Consolidated Statements of Changes in Net Assets - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)	170,028,584	167,259,511	132,658,200
Beginning balance	$ 2,582,692	$ 2,396,193	$ 2,222,854
Issuance of common stock, net of offering and underwriting costs			300,427
Net investment income after taxes	196,119	167,672	139,059
Net realized gain (loss) on investments and foreign currency transactions	20,376	7,905	(18,660)
Net change in unrealized appreciation (depreciation) on investments, foreign currency translation and forward currency contracts	(61,898)	165,246	(65,527)
Provision for taxes on realized gain on investments	(302)	0	0
Provision for taxes on unrealized appreciation on investments	(855)	(543)	0
Stock issued in connection with dividend reinvestment plan	13,174	41,071	20,230
Distributions from distributable earnings	(204,806)	(194,852)	(195,565)
Distributions from return of capital	0	0	(6,625)
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles	0	0
Total increase (decrease) common stock, outstanding value	$ (38,192)	$ 186,499	$ 173,339
Ending balance (in shares)	170,895,670	170,028,584	167,259,511
Ending balance	$ 2,544,500	$ 2,582,692	$ 2,396,193
Common Stock
Investment Company, Net Assets [Roll Forward]
Beginning balance (in shares)	170,028,584	167,259,511	132,658,200
Beginning balance	$ 170	$ 167	$ 133
Issuance of common stock, net of offering and underwriting costs (in shares)			33,451,902
Issuance of common stock, net of offering and underwriting costs			$ 33
Stock issued in connection with dividend reinvestment plan (in shares)	867,086	2,769,073	1,149,409
Stock issued in connection with dividend reinvestment plan	$ 1	$ 3	$ 1
Total increase (decrease) common stock, outstanding (in shares)	867,086	2,769,073	34,601,311
Total increase (decrease) common stock, outstanding value	$ 1	$ 3	$ 34
Ending balance (in shares)	170,895,670	170,028,584	167,259,511
Ending balance	$ 171	$ 170	$ 167
Paid in Capital in Excess of Par
Investment Company, Net Assets [Roll Forward]
Beginning balance	2,664,251	2,624,608	2,310,610
Issuance of common stock, net of offering and underwriting costs			300,394
Stock issued in connection with dividend reinvestment plan	13,173	41,068	20,229
Distributions from return of capital			(6,625)
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles	(750)	(1,425)
Total increase (decrease) common stock, outstanding value	12,423	39,643	313,998
Ending balance	2,676,674	2,664,251	2,624,608
Distributable Earnings (Losses)
Investment Company, Net Assets [Roll Forward]
Beginning balance	(81,729)	(228,582)	(87,889)
Net investment income after taxes	196,119	167,672	139,059
Net realized gain (loss) on investments and foreign currency transactions	20,376	7,905	(18,660)
Net change in unrealized appreciation (depreciation) on investments, foreign currency translation and forward currency contracts	(61,898)	165,246	(65,527)
Provision for taxes on realized gain on investments	(302)
Provision for taxes on unrealized appreciation on investments	(855)	(543)
Distributions from distributable earnings	(204,806)	(194,852)	(195,565)
Tax reclassification of stockholders' equity in accordance with generally accepted accounting principles	750	1,425
Total increase (decrease) common stock, outstanding value	(50,616)	146,853	(140,693)
Ending balance	$ (132,345)	$ (81,729)	$ (228,582)