v3.25.0.1
Borrowings - GBDC 3 DB Credit Facility (Details) - USD ($)
$ in Thousands
 3 Months Ended
Dec. 31, 2024
Sep. 30, 2024
Debt Instrument [Line Items]    
Long-term debt $ 4,890,564 $ 4,599,430
Revolving Credit Facility | GBDC 3 DB Credit Facility | Line of Credit    
Debt Instrument [Line Items]    
Line of credit facility, maximum borrowing capacity $ 625,000  
Line of credit facility, commitment fee percentage 0.06944%  
Line of credit facility, non-usage fee, percentage 0.25%  
Line of credit facility, prepayment fee, first year after effective date, percentage 0.50%  
Line of credit facility, prepayment fee, second year after effective date, percentage 0.25%  
Long-term debt   $ 411,489
Revolving Credit Facility | GBDC 3 DB Credit Facility | Line of Credit | Base Rate    
Debt Instrument [Line Items]    
Line of credit facility, interest rate during period 2.30%  
Line of credit facility, interest rate at period end 2.80%  
Revolving Credit Facility | GBDC 3 DB Credit Facility | Line of Credit | Canadian Overnight Repo Rate Average    
Debt Instrument [Line Items]    
Debt instrument, basis spread on variable rate 0.32138%