquidation preference of such share plus accumulated dividends, that the Fund may tender for or purchase Preferred Shares and that the Fund may subsequently resell any shares so tendered for or purchased. Any redemption or purchase of Preferred Shares by the Fund would reduce the leverage applicable to Common Shares, while any resale of such shares by the Fund would increase such leverage.

Entity Address, Address Line One

333 West Wacker Drive

Entity Address, State or Province

IL

Contact Personnel Name

Mark L. Winget

General Description of Registrant [Abstract]

NAV Per Share

$ 12.88

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Security Voting Rights [Text Block]

Each whole Common Share has one vote with respect to matters submitted for a vote by the Fund’s Common Shareholders and on which the shareholder is entitled to vote, and each fractional share shall be entitled to a proportional fractional vote consistent with the requirements of the 1940 Act and the rules promulgated thereunder, and will vote together as a single class.

Outstanding Security, Not Held [Shares]

47,561,542

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Security Voting Rights [Text Block]

Voting Rights

In connection with any issuance of Preferred Shares, the Fund must comply with Section 18(i) of the 1940 Act, which requires, among other things, that Preferred Shares be voting shares and have equal voting rights with Common Shares. Except with respect to certain matters affecting only the holders of the Preferred Shares and except as discussed further below, holders of Preferred Shares vote together with Common Shareholders as a single class on matters submitted to Fund shareholders.

In connection with the election of the Fund’s trustees, holders of Preferred Shares, voting as a separate class, are entitled to elect two of the Fund’s trustees, and the remaining trustees are elected by Common Shareholders and holders of Preferred Shares, voting together as a single class. In addition, if at any time dividends on the Fund’s outstanding Preferred Shares are unpaid in an amount equal to two full years’ dividends thereon, the holders of all outstanding Preferred Shares, voting as a separate class, would be entitled to elect a majority of the Fund’s trustees until all dividends in arrears have been paid or declared and set apart for payment.

The Statement with respect to each series of the Fund’s Preferred Shares sets forth certain voting and consent rights of the holders of such Shares, including with respect to certain actions that would affect the preferences, rights, or powers of such class or series or the authorization or issuance of any class or series ranking prior to the Preferred Shares. Except as may otherwise be required by law, the Fund’s Declaration of Trust requires that (1) the affirmative vote of the holders of at least two-thirds of the Fund’s Preferred Shares outstanding at the time, voting as a separate class, would be required to approve any conversion of the Fund from a closed-end to an open-end investment company and (2) the affirmative vote of the holders of at least two-thirds of the outstanding Preferred Shares, voting as a separate class, would be required to approve any plan of reorganization (as such term is used in the 1940 Act) adversely affecting such shares; provided however, that such separate class vote would be a majority vote if the action in question has previously been approved, adopted or authorized by the affirmative vote of two-thirds of the total number of trustees fixed in accordance with the Declaration of Trust or the By-laws. The affirmative vote of the holders of a majority of the outstanding Preferred Shares, voting as a separate class, would be required to approve any action not described in the preceding sentence requiring a vote of security holders under Section 13(a) of the 1940 Act including, among other things, changes in the Fund’s investment objectives or changes in the investment restrictions described as fundamental policies under “Investment Restrictions” in the SAI. The class or series vote of holders of Preferred Shares described above would in each case be in addition to any separate vote of the requisite percentage of Common Shares and Preferred Shares necessary to authorize the action in question.

The foregoing voting provisions would not apply with respect to the Fund’s Preferred Shares if, at or prior to the time when a vote was required, such shares have been (1) redeemed or (2) called for redemption and sufficient funds would have been deposited in trust to effect such redemption.

Preferred Stock Restrictions, Other [Text Block]

Redemption, Purchase and Sale of Preferred Shares

The terms of the Preferred Shares may provide that they are redeemable by the Fund at certain times, in whole or in part, at the liquidation preference of such share plus accumulated dividends, that the Fund may tender for or purchase Preferred Shares and that the Fund may subsequently resell any shares so tendered for or purchased. Any redemption or purchase of Preferred Shares by the Fund would reduce the leverage applicable to Common Shares, while any resale of such shares by the Fund would increase such leverage.

Outstanding Security, Not Held [Shares]

0

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Outstanding Security, Held [Shares]

0

Outstanding Security, Authorized [Shares]

427

Outstanding Security, Not Held [Shares]

427

Capital Stock, Long-Term Debt, and Other Securities [Abstract]

Outstanding Security, Held [Shares]

0

Outstanding Security, Authorized [Shares]

1,050

Outstanding Security, Not Held [Shares]

1,050