Borrowings - GBDC 3 2021 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 2021 Debt Securitization(1)
Debt Instrument [Line Items]
Debt instrument, face amount	$ 398,850
Number of investment portfolio company | portfolioCompany		70			70
Investments, at fair value		$ 381,641			$ 381,641
GBDC 3 2021 Debt Securitization(1) | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					5.30%
Credit spread adjustment		0.26161%			0.26161%
GBDC 3 2021 Debt Securitization(1) | AAA Class A 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 224,000
GBDC 3 2021 Debt Securitization(1) | AAA Class A 2021 Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.60%
GBDC 3 2021 Debt Securitization(1) | AA Class B 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 28,000
GBDC 3 2021 Debt Securitization(1) | AA Class B 2021 Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.85%
GBDC 3 2021 Debt Securitization(1) | A Class C-1 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 36,000
GBDC 3 2021 Debt Securitization(1) | A Class C-1 2021 Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.80%
GBDC 3 2021 Debt Securitization(1) | A Class C-2 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 10,000
Interest rate	3.91%
GBDC 3 2021 Debt Securitization(1) | BBB Class D 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 28
GBDC 3 2021 Debt Securitization(1) | Secured 2021 Notes | Secured Overnight Financing Rate (SOFR) | Maximum
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	5.00%

[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”.