| FINANCIAL HIGHLIGHTS (Details) $ / shares in Units, $ in Thousands | 12 Months Ended | ||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Apr. 04, 2024 | Sep. 30, 2024  USD ($)  $ / shares  shares | Sep. 30, 2023  USD ($)  $ / shares  shares | Sep. 30, 2022  USD ($)  $ / shares  shares | Sep. 30, 2021  USD ($)  $ / shares  shares | Sep. 30, 2020  USD ($)  $ / shares  shares | Sep. 30, 2019  USD ($)  $ / shares  shares | Sep. 30, 2018  USD ($)  $ / shares  shares | Sep. 30, 2017  USD ($)  $ / shares  shares | Sep. 30, 2016  USD ($)  $ / shares  shares | Sep. 30, 2015  USD ($)  $ / shares  shares | |||||
| Investment Company, Financial Highlights [Roll Forward] | |||||||||||||||
| Net asset value at beginning of period / year (in USD per share) | $ 18.79 | [1] | $ 18.16 | $ 18.56 | $ 14.80 | $ 16.44 | $ 16.64 | $ 16.80 | $ 17.24 | $ 18.12 | $ 19.02 | ||||
| Income from operations | |||||||||||||||
| Net investment income (in USD per share) | 2.11 | 2.20 | 1.88 | 1.58 | 1.62 | 1.68 | 1.70 | 1.68 | 1.68 | 1.68 | |||||
| Net realized and unrealized gain (loss) on investments (in USD per share) | 2.05 | 0.07 | (0.70) | 3.58 | (1.68) | (0.30) | (0.32) | (0.24) | (0.70) | (1.00) | |||||
| Net realized and unrealized gain (loss) on other (in USD per share) | 0.18 | 0.02 | (0.02) | (0.08) | (0.06) | (0.02) | 0 | (0.10) | 0 | 0.12 | |||||
| Total from operations (in USD per share) | 4.34 | 2.29 | 1.16 | 5.08 | (0.12) | 1.36 | 1.38 | 1.34 | 0.98 | 0.80 | |||||
| Distributions to common stockholders from | |||||||||||||||
| Net investment income (in USD per share) | (1.98) | (1.89) | (1.52) | (1.50) | (1.60) | (1.64) | (1.68) | (1.68) | (1.40) | (1.68) | |||||
| Realized gains (in USD per share) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | (0.28) | 0 | |||||
| Return of capital (in USD per share) | (0.00) | (0.00) | (0.08) | (0.06) | (0.02) | (0.04) | 0 | 0 | 0 | 0 | |||||
| Total distributions (in USD per share) | (1.98) | (1.89) | (1.60) | (1.56) | (1.62) | (1.68) | (1.68) | (1.68) | (1.68) | (1.68) | |||||
| Issuance of common stock (in USD per share) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.12 | |||||
| Discounts, commissions, and offering costs (in USD per share) | 0 | 0 | 0 | (0.02) | 0 | (0.02) | (0.02) | (0.08) | (0.10) | (0.02) | |||||
| Repurchase of common stock (in USD per share) | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0.04 | 0 | |||||
| Net anti-dilutive (dilutive) effect of equity offering (in USD per share) | 0.05 | 0.26 | 0.04 | 0.30 | 0.10 | 0.14 | 0.16 | (0.04) | (0.10) | (0.12) | |||||
| Total capital share transactions (in USD per share) | 0.05 | 0.26 | 0.04 | 0.28 | 0.10 | 0.12 | 0.14 | (0.12) | (0.16) | (0.02) | |||||
| Other, net (in USD per share) | (0.02) | (0.03) | 0 | (0.04) | 0 | 0 | 0 | 0.02 | (0.02) | 0 | |||||
| Net asset value at end of period / year (in USD per share) | 21.18 | [1] | 18.79 | [1] | 18.16 | 18.56 | 14.80 | 16.44 | 16.64 | 16.80 | 17.24 | 18.12 | |||
| Per common share market value at beginning of year (in USD per share) | 19.28 | 16.98 | 22.60 | 14.82 | 19.50 | 19.00 | 19.00 | 16.26 | 16.26 | 17.54 | |||||
| Per common share market value at end of year (in USD per share) | $ 24.05 | $ 19.28 | $ 16.98 | $ 22.60 | $ 14.82 | $ 19.50 | $ 19.00 | $ 19.00 | $ 16.26 | $ 16.26 | |||||
| Total return | 36.61% | 24.85% | (19.16%) | 64.93% | (15.75%) | 12.55% | 9.53% | 27.90% | 11.68% | 2.40% | |||||
| Common stock outstanding at end of year (in shares) | shares | 22,230,587 | 21,754,449 | 17,367,398 | 17,152,186 | 15,783,425 | 15,172,962 | 14,250,990 | 13,080,342 | 11,672,211 | 10,565,811 | |||||
| Statement of Assets and Liabilities Data: | |||||||||||||||
| Net assets at end of year | $ | $ 470,895 | $ 408,695 | $ 315,487 | $ 318,439 | $ 233,743 | $ 249,330 | $ 237,092 | $ 219,650 | $ 201,207 | $ 191,444 | |||||
| Average net assets | $ | 431,796 | 349,518 | 320,838 | 275,509 | 235,266 | 239,851 | 234,092 | 215,421 | 193,228 | 198,864 | |||||
| Senior Securities Data: | |||||||||||||||
| Borrowings under line of credit, at cost | $ | 70,600 | 47,800 | 141,800 | 50,500 | 128,000 | 66,900 | 110,000 | 93,000 | 71,300 | 127,300 | |||||
| Mandatorily redeemable preferred stock | $ | 8,748 | 0 | 0 | 0 | 0 | 51,750 | 51,750 | 51,750 | 61,000 | 61,000 | |||||
| Notes Payable | $ | $ 257,000 | $ 257,000 | $ 200,000 | $ 188,813 | $ 96,313 | $ 57,500 | $ 0 | $ 0 | $ 0 | $ 0 | |||||
| Ratios/Supplemental Data: | |||||||||||||||
| Ratio of net expenses to average net assets | 11.71% | 12.99% | 9.62% | 10.04% | 9.69% | 10.61% | 9.61% | 8.26% | 10.16% | 10.24% | |||||
| Ratio of net investment income to average net assets | 10.67% | 11.74% | 10.06% | 9.48% | 10.70% | 10.25% | 9.86% | 9.95% | 10.08% | 8.90% | |||||
| Reverse stock split, conversion ratio | 0.5 | ||||||||||||||
| Ratio of expenses to average net assets - annualized | 14.53% | 16.31% | 13.13% | 13.17% | 14.36% | 14.18% | 13.12% | 12.14% | 13.40% | 13.65% | |||||
| Ratio of net investment income (loss) to average net assets – annualized | 7.90% | 8.49% | 6.61% | 6.40% | 6.11% | 6.74% | 6.41% | 6.13% | 6.90% | 5.55% | |||||
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