| SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Thousands | 12 Months Ended | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| Sep. 30, 2023  USD ($)  investment  loan | Sep. 30, 2022  USD ($)  investment  loan | Sep. 30, 2021  USD ($) | |||||||
| Summary of Investment Holdings [Line Items] | |||||||||
| Cost | $ 722,269 | [1] | $ 656,053 | [2] | |||||
| Fair Value | 704,815 | [1] | 649,615 | [2] | |||||
| Total interest income (excluding PIK interest income) | 79,480 | 49,759 | $ 47,414 | ||||||
| PIK interest income | $ 3,550 | 4,229 | 2,545 | ||||||
| Investment company, target percentage of taxable income to distribute to shareholders (up to) | 100.00% | ||||||||
| Edge Adhesives Holdings, Inc. | |||||||||
| Summary of Investment Holdings [Line Items] | |||||||||
| Cost | $ 6,100 | ||||||||
| Cost percentage | 0.90% | ||||||||
| Fair Value | $ 2,900 | ||||||||
| Fair value percentage | 0.50% | ||||||||
| OID | |||||||||
| Summary of Investment Holdings [Line Items] | |||||||||
| Fair Value | $ 700 | $ 900 | |||||||
| Number of loans | loan | 4 | 4 | |||||||
| Total interest income (excluding PIK interest income) | $ 200 | $ 500 | 300 | ||||||
| PIK | |||||||||
| Summary of Investment Holdings [Line Items] | |||||||||
| Number of investments | investment | 8 | 6 | |||||||
| Proceeds from interest received | $ 1,100 | $ 2,400 | $ 3,400 | ||||||
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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