Quarterly report pursuant to Section 13 or 15(d)

BORROWINGS - Revolving Line of Credit Narrative (Details)

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BORROWINGS - Revolving Line of Credit Narrative (Details) - Credit Facility - Line of Credit
9 Months Ended
Jun. 30, 2024
USD ($)
obligor
Jun. 27, 2024
USD ($)
Jun. 26, 2024
USD ($)
Mar. 28, 2024
USD ($)
Mar. 27, 2024
USD ($)
Dec. 12, 2023
USD ($)
Dec. 11, 2023
USD ($)
Sep. 30, 2023
USD ($)
Revolving Line of Credit                
Debt Instrument [Line Items]                
Commitment amount $ 268,659,000             $ 223,659,000
Line of credit facility, accordion feature maximum borrowing capacity $ 350,000,000              
Debt instrument, covenant, borrowing base, minimum number of obligors | obligor 25              
Covenant, required minimum net worth of mandatory redeemable term preferred stock $ 325,000,000.0              
Covenant, required minimum net worth of mandatory redeemable term preferred stock, plus percent of all equity and subordinated debt raised 50.00%              
Covenant, required minimum net worth of mandatory redeemable term preferred stock, minus percent of any equity or subordinated debt redeemed or retired 50.00%              
Covenant, required minimum net worth of mandatory redeemable term preferred stock, plus percent of all equity and subordinated debt raised, minus percent of any equity or subordinated debt redeemed or retired $ 412,000,000.0              
Debt instrument, covenant, senior security, indebtedness, required minimum asset coverage ratio 150.00%              
Senior security, indebtedness, asset coverage amount $ 691,800,000              
Indebtedness asset coverage on our senior securities 234.60%              
Investment company, number of obligor | obligor 34              
Revolving Line of Credit | SOFR                
Debt Instrument [Line Items]                
Debt instrument, basis spread on variable rate 0.10%              
Revolving Line of Credit | Secured Overnight Financing Rate (SOFR), Tranche One                
Debt Instrument [Line Items]                
Debt instrument, basis spread on variable rate 3.00%              
Revolving Line of Credit | Secured Overnight Financing Rate (SOFR), Thereafter                
Debt Instrument [Line Items]                
Debt instrument, basis spread on variable rate 3.50%              
Revolving Line of Credit - Amendment No. 5                
Debt Instrument [Line Items]                
Commitment amount           $ 233,700,000 $ 223,700,000  
Revolving Line of Credit - Amendment No. 6                
Debt Instrument [Line Items]                
Commitment amount       $ 243,700,000 $ 233,700,000      
Revolving Line of Credit - Amendment No. 7                
Debt Instrument [Line Items]                
Commitment amount   $ 268,700,000 $ 243,700,000