Quarterly report pursuant to Section 13 or 15(d)

INVESTMENTS - Summary Investment Holdings (Details)

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INVESTMENTS - Summary Investment Holdings (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Jun. 30, 2024
Sep. 30, 2023
Summary of Investment Holdings [Line Items]    
Cost $ 753,263 $ 722,269 [1]
Fair Value 757,745 704,815 [1]
South    
Summary of Investment Holdings [Line Items]    
Fair Value 289,191 273,181
West    
Summary of Investment Holdings [Line Items]    
Fair Value 248,443 224,235
Midwest    
Summary of Investment Holdings [Line Items]    
Fair Value 175,923 145,122
Northeast    
Summary of Investment Holdings [Line Items]    
Fair Value 44,188 62,277
Diversified/Conglomerate Service    
Summary of Investment Holdings [Line Items]    
Fair Value 180,136 135,060
Diversified/Conglomerate Manufacturing    
Summary of Investment Holdings [Line Items]    
Fair Value 164,251 158,061
Aerospace and Defense    
Summary of Investment Holdings [Line Items]    
Fair Value 128,782 97,836
Healthcare, Education, and Childcare    
Summary of Investment Holdings [Line Items]    
Fair Value 93,853 146,438
Beverage, Food, and Tobacco    
Summary of Investment Holdings [Line Items]    
Fair Value 90,843 78,788
Automobile    
Summary of Investment Holdings [Line Items]    
Fair Value 28,040 27,571
Oil and Gas    
Summary of Investment Holdings [Line Items]    
Fair Value 21,600 27,830
Cargo Transportation    
Summary of Investment Holdings [Line Items]    
Fair Value 20,000 0
Personal and Non-Durable Consumer Products    
Summary of Investment Holdings [Line Items]    
Fair Value 13,617 14,576
Machinery    
Summary of Investment Holdings [Line Items]    
Fair Value 6,389 6,411
Other    
Summary of Investment Holdings [Line Items]    
Fair Value $ 10,234 $ 12,244
Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Investment Owned, At Fair Value | Industry Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Investment Owned, At Fair Value | Industry Concentration Risk | Diversified/Conglomerate Service    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 23.80% 19.20%
Investment Owned, At Fair Value | Industry Concentration Risk | Diversified/Conglomerate Manufacturing    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 21.70% 22.40%
Investment Owned, At Fair Value | Industry Concentration Risk | Aerospace and Defense    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 17.00% 13.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Healthcare, Education, and Childcare    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 12.40% 20.80%
Investment Owned, At Fair Value | Industry Concentration Risk | Beverage, Food, and Tobacco    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 12.00% 11.20%
Investment Owned, At Fair Value | Industry Concentration Risk | Automobile    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 3.70% 3.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Oil and Gas    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 2.90% 3.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Cargo Transportation    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 2.60% 0.00%
Investment Owned, At Fair Value | Industry Concentration Risk | Personal and Non-Durable Consumer Products    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 1.80% 2.10%
Investment Owned, At Fair Value | Industry Concentration Risk | Machinery    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 0.80% 0.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Other    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 1.30% 1.70%
Investment Owned, At Fair Value | Geographic Regions Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 100.00% 100.00%
Investment Owned, At Fair Value | Geographic Regions Risk | South    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 38.20% 38.80%
Investment Owned, At Fair Value | Geographic Regions Risk | West    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 32.80% 31.80%
Investment Owned, At Fair Value | Geographic Regions Risk | Midwest    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 23.20% 20.60%
Investment Owned, At Fair Value | Geographic Regions Risk | Northeast    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 5.80% 8.80%
Debt Securities    
Summary of Investment Holdings [Line Items]    
Cost $ 680,862 $ 659,826
Fair Value $ 660,191 $ 638,579
Debt Securities | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 90.40% 91.40%
Debt Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 87.10% 90.60%
Secured first lien debt    
Summary of Investment Holdings [Line Items]    
Cost $ 562,760 $ 529,376
Fair Value $ 542,283 $ 510,701
Secured first lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 74.70% 73.30%
Secured first lien debt | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 71.60% 72.50%
Secured second lien debt    
Summary of Investment Holdings [Line Items]    
Cost $ 117,904 $ 130,252
Fair Value $ 117,875 $ 127,854
Secured second lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 15.70% 18.10%
Secured second lien debt | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 15.50% 18.10%
Unsecured debt    
Summary of Investment Holdings [Line Items]    
Cost $ 198 $ 198
Fair Value $ 33 $ 24
Unsecured debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 0.00% 0.00%
Unsecured debt | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 0.00% 0.00%
Equity Securities    
Summary of Investment Holdings [Line Items]    
Cost $ 72,401 $ 62,443
Fair Value $ 97,554 $ 66,236
Equity Securities | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 9.60% 8.60%
Equity Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 12.90% 9.40%
Preferred Equity    
Summary of Investment Holdings [Line Items]    
Cost $ 44,867 $ 35,617
Fair Value $ 32,965 $ 26,855
Preferred Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 6.00% 4.90%
Preferred Equity | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 4.40% 3.80%
Common Equity/ Equivalents    
Summary of Investment Holdings [Line Items]    
Cost $ 27,534 $ 26,826
Fair Value $ 64,589 $ 39,381
Common Equity/ Equivalents | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 3.60% 3.70%
Common Equity/ Equivalents | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 8.50% 5.60%
[1] Cumulative gross unrealized depreciation for federal income tax purposes is $56.9 million; cumulative gross unrealized appreciation for federal income tax purposes is $33.7 million. Cumulative net unrealized depreciation is $23.2 million, based on a tax cost of $728.0 million.