Financial Highlights - Narrative (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	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	Feb. 06, 2019	Feb. 05, 2019
Investment Company, Financial Highlights [Roll Forward]
Net asset value at beginning of period (in dollars per share)	$ 15.19	$ 15.02	$ 14.89	$ 15.19	$ 14.33
Net increase (decrease) in net assets as a result of issuance of shares (in dollars per share)	0.00	0.51	0	0	0
Net increase in net assets as a result of repurchases of shares (in dollars per share)	0.01	0.00	0.01	0	0
Distributions declared:
Net investment income - after tax (in dollars per share)	1.49	1.90	1.70	1.15	0.99
Net realized gain (loss) on investment transactions (in dollars per share)	(0.27)	(0.40)	(0.26)	0.12	0.05
Net realized gain (loss) on extinguishment of debt (in dollars per share)	0.00	0	0	0	0
Net change in unrealized appreciation (depreciation) on investment transactions (in dollars per share)	0.20	0.05	0.08	(0.37)	0.98
Net asset value at end of period (in dollars per share)	14.97	15.19	15.02	14.89	15.19
Per share market value at end of period (in dollars per share)	$ 13.69	$ 15.11	$ 14.67	$ 12.39	$ 15.81
Total return based on market value	0.79%	15.77%	30.50%	(14.80%)	28.90%
Number of common shares outstanding (in shares) | shares	266,008,083	264,277,128	169,594,742	170,895,670	170,028,584
Ratio of net investment income - after tax to average net assets	9.87%	12.55%	11.46%	7.53%	6.73%
Ratio of total expenses to average net assets	11.77%	12.12%	12.45%	7.43%	5.78%
Ratio of incentive fee waiver to average net assets	0.00%	(0.90%)	0.00%	0.00%	0.00%
Ratio of incentive fees to average net assets	1.82%	2.53%	2.94%	0.68%	0.13%
Ratio of income and excise taxes to average net assets	(0.01%)	0.04%	0.15%	0.00%	0.00%
Ratio of net expenses (without incentive fees) to average net assets	9.95%	9.59%	9.51%	6.68%	5.49%
Total return based on average net asset value	9.36%	8.98%	10.25%	5.89%	13.70%
Net assets at end of period | $	$ 3,982,567	$ 4,014,529	$ 2,547,878	$ 2,544,500	$ 2,582,692
Average debt outstanding | $	$ 4,892,712	$ 3,492,575	$ 3,069,412	$ 2,935,846	$ 2,184,010
Average debt outstanding per share (in dollars per share)	$ 18.39	$ 13.22	$ 18.10	$ 17.18	$ 12.84
Portfolio Turnover	17.55%	20.29%	12.19%	23.56%	35.58%
Asset coverage ratio	180.21%	185.96%	180.68%	181.70%	200.04%
Minimum asset coverage ratio required						1.50	1.50
Ratio of management fee waiver to average net assets	0.00%	0.00%	0.00%	(0.07%)	(0.16%)
From net investment income - after tax
Distributions declared:
From net investment income - after tax (in dollars per share)	$ (1.36)	$ (1.89)	$ (1.23)	$ (1.20)	$ (1.13)
From capital gains
Distributions declared:
From net investment income - after tax (in dollars per share)	0	0	(0.17)	0	(0.03)
From return of capital
Distributions declared:
From net investment income - after tax (in dollars per share)	(0.29)	0	0	0	0
Dividend Reinvestment Plan (DRIP)
Distributions declared:
Net increase in net assets as a result of issuance of DRIP shares (in dollars per share)	$ 0.00	$ 0.00	$ 0	$ 0.00	$ 0.00