Borrowings - 2022-2 GBDC 3 2022-2 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]
Debt instrument, interest rate during period		6.50%		5.10%	5.90%		4.80%
Investments, at fair value		$ 7,867,514	[1]		$ 7,867,514	[1]		$ 5,516,613
GBDC 3 2022-2 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 386,600
Number of investment portfolio company | portfolioCompany		72			72
Investments, at fair value		$ 372,528			$ 372,528
GBDC 3 2022-2 Debt Securitization | AAA Class A Senior Secured Floating Rate Notes Due 2034
Debt Instrument [Line Items]
Debt instrument, face amount	140,000
GBDC 3 2022-2 Debt Securitization | AA Class B Senior Secured Floating Rate Notes Due 2034
Debt Instrument [Line Items]
Debt instrument, face amount	38,800
GBDC 3 2022-2 Debt Securitization | AAA Class A Senior Secured Floating Rate Loans Due 2034
Debt Instrument [Line Items]
Debt instrument, face amount	$ 85,000
GBDC 3 2022-2 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					5.30%
GBDC 3 2022-2 Debt Securitization | Secured Overnight Financing Rate (SOFR) | AAA Class A Senior Secured Floating Rate Notes Due 2034
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.60%
GBDC 3 2022-2 Debt Securitization | Secured Overnight Financing Rate (SOFR) | AA Class B Senior Secured Floating Rate Notes Due 2034
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	3.09%
GBDC 3 2022-2 Debt Securitization | Secured Overnight Financing Rate (SOFR) | AAA Class A Senior Secured Floating Rate Loans Due 2034
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.60%
GBDC 3 2022-2 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	$ 122,800

[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”.