Borrowings - 2018 Debt Securitization (Narrative) (Details) $ in Thousands		12 Months Ended
	Nov. 16, 2018 USD ($)	Sep. 30, 2023 USD ($) portfolioCompany		Sep. 30, 2022 USD ($) portfolioCompany		Sep. 30, 2021 USD ($)
Debt Instrument [Line Items]
Investments, at fair value		$ 5,516,613	[1]	$ 5,446,356	[2]
2018 Debt Securitization
Debt Instrument [Line Items]
Debt instrument, face amount	$ 602,400
Repayments of debt		$ 19,503		$ 0		$ 0
Number of investment portfolio company | portfolioCompany		65		74
Investments, at fair value		$ 555,699		$ 568,310
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 | AAA/AAA Class A 2018 Notes | London Interbank Offered Rate (LIBOR) 1 [Member]
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 | London Interbank Offered Rate (LIBOR) 1 [Member]
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 | London Interbank Offered Rate (LIBOR) 1 [Member]
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 | London Interbank Offered Rate (LIBOR) 1 [Member]
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 | London Interbank Offered Rate (LIBOR) 1 [Member]
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

[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.