Consolidated Statements of Changes in Net Assets shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2023 USD ($) $ / shares shares
Operations:
Net investment income	$ 152,640	$ 175,052	$ 180,697
Net unrealized appreciation (depreciation)	(101,229)	19,101	(28,555)
Net realized gains (losses)	(17,097)	(136,356)	(33,155)
(Provision) benefit for taxes on realized and unrealized gains (losses)	(394)	108	(1,656)
Net increase (decrease) in net assets resulting from operations	33,920	57,905	117,331
Stockholder transactions:
Distributions to stockholders	(141,603)	(184,027)	(185,900)
Return of capital	(17,262)	0	0
Net increase (decrease) in net assets from stockholder transactions	(158,865)	(184,027)	(185,900)
Capital share transactions:
Issuance of common stock in connection with the OSI2 Merger	0	0	334,034
Issuance of common stock under dividend reinvestment plan	10,666	7,213	5,854
Repurchase of common stock under dividend reinvestment plan	(10,666)	(1,551)	(2,418)
Issuance of common stock in private placement	100,000	0	0
Issuance of common stock in connection with the "at the market" offering	2,947	92,507	1,300
Net increase (decrease) in net assets from capital share transactions	102,947	98,169	338,770
Total increase (decrease) in net assets	(21,998)	(27,953)	270,201
Net assets at beginning of period	1,487,811	1,515,764	1,245,563
Net assets at end of period	$ 1,465,813	$ 1,487,811	$ 1,515,764
Net asset value per common share (in dollars per share) | $ / shares	$ 16.64	$ 18.09	$ 19.63
Common shares outstanding at end of period (in shares) | shares	88,086	82,245	77,225