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
 6 Months Ended 12 Months Ended
Mar. 31, 2024
Sep. 30, 2023
Summary of Investment Holdings [Line Items]    
Cost $ 790,542 $ 722,269 [1]
Fair Value 791,588 704,815 [1]
South    
Summary of Investment Holdings [Line Items]    
Fair Value 311,428 273,181
West    
Summary of Investment Holdings [Line Items]    
Fair Value 265,286 224,235
Midwest    
Summary of Investment Holdings [Line Items]    
Fair Value 153,901 145,122
Northeast    
Summary of Investment Holdings [Line Items]    
Fair Value 60,973 62,277
Diversified/Conglomerate Service    
Summary of Investment Holdings [Line Items]    
Fair Value 179,228 135,060
Diversified/Conglomerate Manufacturing    
Summary of Investment Holdings [Line Items]    
Fair Value 165,273 158,061
Healthcare, Education, and Childcare    
Summary of Investment Holdings [Line Items]    
Fair Value 160,081 146,438
Aerospace and Defense    
Summary of Investment Holdings [Line Items]    
Fair Value 120,437 97,836
Beverage, Food, and Tobacco    
Summary of Investment Holdings [Line Items]    
Fair Value 85,389 78,788
Automobile    
Summary of Investment Holdings [Line Items]    
Fair Value 27,595 27,571
Oil and Gas    
Summary of Investment Holdings [Line Items]    
Fair Value 22,343 27,830
Personal and Non-Durable Consumer Products    
Summary of Investment Holdings [Line Items]    
Fair Value 14,647 14,576
Machinery    
Summary of Investment Holdings [Line Items]    
Fair Value 6,473 6,411
Other    
Summary of Investment Holdings [Line Items]    
Fair Value $ 10,122 $ 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 22.60% 19.20%
Investment Owned, At Fair Value | Industry Concentration Risk | Diversified/Conglomerate Manufacturing    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 20.90% 22.40%
Investment Owned, At Fair Value | Industry Concentration Risk | Healthcare, Education, and Childcare    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 20.20% 20.80%
Investment Owned, At Fair Value | Industry Concentration Risk | Aerospace and Defense    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 15.20% 13.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Beverage, Food, and Tobacco    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 10.80% 11.20%
Investment Owned, At Fair Value | Industry Concentration Risk | Automobile    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 3.50% 3.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Oil and Gas    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 2.80% 3.90%
Investment Owned, At Fair Value | Industry Concentration Risk | Personal and Non-Durable Consumer Products    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 1.90% 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 39.30% 38.80%
Investment Owned, At Fair Value | Geographic Regions Risk | West    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 33.50% 31.80%
Investment Owned, At Fair Value | Geographic Regions Risk | Midwest    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 19.50% 20.60%
Investment Owned, At Fair Value | Geographic Regions Risk | Northeast    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 7.70% 8.80%
Debt Securities    
Summary of Investment Holdings [Line Items]    
Cost $ 718,850 $ 659,826
Fair Value $ 704,291 $ 638,579
Debt Securities | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 90.90% 91.40%
Debt Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 89.00% 90.60%
Secured first lien debt    
Summary of Investment Holdings [Line Items]    
Cost $ 577,545 $ 529,376
Fair Value $ 563,849 $ 510,701
Secured first lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 73.10% 73.30%
Secured first lien debt | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 71.20% 72.50%
Secured second lien debt    
Summary of Investment Holdings [Line Items]    
Cost $ 141,107 $ 130,252
Fair Value $ 140,409 $ 127,854
Secured second lien debt | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 17.80% 18.10%
Secured second lien debt | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 17.80% 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 $ 71,692 $ 62,443
Fair Value $ 87,297 $ 66,236
Equity Securities | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 9.10% 8.60%
Equity Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 11.00% 9.40%
Preferred Equity    
Summary of Investment Holdings [Line Items]    
Cost $ 44,867 $ 35,617
Fair Value $ 32,598 $ 26,855
Preferred Equity | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 5.70% 4.90%
Preferred Equity | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 4.10% 3.80%
Common Equity/ Equivalents    
Summary of Investment Holdings [Line Items]    
Cost $ 26,825 $ 26,826
Fair Value $ 54,699 $ 39,381
Common Equity/ Equivalents | Investment Owned, At Cost | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 3.40% 3.70%
Common Equity/ Equivalents | Investment Owned, At Fair Value | Investment Type Concentration Risk    
Summary of Investment Holdings [Line Items]    
Concentration risk, percentage 6.90% 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.