Consolidated Statements of Changes in Net Assets (unaudited) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	Jun. 30, 2023	Apr. 30, 2023	Mar. 31, 2023	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)						170,895,670
Beginning balance		$ 2,506,145		$ 2,506,145	$ 2,623,549	$ 2,544,500	$ 2,582,692
Repurchases of common stock, net of commission costs (in shares)	(299,794)	(56,130)	(751,544)			(1,295,678)
Repurchases of common stock, net of commission costs	$ (3,975)	$ (731)	$ (9,647)	(7,144)		$ (16,791)
Net investment income				73,778	53,928	205,647	142,158
Net realized gain (loss) on investment transactions				(41,923)	2,511	(43,391)	17,431
Net change in unrealized appreciation (depreciation) on investment transactions				41,050	(40,555)	(5,558)	(13,162)
(Provision) benefit for taxes on realized gain on investments				0	0	(207)	0
(Provision) benefit for taxes on unrealized appreciation on investments				78	(478)	576	(1,070)
Stock issued in connection with dividend reinvestment plan							13,174
Distributions from distributable earnings				(56,066)	(51,269)	(168,858)	(153,537)
Total increase (decrease) common stock, outstanding value				$ 9,773	$ (35,863)	$ (28,582)	$ 4,994
Ending balance (in shares)	169,599,992			169,599,992	170,895,670	169,599,992	170,895,670
Ending balance	$ 2,515,918		$ 2,506,145	$ 2,515,918	$ 2,587,686	$ 2,515,918	$ 2,587,686
Common Stock
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)		170,144,126		170,144,126	170,895,670	170,895,670	170,028,584
Beginning balance		$ 170		$ 170	$ 171	$ 171	$ 170
Repurchases of common stock, net of commission costs (in shares)				(544,134)		(1,295,678)
Repurchases of common stock, net of commission costs						$ (1)
Stock issued in connection with dividend reinvestment plan (in shares)							867,086
Stock issued in connection with dividend reinvestment plan							$ 1
Total increase (decrease) common stock, outstanding (in shares)				(544,134)	0	(1,295,678)	867,086
Total increase (decrease) common stock, outstanding value				$ 0	$ 0	$ (1)	$ 1
Ending balance (in shares)	169,599,992		170,144,126	169,599,992	170,895,670	169,599,992	170,895,670
Ending balance	$ 170		$ 170	$ 170	$ 171	$ 170	$ 171
Paid in Capital in Excess of Par
Investment Company, Net Assets [Roll Forward]
Beginning balance		2,667,028		2,667,028	2,677,424	2,676,674	2,664,251
Repurchases of common stock, net of commission costs				(7,144)		(16,790)
Stock issued in connection with dividend reinvestment plan							13,173
Total increase (decrease) common stock, outstanding value				(7,144)	0	(16,790)	13,173
Ending balance	2,659,884		2,667,028	2,659,884	2,677,424	2,659,884	2,677,424
Distributable Earnings (Losses)
Investment Company, Net Assets [Roll Forward]
Beginning balance		$ (161,053)		(161,053)	(54,046)	(132,345)	(81,729)
Net investment income				73,778	53,928	205,647	142,158
Net realized gain (loss) on investment transactions				(41,923)	2,511	(43,391)	17,431
Net change in unrealized appreciation (depreciation) on investment transactions				41,050	(40,555)	(5,558)	(13,162)
(Provision) benefit for taxes on realized gain on investments						(207)
(Provision) benefit for taxes on unrealized appreciation on investments				78	(478)	576	(1,070)
Distributions from distributable earnings				(56,066)	(51,269)	(168,858)	(153,537)
Total increase (decrease) common stock, outstanding value				16,917	(35,863)	(11,791)	(8,180)
Ending balance	$ (144,136)		$ (161,053)	$ (144,136)	$ (89,909)	$ (144,136)	$ (89,909)