v3.25.2
Borrowings - GCIC 2018 Debt Securitization (Narrative) (Details)
$ in Thousands
 9 Months Ended
Dec. 21, 2020
USD ($)
Jun. 30, 2025
USD ($)
Sep. 30, 2024
USD ($)
portfolioCompany
Sep. 16, 2019
USD ($)
Dec. 13, 2018
USD ($)
Debt Instrument [Line Items]          
Investments, at fair value   $ 8,961,549 [1] $ 8,235,411 [2]    
GCIC 2018 Debt Securitization          
Debt Instrument [Line Items]          
Debt instrument, face amount       $ 908,195  
Debt instrument, basis spread on variable rate   0.26161%      
Number of investment portfolio company | portfolioCompany     58    
Investments, at fair value     $ 524,168    
GCIC 2018 Debt Securitization | Subordinated debt          
Debt Instrument [Line Items]          
Debt instrument, face amount         $ 179,695
GCIC 2018 Debt Securitization | AAA/AAA Class A-1 GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, face amount         490,000
GCIC 2018 Debt Securitization | AAA Class A-2 GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, face amount         38,500
GCIC 2018 Debt Securitization | AA Class B-1 GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, face amount         18,000
GCIC 2018 Debt Securitization | Class B-2 GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, received and retained, amount         27,000
GCIC 2018 Debt Securitization | Class C GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, received and retained, amount         95,000
GCIC 2018 Debt Securitization | Class D GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, received and retained, amount         $ 60,000
GCIC 2018 Debt Securitization | Class A-2 GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, repurchased face amount $ 38,500        
Debt instrument, basis spread on variable rate 4.665%        
GCIC 2018 Debt Securitization | Class A-2-R GCIC 2018 Notes          
Debt Instrument [Line Items]          
Debt instrument, issued, principal $ 38,500        
Debt instrument, basis spread on variable rate 2.498%        
[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6.
[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6.