Significant Accounting Policies and Recent Accounting Updates - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	May 28, 2021	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022	Aug. 05, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, operating, accretion of discounts		$ 4,660	$ 4,822	$ 15,351	$ 16,605
Amortization of deferred loan origination fees, net		2,663	6,857	14,049	22,958
Payment-in-kind interest income		9,587	5,382	27,270	14,325
Payment-in-kind interest capitalized		10,405	6,095	27,660	14,692
Investment company, premium prepayment fees		0	721	79	1,516
Interest and fee income		139,293	90,876	380,046	252,899
Dividend income, operating, paid in kind		3,723	0	10,767	0
Non-cash preferred dividends		3,723	0	10,767	0
Proceeds from dividends received		0	0	0	0
Fair value non-accrual loan		80,362		80,362		$ 65,125
Amortization of debt premium		1,570	3,036	5,357	13,071
Excise tax expense (reduction), net		0	0	2,432	0
Stock repurchase program, authorized amount							$ 150,000
Offering costs, partnership interests	$ 250,000
Maximum percentage of gross sales, commission received, percentage	1.25%
Deferred debt issuance cost		17,217		17,217		$ 17,211
Amortization of deferred debt issuance costs		1,937	1,798	5,362	5,241
Non-Senior Loan Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend income		334	52	1,329	372
Return of Capital		$ 212	$ 420	$ 295	$ 1,146