Borrowings - GCIC 2018 Debt Securitization (Narrative) (Details) $ in Thousands		12 Months Ended
	Dec. 21, 2020 USD ($)	Sep. 30, 2023 USD ($) portfolioCompany		Sep. 30, 2022 USD ($) portfolioCompany		Sep. 30, 2021 USD ($)	Sep. 16, 2019 USD ($)	Dec. 13, 2018 USD ($)
Debt Instrument [Line Items]
Investments, at fair value		$ 5,516,613	[1]	$ 5,446,356	[2]
2018 GCIC Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount							$ 908,195
Repayments of debt		$ 32,973		$ 0		$ 0
Number of investment portfolio company | portfolioCompany		87		91
Investments, at fair value		$ 841,241		$ 885,171
Debt instrument, interest rate during period		5.30%
2018 GCIC Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		0.26161%
2018 GCIC Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount								$ 179,695
2018 GCIC Debt Securitization | AAA/AAA Class A-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								490,000
2018 GCIC Debt Securitization | AAA Class A-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								38,500
2018 GCIC Debt Securitization | AA Class B-1 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, face amount								18,000
2018 GCIC Debt Securitization | Class B-2 GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								27,000
2018 GCIC Debt Securitization | Class C GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								95,000
2018 GCIC Debt Securitization | Class D GCIC 2018 Notes
Debt Instrument [Line Items]
Debt instrument, received and retained, amount								$ 60,000
2018 GCIC 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%
2018 GCIC 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.