Borrowings - GBDC 3 2022 Debt Securitization (Narrative) (Details) $ in Thousands		4 Months Ended
	Jun. 03, 2024 USD ($)	Sep. 30, 2024 USD ($) portfolioCompany		Sep. 30, 2025 USD ($)	[2]
Debt Instrument [Line Items]
Investments, at fair value		$ 8,235,411	[1]	$ 8,769,389
GBDC 3 2022 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 401,750
Repayments of debt		$ 14,751
Number of investment portfolio company | portfolioCompany		55
Investments, at fair value		$ 377,755
GBDC 3 2022 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	149,750
GBDC 3 2022 Debt Securitization | Class A Senior Secured Floating Rate Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 252,000
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. 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.