Consolidated Statements of Changes in Net Assets - USD ($) $ in Thousands	2 Months Ended	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)		170,895,670	170,028,584	167,259,511
Beginning balance		$ 2,544,500	$ 2,582,692	$ 2,396,193
Repurchases of common stock, net of commission costs (in shares)	0	(1,300,928)
Repurchases of common stock, net of commission costs		$ (16,861)
Net increase (decrease) in net assets resulting from operations
Net investment income after taxes		289,088	196,119	167,672
Net realized gain (loss) on investment transactions		(44,113)	20,376	7,905
Net change in unrealized appreciation (depreciation) on investment transactions		13,555	(61,898)	165,246
(Provision) benefit for taxes on realized gain on investments		(207)	(302)	0
(Provision) benefit for taxes on unrealized appreciation on investments		308	(855)	(543)
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan			13,174	41,071
Distributions from distributable earnings		(238,392)	(204,806)	(194,852)
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles		0	0	0
Total increase (decrease) common stock, outstanding value		$ 3,378	$ (38,192)	$ 186,499
Ending balance (in shares)	169,594,742	169,594,742	170,895,670	170,028,584
Ending balance	$ 2,547,878	$ 2,547,878	$ 2,544,500	$ 2,582,692
Common Stock
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)		170,895,670	170,028,584	167,259,511
Beginning balance		$ 171	$ 170	$ 167
Repurchases of common stock, net of commission costs (in shares)		(1,300,928)
Repurchases of common stock, net of commission costs		$ (1)
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan (in shares)			867,086	2,769,073
Stock issued in connection with dividend reinvestment plan			$ 1	$ 3
Total increase (decrease) common stock, outstanding (in shares)		(1,300,928)	867,086	2,769,073
Total increase (decrease) common stock, outstanding value		$ (1)	$ 1	$ 3
Ending balance (in shares)	169,594,742	169,594,742	170,895,670	170,028,584
Ending balance	$ 170	$ 170	$ 171	$ 170
Paid in Capital in Excess of Par
Investment Company, Net Assets [Roll Forward]
Beginning balance		2,676,674	2,664,251	2,624,608
Repurchases of common stock, net of commission costs		(16,860)
Distributions to stockholders:
Stock issued in connection with dividend reinvestment plan			13,173	41,068
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles		(12,902)	(750)	(1,425)
Total increase (decrease) common stock, outstanding value		(29,762)	12,423	39,643
Ending balance	2,646,912	2,646,912	2,676,674	2,664,251
Distributable Earnings (Losses)
Investment Company, Net Assets [Roll Forward]
Beginning balance		(132,345)	(81,729)	(228,582)
Net increase (decrease) in net assets resulting from operations
Net investment income after taxes		289,088	196,119	167,672
Net realized gain (loss) on investment transactions		(44,113)	20,376	7,905
Net change in unrealized appreciation (depreciation) on investment transactions		13,555	(61,898)	165,246
(Provision) benefit for taxes on realized gain on investments		(207)	(302)
(Provision) benefit for taxes on unrealized appreciation on investments		308	(855)	(543)
Distributions to stockholders:
Distributions from distributable earnings		(238,392)	(204,806)	(194,852)
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles		12,902	750	1,425
Total increase (decrease) common stock, outstanding value		33,141	(50,616)	146,853
Ending balance	$ (99,204)	$ (99,204)	$ (132,345)	$ (81,729)