Borrowings - GBDC 3 2022 Debt Securitization (Narrative) (Details) $ in Thousands		3 Months Ended			9 Months Ended
	Jun. 03, 2024 USD ($)	Jun. 30, 2024 USD ($) portfolioCompany		Jun. 30, 2023	Jun. 30, 2024 USD ($) portfolioCompany		Jun. 30, 2023	Sep. 30, 2023 USD ($)	[2]
Debt Instrument [Line Items]
Investments, at fair value		$ 7,867,514	[1]		$ 7,867,514	[1]		$ 5,516,613
Debt instrument, interest rate during period		6.50%		5.10%	5.90%		4.80%
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 401,750
Repayments of debt		$ 413			$ 413
Number of investment portfolio company | portfolioCompany		56			56
Investments, at fair value		$ 383,300			$ 383,300
GBDC 3 2022 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	149,750
GBDC 3 2022 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					5.30%
GBDC 3 2022 Debt Securitization | Class A Senior Secured Floating Rate Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 252,000
GBDC 3 2022 Debt Securitization | Class A Senior Secured Floating Rate Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.00%

[1] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.