Federal Income Tax Matters - Reconciliation of Taxable Income Increase (Decrease) in Net Assets from Operation (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Schedule of Investments [Abstract]
Net increase (decrease) in net assets resulting from operations	$ 376,646	$ 273,785	$ 258,631
Net change in unrealized (appreciation) depreciation on investment transactions	(51,232)	29,371	(13,555)
Other income not currently taxable	(52,646)	(30,908)	(22,437)
Expenses not currently deductible	20,944	7,647	11,198
Other income for tax but not book	6,544	9,460	3,773
Other deductions/losses for tax not book	(30,184)	(17,624)	(39,639)
Other realized gain/loss differences	73,034	63,491	29,175
Taxable income before deductions for distributions	$ 343,106	$ 335,222	$ 227,146