Borrowings - GBDC 3 2022-2 Debt Securitization (Narrative) (Details) $ in Thousands	Jun. 03, 2024 USD ($)	Sep. 30, 2025 USD ($)	[1]	Sep. 30, 2024 USD ($) portfolioCompany
Debt Instrument [Line Items]
Investments, at fair value		$ 8,769,389		$ 8,235,411	[2]
GBDC 3 2022-2 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 386,600
Number of investment portfolio company | portfolioCompany				73
Investments, at fair value				$ 377,471
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
Debt instrument, basis spread on variable rate	2.60%
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
Debt instrument, basis spread on variable rate	3.09%
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
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. 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.