Borrowings - GBDC 3 2021 Debt Securitization (Narrative) (Details) $ in Thousands		3 Months Ended	12 Months Ended
	Jun. 03, 2024 USD ($)	Dec. 31, 2024 USD ($)	Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($) portfolioCompany		Sep. 30, 2023 USD ($)
Debt Instrument [Line Items]
Realized loss on extinguishment of debt			$ 48		$ 0		$ 0
Investments, at fair value			$ 8,769,389	[1]	$ 8,235,411	[2]
GBDC 3 2021 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 398,850
Debt instrument, basis spread on variable rate			0.26161%
Realized loss on extinguishment of debt		$ 48
Number of investment portfolio company | portfolioCompany					81
Investments, at fair value					$ 395,682
GBDC 3 2021 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	100,850
GBDC 3 2021 Debt Securitization | AAA Class A 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 224,000
Debt instrument, basis spread on variable rate	1.60%
GBDC 3 2021 Debt Securitization | AA Class B 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 28,000
Debt instrument, basis spread on variable rate	1.85%
GBDC 3 2021 Debt Securitization | A Class C-1 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 36,000
Debt instrument, basis spread on variable rate	2.80%
GBDC 3 2021 Debt Securitization | 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 | BBB Class D 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 28,000
GBDC 3 2021 Debt Securitization | Secured 2021 Notes | 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. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See Note 6. The fair value of loan investments may include the impact of the unfunded commitment being valued below par.