Borrowings - GCIC 2018 Debt Securitization (Narrative) (Details) $ in Thousands		12 Months Ended
	Dec. 21, 2020 USD ($)	Sep. 30, 2025 USD ($)		Sep. 30, 2024 USD ($) portfolioCompany		Sep. 30, 2023 USD ($)	Sep. 16, 2019 USD ($)	Dec. 13, 2018 USD ($)
Debt Instrument [Line Items]
Investments, at fair value		$ 8,769,389	[1]	$ 8,235,411	[2]
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount							$ 908,195
Debt instrument, basis spread on variable rate		0.26161%
Repayments of debt				$ 260,755		$ 32,973
Number of investment portfolio company | portfolioCompany				58
Investments, at fair value				$ 524,168
GCIC 2018 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount								$ 179,695
GCIC 2018 Debt Securitization | AAA/AAA Class A-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								490,000
GCIC 2018 Debt Securitization | AAA Class A-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								38,500
GCIC 2018 Debt Securitization | AA Class B-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								18,000
GCIC 2018 Debt Securitization | Class B-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								27,000
GCIC 2018 Debt Securitization | Class C GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								95,000
GCIC 2018 Debt Securitization | Class D GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								$ 60,000
GCIC 2018 Debt Securitization | Class A-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, repurchased face amount	$ 38,500
Debt instrument, basis spread on variable rate	4.665%
GCIC 2018 Debt Securitization | Class A-2-R GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, issued, principal	$ 38,500
Debt instrument, basis spread on variable rate	2.498%

[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.