| INVESTMENTS - Investments Classified by Contractual Maturity Date (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Sep. 30, 2022 | 
|---|---|---|
| Investments, Debt and Equity Securities [Abstract] | ||
| 2023 | $ 4,806 | |
| 2024 | 28,818 | |
| 2025 | 86,790 | |
| 2026 | 160,943 | |
| 2027 | 241,385 | |
| Thereafter | 151,330 | |
| Total contractual repayments | 674,072 | |
| Adjustments to cost basis of debt investments | (1,333) | |
| Investments in equity securities | 62,443 | |
| Investment owned, at cost | 735,182 | $ 656,053 | 
| Debt investments with maturity dates prior to period end | $ 200 | 
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- Definition Debt Investments With Maturity Dates Prior To Period End, Amount No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Adjustments to Cost Basis of Debt Investments No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, After Year Four No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Remainder of Fiscal Year No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Year Four No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Year One No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Year Three No definition available. 
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- Definition Principal Repayment And Maturity Of Investments, Year Two No definition available. 
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- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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- Definition Cost of the investment. Reference 1: http://www.xbrl.org/2003/role/exampleRef 
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- References No definition available. 
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