Significant Accounting Policies and Recent Accounting Updates - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
	May 28, 2021	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Aug. 03, 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest income, operating, accretion of discounts		$ 19,951	$ 24,679	$ 21,399
Amortization of deferred loan origination fees, net		17,692	27,023	34,215
Payment-in-kind interest income		40,590	23,354	13,931
Payment-in-kind interest capitalized		38,844	21,506	16,092
Investment company, premium prepayment fees		79	3,040	3,200
Interest and fee income		496,114	354,032	301,687
Dividend income, operating, paid in kind		14,901	0	0
Non-cash dividend income		14,901	0	0
Proceeds from dividends received		10	0	0
Fair value non-accrual loan		62,937	65,125
Amortization of debt premium		7,073	15,632	30,793
Excise tax expense (reduction), net		3,682	0	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		15,613	17,211
Amortization of deferred debt issuance costs		7,380	7,337	10,203
Non-Senior Loan Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Dividend income		1,340	684	1,713
Return of capital		$ 373	$ 1,146	$ 542