v3.23.3
Realized Gains or Losses and Net Unrealized Appreciation or Depreciation - Aggregate Net Realized Gains (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
SIO2 Medical Products Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (13,900)    
Convergeone Holdings Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (6,000)    
Foreign currency forward contracts      
Schedule of Investments [Line Items]      
Net realized gains (losses) (5,800) 13,700  
Aden & Anais Merger Sub Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (5,200)    
Radiology Partners Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (4,200)    
Carvana Co.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (2,800)    
Impel Neuropharma Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (2,300)    
ASP Unifrax Holdings Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (2,100)    
WP CPP Holdings LLC      
Schedule of Investments [Line Items]      
Net realized gains (losses) (1,300) (1,700)  
Global Medical Response Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) (1,000)    
Athenex Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses) 6,500    
Tersera Therapeutics LLC      
Schedule of Investments [Line Items]      
Net realized gains (losses) 5,200    
CorEvitas Group Holdings LP      
Schedule of Investments [Line Items]      
Net realized gains (losses) 4,000    
OmniSYS Acquisition Corporation      
Schedule of Investments [Line Items]      
Net realized gains (losses)   2,200  
First Star Speir Aviation Limited      
Schedule of Investments [Line Items]      
Net realized gains (losses)   1,900  
TigerConnect Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses)   1,800  
PLATO Learning Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses)     7,800
Keypath Education Holdings, LLC      
Schedule of Investments [Line Items]      
Net realized gains (losses)     6,800
L Squared Capital Partners LLC      
Schedule of Investments [Line Items]      
Net realized gains (losses)     3,400
LTI Holdings, Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses)     2,600
BX Commercial Mortgage Trust 2020-VIVA      
Schedule of Investments [Line Items]      
Net realized gains (losses)     2,600
California Pizza Kitchen Inc.      
Schedule of Investments [Line Items]      
Net realized gains (losses)     (1,800)
Refac Optical Group      
Schedule of Investments [Line Items]      
Net realized gains (losses)     (1,300)
Other, net      
Schedule of Investments [Line Items]      
Net realized gains (losses) $ (4,300) $ (700) $ 6,300