Borrowings - 2018 Debt Securitization (Narrative) (Details) $ in Thousands			3 Months Ended			9 Months Ended
	Jun. 03, 2024 USD ($)	Nov. 16, 2018 USD ($)	Jun. 30, 2024 USD ($) portfolioCompany		Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) portfolioCompany		Jun. 30, 2023 USD ($)	Sep. 30, 2023 USD ($) portfolioCompany
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%
2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount		$ 602,400
Repayments of debt			$ 38,989		$ 4,820	$ 80,703		$ 4,820
Number of investment portfolio company | portfolioCompany			52			52			65
Investments, at fair value			$ 425,279			$ 425,279			$ 555,699
Debt instrument, interest rate during period						5.30%
2018 Debt Securitization | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount		$ 113,400
2018 Debt Securitization | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						0.26161%
2018 Debt Securitization | AAA/AAA Class A 2018 Notes | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		1.48%
2018 Debt Securitization | AAA/AAA Class A 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction		$ 327,000
2018 Debt Securitization | AA Class B 2018 Notes | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.10%
2018 Debt Securitization | AA Class B 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction		$ 61,200
2018 Debt Securitization | A Class C-1 2018 Notes | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.80%
2018 Debt Securitization | A Class C-1 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction		$ 20,000
2018 Debt Securitization | A Class C-2 2018 Notes | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.65%
2018 Debt Securitization | A Class C-2 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction		$ 38,800
2018 Debt Securitization | BBB- Class D 2018 Notes | LIBOR
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate		2.95%
2018 Debt Securitization | BBB- Class D 2018 Notes | Private Placement
Debt Instrument [Line Items]
Sale of stock, consideration received on transaction		$ 42,000
GBDC 3 2021 Debt Securitization(1)
Debt Instrument [Line Items]
Debt instrument, face amount	$ 398,850
Number of investment portfolio company | portfolioCompany			70			70
Investments, at fair value			$ 381,641			$ 381,641
GBDC 3 2021 Debt Securitization(1) | Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount	100,850
GBDC 3 2021 Debt Securitization(1) | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate						5.30%
GBDC 3 2021 Debt Securitization(1) | AA Class B 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 28,000
GBDC 3 2021 Debt Securitization(1) | AA Class B 2021 Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	1.85%
GBDC 3 2021 Debt Securitization(1) | A Class C-1 2021 Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 36,000
GBDC 3 2021 Debt Securitization(1) | A Class C-1 2021 Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate	2.80%

[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”.