Portfolio Investments - Summarized Balance Sheet Information (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2024	Sep. 30, 2023
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value	$ 3,047,445	$ 2,892,420
Cash and cash equivalents	125,031	136,450
Restricted cash	12,461	9,089
Other assets	2,496	1,681
Total assets	3,297,939	3,217,839
Senior credit facility payable	730,000	710,000
Secured borrowings	9,380	18,106
Other liabilities	3,773	4,310
Total liabilities	1,773,840	1,702,075
Total liabilities and net assets	3,297,939	3,217,839
Investment owned, at cost	3,243,772	3,044,119
SLF JV I
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value	362,545	322,179
Cash and cash equivalents	29,039	31,950
Restricted cash	3,450	2,987
Other assets	3,688	18,988
Total assets	398,722	376,104
Senior credit facility payable	190,000	149,000
Secured borrowings	17,705	38,845
Notes payable at fair value	128,750	128,750
Other liabilities	28,354	26,630
Total liabilities	364,809	343,225
Members' equity	33,913	32,879
Total liabilities and net assets	398,722	376,104
Investment owned, at cost	370,558	331,808
Net proceeds	128,750	128,750
OCSI Glick JV LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value	141,454	123,343
Cash and cash equivalents	9,565	12,119
Restricted cash	1,406	184
Other assets	2,268	5,521
Total assets	154,693	141,167
Senior credit facility payable	74,000	53,000
Notes payable at fair value	58,589	57,201
Other liabilities	12,724	12,860
Total liabilities	154,693	141,167
Members' equity	0	0
Total liabilities and net assets	154,693	141,167
Investment owned, at cost	145,162	127,788
Net proceeds	$ 66,685	$ 66,685