Senior Securities [Table Text Block] | | Year Ended May 31, 2024 2023 2022 2021 2020 Net asset value – Beginning of year (Common shares) $ 12.70 $ 13.24 $ 14.56 $ 13.03 $ 15.21 Income (Loss) From Operations Net investment income (1) $ 1.40 $ 1.15 $ 0.71 $ 0.72 $ 0.84 Net realized and unrealized gain (loss) 0.62 (0.64 ) (1.16 ) 1.54 (2.01 ) Total income (loss) from operations $ 2.02 $ 0.51 $ (0.45 ) $ 2.26 $ (1.17 ) Less Distributions to Common Shareholders From net investment income $ (1.43 ) $ (1.05 ) $ (0.80 ) $ (0.73 ) $ (1.01 ) Tax return of capital — — (0.13 ) — — Total distributions to common shareholders $ (1.43 ) $ (1.05 ) $ (0.93 ) $ (0.73 ) $ (1.01 ) Premium from common shares sold through shelf offering (1) $ — $ — $ 0.01 $ — $ — Discount on tender offer (1) $ 0.03 $ — $ 0.05 $ — $ — Net asset value – End of year (Common shares) $ 13.32 $ 12.70 $ 13.24 $ 14.56 $ 13.03 Market value – End of year (Common shares) $ 13.38 $ 11.24 $ 12.28 $ 14.28 $ 11.24 Total Investment Return on Net Asset Value (2) 17.42 % 4.87 % (2.81 )% 18.25 % (7.36 )% Total Investment Return on Market Value (2) 33.26 % 0.14 % (8.10 )% 34.36 % (9.83 )% Ratios/Supplemental Data Net assets applicable to common shares, end of year (000’s omitted) $ 348,989 $ 369,557 $ 385,295 $ 580,590 $ 519,465 Ratios (as a percentage of average daily net assets applicable to common shares): † Expenses excluding interest and fees 1.39 % (3) 1.32 % (3) 1.25 % 1.33 % 1.26 % Interest and fee expense (4) 3.87 % 2.89 % 0.92 % 0.91 % 1.79 % Total expenses 5.26 % (3) 4.21 % (3) 2.17 % 2.24 % 3.05 % Net investment income 10.57 % 8.98 % 4.91 % 5.08 % 5.85 % Portfolio Turnover 29 % 16 % 53 % 32 % 34 % Senior Securities: Total notes payable outstanding (in 000’s) $ 112,000 $ 118,000 $ 147,000 $ 250,000 $ 190,000 Asset coverage per $1,000 of notes payable (5) $ 4,830 $ 4,810 $ 4,165 $ 3,642 $ 4,155 Total preferred shares outstanding 800 800 800 800 800 Asset coverage per preferred share (6) $ 281,765 $ 286,645 $ 269,734 $ 275,936 $ 292,394 Involuntary liquidation preference per preferred share (7) $ 100,000 $ 100,000 $ 100,000 $ 100,000 $ 100,000 Approximate market value per preferred share (7) $ 100,000 $ 100,000 $ 100,000 $ 100,000 $ 100,000 Year Ended May 31, 2019 2018 2017 2016 2015 Net asset value – Beginning of year (Common shares) $ 15.61 $ 15.57 $ 14.68 $ 15.64 $ 16.08 Income (Loss) From Operations Net investment income (1) $ 0.85 $ 0.79 $ 0.86 $ 0.91 $ 0.88 Net realized and unrealized gain (loss) (0.37 ) 0.08 0.90 (0.96 ) (0.43 ) Total income (loss) from operations $ 0.47 $ 0.87 $ 1.76 $ (0.06 ) $ 0.45 Less Distributions to Common Shareholders From net investment income $ (0.87 ) $ (0.83 ) $ (0.87 ) $ (0.90 ) $ (0.89 ) Total distributions to common shareholders $ (0.87 ) $ (0.83 ) $ (0.87 ) $ (0.90 ) $ (0.89 ) Net asset value – End of year (Common shares) $ 15.21 $ 15.61 $ 15.57 $ 14.68 $ 15.64 Market value – End of year (Common shares) $ 13.48 $ 14.85 $ 15.15 $ 13.56 $ 14.36 Total Investment Return on Net Asset Value (2) 3.77 % 6.03 % 12.65 % 0.46 % 3.43 % Total Investment Return on Market Value (2) (3.32 )% 3.67 % 18.58 % 1.14 % 0.59 % Ratios/Supplemental Data Net assets applicable to common shares, end of year (000’s omitted) $ 606,408 $ 622,241 $ 620,772 $ 585,101 $ 623,439 Ratios (as a percentage of average daily net assets applicable to common shares): † Expenses excluding interest and fees (8) 1.28 % 1.28 % 1.32 % 1.36 % 1.37 % Interest and fee expense (4) 2.00 % 1.52 % 1.16 % 0.93 % 0.80 % Total expenses 3.28 % 2.80 % 2.48 % 2.29 % 2.17 % Net investment income 5.49 % 5.09 % 5.68 % 6.22 % 5.60 % Portfolio Turnover 24 % 34 % 47 % 29 % 32 % Senior Securities: Total notes payable outstanding (in 000’s) $ 248,000 $ 254,000 $ 246,000 $ 232,000 $ 290,000 Asset coverage per $1,000 of notes payable (5) $ 3,768 $ 3,765 $ 3,849 $ 3,867 $ 3,426 Total preferred shares outstanding 800 800 800 800 800 Asset coverage per preferred share (6) $ 284,880 $ 286,300 $ 290,421 $ 287,532 $ 268,497 Involuntary liquidation preference per preferred share (7) $ 100,000 $ 100,000 $ 100,000 $ 100,000 $ 100,000 Approximate market value per preferred share (7) $ 100,000 $ 100,000 $ 100,000 $ 100,000 $ 100,000 (1) Computed using average common shares outstanding. (2) Returns are historical and are calculated by determining the percentage change in net asset value or market value with all distributions reinvested. Distributions are assumed to be reinvested at prices obtained under the Trust’s dividend reinvestment plan. (3) Includes a reduction by the investment adviser of a portion of its adviser fee due to the Trust’s investment in the Liquidity Fund (equal to less than 0.005% of average daily net assets for the years ended May 31, 2024 and 2023). (4) Interest and fee expense relates to variable rate term preferred shares and the notes payable. (5) Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, and dividing the result by the notes payable balance in thousands. (6) Calculated by subtracting the Trust’s total liabilities (not including the notes payable and preferred shares) from the Trust’s total assets, dividing the result by the sum of the value of the notes payable and liquidation value of the preferred shares, and multiplying the result by the liquidation value of one preferred share. Such amount equates to 282%, 287%, 270%, 276%, 292%, 285%, 286%, 290%, 288% and 268% at May 31, 2024, 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016 and 2015, respectively. (7) Plus accumulated and unpaid dividends. (8) Excludes the effect of custody fee credits, if any, of less than 0.005%. Effective September 1, 2015, custody fee credits, which were earned on cash deposit balances, were discontinued by the custodian. † Ratios based on net assets applicable to common shares plus preferred shares and borrowings are presented below. | | | | | | | | | | |