Significant Accounting Policies and Recent Accounting Updates - Narrative (Details) $ in Thousands		12 Months Ended			24 Months Ended
	May 16, 2025 USD ($)	Sep. 30, 2025 USD ($) security segment	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($) shares	Sep. 30, 2025 USD ($) security shares	Aug. 01, 2025 USD ($)	Aug. 02, 2024 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Amortization of deferred loan origination fees, net		$ 24,982	$ 19,817	$ 17,692
Interest income, operating, accretion of discounts		26,329	22,074	19,951
Payment-in-kind interest income		52,323	49,715	40,590
Payment-in-kind interest capitalized		55,390	52,552	38,844
Investment company, premium prepayment fees		2,418	606	79
Interest and fee income		782,906	626,622	496,114
Dividend income, operating, paid in kind		30,707	21,263	14,901
Non-cash dividend income		30,707	21,263	14,901
Proceeds from dividends received		6,436	47	10
Dividend income		1,940	691	1,340
Return of capital		2,322	1,622	373
Fair value non-accrual loan		$ 27,321	93,204		$ 27,321
Number of preferred equity securities on non-accrual status | security		1			1
Preferred equity securities aggregate amount in nonaccrual status		$ 0	0		$ 0
Amortization of debt premium		17,423	11,671	7,073
Excise tax		(475)	1,124	3,682
Income tax		0	71	0
Stock repurchase program, authorized amount						$ 150,000	$ 150,000
Offering costs, partnership interests	$ 288,043
Maximum percentage of gross sales, commission received, percentage	1.50%
Deferred debt issuance cost		26,005	25,361		26,005
Amortization of deferred debt issuance costs		$ 9,995	$ 9,237	$ 7,380
Number of operating segment | segment		1
Number of reporting segment | segment		1
Equity Distribution Agreement Plan 2023
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Consideration received					$ 38,043
Number of shares issued under Equity Distribution Agreements (in shares) | shares			0	0	2,408,940