Consolidated Statements of Changes in Net Assets (unaudited) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)				170,895,670
Beginning balance		$ 2,513,685	$ 2,607,479	$ 2,544,500	$ 2,582,692
Repurchases of common stock, net of commission costs		(9,647)		(9,647)
Net investment income		70,206	43,440	131,869	88,230
Net realized gain (loss) on investment transactions		(4,762)	372	(1,468)	14,920
Net change in unrealized appreciation (depreciation) on investment transactions		(6,986)	23,158	(46,608)	27,393
(Provision) benefit for taxes on realized gain on investments		0	0	(207)	0
(Provision) benefit for taxes on unrealized appreciation on investments		45	(97)	498	(592)
Stock issued in connection with dividend reinvestment plan			457		13,174
Distributions from distributable earnings		(56,396)	(51,260)	(112,792)	(102,268)
Total increase (decrease) common stock, outstanding value		$ (7,540)	$ 16,070	$ (38,355)	$ 40,857
Ending balance (in shares)	170,144,126	170,144,126	170,895,670	170,144,126	170,895,670
Ending balance	$ 2,506,145	$ 2,506,145	$ 2,623,549	$ 2,506,145	$ 2,623,549
Common Stock
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)		170,895,670	170,865,742	170,895,670	170,028,584
Beginning balance		$ 171	$ 171	$ 171	$ 170
Repurchases of common stock, net of commission costs (in shares)	(751,544)	(751,544)		(751,544)
Repurchases of common stock, net of commission costs	$ (9,647)	$ (1)		$ (1)
Stock issued in connection with dividend reinvestment plan (in shares)			29,928		867,086
Stock issued in connection with dividend reinvestment plan					$ 1
Total increase (decrease) common stock, outstanding (in shares)		(751,544)	29,928	(751,544)	867,086
Total increase (decrease) common stock, outstanding value		$ (1)	$ 0	$ (1)	$ 1
Ending balance (in shares)	170,144,126	170,144,126	170,895,670	170,144,126	170,895,670
Ending balance	$ 170	$ 170	$ 171	$ 170	$ 171
Paid in Capital in Excess of Par
Investment Company, Net Assets [Roll Forward]
Beginning balance		2,676,674	2,676,967	2,676,674	2,664,251
Repurchases of common stock, net of commission costs		(9,646)		(9,646)
Stock issued in connection with dividend reinvestment plan			457		13,173
Total increase (decrease) common stock, outstanding value		(9,646)	457	(9,646)	13,173
Ending balance	2,667,028	2,667,028	2,677,424	2,667,028	2,677,424
Distributable Earnings (Losses)
Investment Company, Net Assets [Roll Forward]
Beginning balance		(163,160)	(69,659)	(132,345)	(81,729)
Net investment income		70,206	43,440	131,869	88,230
Net realized gain (loss) on investment transactions		(4,762)	372	(1,468)	14,920
Net change in unrealized appreciation (depreciation) on investment transactions		(6,986)	23,158	(46,608)	27,393
(Provision) benefit for taxes on realized gain on investments				(207)
(Provision) benefit for taxes on unrealized appreciation on investments		45	(97)	498	(592)
Distributions from distributable earnings		(56,396)	(51,260)	(112,792)	(102,268)
Total increase (decrease) common stock, outstanding value		2,107	15,613	(28,708)	27,683
Ending balance	$ (161,053)	$ (161,053)	$ (54,046)	$ (161,053)	$ (54,046)