v3.23.3
Realized Gains or Losses and Net Unrealized Appreciation or Depreciation - Narrative (Details) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2021
Schedule of Investments [Line Items]      
Net realized gains (losses) $ (33,155) $ 17,179 $ 26,420
Net unrealized appreciation (depreciation) (28,555) (136,248) 114,519
OSI2 Merger      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) (20,700)    
OCSI Merger      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) 34,100    
Investments in debt securities      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) (49,100) (94,100) 70,000
Investments in equity securities      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) (4,900) (35,400) 36,300
Exited Investments      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) 25,400 (11,700) 6,600
Foreign currency forward contract      
Schedule of Investments [Line Items]      
Net unrealized appreciation (depreciation) $ 100 $ 4,900 $ 1,700