Borrowings - GCIC 2018 Debt Securitization (Narrative) (Details) $ in Thousands		3 Months Ended			9 Months Ended
	Dec. 21, 2020 USD ($)	Jun. 30, 2024 USD ($) portfolioCompany agreement		Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) portfolioCompany agreement		Jun. 30, 2023 USD ($)	Sep. 30, 2023 USD ($) portfolioCompany		Sep. 16, 2019 USD ($)	Dec. 13, 2018 USD ($)
Debt Instrument [Line Items]
Investments, at fair value		$ 7,867,514	[1]		$ 7,867,514	[1]		$ 5,516,613	[2]
Debt instrument, interest rate during period		6.50%		5.10%	5.90%		4.80%
GCIC 2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount										$ 908,195
Repayments of debt		$ 87,299		$ 8,628	$ 133,510		$ 8,628
Number of loan sale agreements | agreement		2			2
Number of investment portfolio company | portfolioCompany		67			67			87
Investments, at fair value		$ 615,268			$ 615,268			$ 841,241
Debt instrument, interest rate during period					5.30%
GCIC 2018 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					0.26161%
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. Fair Value Measurements”.

[2] The fair values of investments were valued using significant unobservable inputs, unless noted otherwise. See “Note 6. Fair Value Measurements”.