Financial Highlights - Schedule of Portfolio Composition (Details) $ / shares in Units, shares in Thousands, $ in Thousands		12 Months Ended
	Jan. 20, 2023	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Investment Company, Financial Highlights [Roll Forward]
Net asset value per share at beginning of period (in dollars per share)		$ 18.09	$ 19.63	$ 20.38	$ 21.84	$ 19.47
Net investment income (in dollars per share)		1.77	2.18	2.51	2.45	1.80
Net unrealized appreciation (depreciation) (in dollars per share)		(1.18)	0.25	(0.17)	(2.23)	2.19
Net realized gains (losses) (in dollars per share)		(0.20)	(1.70)	(0.46)	0.28	0.49
(Provision) benefit for taxes on realized and unrealized gains (losses) (in dollars Per share)		0	0	(0.02)	(0.01)	(0.02)
Distributions of net investment income to stockholders (dollars per share)		(1.64)	(2.27)	(2.61)	(1.95)	(1.52)
Issuance of common stock (in dollars per share)		0	0	0	0	(0.57)
Tax return of capital (in dollars per share)		(0.20)	0	0	0	0
Net asset value per share at end of period (in dollars per share)		16.64	18.09	19.63	20.38	21.84
Per share market value at beginning of period (in dollars per share)		16.31	20.12	18.00	21.18	14.52
Per share market value at end of period (in dollars per share)		$ 13.05	$ 16.31	$ 20.12	$ 18.00	$ 21.18
Total return		(9.32%)	(8.47%)	27.30%	(6.71%)	57.61%
Common shares outstanding at beginning of period (in shares) | shares		82,245	77,225	61,125	60,120	46,987
Common shares outstanding at end of period (in shares) | shares		88,086	82,245	77,225	61,125	60,120
Net assets at beginning of period | $		$ 1,487,811	$ 1,515,764	$ 1,245,563	$ 1,312,823	$ 914,879
Net assets at end of period | $		1,465,813	1,487,811	1,515,764	1,245,563	1,312,823
Average net assets | $		$ 1,486,192	$ 1,520,016	$ 1,437,728	$ 1,308,518	$ 1,150,662
Ratio of net investment income to average net assets		10.27%	11.52%	12.57%	11.36%	8.44%
Ratio of total expenses to average net assets		12.42%	14.23%	14.19%	8.68%	9.65%
Ratio of net expenses to average net assets		10.99%	13.59%	13.81%	8.45%	9.51%
Ratio of portfolio turnover to average investments at fair value		33.27%	35.96%	26.12%	26.99%	39.66%
Weighted average outstanding debt | $		$ 1,559,548	$ 1,683,757	$ 1,659,701	$ 1,361,151	$ 964,390
Average debt per share (dollars per share)		$ 18.12	$ 20.94	$ 23.01	$ 22.41	$ 17.85
Asset coverage ratio		197.50%	188.14%	187.74%	188.64%	201.68%
Outstanding senior securities | $		$ 1,495,000	$ 1,663,800	$ 1,660,000	$ 1,350,000	$ 1,280,000	$ 714,825	$ 476,075	$ 643,389	$ 680,734	$ 946,474	$ 1,660,000
Stock split, conversion ratio	0.3333